Metals

22 pages, 40292 KB

Open AccessEditor’s ChoiceArticle

Maritime Applications as Motivation for Analytical Calculation of Thermal History in Low-Carbon Mild Steel WAAM Cylinders

by Eleftherios Lampros and Anna D. Zervaki

Metals 2026, 16(2), 192; https://doi.org/10.3390/met16020192 - 5 Feb 2026

Cited by 2 | Viewed by 828

Abstract

This study reviews the application of wire arc additive manufacturing (WAAM) technology in maritime engineering and investigates an experimentally driven analytical approach for prediction of thermal distributions based on the Rosenthal solution. Two ER70S-6 low-carbon steel WAAM cylinders were fabricated using gas metal [...] Read more.

This study reviews the application of wire arc additive manufacturing (WAAM) technology in maritime engineering and investigates an experimentally driven analytical approach for prediction of thermal distributions based on the Rosenthal solution. Two ER70S-6 low-carbon steel WAAM cylinders were fabricated using gas metal arc welding (GMAW) and plasma arc welding (PAW) processes, with interlayer temperatures of 453 °C and 250 °C, respectively. Accurately measuring the temperature field to tailor the microstructure has long been a challenge. The results indicated a significant deviation between the analytical predictions and the experimental data. To address this discrepancy, a hybrid approach combining analytical and experimental results was implemented. Time intervals between layers, extracted from the experimental data, were incorporated into the Rosenthal equation to improve the accuracy of temperature field predictions. The microstructure at the bottom, middle, and top regions of the WAAM components was examined using optical microscopy. Tensile testing and Vickers microhardness measurements were conducted to evaluate mechanical properties. Scanning electron microscopy (SEM) was used to analyze fracture surfaces and identify fracture modes. The results were consistent with those reported for other ER70S-6 cylindrical WAAM components. This work highlights limitations of the Rosenthal solution and emphasizes the need for thermal models in WAAM applications. Full article

(This article belongs to the Special Issue Advanced Additive Manufacturing of Metallic Materials)

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13 pages, 4564 KB

Open AccessEditor’s ChoiceArticle

Microstructure and Mechanical Properties of Ultrafine-Grained CrMnFeCoNi High-Entropy Alloy Prepared via Powder Metallurgy

by Sunghyuk Jang, Seonghyun Park and Jae-Gil Jung

Metals 2026, 16(2), 170; https://doi.org/10.3390/met16020170 - 1 Feb 2026

Viewed by 826

Abstract

We studied the microstructural evolution and mechanical properties of ultrafine-grained CrMnFeCoNi high-entropy alloys fabricated by mechanical alloying of various additives and spark plasma sintering. The additives were 1 wt.% process control agent (stearic acid) + 1 wt.% graphene nanofiber (GNF) (PG) or 1 [...] Read more.

We studied the microstructural evolution and mechanical properties of ultrafine-grained CrMnFeCoNi high-entropy alloys fabricated by mechanical alloying of various additives and spark plasma sintering. The additives were 1 wt.% process control agent (stearic acid) + 1 wt.% graphene nanofiber (GNF) (PG) or 1 wt.% Y₂O₃ + 1 wt.% GNF (YG) to modify the constituting phase of the sintered alloy. The PG and YG powders exhibited a single FCC phase. The YG powders had a larger powder size and a smaller crystallite size than the PG powders. Ultrafine-grained FCC matrices with average particle sizes of 0.57 μm and 0.71 μm, respectively, were formed through the SPS process of PG and YG powders. The absence of PCA in YG alloys resulted in a bimodal distribution of fine and coarse grains (due to incomplete mechanical alloying) and formation of a lesser and finer Cr₇C₃ phase (due to reduced C content). The sintered PG alloy contained coarse (~60 nm) spinel Mn₃O₄ oxides along grain boundaries, whereas the YG alloy exhibited coarse Mn₃O₄ and fine (~17 nm) Y₂O₃ oxide particles along grain boundaries. Additionally, the YG alloy contained tiny (~5 nm) Y₂O₃ oxide particles with a cube-on-cube orientation relationship within the FCC matrix. YG alloy exhibited higher hardness and compressive yield strength than PG alloy, mainly due to the oxide dispersion strengthening of finely dispersed Y₂O₃ particles. The addition of Y₂O₃ reinforcing particles had a minimal effect on the ultimate compressive strength and fracture strain of the sintered alloy. Full article

(This article belongs to the Special Issue Feature Papers in Entropic Alloys and Meta-Metals)

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34 pages, 10560 KB

Open AccessEditor’s ChoiceReview

Large Language Models for High-Entropy Alloys: Literature Mining, Design Orchestration, and Evaluation Standards

by Yutong Guo and Chao Yang

Metals 2026, 16(2), 162; https://doi.org/10.3390/met16020162 - 29 Jan 2026

Viewed by 1365

Abstract

High-entropy alloys (HEAs) present a fundamental design paradox: their exceptional properties arise from complex, high-dimensional composition–process–microstructure–property (CPMP) relationships, yet the knowledge needed to navigate this space is fragmented across a vast and unstructured literature. Large language models (LLMs) offer a transformative interface to [...] Read more.

High-entropy alloys (HEAs) present a fundamental design paradox: their exceptional properties arise from complex, high-dimensional composition–process–microstructure–property (CPMP) relationships, yet the knowledge needed to navigate this space is fragmented across a vast and unstructured literature. Large language models (LLMs) offer a transformative interface to this complexity. By extracting structured facts from text, they can convert dispersed and heterogeneous evidence (i.e., findings scattered across many studies and reported with inconsistent test protocols or characterization standards) into queryable knowledge graphs. Through code generation and tool composition, they can automate simulation pipelines, surrogate model construction, and inverse design workflows. This review analyzes how LLMs can augment key stages of HEA research—from intelligent literature mining and multimodal data integration (using LLMs to automatically extract and structure data from texts and to combine information across text, images, and other data sources) to model-driven design and closed-loop experimentation—illustrated by emerging case studies. We propose concrete evaluation protocols that measure direct scientific utility, including knowledge-graph completeness, workflow setup efficiency, and experimental validation hit rates. We also confront practical limitations: data sparsity and noise, model hallucination, domain bias (where models may exhibit superior predictive performance for specific, well-represented alloy systems over others due to imbalances in training data), and the imperative for reproducible infrastructure. We argue that domain-specialized LLMs, embedded within grounded, verifiable research systems, can not only accelerate HEA discovery but also standardize the representation, sharing, and reuse of community knowledge. Full article

(This article belongs to the Special Issue Advances in High-Entropy Alloys’ Microstructure, Properties and Preparation)

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20 pages, 7671 KB

Open AccessEditor’s ChoiceArticle

Impact of Aggressive Environments and Processing Orientation on the Mechanical Performance of L-PBF 316L Stainless Steel

by Najib Abu-warda, Javier Bedmar, Sonia García-Rodríguez, Belén Torres and Joaquin Rams

Metals 2026, 16(1), 86; https://doi.org/10.3390/met16010086 - 13 Jan 2026

Cited by 1 | Viewed by 517

Abstract

This study addresses the limited understanding of how build orientation and aggressive environments jointly affect the mechanical reliability of L-PBF 316L stainless steel. Specimens were fabricated in vertical, edge, and flat orientations and exposed for 360 h to 1 M H₂SO [...] Read more.

This study addresses the limited understanding of how build orientation and aggressive environments jointly affect the mechanical reliability of L-PBF 316L stainless steel. Specimens were fabricated in vertical, edge, and flat orientations and exposed for 360 h to 1 M H₂SO₄, 3.5 wt.% NaCl, and dry air oxidation at 800 °C. Tensile tests and microstructural analyses revealed strong anisotropy: edge and flat builds showed higher tensile and yield strength, while vertical builds exhibited greater ductility. Aqueous environments caused surface degradation and moderate strength loss, most severe in vertical samples. High-temperature oxidation induced σ-phase precipitation, increasing tensile strength (~20%) but reducing ductility and yield strength. These findings highlight the critical role of building orientation and service conditions in ensuring long-term performance of L-PBF 316L stainless steel. Full article

(This article belongs to the Special Issue Recent Advances in Powder-Based Additive Manufacturing of Metals)

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20 pages, 6158 KB

Open AccessEditor’s ChoiceArticle

Improving Surface Roughness and Printability of LPBF Ti6246 Components Without Affecting Their Structure, Mechanical Properties and Building Rate

by Thibault Mouret, Aurore Leclercq, Patrick K. Dubois and Vladimir Brailovski

Metals 2026, 16(1), 32; https://doi.org/10.3390/met16010032 - 27 Dec 2025

Cited by 1 | Viewed by 839

Abstract

Laser powder bed fusion (LPBF) is the best suited technology to manufacture temperature-resistant Ti-6Al-2Sn-4Zr-6Mo parts with complex geometrical features for high-end applications. Improving printing accuracy by reducing the layer thickness (t) generally requires repeating a tedious and time-consuming process optimization routine. [...] Read more.

Laser powder bed fusion (LPBF) is the best suited technology to manufacture temperature-resistant Ti-6Al-2Sn-4Zr-6Mo parts with complex geometrical features for high-end applications. Improving printing accuracy by reducing the layer thickness (t) generally requires repeating a tedious and time-consuming process optimization routine. To simplify this endeavour, the present work proposes three process equivalence criteria allowing to transfer optimized process conditions from one printing parameter set to another. This approach recommends keeping the volumetric laser energy density (VED) and hatching space-to-layer thickness ratio (h/t) constant, while adjusting the scanning speed (v) and hatching space (h) accordingly. To validate this approach, Ti6246 parts were printed with 50 µm and 25 µm layer thicknesses, while keeping VED = 100 J/mm³ and h/t = 3 constant for both cases. The printed samples were analyzed in terms of their density, microstructure and mechanical properties, as well as the geometric compliance of wall-, gap- and channel-containing artefacts. Highly dense samples exhibiting comparable microstructures and mechanical properties were obtained with both parameters sets investigated. However, they induced markedly differing geometric characteristics. Notably, using 25 µm layers allowed printing walls as thin as 0.2 mm as compared to 1.0 mm for 50 µm layers. Full article

(This article belongs to the Special Issue Recent Advances in Powder-Based Additive Manufacturing of Metals)

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20 pages, 6996 KB

Open AccessEditor’s ChoiceArticle

Influence of Surface Finishing on the Corrosion and Wear Behaviour of AISI 304 and AISI 436 Stainless Steels

by Silvia Gómez, Ismael Lamas, Alejandro Pereira and M. Consuelo Pérez

Metals 2025, 15(12), 1390; https://doi.org/10.3390/met15121390 - 18 Dec 2025

Viewed by 717

Abstract

The pitting corrosion resistance and the tribological behaviour of a ferritic stainless steel with high Mo content (AISI 436) and a commonly employed austenitic stainless steel (AISI 304) are compared. Special attention was paid to the role of Mo in improving corrosion resistance [...] Read more.

The pitting corrosion resistance and the tribological behaviour of a ferritic stainless steel with high Mo content (AISI 436) and a commonly employed austenitic stainless steel (AISI 304) are compared. Special attention was paid to the role of Mo in improving corrosion resistance of ferritic stainless steels. Since the surface condition is an important parameter related to the onset of pitting corrosion in the presence of chlorides, three different surface finishes were tested for both steels. Two commercial finishing grades and laboratory polishing down to 1 µm were compared. Moreover, the influence of surface condition on the tribological properties for both steels was also evaluated. The study demonstrates that surface finishing plays a decisive role in both the electrochemical and mechanical response of stainless steels. A comprehensive microstructural and tribological analysis reveals not only how commercial finishing treatments modify passive film behaviour, but also how they affect friction stability and wear mechanisms. Special emphasis is placed on the synergistic effect between molybdenum content, passive film integrity and manufacturing processes. The obtained results provide valuable insight for industrial applications where durability against chloride exposure and abrasion is critical. Full article

(This article belongs to the Special Issue Corrosion Behavior of Carbon Steels in Natural and Industrial Environments—3rd Edition)

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29 pages, 12009 KB

Open AccessFeature PaperEditor’s ChoiceReview

Anode Protection Strategies for Next-Generation Lithium–Oxygen Batteries: Toward Dendrite-Free Lithium Metal at Practical Current Densities

by Myeong-Chang Sung, Minhe Kim, Jiyoon Yu and Changhoon Choi

Metals 2025, 15(12), 1373; https://doi.org/10.3390/met15121373 - 15 Dec 2025

Cited by 1 | Viewed by 1265

Abstract

The promise of lithium–oxygen batteries lie not merely in their record-breaking theoretical energy density, but in the challenge of making such energy truly reversible. Rising as the key obstacle is the lithium metal anode, whose remarkable capacity and low potential come at the [...] Read more.

The promise of lithium–oxygen batteries lie not merely in their record-breaking theoretical energy density, but in the challenge of making such energy truly reversible. Rising as the key obstacle is the lithium metal anode, whose remarkable capacity and low potential come at the cost of dendritic growth, unstable solid electrolyte interphases, and relentless reactions with oxygen species. These instabilities, once overshadowed by cathode-related limitations, now define the frontier of research as current densities and energy demands approach practical levels. This review highlights recent progress in two complementary directions for anode protection: physical approaches, such as artificial protective layers, solid or functional separators, and oxygen-blocking interlayers that isolate and stabilize the surface; and chemical strategies, including electrolyte and additive design that enable in situ formation of LiF- and Li₃N-rich interfaces with high ionic conductivity and chemical robustness. Together, these approaches establish a unified framework for achieving dendrite-free and oxygen-resistant lithium interfaces. Mastering solid electrolyte interfacial stability rather than only cathode catalysis will ultimately determine whether lithium oxygen battery can evolve from laboratory prototypes to truly viable high-energy systems. Full article

(This article belongs to the Special Issue Surface Treatments and Coating of Metallic Materials)

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15 pages, 6689 KB

Open AccessEditor’s ChoiceArticle

Study of Selective Recovery of Lead- and Zinc-Based Products from Leachate After Alkaline Leaching of Copper Shaft Furnace Dust

by Michaela Ružičková, Martina Laubertová and Michal Marcin

Metals 2025, 15(12), 1362; https://doi.org/10.3390/met15121362 - 11 Dec 2025

Viewed by 726

Abstract

A leachate from alkaline leaching of copper shaft furnace (CSF) dust as a hazardous waste was used in this study for performing a chemical precipitation experiment of lead, zinc, and copper. The precipitation processes for lead, zinc, and copper were theoretically optimized based [...] Read more.

A leachate from alkaline leaching of copper shaft furnace (CSF) dust as a hazardous waste was used in this study for performing a chemical precipitation experiment of lead, zinc, and copper. The precipitation processes for lead, zinc, and copper were theoretically optimized based on a thermodynamic study. To determine suitable operating conditions, metal phase stability, reaction mechanisms, and precipitation order were analyzed using the Hydra/Medusa and HSC Chemistry v.10 software packages. In the first experimental stage, treatment of the alkaline leachate resulted in the formation of insoluble lead sulfate (PbSO₄), while zinc remained dissolved for subsequent recovery. In the second stage, the zinc-bearing solution was treated with Na₂CO₃, producing a mixed zinc precipitate consisting of Zn₅(OH)₆(CO₃)_2(s). This study determined that the optimal conditions for chemically precipitating lead as PbSO₄ from alkaline leachate (pH 13.5) are the use of 1 mol/L H₂SO₄ at pH 3.09 and Eh 0.22 V at 25 °C, while optimal zinc precipitation from this solution (pH 3.02) is achieved with 2 mol/L Na₂CO₃ at pH 9.39 and Eh –0.14 V at 25 °C. A small amount of copper present in the solution co-precipitated and was identified as an impurity in the zinc product. The chemical composition of the resulting precipitates was confirmed by SEM–EDX analysis. Full article

(This article belongs to the Special Issue Studies on Metal Leaching, Extraction and Recovery)

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18 pages, 9799 KB

Open AccessEditor’s ChoiceArticle

Influence of Experimental Parameters on the Determination of Copper Dissolution in Corrosion Processes Using Gold Microelectrodes

by Javier Izquierdo, Adrián Méndez-Guerra, Raquel Rodríguez-Raposo and Ricardo M. Souto

Metals 2025, 15(12), 1278; https://doi.org/10.3390/met15121278 - 21 Nov 2025

Viewed by 659

Abstract

In situ electrochemical imaging of corrosion reactions is performed directly by scanning electrochemical microscopy (SECM) in generation-collection mode. This method involves redox conversion of soluble metal ions at the amperometric tip for quantification. Unfortunately, many metals, such as copper, do not undergo redox [...] Read more.

In situ electrochemical imaging of corrosion reactions is performed directly by scanning electrochemical microscopy (SECM) in generation-collection mode. This method involves redox conversion of soluble metal ions at the amperometric tip for quantification. Unfortunately, many metals, such as copper, do not undergo redox conversion to a soluble state and are deposited on the SECM tip. They therefore modify the electrochemical behavior of the tip and require consideration of metal stripping processes. In addition, the miniaturization of the electrode to operate as a microelectrode tip can be accompanied by variations in the potential range and distribution of the redox processes related to copper deposition and redissolution, thus complicating the adequate choice of electrochemical conditions applied to the tip for the unambiguous operation of SECM in the generation-collection mode to study the corrosion of copper-based materials. Therefore, in this work, a study of different parameters for the amperometric determination of Cu²⁺ ions was conducted using gold disk electrodes of 500 and 10 μm diameter to represent the typical sizes employed in conventional and scanning microelectrochemical measurements. The investigation was performed to analyze the effect of underpotential deposition (UPD) and overpotential deposition (ODP) on the voltammetric characteristics of copper deposition and redissolution resulting from variations in the solution composition, i.e., the nature of anions and pH. The dependence and limits of the reduction and reoxidation waves were analyzed as functions of the Cu²⁺ ion concentration, the ionic strength of the electrolyte, and the pH of the solution. The results were interpreted as UPD and OPD. Under conditions close to the marine environment, the release of Cu²⁺ ions can be unambiguously detected and quantified from potentials above −0.1 V vs. Ag/AgCl. Full article

(This article belongs to the Section Corrosion and Protection)

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14 pages, 2656 KB

Open AccessEditor’s ChoiceArticle

Evaluation of Filter Cake Washing Processes in Hydrometallurgical Battery Recycling of Lithium-Ion Batteries to Optimize Recoveries

by Dominic Dittmer, Maya Andary, Fabian Diaz and Bernd Friedrich

Metals 2025, 15(11), 1262; https://doi.org/10.3390/met15111262 - 19 Nov 2025

Cited by 1 | Viewed by 1484

Abstract

Due to climate change, electromobility and thus lithium-ion batteries are attracting increased interest. With a simultaneous increase in demand for raw materials like Li, Ni, Co, and Mn, their hydrometallurgical recycling is also gaining attention. The associated recoveries must be improved due to [...] Read more.

Due to climate change, electromobility and thus lithium-ion batteries are attracting increased interest. With a simultaneous increase in demand for raw materials like Li, Ni, Co, and Mn, their hydrometallurgical recycling is also gaining attention. The associated recoveries must be improved due to EU regulations. In a lab scale, metals are lost to the wrong filter cakes after leaching, cementation, and precipitations. Therefore, this work investigates the question of how many wash steps are suitable after each process step to optimize the recoveries and purity of filter cakes by comparing a reference process and a process with extended washing. The comparison showed that it is possible to recover up to 3.5% of Ni, Co, and Mn by extended washing at each step and in total nearly 100% of Li if wash water is recirculated. An investigation of the substeps of washing demonstrated that single wash steps are able to recover from 0.5% to 3.5% of Ni, Co, and Mn and from 1.6% to 8.7% of Li. The impact of extended washing on purity is shown by the analysis of filter cakes, where the purity of Fe and Al could be improved by 43.0% and for Ni, Co, and Mn by 48.0%. The paper closes with recommendations on how many wash steps are suitable after each process step. Full article

(This article belongs to the Special Issue New Science-Based Concepts for Enhancing Efficiency in Battery Recycling)

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28 pages, 33891 KB

Open AccessEditor’s ChoiceArticle

Influence of Substrate Preheating on Processing Dynamics and Microstructure of Alloy 718 Produced by Directed Energy Deposition Using a Laser Beam and Wire

by Atieh Sahraeidolatkhaneh, Achmad Ariaseta, Gökçe Aydin, Morgan Nilsen and Fredrik Sikström

Metals 2025, 15(11), 1184; https://doi.org/10.3390/met15111184 - 25 Oct 2025

Cited by 3 | Viewed by 1375

Abstract

Effective thermal management is essential in metal additive manufacturing to ensure process stability and desirable material properties. Directed energy deposition using a laser beam and wire (DED-LB/w) enables the production of large, high-performance components but remains sensitive to adverse thermal effects during multi-layer [...] Read more.

Effective thermal management is essential in metal additive manufacturing to ensure process stability and desirable material properties. Directed energy deposition using a laser beam and wire (DED-LB/w) enables the production of large, high-performance components but remains sensitive to adverse thermal effects during multi-layer deposition due to heat accumulation. While prior studies have investigated interlayer temperature control and substrate preheating in DED modalities, including laser-powder and arc-based systems, the influence of substrate preheating in DED-LB/w has not been thoroughly examined. This study employs substrate preheating to simulate heat accumulation and assess its effects on melt pool geometry, wire–melt pool interaction, and the microstructural evolution of Alloy 718. Experimental results demonstrate that increased substrate temperatures lead to a gradual expansion of the melt pool, with a notable transition occurring beyond 400 °C. Microstructural analysis reveals that elevated preheat temperatures promote coarser secondary dendrite arm spacing and the development of wider columnar grains. Moreover, Nb-rich secondary phases, including the Laves phase, exhibit increased size but relatively unchanged area fractions. Observations from electrical conductance measurements and coaxial visual imaging show that preheat temperature significantly affects the process dynamics and microstructural evolution, providing a basis for advanced process control strategies. Full article

(This article belongs to the Special Issue Advanced Manufacturing Technologies: Friction Stir Welding and Additive Manufacturing of Metals)

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14 pages, 21454 KB

Open AccessEditor’s ChoiceArticle

Microstructure and Mechanical Properties of Y-Doped AlCoCrFeNi_2.1 Eutectic High-Entropy Alloy Fabricated by PBF-LB/M

by Gang Wang, Xiangyu Xu, Runbo Zhang, Ren Yuan and Xuteng Lv

Metals 2025, 15(10), 1130; https://doi.org/10.3390/met15101130 - 11 Oct 2025

Viewed by 1020

Abstract

A Y-doped AlCoCrFeNi_2.1 eutectic high-entropy alloy was fabricated via powder bed fusion-laser melting/metal (PBF-LB/M), and the effects of the rare-earth element Y on its microstructure and mechanical properties were investigated. The results indicate that Y addition preserves the fine eutectic microstructure inherent [...] Read more.

A Y-doped AlCoCrFeNi_2.1 eutectic high-entropy alloy was fabricated via powder bed fusion-laser melting/metal (PBF-LB/M), and the effects of the rare-earth element Y on its microstructure and mechanical properties were investigated. The results indicate that Y addition preserves the fine eutectic microstructure inherent to the PBF-LB/M process, while inducing lattice distortion within the face-centered cubic (FCC) matrix and promoting grain refinement. During solidification, Y facilitates heterogeneous nucleation and, due to its strong affinity with Al, increases both the volume fraction of the body-centered cubic (BCC) phase and the proportion of high-angle grain boundaries. X-ray diffraction (XRD) analysis further confirms that Y suppresses the formation of the ordered B2 phase. Tensile testing reveals that Y doping improves the tensile strength from 1383 MPa to 1475 MPa and enhances the elongation from 13.0% to 16.3%. Fractography shows a transition from quasi-cleavage to ductile fracture mode, indicating that Y significantly enhances the strength–ductility synergy of the alloy. Full article

(This article belongs to the Section Additive Manufacturing)

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27 pages, 3885 KB

Open AccessEditor’s ChoiceArticle

Experimental and Machine Learning-Based Assessment of Fatigue Crack Growth in API X60 Steel Under Hydrogen–Natural Gas Blending Conditions

by Nayem Ahmed, Ramadan Ahmed, Samin Rhythm, Andres Felipe Baena Velasquez and Catalin Teodoriu

Metals 2025, 15(10), 1125; https://doi.org/10.3390/met15101125 - 10 Oct 2025

Cited by 2 | Viewed by 2036

Abstract

Hydrogen-assisted fatigue cracking presents a critical challenge to the structural integrity of legacy carbon steel natural gas pipelines being repurposed for hydrogen transport, posing a major barrier to the deployment of hydrogen infrastructure. This study systematically evaluates the fatigue crack growth (FCG) behavior [...] Read more.

Hydrogen-assisted fatigue cracking presents a critical challenge to the structural integrity of legacy carbon steel natural gas pipelines being repurposed for hydrogen transport, posing a major barrier to the deployment of hydrogen infrastructure. This study systematically evaluates the fatigue crack growth (FCG) behavior of API 5L X60 pipeline steel under varying hydrogen–natural gas (H₂–NG) blending conditions to assess its suitability for long-term hydrogen service. Experiments are conducted using a custom-designed autoclave to replicate field-relevant environmental conditions. Gas mixtures range from 0% to 100% hydrogen by volume, with tests performed at a constant pressure of 6.9 MPa and a temperature of 25 °C. A fixed loading frequency of 8.8 Hz and load ratio (R) of 0.60 ± 0.1 are applied to simulate operational fatigue loading. The test matrix is designed to capture FCG behavior across a broad range of stress intensity factor values (ΔK), spanning from near-threshold to moderate levels consistent with real-world pipeline pressure fluctuations. The results demonstrate a clear correlation between increasing hydrogen concentration and elevated FCG rates. Notably, at 100% hydrogen, API X60 specimens exhibit crack propagation rates up to two orders of magnitude higher than those in 0% hydrogen (natural gas) conditions, particularly within the Paris regime. In the lower threshold region (ΔK ≈ 10 MPa·√m), the FCG rate (da/dN) increased nonlinearly with hydrogen concentration, indicating early crack activation and reduced crack initiation resistance. In the upper Paris regime (ΔK ≈ 20 MPa·√m), da/dNs remained significantly elevated but exhibited signs of saturation, suggesting a potential limiting effect of hydrogen concentration on crack propagation kinetics. Fatigue life declined substantially with hydrogen addition, decreasing by ~33% at 50% H₂ and more than 55% in pure hydrogen. To complement the experimental investigation and enable predictive capability, a modular machine learning (ML) framework was developed and validated. The framework integrates sequential models for predicting hydrogen-induced reduction of area (RA), fracture toughness (FT), and FCG rate (da/dN), using CatBoost regression algorithms. This approach allows upstream degradation effects to be propagated through nested model layers, enhancing predictive accuracy. The ML models accurately captured nonlinear trends in fatigue behavior across varying hydrogen concentrations and environmental conditions, offering a transferable tool for integrity assessment of hydrogen-compatible pipeline steels. These findings confirm that even low-to-moderate hydrogen blends significantly reduce fatigue resistance, underscoring the importance of data-driven approaches in guiding material selection and infrastructure retrofitting for future hydrogen energy systems. Full article

(This article belongs to the Special Issue Failure Analysis and Evaluation of Metallic Materials)

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22 pages, 16895 KB

Open AccessEditor’s ChoiceArticle

Surface Characterization of Hot-Rolled AISI 440C Round Wire at the Different Steps of the Typical Production Process

by Alessio Malandruccolo, Stefano Rossi and Cinzia Menapace

Metals 2025, 15(10), 1102; https://doi.org/10.3390/met15101102 - 2 Oct 2025

Viewed by 857

Abstract

This study investigates the surface characteristics and corrosion behavior of a high-C martensitic stainless steel (AISI 440C) at different stages of its manufacturing process. As a class, these steels prioritize high mechanical properties and wear resistance over superior corrosion resistance. Hot working operations, [...] Read more.

This study investigates the surface characteristics and corrosion behavior of a high-C martensitic stainless steel (AISI 440C) at different stages of its manufacturing process. As a class, these steels prioritize high mechanical properties and wear resistance over superior corrosion resistance. Hot working operations, such as rolling, create a surface oxide scale that must be removed via pickling to restore the material’s inherent corrosion resistance. This process also eliminates the underlying Cr-depleted layer, allowing for the re-establishment of a protective passive film. Using potentiodynamic polarization curves and micrographic analysis, the material’s behavior in different conditions, as-rolled, with a post-heat treatment oxide scale, and in a bare, oxide-free state, has been assessed. The results showed that the material lacks stable passive behavior under all conditions. The as-rolled and heat-treated conditions both exhibited active behavior and formed thick, non-adherent corrosion products. The oxide layer formed after heat treatment performed the worst, showing a significant increase in corrosion current density. These findings confirm the material’s susceptibility to corrosion in Cl⁻ ion-rich environments, highlighting the need for limited storage in such conditions and rapid pickling after thermal processing to mitigate surface damage. Full article

(This article belongs to the Section Corrosion and Protection)

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29 pages, 4438 KB

Open AccessEditor’s ChoiceReview

AI Design for High Entropy Alloys: Progress, Challenges and Future Prospects

by Enzhi Xie and Chao Yang

Metals 2025, 15(9), 1012; https://doi.org/10.3390/met15091012 - 11 Sep 2025

Cited by 8 | Viewed by 5967

Abstract

High-entropy alloys have demonstrated significant application potential in many industrial fields due to their outstanding comprehensive properties. However, the complex multi-component compositions pose challenges for traditional design approaches. In recent years, artificial intelligence (AI) technology, with its powerful capabilities in data analysis, prediction, [...] Read more.

High-entropy alloys have demonstrated significant application potential in many industrial fields due to their outstanding comprehensive properties. However, the complex multi-component compositions pose challenges for traditional design approaches. In recent years, artificial intelligence (AI) technology, with its powerful capabilities in data analysis, prediction, and optimization, has provided new pathways for rapid discovery and performance modulation of high-entropy alloys. This paper systematically reviews the latest advancements in AI applications for high-entropy alloy design, covering key technologies such as machine learning models (e.g., active learning, generative models, transfer learning), high-throughput computing and experimental data processing, phase structure and property prediction. It also presents typical application cases, including compositional optimization, phase structure prediction, performance synergistic regulation, and novel material discovery. Although AI has significantly improved design efficiency and accuracy, challenges remain, such as the scarcity of high-quality data, insufficient model interpretability, and interdisciplinary integration. Future efforts should focus on building a more robust data ecosystem, enhancing model transparency, and strengthening closed-loop validation between AI and experimental science to advance intelligent design and engineering applications of high-entropy alloys. Full article

(This article belongs to the Special Issue Advances in High-Entropy Alloys’ Microstructure, Properties and Preparation)

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15 pages, 8787 KB

Open AccessEditor’s ChoiceArticle

Point Defects in MoNbTi-Based Refractory Multi-Principal-Element Alloys

by Thai hang Chung, Maciej Oskar Liedke, Saikumaran Ayyappan, Maik Butterling, Riley Craig Ferguson, Adric C. L. Jones, Andreas Wagner, Khalid Hattar, Djamel Kaoumi and Farida A. Selim

Metals 2025, 15(9), 989; https://doi.org/10.3390/met15090989 - 6 Sep 2025

Cited by 1 | Viewed by 1224

Abstract

As emergent material candidates for extreme environments, refractory high-entropy alloys (HEAs) or refractory multi-principal-element alloys (RMPEAs) comprising refractory metals feature qualities such as high radiation tolerance, corrosion resistance, and mechanical strength. A set of MoNbTi-based RMPEA samples with Al, Cr, V, and Zr [...] Read more.

As emergent material candidates for extreme environments, refractory high-entropy alloys (HEAs) or refractory multi-principal-element alloys (RMPEAs) comprising refractory metals feature qualities such as high radiation tolerance, corrosion resistance, and mechanical strength. A set of MoNbTi-based RMPEA samples with Al, Cr, V, and Zr additions are prepared by spark plasma sintering and investigated for their response to irradiation using 10 MeV Si⁺ ions with a dose of

1.43 \times 10^{15}

ions/cm². Positron annihilation spectroscopy and transmission electron microscopy are employed as atomic- and meso- scale techniques to reveal how chemical complexity, nanotwinning, and phase fractions play an important role in radiation-induced defect accumulation and damage tolerance. The study provides experimental evidence of nanotwinning acting as an effective sink for radiation-induced point defects. Full article

(This article belongs to the Special Issue Advances in High-Entropy Alloys’ Microstructure, Properties and Preparation)

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24 pages, 6760 KB

Open AccessEditor’s ChoiceArticle

Influence of Microstructure and Heat Treatment on the Corrosion Resistance of Mg-1Zn Alloy Produced by Laser Powder Bed Fusion

by Raúl Reyes-Riverol, Ángel Triviño-Peláez, Federico García-Galván, Marcela Lieblich, José Antonio Jiménez and Santiago Fajardo

Metals 2025, 15(8), 853; https://doi.org/10.3390/met15080853 - 30 Jul 2025

Cited by 2 | Viewed by 1890

Abstract

The corrosion behavior of an additively manufactured Mg-1Zn alloy was investigated in both the transverse and longitudinal directions relative to the build direction, in the as-built condition and after annealing at 350 °C for 24 h under high vacuum. Microstructural characterization using XRD [...] Read more.

The corrosion behavior of an additively manufactured Mg-1Zn alloy was investigated in both the transverse and longitudinal directions relative to the build direction, in the as-built condition and after annealing at 350 °C for 24 h under high vacuum. Microstructural characterization using XRD and SEM revealed the presence of magnesium oxide (MgO) and the absence of intermetallic second-phase particles. Optical microscopy (OM) images and Electron Backscatter Diffraction (EBSD) maps showed a highly complex grain morphology with anomalous, anisotropic shapes and a heterogeneous grain size distribution. The microstructure includes grains with a pronounced columnar morphology aligned along the build direction and is therefore characterized by a strong crystallographic texture. Electrochemical techniques, including PDP and EIS, along with gravimetric H₂ collection, concluded that the transverse plane exhibited greater corrosion resistance compared to the longitudinal plane. Additionally, an increase in cathodic kinetics was observed when comparing as-built with heat-treated samples. Full article

(This article belongs to the Section Corrosion and Protection)

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19 pages, 3512 KB

Open AccessEditor’s ChoiceReview

Data Science in Order and Disorder of High-Entropy Materials

by Jiasheng Wang, Jianzhong Jiang, Peter K. Liaw, Guihong Geng and Yong Zhang

Metals 2025, 15(6), 632; https://doi.org/10.3390/met15060632 - 3 Jun 2025

Cited by 5 | Viewed by 2738

Abstract

In recent years, high-entropy materials (HEMs) have garnered significant attention due to their unique multi-principal element compositions, which endow them with remarkable properties distinct from traditional materials. The order and disorder in HEMs are particularly complex, influenced by factors such as temperature, pressure, [...] Read more.

In recent years, high-entropy materials (HEMs) have garnered significant attention due to their unique multi-principal element compositions, which endow them with remarkable properties distinct from traditional materials. The order and disorder in HEMs are particularly complex, influenced by factors such as temperature, pressure, and composition, and are closely related to their mechanical and physical properties. This review systematically summarizes the progress in understanding the order and disorder in HEMs, with a focus on the role of data science in this field. We introduce the basic concepts of order and disorder and the related research in HEMs, discuss the nonlinear behaviors of HEMs, and elaborate on the relevant applications of data science, including analysis by machine learning, molecular dynamics simulations, and Monte Carlo simulations. Challenges and future directions are also explored, aiming to provide comprehensive insights into materials science. Full article

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18 pages, 8811 KB

Open AccessEditor’s ChoiceArticle

Lightweight Ti₃VNbAl_0.5Zr_x (x = 0, 0.1, 0.5, and 1) Refractory High-Entropy Alloys with an Optimized Balance of Strength and Ductility

by Haoyu Fang, Xuejiao Wang, Aidong Lan, Xi Jin and Junwei Qiao

Metals 2025, 15(5), 503; https://doi.org/10.3390/met15050503 - 30 Apr 2025

Cited by 2 | Viewed by 1743

Abstract

Achieving a balance between strength and room-temperature ductility remains an urgent need and a significant challenge for body-centered cubic (BCC) structure materials. In this paper, a good combination of strength and ductility in single-phase BCC-structured Ti₃VNbAl_0.5Zr_x (x = [...] Read more.

Achieving a balance between strength and room-temperature ductility remains an urgent need and a significant challenge for body-centered cubic (BCC) structure materials. In this paper, a good combination of strength and ductility in single-phase BCC-structured Ti₃VNbAl_0.5Zr_x (x = 0, 0.1, 0.5, and 1) lightweight high-entropy alloys (LHEAs) was designed by reducing the valence-electron concentration in combination with the d-electron theory. The influences of Zr on the microstructures and mechanical properties of the alloys were systematically studied. The yield strengths of Zr0, Zr0.1, Zr0.5, and Zr1 alloys were 644 MPa, 703 MPa, 827 MPa, and 904 Mpa, respectively. The tensile strains of Zr0, Zr0.1, Zr0.5, and Zr1 alloys were 29%, 30%, 20%, and 16%, respectively. The deformation mechanism was studied using transmission electron microscopy (TEM). The results demonstrate that the alloys could still maintain single-phase BCC structure after deformation, and neither phase transformation nor twinning was detected during the deformation process. The main deformation mechanism of the Zr1 alloy is dislocation slip. The current work has great significance for developing high-strength, ductile, and low-density structural materials. Full article

(This article belongs to the Special Issue Feature Papers in Entropic Alloys and Meta-Metals)

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16 pages, 4063 KB

Open AccessEditor’s ChoiceArticle

Kinetics of Oxidation of Binary Ti-Cu Alloys in the 600–800 °C Temperature Range

by Fatemah Alqattan, Fei Yang and Leandro Bolzoni

Metals 2025, 15(2), 222; https://doi.org/10.3390/met15020222 - 18 Feb 2025

Cited by 3 | Viewed by 2442

Abstract

The oxidation behaviour of Ti alloys is a crucial aspect for structural components operating at high service temperature. The aim of this study is to identify the oxidation kinetics and mechanism of binary Ti-Cu alloys with a progressively higher amount of Cu with [...] Read more.

The oxidation behaviour of Ti alloys is a crucial aspect for structural components operating at high service temperature. The aim of this study is to identify the oxidation kinetics and mechanism of binary Ti-Cu alloys with a progressively higher amount of Cu with the alloys having a α + β lamellar structure. It is found that all the alloys followed a non-ideal (i.e., n ≠ 2) parabolic relationship, as controlled by anionic oxygen diffusion, with a distinct effect from both oxidation temperature and alloy chemistry. Specifically, faster oxidation kinetics are found both at higher temperatures and for higher Cu contents, resulting in the formation of thicker oxide scale layers. The oxidation mechanism primarily entails the formation of the stable TiO₂ rutile polymorph. However, transitions through metastable phases (e.g., anatase) and texturing of rutile are also revealed as dictated by the composition of the alloy at specific oxidation temperature/time pairs. Full article

(This article belongs to the Special Issue Powder Metallurgy of Metals and Alloys)

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14 pages, 5744 KB

Open AccessEditor’s ChoiceArticle

Effect of Multi-Phase Composite Structure on the Mechanical Properties of Al_xFe_1.5CoNiC_0.12 High Entropy Alloys

by Yiteng Jiang, Aoxiang Li, Kaiwen Kang, Jinshan Zhang, Di Huang, Chunning Che, Saike Liu, Mingkun Xu, Yaqing Li, Borui Zhang and Gong Li

Metals 2025, 15(2), 203; https://doi.org/10.3390/met15020203 - 14 Feb 2025

Cited by 3 | Viewed by 1690

Abstract

Single-phase high entropy alloys (HEAs) exhibit limited mechanical properties while dual-phase and multi-phase HEAs offer better strength, toughness, and stability. In this paper, the as-cast Al_xFe_1.5CoNiC_0.12 HEAs with triple-phase dendritic composite structure is studied, and the influence of [...] Read more.

Single-phase high entropy alloys (HEAs) exhibit limited mechanical properties while dual-phase and multi-phase HEAs offer better strength, toughness, and stability. In this paper, the as-cast Al_xFe_1.5CoNiC_0.12 HEAs with triple-phase dendritic composite structure is studied, and the influence of the composite structure on the mechanical properties is discussed. The interdendrite (ID) of this structure is composed of a uniformly distributed high-density ordered face-centered cubic structure (L1₂) precipitate phase and face-centered cubic (FCC) matrix, while the dendrite (DR) consists of an ordered body-centered cubic (B2) single-phase. The high density L1₂ precipitate phase leads to a higher hardness in the FCC+L1₂ dual-phase region compared to the B2 single-phase region. The decrease in Al content can greatly improve mechanical performance. The improvement was attributed to the higher volume fraction of the ID and the smaller particle size of the precipitates. The L1₂ phase nano-precipitates exhibit minimal lattice mismatch with the FCC matrix, thereby significantly enhancing the stability of the alloy at the nanoscale. This stability is reflected in the fracture morphology. Modulating the triple-phase dendritic composite structure effectively improves the mechanical properties of the alloy. Full article

(This article belongs to the Special Issue Advances in High-Entropy Alloys’ Microstructure, Properties and Preparation)

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22 pages, 6744 KB

Open AccessEditor’s ChoiceArticle

Magnetic Pulse Powder Compaction

by Viktors Mironovs, Jekaterina Nikitina, Matthias Kolbe, Irina Boiko and Yulia Usherenko

Metals 2025, 15(2), 155; https://doi.org/10.3390/met15020155 - 4 Feb 2025

Cited by 1 | Viewed by 3641

Abstract

Powder metallurgy (PM) offers several advantages over conventional melt metallurgy, including improved homogeneity, fine grain size, and pseudo-alloying capabilities. Transitioning from conventional methods to PM can result in significant enhancements in material properties and production efficiency by eliminating unnecessary process steps. Dynamic compaction [...] Read more.

Powder metallurgy (PM) offers several advantages over conventional melt metallurgy, including improved homogeneity, fine grain size, and pseudo-alloying capabilities. Transitioning from conventional methods to PM can result in significant enhancements in material properties and production efficiency by eliminating unnecessary process steps. Dynamic compaction techniques, such as impulse and explosive compaction, aim to achieve higher powder density without requiring sintering, further improving PM efficiency. Among these techniques, magnetic pulse compaction (MPC) has gained notable interest due to its unique process mechanics and distinct advantages. MPC utilizes the rapid discharge of energy stored in capacitors to generate a pulsed electromagnetic field, which accelerates a tool to compress the powder. This high-speed process is particularly well-suited for compacting complex geometries and finds extensive application in industries such as powder metallurgy, welding, die forging, and advanced material manufacturing. This paper provides an overview of recent advancements and applications of MPC technology, highlighting its capabilities and potential for broader integration into modern manufacturing processes. Full article

(This article belongs to the Special Issue Powder Metallurgy of Metallic Materials)

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15 pages, 3642 KB

Open AccessEditor’s ChoiceArticle

Effect of Annealing after Casting and Cold Rolling on Microstructure and Electrochemical Behavior of High-Entropy Alloy, Cantor

by Jinsurang Lim, Byung-Hyun Shin, Doo-In Kim, Jong-Seong Bae, Jung-Woo Ok, Seongjun Kim, Jinyong Park, Je In Lee and Jang-Hee Yoon

Metals 2024, 14(8), 846; https://doi.org/10.3390/met14080846 - 24 Jul 2024

Cited by 7 | Viewed by 3389

Abstract

High-entropy alloys (HEAs), a relatively new class of materials, have attracted significant attention in materials science owing to their unique properties and potential applications. High entropy stabilizes the phase of a solid solution over a wide range of chemical compositions, yielding unique properties [...] Read more.

High-entropy alloys (HEAs), a relatively new class of materials, have attracted significant attention in materials science owing to their unique properties and potential applications. High entropy stabilizes the phase of a solid solution over a wide range of chemical compositions, yielding unique properties superior to those of conventional alloys. Therefore, this study analyzed the microstructure and electrochemical behavior of HEAs (Cantor) to evaluate their corrosion resistance, according to their manufacturing process (casting, cold rolling, and annealing). The microstructural morphologies and sizes were analyzed using electron backscatter diffraction. The electrochemical behavior was examined using open circuit potential measurements, electrochemical impedance spectroscopy, potentiodynamic polarization tests, and critical pitting temperature measurements using a potentiostat. The casting process formed a nonuniform microstructure (average grain size = 19 μm). The cold rolling process caused the formation of fine grains (size = 4 μm). A uniform microstructure (grain size > 151 μm) was formed after heat treatment. The corrosion resistance of the HEAs was determined from the passivation layer formed by Cr oxidation. These microstructural differences resulted in variations in the electrochemical behavior. Microstructural and electrochemical analyses are crucial because HEAs have diverse potential applications. Therefore, this study contributes to future improvements in HEA manufacturing processes. Full article

(This article belongs to the Special Issue Casting Alloy Design and Characterization—2nd Edition)

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19 pages, 13655 KB

Open AccessFeature PaperEditor’s ChoiceArticle

High-Entropy Alloy Al_0.2Co_1.5CrFeNi_1.5Ti_0.5 Prepared from High-Entropy Oxide (Al_0.2Co_1.5CrFeNi_1.5Ti_0.5)₃O₄ by a Deoxidation Process via a CaH₂-Assisted Molten Salt Method

by Yasukazu Kobayashi, Shota Yokoyama and Ryo Shoji

Metals 2024, 14(4), 443; https://doi.org/10.3390/met14040443 - 10 Apr 2024

Cited by 5 | Viewed by 3764

Abstract

High-entropy alloys (HEAs) have attracted a great deal of research interest these days because of their attractive properties. Low-temperature chemical synthesis methods are being developed to obtain nanoscale HEAs with low energy consumption. In this study, we prepared HEA Al_0.2Co_1.5 [...] Read more.

High-entropy alloys (HEAs) have attracted a great deal of research interest these days because of their attractive properties. Low-temperature chemical synthesis methods are being developed to obtain nanoscale HEAs with low energy consumption. In this study, we prepared HEA Al_0.2Co_1.5CrFeNi_1.5Ti_0.5 nanoparticles from high-entropy oxide (HEO) (Al_0.2Co_1.5CrFeNi_1.5Ti_0.5)₃O₄ by a deoxidation process via a CaH₂-assisted molten salt method at 600 °C. X-ray diffraction measurements demonstrated that the oxide precursor and the reduced product have single-phases of spinel structure and face-centered cubic structures, indicating the formation of HEO and HEA, respectively. The HEA nanoparticles exhibited superior catalytic performance in the liquid-phase hydrogenation of p-nitrophenol at room temperature with little leaching of the component elements. Scanning electron microscopy (SEM) with energy-dispersive X-ray spectrometry (EDX) exhibited a good distribution of constituent elements over the HEA nanoparticles in a micro-sized range. However, transmission electron microscopy (TEM) with EDX revealed a slight deviation of elemental distributions of Al and Ti from those of Co, Cr, Fe, and Ni in a nano-sized range, probably due to the incomplete reduction of aluminum and titanium oxides. The elemental homogeneity in the HEA nanoparticles could be improved by taking advantage of the HEO precursor with homogeneous elemental distributions, but the experimental results suggested the importance of the total reduction of oxide precursors to prepare homogeneous HEAs from HEOs. Full article

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13 pages, 9623 KB

Open AccessEditor’s ChoiceArticle

Effect of Tropical Marine Atmospheric Environment on Corrosion Behaviour of the 7B04-T74 Aluminium Alloy

by Ning Li, Weifang Zhang, Xiaojun Yan, Meng Zhang, Lu Han and Yikun Cai

Metals 2023, 13(5), 995; https://doi.org/10.3390/met13050995 - 21 May 2023

Cited by 10 | Viewed by 4404

Abstract

In this work, the effects of the tropical marine atmospheric environment on the corrosion behaviour of the 7B04-T74 aluminium alloy were systematically investigated by using accelerated testing, together with corrosion kinetic analysis, microstructure observation, product composition analysis, and potentiodynamic polarization curve tests. The [...] Read more.

In this work, the effects of the tropical marine atmospheric environment on the corrosion behaviour of the 7B04-T74 aluminium alloy were systematically investigated by using accelerated testing, together with corrosion kinetic analysis, microstructure observation, product composition analysis, and potentiodynamic polarization curve tests. The weight loss method was used for the corrosion kinetics analysis. The surface morphology and corrosion products transformation law were investigated by OM, SEM, EDS, and XPS. The electrochemical characteristics were studied using potentiodynamic polarization curves. The research indicated that the 7B04-T74 aluminium alloy has eminent corrosion resistance in the tropical marine atmospheric environment. Localized pitting corrosion occurred rapidly in the tropical marine atmosphere. In the later stage of corrosion, the corrosion of aluminium alloy did not become serious. Specifically, no obvious intergranular corrosion was found, which is related to the thermal treatment method. Corrosion products included Al(OH)₃, Al₂O₃, and AlCl₃, of which Al(OH)₃ is the most notable. Full article

(This article belongs to the Special Issue Corrosion Prediction in Different Environment)

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11 pages, 1840 KB

Open AccessEditor’s ChoiceArticle

Corrosion of Copper in a Tropical Marine Atmosphere Rich in H₂S Resulting from the Decomposition of Sargassum Algae

by Mahado Said Ahmed, Mounim Lebrini, Benoit Lescop, Julien Pellé, Stéphane Rioual, Olivia Amintas, Carole Boullanger and Christophe Roos

Metals 2023, 13(5), 982; https://doi.org/10.3390/met13050982 - 19 May 2023

Cited by 12 | Viewed by 4482

Abstract

The atmospheric corrosion of copper exposed in Martinique (Caribbean Sea) for 1 year was reported. This island suffered the stranding of sargassum algae, which decompose and release toxic gases such as hydrogen sulfide (H₂S) or ammonia (NH₃). Four sites [...] Read more.

The atmospheric corrosion of copper exposed in Martinique (Caribbean Sea) for 1 year was reported. This island suffered the stranding of sargassum algae, which decompose and release toxic gases such as hydrogen sulfide (H₂S) or ammonia (NH₃). Four sites in Martinique (France) more or less impacted by sargassum algae strandings were selected. The corrosion rate was studied via mass loss determination. The morphology and properties of the corrosion products were determined using Scanning Electron Microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The samples were exposed for up to 12 months. The mass loss results after 1-year exposure were from 4.8 µm for the least impacted site to 325 µm for the site most affected by sargassum algae. This very high value proves that the presence of sargassum algae caused a significant degradation of copper. The morphological structures and properties of the corrosion products obtained at the impacted and non-impacted sites differed significantly. In the absence of sargassum algae, classical corrosion products of copper were reported such as Cu₂O and Cu₂Cl(OH)₃. In the sites near the stranding of the sargassum algae, the CuS product is the main corrosion product obtained, but copper hydroxylsulfate is created. Full article

(This article belongs to the Section Corrosion and Protection)

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13 pages, 80879 KB

Open AccessEditor’s ChoiceArticle

Corrosion Behavior of the AZ31 Mg Alloy in Neutral Aqueous Solutions Containing Various Anions

by Duyoung Kwon, Hien Van Pham, Pungkeun Song and Sungmo Moon

Metals 2023, 13(5), 962; https://doi.org/10.3390/met13050962 - 16 May 2023

Cited by 10 | Viewed by 3861

Abstract

This work demonstrates the corrosion behavior of the AZ31 Mg alloy as a function of an immersion time of 48 h in 0.1 M HCl, H₂SO₄, H₃PO₄ and HF solutions, in which pH was adjusted to [...] Read more.

This work demonstrates the corrosion behavior of the AZ31 Mg alloy as a function of an immersion time of 48 h in 0.1 M HCl, H₂SO₄, H₃PO₄ and HF solutions, in which pH was adjusted to 6 to exclude the contribution of hydrogen ions (H⁺) and hydroxide ions (OH⁻). In situ observations, open circuit potential (OCP), weight changes and AC impedance measurements were performed with an immersion time of 48 h and the morphologies and chemical compositions of the surface products after 48 h of immersion were analyzed by SEM, EDS and XPS. In the chloride ion (Cl⁻)-containing solution, the corrosion of the AZ31 Mg alloy initiated locally and propagated discontinuously over the surface with immersion time. The OCP value of the AZ31 Mg alloy showed an initial increase from −1.51 V_Ag/AgCl to −1.47 V_Ag/AgCl after about 5 h of immersion and then a decrease to −1.51 V_Ag/AgCl due to corrosion initiation. In the F⁻-containing solution, after 48 h of immersion, the OCP showed an extremely large value of −0.6 V_Ag/AgCl, while the relatively lower values of −1.52 V_Ag/AgCl, −1.59 V_Ag/AgCl were seen in the solutions containing SO₄²⁻ and PO₄³, respectively. In the sulfate ion (SO₄²⁻)-containing neutral aqueous solution, needle-like surface films were formed and there were no changes in the weight of the AZ31 Mg alloy with immersion time. In the phosphate ion (PO₄³⁻)-containing neutral aqueous solution, a vigorous gas evolution occurred, together with the formation of black surface films with cracks, and a high corrosion rate of −13.8018 × 10⁻³ g·cm⁻²·day⁻¹ was obtained. In the fluoride ion (F⁻)-containing neutral aqueous solution, a surface film with crystalline grains of MgF₂ was formed and the weight of the AZ31 Mg alloy increased continuously with immersion time. In conclusion, the corrosion of the AZ31 Mg alloy occurred uniformly in neutral phosphate solution but locally in chloride solution. No corrosion was observed in either the neutral sulfate or fluoride solutions. Full article

(This article belongs to the Special Issue Corrosion and Protection of Lightweight Engineering Materials: Mg Alloys, Al Alloys, Ti Alloys and Other Related Metals)

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14 pages, 4704 KB

Open AccessEditor’s ChoiceArticle

Electrochemical Corrosion Behavior of 310S Stainless Steel in Hot Concentrated Tap Water

by Wen Xian, Zhong Yin, Lele Liu and Moucheng Li

Metals 2023, 13(4), 713; https://doi.org/10.3390/met13040713 - 5 Apr 2023

Cited by 8 | Viewed by 3786

Abstract

The corrosion behavior of 310S stainless steel was investigated in synthetic tap water and Ca²⁺ and Mg²⁺-free solutions with different concentration ratios at 80 °C using electrochemical measurement techniques and surface analysis methods. The main purpose was to obtain the [...] Read more.

The corrosion behavior of 310S stainless steel was investigated in synthetic tap water and Ca²⁺ and Mg²⁺-free solutions with different concentration ratios at 80 °C using electrochemical measurement techniques and surface analysis methods. The main purpose was to obtain the electrochemical corrosion characteristics under carbonate scale conditions. The specimens displayed a spontaneous passivation state in the solutions with or without Ca²⁺ and Mg²⁺ ions. With the enlargement of the concentration ratio of synthetic tap water from 1 to 10 times, the polarization resistance under free corrosion conditions and the pitting potential decreased by about 48% and 327 mV, respectively. The pitting tendency increased with increasing concentration ratio of tap water. The carbonate scales deposited from the synthetic tap water solutions were mainly composed of CaCO₃, which led to a slight increase in the polarization resistance and the pitting potential and decrease in the passive current density. Full article

(This article belongs to the Special Issue Applications of Electrochemistry in Corrosion Science and in Practice)

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38 pages, 7189 KB

Open AccessEditor’s ChoiceReview

High-Entropy Alloy Coatings Deposited by Thermal Spraying: A Review of Strengthening Mechanisms, Performance Assessments and Perspectives on Future Applications

by Rakesh Bhaskaran Nair, Raunak Supekar, Seyyed Morteza Javid, Wandong Wang, Yu Zou, André McDonald, Javad Mostaghimi and Pantcho Stoyanov

Metals 2023, 13(3), 579; https://doi.org/10.3390/met13030579 - 13 Mar 2023

Cited by 63 | Viewed by 12910

Abstract

Thermal spray deposition techniques have been well-established, owing to their flexibility in addressing degradation due to wear and corrosion issues faced due to extreme environmental conditions. With the adoption of these techniques, a broad spectrum of industries is experiencing continuous improvement in resolving [...] Read more.

Thermal spray deposition techniques have been well-established, owing to their flexibility in addressing degradation due to wear and corrosion issues faced due to extreme environmental conditions. With the adoption of these techniques, a broad spectrum of industries is experiencing continuous improvement in resolving these issues. To increase industrial-level implementation, state-of-the-art advanced materials are required. High-entropy alloys (HEAs) have recently gained considerable attention within the scientific community as advanced materials, mainly due to their exceptional properties and desirable microstructural features. Unlike traditional material systems, high-entropy alloys are composed of multi-component elements (at least five elements) with equimolar or nearly equimolar concentrations. This allows for a stable microstructure that is associated with high configurational entropy. This review article provides a critical assessment of different strengthening mechanisms observed in various high-entropy alloys developed by means of deposition techniques. The wear, corrosion, and oxidation responses of these alloys are reviewed in detail and correlated to microstructural and mechanical properties and behavior. In addition, the review focused on material design principles for developing next-generation HEAs that can significantly benefit the aerospace, marine, oil and gas, nuclear sector, etc. Despite having shown exceptional mechanical properties, the article describes the need to further evaluate the tribological behavior of these HEAs in order to show proof-of-concept perspectives for several industrial applications in extreme environments. Full article

(This article belongs to the Special Issue Wear and Corrosion Behavior of High-Entropy Alloy)

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17 pages, 5148 KB

Open AccessEditor’s ChoiceArticle

Stress Corrosion Cracking Mechanisms of UNS S32205 Duplex Stainless Steel in Carbonated Solution Induced by Chlorides

by Ulises Martin and David M. Bastidas

Metals 2023, 13(3), 567; https://doi.org/10.3390/met13030567 - 11 Mar 2023

Cited by 8 | Viewed by 6948

Abstract

Herein, the chloride-induced stress corrosion cracking (SCC) mechanisms of UNS S32205 duplex stainless steel (DSS) reinforcing bars in alkaline and carbonated solutions are studied. Electrochemical monitoring and mechanical properties were tested using linear polarization resistance and electrochemical impedance spectroscopy, coupled with the slow [...] Read more.

Herein, the chloride-induced stress corrosion cracking (SCC) mechanisms of UNS S32205 duplex stainless steel (DSS) reinforcing bars in alkaline and carbonated solutions are studied. Electrochemical monitoring and mechanical properties were tested using linear polarization resistance and electrochemical impedance spectroscopy, coupled with the slow strain rate tensile test (SSRT) to evaluate the SCC behavior and unravel the pit-to-crack mechanisms. Pit initiation and crack morphology were identified by fractographic analysis, which revealed the transgranular (TG) SCC mechanism. HCO₃⁻ acidification enhanced the anodic dissolution kinetics, thus promoting a premature pit-to-crack transition, seen by the decrease in the maximum phase angle in the Bode plot at low frequencies (≈ 1 Hz) for the carbonated solution. The crack propagation rate for the carbonated solution increased by over 100% compared to the alkaline solution, coinciding with the lower phase angle from the Bode plots, as well as with the lower charge transfer resistance. Pit initiation was found at the TiN nonmetallic inclusion inside the ferrite phase cleavage facet, which developed TG-SCC. Full article

(This article belongs to the Special Issue Corrosion and Protection of Stainless Steels)

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21 pages, 14270 KB

Open AccessEditor’s ChoiceArticle

Effect of Laser Shock Peening on the Stress Corrosion Cracking of 304L Stainless Steel

by Young-Ran Yoo, Seung-Heon Choi and Young-Sik Kim

Metals 2023, 13(3), 516; https://doi.org/10.3390/met13030516 - 3 Mar 2023

Cited by 24 | Viewed by 5345

Abstract

Storage canisters used in nuclear power plants operating in seaside areas—where the salt content in the atmosphere is high—may be susceptible to chloride-induced stress corrosion cracking (CISCC). Chloride-induced stress corrosion cracking is one of the ways in which dry storage canisters made of [...] Read more.

Storage canisters used in nuclear power plants operating in seaside areas—where the salt content in the atmosphere is high—may be susceptible to chloride-induced stress corrosion cracking (CISCC). Chloride-induced stress corrosion cracking is one of the ways in which dry storage canisters made of stainless steel can degrade. Stress corrosion cracking depends on the microstructure and residual stress, and it is therefore very important to improve the surface properties of materials. Laser shock peening both greatly deforms the material surface and refines grains, and it generates compressive residual stress in the deep part from the surface of the material. This study focused on the effect of laser shock peening on the stress corrosion cracking of 304L stainless steel. The laser shock peening was found to induce compressive residual stress from the surface to a 1 mm depth, and the SCC properties were evaluated by a U-bend test. The results showed that the SCC resistance of laser-peened 304L stainless steel in a chloride environment was enhanced, and that it was closely related to grain size, the pitting potential of the cross section, and residual stress. Full article

(This article belongs to the Special Issue Feature Paper Collection of “Current Challenges in Corrosion Research”)

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29 pages, 15823 KB

Open AccessEditor’s ChoiceReview

Corrosion Behavior of High Entropy Alloys and Their Application in the Nuclear Industry—An Overview

by Tianrun Li, Debin Wang, Suode Zhang and Jianqiang Wang

Metals 2023, 13(2), 363; https://doi.org/10.3390/met13020363 - 10 Feb 2023

Cited by 44 | Viewed by 11041

Abstract

With multiple principal components, high entropy alloys (HEAs) have aroused great interest due to their unique microstructures and outstanding properties. Recently, the corrosion behavior of HEAs has become a scientific hotspot in the area of material science and engineering, and HEAs can exhibit [...] Read more.

With multiple principal components, high entropy alloys (HEAs) have aroused great interest due to their unique microstructures and outstanding properties. Recently, the corrosion behavior of HEAs has become a scientific hotspot in the area of material science and engineering, and HEAs can exhibit good protection against corrosive environments. A comprehensive understanding of the corrosion mechanism of HEAs is important for further design of HEAs with better performance. This paper reviews the corrosion properties and mechanisms of HEAs (mainly Cantor alloy and its variants) in various environments. More crucially, this paper is focused on the influences of composition and microstructure on the evolution of the corrosion process, especially passive film stability and localized corrosion resistance. The corrosion behavior of HEAs as structural materials in nuclear industry applications is emphasized. Finally, based on this review, the possible perspectives for scientific research and engineering applications of HEAs are proposed. Full article

(This article belongs to the Special Issue Feature Paper Collection of “Current Challenges in Corrosion Research”)

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13 pages, 13119 KB

Open AccessEditor’s ChoiceArticle

Effect of Superhydrophobic Surface on Corrosion Resistance of Magnesium-Neodymium Alloy in Artificial Hand Sweat

by Changyang Liu, Jiapeng Sun and Guosong Wu

Metals 2023, 13(2), 219; https://doi.org/10.3390/met13020219 - 24 Jan 2023

Cited by 12 | Viewed by 3029

Abstract

A superhydrophobic surface can endow metals with some intriguing characteristics such as self-cleaning behavior. In this study, a simple solution-immersion method based on the concept of predesigned corrosion is developed to enhance the corrosion resistance of a magnesium-neodymium alloy. The Mg alloy is [...] Read more.

A superhydrophobic surface can endow metals with some intriguing characteristics such as self-cleaning behavior. In this study, a simple solution-immersion method based on the concept of predesigned corrosion is developed to enhance the corrosion resistance of a magnesium-neodymium alloy. The Mg alloy is directly soaked in potassium dihydrogen phosphate solution with the addition of ultrasound, and a layer of rough but dense coating is uniformly formed on the Mg-Nd alloy after the immersion process, which is mainly composed of MgHPO₄∙3H₂O. A superhydrophobic surface with an average wetting angle of 150.5° and a sliding angle of about 4.5° can be obtained on the Mg alloy by further chemical surface modification with perfluorodecyltriethoxysilane. This superhydrophobic surface has an interesting self-cleaning effect as well as good corrosion resistance in artificial hand sweat. In brief, this study provides a feasible way to prepare a superhydrophobic surface on the Mg-Nd alloy and reveals the effect of a superhydrophobic surface on the corrosion behavior of the Mg-Nd alloy, offering new technical insights into the corrosion protection of magnesium alloys. Full article

(This article belongs to the Special Issue Corrosion and Protection of Lightweight Engineering Materials: Mg Alloys, Al Alloys, Ti Alloys and Other Related Metals)

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15 pages, 5943 KB

Open AccessFeature PaperEditor’s ChoiceArticle

A Survey on the Oxidation Behavior of a Nickel-Based Alloy Used in Natural Gas Engine Exhaust Valve Seats

by José Henrique Alano, Renato Luiz Siqueira, Claudio Beserra Martins Júnior, Rodrigo Silva, Guilherme dos Santos Vacchi and Carlos Alberto Della Rovere

Metals 2023, 13(1), 49; https://doi.org/10.3390/met13010049 - 24 Dec 2022

Cited by 6 | Viewed by 3544

Abstract

This study reports the oxidation behavior of a Ni-based alloy used in the manufacture of valve seats for automotive engine exhaust systems. Isothermal thermogravimetric analyses were carried out at temperatures of 660, 740, 860, and 900 °C under an oxygen atmosphere for up [...] Read more.

This study reports the oxidation behavior of a Ni-based alloy used in the manufacture of valve seats for automotive engine exhaust systems. Isothermal thermogravimetric analyses were carried out at temperatures of 660, 740, 860, and 900 °C under an oxygen atmosphere for up to 1 h. At 660 and 740 °C, only one stage was observed during the whole time studied. At this stage, the oxide layer was formed mainly by NiO + Cr₂O₃, following a linear oxidation law with a rate constant (K_l) on the order of magnitude of 10⁻⁶ kg/m²s and an apparent activation energy (E_a) of ~47 kJ/mol. At 860 and 900 °C, an identical first stage was observed with a transition to a different stage. In the second stage, the oxidation layer was composed of Cr₂O₃, and a parabolic oxidation law was followed with a rate constant (K_p) on the order of 10⁻⁸ kg²/m⁴s and E_a of ~128 kJ/mol. Moreover, the Ni-based alloy formed a dense and compact oxide layer after oxidation, with no apparent cavities, pores, or microcracks. Characterization techniques such as Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS), Fourier Transform Infrared Spectroscopy (FTIR), and Raman Spectroscopy were carried out to characterize the formed oxide layer. Full article

(This article belongs to the Special Issue Composition Design and Damage Mechanism of Crystal Superalloys)

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37 pages, 17666 KB

Open AccessFeature PaperEditor’s ChoiceArticle

Effect of σ-Phase on the Strength, Stress Relaxation Behavior, and Corrosion Resistance of an Ultrafine-Grained Austenitic Steel AISI 321

by Vladimir I. Kopylov, Aleksey V. Nokhrin, Natalia A. Kozlova, Mikhail K. Chegurov, Mikhail Yu. Gryaznov, Sergey V. Shotin, Nikolay V. Melekhin, Nataliya Yu. Tabachkova, Ksenia E. Smetanina and Vladimir N. Chuvil’deev

Metals 2023, 13(1), 45; https://doi.org/10.3390/met13010045 - 24 Dec 2022

Cited by 13 | Viewed by 5075

Abstract

This paper reported the results of research into the effect of Equal Channel Angular Pressing (ECAP) temperature and 1-h annealing temperature on mechanical properties, stress-relaxation resistance, and corrosion resistance of austenitic steel AISI 321L with strongly elongated thin δ-ferrite particles in its microstructure. [...] Read more.

This paper reported the results of research into the effect of Equal Channel Angular Pressing (ECAP) temperature and 1-h annealing temperature on mechanical properties, stress-relaxation resistance, and corrosion resistance of austenitic steel AISI 321L with strongly elongated thin δ-ferrite particles in its microstructure. The formation of α′-martensite and fragmentation of austenite grains takes place during ECAP. Ultrafine-grained (UFG) steels demonstrate increased strength. However, we observed a reduced Hall–Petch coefficient as compared with coarse-grained (CG) steels due to the fragmentation of δ-ferrite particles. UFG steel specimens were found to have 2–3 times higher stress-relaxation resistance as compared with CG steels. For the first time, the high stress-relaxation resistance of UFG steels was shown to stem from a internal stress-relaxation mechanism, i.e., the interaction of lattice dislocations with non-equilibrium grain boundaries. Short-time 1-h annealing of UFG steel specimens at 600–800 °C was found to result in the nucleation of σ-phase nanoparticles. These nanoparticles affect the grain boundary migration, raise strength, and stress-relaxation resistance of steel but reduce the corrosion resistance of UFG steel. Lower corrosion resistance of UFG steel was shown to be related to the formation of α′-martensite during ECAP and the nucleation of σ-phase particles during annealing. Full article

(This article belongs to the Special Issue Environmentally-Assisted Degradation of Metals and Alloys)

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18 pages, 8364 KB

Open AccessEditor’s ChoiceArticle

Challenges and Latest Developments in Diffusion Bonding of High-Magnesium Aluminium Alloy (Al-5056/Al-5A06) to Stainless Steels

by Amir A. Shirzadi, Chengcong Zhang, Muhammad Zeeshan Mughal and Peiyun Xia

Metals 2022, 12(7), 1193; https://doi.org/10.3390/met12071193 - 13 Jul 2022

Cited by 11 | Viewed by 6382

Abstract

The aim of this work was to investigate the challenges associated with bonding Al-Mg alloys and develop a new method for bonding these alloys to steels. During an extensive R&D project, over 80 attempts, using 11 methods, were made to bond Al-6 wt.% [...] Read more.

The aim of this work was to investigate the challenges associated with bonding Al-Mg alloys and develop a new method for bonding these alloys to steels. During an extensive R&D project, over 80 attempts, using 11 methods, were made to bond Al-6 wt.% Mg alloy (Al-5056/Al-5A06) to two types of stainless steels (heat-resistant 1Cr18Ni9Ti and conventional 316). Wide ranges of temperature (500 °C to 580 °C), pressure (0.5 MPa to 10 MPa) and time (1 min to 2 h) were used when direct diffusion bonding of these alloys. Then, effects of using various interlayers and brazing foils were investigated. The interlayers used in this work were gallium, pure titanium, copper and aluminium foils, aluminium 6061 alloy sheets, aluminium-silicon brazing foils, zinc and zinc alloy foils as well as an active brazing foil (known as Incusil-ABA containing silver, copper, indium and titanium). Several complex and multi-stage processes, using up to 3 different interlayers in the same joint, were also developed and assessed. Examination and assessment of the bonded samples, including failed attempts, paved the way of developing new methods for bonding these dissimilar materials. A number of samples with tensile strengths from 200 MPa to 226 MPa were made by using complex combinations of 2 or 3 interlayers and triple-stage bonding cycles. The highest recorded bond strength was 226 MPa in the as-bonded condition. This value is above the measured yield strength (134 MPa) and about 93% of the measured ultimate strength (243 MPa) of the parent Al-Mg alloy after it was subjected to the same bonding cycle. Since the use of complex processes was not feasible for bonding large components, a simpler and more practical bonding cycle was also developed in the project. Using the simpler process, joints with tensile strengths around 90 MPa could be made. This article also sheds light on the difficulties associated with brazing and soldering aluminium alloys with a high magnesium content. Full article

(This article belongs to the Special Issue Advances in Technology and Applications of Diffusion Bonding)

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13 pages, 6920 KB

Open AccessEditor’s ChoiceArticle

Effect of Process Control Agent on Microstructures and High-Temperature Oxidation Behavior of a Nickel-Based ODS Alloy

by Zhe Mao, Jing Li, Shi Liu and Liangyin Xiong

Metals 2022, 12(6), 1029; https://doi.org/10.3390/met12061029 - 17 Jun 2022

Cited by 4 | Viewed by 3392

Abstract

Two nickel-based oxide-dispersion-strengthened (ODS) alloys supplemented with different amounts of process control agent (PCA) were prepared. The microstructures including grains and nanometric oxides and the subsequent oxidation behavior of these ODS alloys were investigated. It was found that the distribution of nanometric oxides [...] Read more.

Two nickel-based oxide-dispersion-strengthened (ODS) alloys supplemented with different amounts of process control agent (PCA) were prepared. The microstructures including grains and nanometric oxides and the subsequent oxidation behavior of these ODS alloys were investigated. It was found that the distribution of nanometric oxides in the nickel-based ODS alloy is uniform and the grains are refined by adding a proper amount of PCA in the mechanical milling, while the blocking effect on the diffusion of active elements Y, Al and Ti among powders takes place with an excessive amount of PCA, resulting in the precipitation of large-size oxides in local areas of the alloy. After oxidation in air at 1000 °C for 200 h, the oxide scales on the surface of both nickel-based ODS alloys are composed of Cr₂O₃. As Y-rich oxide particles are precipitated in the matrix, the thickness of the oxide scale is significantly reduced compared with non-ODS alloys. However, due to the influence of grain boundaries on the diffusion of elements, the oxide scale on the surface of an alloy with finer grain size is thicker. The oxidation resistance of ODS alloys strongly depends on the exact manufacturing process. Full article

(This article belongs to the Special Issue High Temperature Corrosion or Oxidation of Metals and Alloys)

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13 pages, 3661 KB

Open AccessEditor’s ChoiceArticle

Effect of Austenitizing Temperature on the Work Hardening Behavior of Air-Hardening Steel LH800

by Xiang Luo, Zhenli Mi, Yanxin Wu, Yonggang Yang, Haitao Jiang and Kuanhui Hu

Metals 2022, 12(6), 1026; https://doi.org/10.3390/met12061026 - 16 Jun 2022

Cited by 5 | Viewed by 4105

Abstract

In this paper, we present the effect of austenitizing temperature on the work hardening behavior of air-hardening steel LH800 by evaluating the influence of austenitizing temperature on microstructure evolution and mechanical properties, using Hollomon, Differential Crussard–Jaoul (D_C-J), and Modified C-J (M [...] Read more.

In this paper, we present the effect of austenitizing temperature on the work hardening behavior of air-hardening steel LH800 by evaluating the influence of austenitizing temperature on microstructure evolution and mechanical properties, using Hollomon, Differential Crussard–Jaoul (D_C-J), and Modified C-J (M_C-J) work hardening models. The results reveal that with an increase in austenitizing temperature, there is an increase in the percentage of martensite, along with an increase in the strength and hardness of the LH800 steel; on the other hand, there is a decrease in the plasticity. Austenitized at 825 °C, LH800 steel exhibits its highest strength and good plasticity, with a tensile strength of 897 MPa and an elongation of 13.6%. The comparison between the three strain hardening models revealed that the Hollomon model was the finest fit for the experimental data utilized and could illustrate the work hardening behavior of LH800 steel most suitably. This model manifests a two-stage work hardening mechanism; the first stage is related to the plastic deformation of ferrite phase, while the second stage deals with the co-deformation of ferrite and martensite/bainite phase. As austenitizing temperature increases, the work hardening ability of LH800 steel diminishes at each stage, the transition strain decreases, and the plastic deformation of martensite starts earlier. Full article

(This article belongs to the Special Issue Development and Performance Optimization of High-Strength Steels)

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12 pages, 6416 KB

Open AccessEditor’s ChoiceArticle

Structure, Mechanical Properties and Friction Characteristics of the Al-Mg-Sc Alloy Modified by Friction Stir Processing with the Mo Powder Addition

by Tatiana Kalashnikova, Evgeny Knyazhev, Denis Gurianov, Andrey Chumaevskii, Andrey Vorontsov, Kirill Kalashnikov, Natalya Teryukalova and Evgeny Kolubaev

Metals 2022, 12(6), 1015; https://doi.org/10.3390/met12061015 - 15 Jun 2022

Cited by 5 | Viewed by 3348

Abstract

In this study, samples of Al-Mg-Sc alloy were investigated after friction stir processing with the addition of Mo powder. Holes were drilled into 5 mm-thick aluminum alloy sheets into which Mo powder was added at percentages of 5, 10, and 15 wt%. The [...] Read more.

In this study, samples of Al-Mg-Sc alloy were investigated after friction stir processing with the addition of Mo powder. Holes were drilled into 5 mm-thick aluminum alloy sheets into which Mo powder was added at percentages of 5, 10, and 15 wt%. The workpieces with different powder contents were then subjected to four passes of friction stir processing. Studies have shown that at least three tool passes are necessary and sufficient for a uniform Mo powder distribution in the stir zone, but the number of required passes is higher with an increase in the Mo content. Due to the temperature specifics of the processing, no intermetallic compounds are formed in the stir zone, and Mo is distributed as separate particles of different sizes. The average ultimate strength of the composite materials after four passes is approximately 387 MPa in the stir zone, and the relative elongation of the material changes from 15 to 24%. The dry sliding friction test showed that the friction coefficient of the material decreases with the addition of 5 wt% Mo, but with a further increase in Mo content, returns to the original material values. Full article

(This article belongs to the Special Issue Mechanical Properties of Metals Welding Joints)

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25 pages, 8807 KB

Open AccessEditor’s ChoiceArticle

Biofilm Development on Carbon Steel by Iron Reducing Bacterium Shewanella putrefaciens and Their Role in Corrosion

by Sachie Welikala, Saad Al-Saadi, Will P. Gates, Christopher Panter and R. K. Singh Raman

Metals 2022, 12(6), 1005; https://doi.org/10.3390/met12061005 - 12 Jun 2022

Cited by 8 | Viewed by 4666

Abstract

Microscopic, electrochemical and surface characterization techniques were used to investigate the effects of iron reducing bacteria (IRB) biofilm on carbon steel corrosion for 72 and 168 h under batch conditions. The organic nutrient availability for the bacteria was varied to evaluate biofilms formed [...] Read more.

Microscopic, electrochemical and surface characterization techniques were used to investigate the effects of iron reducing bacteria (IRB) biofilm on carbon steel corrosion for 72 and 168 h under batch conditions. The organic nutrient availability for the bacteria was varied to evaluate biofilms formed under nutritionally rich, as compared to nutritionally deficient, conditions. Focused ion beam-scanning electron microscopy (FIB-SEM) was used to investigate the effect of subsurface biofilm structures on the corrosion characteristics of carbon steel. Hydrated biofilms produced by IRB were observed under environmental scanning electron microscope (ESEM) with minimal surface preparation, and the elemental composition of the biofilms was investigated using energy dispersive spectroscopy (EDX). Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) was used to provide information on the organic and inorganic chemical makeup of the biofilms. Electrochemical techniques employed for assessing corrosion, by open circuit potential, linear polarization and potentiodynamic polarization tests indicated no significant difference in the corrosion resistance for carbon steel in IRB-inoculated, compared to the abiotic solutions of common Postgate C after 72 and 168 h. However, the steel was found to be more susceptible to corrosion when the yeast extract was removed from the biotic environment for the 168 h test. In the absence of yeast nutrient, it is postulated that IRB received energy by transforming the protective film of Fe³⁺ into more soluble Fe²⁺ products. Full article

(This article belongs to the Special Issue Advances in Corrosion and Protection of Materials)

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15 pages, 6999 KB

Open AccessEditor’s ChoiceArticle

Effect of Two-Step High Temperature Treatment on Phase Transformation and Microstructure of V-Bearing Bainitic Steel

by Bo Lv, Dongxin Yin, Dongyun Sun, Zhinan Yang, Xiaoyan Long and Zeliang Liu

Metals 2022, 12(6), 983; https://doi.org/10.3390/met12060983 - 7 Jun 2022

Cited by 3 | Viewed by 2892

Abstract

The effects of VC precipitation on phase transformation, microstructure, and mechanical properties were studied by controlling two-step isothermal treatment, i.e., austenization followed by intercritical transformation. The results show that the bainite transformation time of 950 °C–860 °C treatment and 950 °C–848 °C treatment [...] Read more.

The effects of VC precipitation on phase transformation, microstructure, and mechanical properties were studied by controlling two-step isothermal treatment, i.e., austenization followed by intercritical transformation. The results show that the bainite transformation time of 950 °C–860 °C treatment and 950 °C–848 °C treatment is shorter than that of 950 °C single-step treatment. This is related to the isothermal ferrite transformation in the intercritical transformation range. The formation of ferrite nuclei increases the density of medium temperature bainite nucleation sites and decrease the bainite nucleation activation energy. At the same time, a large number of VC particles are precipitated. The additional VC particles provide numbers of preferential nucleation sites. The toughness of the specimen treated at 950~870 °C is improved, which is related to the large proportion of high angle grain boundaries. High angle grain boundaries can hinder crack propagation or change the direction of crack propagation. The specimen treated at 950 °C–848 °C exhibits large proportion of low angle grain boundaries, which is beneficial for the strength improvement. Full article

(This article belongs to the Special Issue High Performance Bainitic Steels)

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10 pages, 2383 KB

Open AccessEditor’s ChoiceArticle

A New Strategy for Dissimilar Material Joining between SiC and Al Alloys through Use of High-Si Al Alloys

by Yongjing Yang, Ayan Bhowmik, Jin Lee Tan, Zehui Du and Wei Zhou

Metals 2022, 12(5), 887; https://doi.org/10.3390/met12050887 - 23 May 2022

Cited by 4 | Viewed by 4153

Abstract

Joining metals and ceramics plays a crucial role in many engineering applications. The current research aims to develop a simple and convenient approach for dissimilar material joining between SiC and Al alloys. In this work, Al alloys with Si contents varying from 7 [...] Read more.

Joining metals and ceramics plays a crucial role in many engineering applications. The current research aims to develop a simple and convenient approach for dissimilar material joining between SiC and Al alloys. In this work, Al alloys with Si contents varying from 7 wt.% to 50 wt.% were bonded with SiC at a high temperature of 1100 °C by a pressure-less bonding process in a vacuum furnace, and shear tests were carried out to study the bonding strength. When using low-Si Al alloys to bond with SiC, the bonding strength was very low. The bonding strength of Al/SiC joints increased significantly through the use of high-Si Al alloys with 30 wt.% and 50 wt.% Si. The shear strength achieved (28.8 MPa) is far higher than those reported previously. The remarkable improvement in bonding strength is attributed to the suppression of brittle interfacial products and reduced thermal stresses. This research provides a new strategy for joining between SiC and a wide range of Al alloys through the use of high-Si Al alloys as the interlayers. Full article

(This article belongs to the Special Issue Advances in Welding, Joining and Surface Coating Technology)

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31 pages, 6813 KB

Open AccessEditor’s ChoiceArticle

The Holes of Zn Phosphate and Hot Dip Galvanizing on Electrochemical Behaviors of Multi-Coatings on Steel Substrates

by Thiago Duarte, Yuri A. Meyer and Wislei R. Osório

Metals 2022, 12(5), 863; https://doi.org/10.3390/met12050863 - 18 May 2022

Cited by 46 | Viewed by 4670

Abstract

The aim of this investigation is focused on the evaluation of distinctive coatings commonly applied in the automotive industry. The resulting corrosion behavior is analyzed by using electrochemical impedance spectroscopy (EIS), equivalent circuit (EC) and potentiodynamic polarization curves. The novelty concerns a comparison [...] Read more.

The aim of this investigation is focused on the evaluation of distinctive coatings commonly applied in the automotive industry. The resulting corrosion behavior is analyzed by using electrochemical impedance spectroscopy (EIS), equivalent circuit (EC) and potentiodynamic polarization curves. The novelty concerns a comparison between tricationic phosphate (TCP), cataphoretic electrodeposition (CED) of an epoxy layer, TCP + CED and HDG (hot-dip galvanized) + TCP + CED multi-coatings. Both the naturally deposited and defect-induced damage (incision) coatings are examined. The experimental impedance parameters and corrosion current densities indicate that multi-coating system (HDG + TCP + CED layers) provides better protection. Both planar and porous electrode behaviors are responsible to predict the corrosion mechanism of the majority of samples examined. Although induced-damage samples reveal that corrosion resistances decreased up to 10×, when compared with no damaged samples, the same trend of the corrosion protection is maintained, i.e., TCP < CED < TCP + CED < HDG + TCP + CED. It is also found that the same trend verified by using electrochemical parameters is also observed when samples are subjected under salt spray condition (500 h). It is also found that porous electrode behavior is not a deleterious aspect to corrosion resistance. It is more intimately associated with initial thickness coating, while corrosion resistance is associated with adhesion of the CED layer on TCP coating. The results of relative cost-to-efficiency to relative coating density ratios are associated with fact that a CED coating is necessary to top and clear coating applications and the TCP + CED and the HDG/TCP + CED coating systems exhibit the best results. Full article

(This article belongs to the Special Issue Investigations on Mechanical Properties and Corrosion Resistance of Alloys and Compounds)

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13 pages, 3133 KB

Open AccessFeature PaperEditor’s ChoiceArticle

An Improved Approach to Manufacture Carbon Nanotube Reinforced Magnesium AZ91 Composites with Increased Strength and Ductility

by Samaneh Nasiri, Guang Yang, Erdmann Spiecker and Qianqian Li

Metals 2022, 12(5), 834; https://doi.org/10.3390/met12050834 - 13 May 2022

Cited by 7 | Viewed by 3646

Abstract

Multiwalled carbon nanotubes (MWCNTs) are decorated with Pt nanoparticles by a “layer-by-layer” approach using poly (sodium 4-styrene sulfonate) (PSS) and poly (diallyl dimethylammonium chloride) (PDDA). Transmission electron microscopy (TEM) images and Energy Dispersive X-ray (EDX) analysis of the samples confirm Pt deposition on [...] Read more.

Multiwalled carbon nanotubes (MWCNTs) are decorated with Pt nanoparticles by a “layer-by-layer” approach using poly (sodium 4-styrene sulfonate) (PSS) and poly (diallyl dimethylammonium chloride) (PDDA). Transmission electron microscopy (TEM) images and Energy Dispersive X-ray (EDX) analysis of the samples confirm Pt deposition on surfaces of CNTs. Dispersibility and dispersion stability of MWCNTs in the solvents are enhanced when MWCNTs are coated with Pt nanoparticles. Mg AZ91 composites reinforced with MWCNTs are then produced by a melt stirring process. Compression tests of the composites show that adding 0.05% wt Pt-coated MWCNTs in AZ91 improves the composite’s mechanical properties compared to the pure AZ91 and pristine MWCNT/AZ91. Fracture surface analysis of the composite using a scanning electron microscope (SEM) shows individual pulled out MWCNTs in the case of the Pt-coated MWCNT/AZ91 composites. This finding can be attributed to the uniform dispersion of Pt-coated MWCNTs in Mg due to the improved wettability of Pt-coated MWCNTs in Mg melts. The study of the pull-out behaviour of pristine and Pt-coated CNTs from an Mg matrix using molecular dynamics simulation supports this interpretation. Full article

(This article belongs to the Special Issue Advances in High Strength–Ductility Synergy Materials)

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14 pages, 4127 KB

Open AccessFeature PaperEditor’s ChoiceArticle

Real-Time Quality Monitoring of Laser Cladding Process on Rail Steel by an Infrared Camera

by Pornsak Srisungsitthisunti, Boonrit Kaewprachum, Zhigang Yang and Guhui Gao

Metals 2022, 12(5), 825; https://doi.org/10.3390/met12050825 - 11 May 2022

Cited by 19 | Viewed by 7254

Abstract

Laser cladding is considered to be a highly complex process to set up and control because it involves several parameters, such as laser power, laser scanning speed, powder flow rate, powder size, etc. It has been widely studied for metal-part coating and repair [...] Read more.

Laser cladding is considered to be a highly complex process to set up and control because it involves several parameters, such as laser power, laser scanning speed, powder flow rate, powder size, etc. It has been widely studied for metal-part coating and repair due to its advantage in controllable deposited materials on a small target substrate with low heat-affected distortion. In this experiment, laser cladding of U75V and U20Mn rail steels with Inconel 625 powder was captured by an infrared camera with image analysis software to monitor the laser cladding process in order to determine the quality of the cladded substrates. The cladding temperature, thermal gradient, spot profile, and cooling rate were determined from infrared imaging of the molten pool. The results showed that cladding temperature and molten pool’s spot closely related to the laser cladding process condition. Infrared imaging provided the cooling rate from a temperature gradient which was used to correctly predict the microhardness and microstructure of the HAZ region. This approach was able to effectively detect disturbance and identify geometry and microstructure of the cladded substrate. Full article

(This article belongs to the Special Issue High Performance Bainitic Steels)

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14 pages, 9661 KB

Open AccessEditor’s ChoiceArticle

Microstructure Characteristics and Wear Performance of a Carburizing Bainitic Ferrite + Martensite Si/Al-Rich Gear Steel

by Yanhui Wang, Qingsong He, Qian Yang, Dong Xu, Zhinan Yang and Fucheng Zhang

Metals 2022, 12(5), 822; https://doi.org/10.3390/met12050822 - 10 May 2022

Cited by 8 | Viewed by 4360

Abstract

In this paper, a new low-carbon alloy gear steel is designed via Si/Al alloying. The carburizing and austempering, at a temperature slightly higher than the martensitic transformation point (Ms) of the surface and much lower than the Ms of the core, for different [...] Read more.

In this paper, a new low-carbon alloy gear steel is designed via Si/Al alloying. The carburizing and austempering, at a temperature slightly higher than the martensitic transformation point (Ms) of the surface and much lower than the Ms of the core, for different times, were carried out on the newly designed gear steel. After heat treatment, a series of different microstructures (superfine bainitic ferrite + retained austenite, superfine bainitic ferrite + martensite + retained austenite, and martensite + retained austenite) were obtained on the surface, whilst the low-carbon lath martensitic microstructure was obtained in the core. The microstructure of the surface was examined using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The phase composition was analyzed using X-ray diffraction (XRD). The hardness and wear resistance of the surface as well as the hardness distribution of carburizing layer of the samples with different microstructures were studied. The results show that the Si/Al-rich gear steel, after carburizing and austempering at 200 °C for 8 h, not only has excellent mechanical properties but also has high wear resistance, which meets the technical requirements of heavy-duty gear steel. The research work in this paper can provide a data reference for the application of carburized steel with mixed microstructures of bainitic ferrite and martensite in the design of heavy-duty gear. Full article

(This article belongs to the Special Issue High Performance Bainitic Steels)

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21 pages, 5807 KB

Open AccessEditor’s ChoiceArticle

Observing the Effect of Grain Refinement on Crystal Growth of Al and Mg Alloys during Solidification Using In-Situ Neutron Diffraction

by Abdallah Elsayed, Francesco D’Elia, Comondore Ravindran and Dimitry Sediako

Metals 2022, 12(5), 793; https://doi.org/10.3390/met12050793 - 4 May 2022

Cited by 4 | Viewed by 3614

Abstract

The present research uses in-situ neutron diffraction to examine the effect of grain refinement on grain growth during solidification of Al-5 wt.% Cu and Mg-5 wt.% Zn alloys. The alloys were grain refined through additions of Al-5Ti-1B and Zr, respectively. The in-situ neutron [...] Read more.

The present research uses in-situ neutron diffraction to examine the effect of grain refinement on grain growth during solidification of Al-5 wt.% Cu and Mg-5 wt.% Zn alloys. The alloys were grain refined through additions of Al-5Ti-1B and Zr, respectively. The in-situ neutron diffraction experiments were carried out by heating the alloys to temperatures above the liquidus and subsequently cooling in 5 or 10 °C temperature steps to temperatures below solidus, while being irradiated by thermal neutrons. With the addition of grain refiners, grain size reductions of 92% were observed for both the Al-5 wt.% Cu and Mg-5 wt.% Zn alloys. The refined and unrefined Al-5 wt.% Cu alloys contained α-Al with Al₂Cu along the grain boundary regions. Differences in Al₂Cu morphology were observed in the grain refined alloys. The Mg-5 wt.% Zn alloy contained MgZn intermetallic phases with primary Mg. The refined Mg-5 wt.% Zn-0.7 wt.% Zr alloy contained Mg, MgZn and Zn₂Zr phases. In-situ neutron diffraction enabled quantification of individual plane solid fraction growth for the α-Al and Al₂Cu phases in the Al-Cu alloys, and for α-Mg in the Mg alloys. For the unrefined Al-5 wt.% Cu, the coarse microstructure resulted in a rapid solid fraction rise at temperatures just below liquidus followed by a gradual increase in solid fraction until the sample was fully solid. The grain-refined Al-5 wt.% Cu alloys showed a columnar to equiaxed microstructure transition and a more gradual growth in fraction solid throughout solidification. For the Mg-5 wt.% Zn alloy, the more packed

(0002)

and

(10 \bar{1} 1)

α-Mg plane intensities grew at a slower rate than the

(10 \bar{1} 0)

plane intensity, resulting in an irregular grain structure. With the addition of the Zr grain refiner, the Mg-5 wt.% Zn-0.7 wt.% Zr alloy had

(10 \bar{1} 0)

,

(0002)

and

(10 \bar{1} 1)

planes intensities all increasing at similar rates, especially at the early stages of solidification. FactSage™ (version 6.4, Montréal, QC, Canada) equilibrium solidification models followed the fraction solid curves developed by tracking the fastest growing planes of the Mg alloys. Full article

(This article belongs to the Special Issue Material Properties—Superalloys, Ferrous and Lightweight Alloys and Metal Matrix Composites)

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8 pages, 5218 KB

Open AccessEditor’s ChoiceArticle

Electrochemical Corrosion Resistance of Mg Alloy ZK60 in Different Planes with Respect to Extrusion Direction

by G. Keerthiga, Dandapani Vijayshankar, MJNV Prasad, Mirco Peron, Jafar Albinmousa and RK Singh Raman

Metals 2022, 12(5), 782; https://doi.org/10.3390/met12050782 - 30 Apr 2022

Cited by 8 | Viewed by 3524

Abstract

The electrochemical corrosion resistance of a Mg-Zn-Zr alloy, ZK60, in different planes with respect to the extrusion direction was investigated in 3.5 wt% NaCl. The motivation of this study lies in the influence of extrusion on the grain size, texture and precipitation characteristics [...] Read more.

The electrochemical corrosion resistance of a Mg-Zn-Zr alloy, ZK60, in different planes with respect to the extrusion direction was investigated in 3.5 wt% NaCl. The motivation of this study lies in the influence of extrusion on the grain size, texture and precipitation characteristics of magnesium alloys, and the profound role of these characteristics in the corrosion resistance of the alloys. Corrosion resistance was found to be considerably superior in the plane transverse to the extrusion direction (TD) than in the extrusion direction (ED) or normal to the extrusion direction (ND). The difference in the corrosion resistance was attributed to the variations in microstructural features in the TD, ED and ND directions. Full article

(This article belongs to the Special Issue Investigations on Mechanical Properties and Corrosion Resistance of Alloys and Compounds)

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3 pages, 162 KB

Open AccessEditor’s ChoiceEditorial

Recent Developments in Medium and High Manganese Steels

by Colin P. Scott

Metals 2022, 12(5), 743; https://doi.org/10.3390/met12050743 - 27 Apr 2022

Cited by 8 | Viewed by 4136

Abstract

A huge amount of intellectual effort is currently being devoted to the study of medium and high manganese steels due to the diverse and impressive mechanical properties that can be achieved with these steels [...] Full article

(This article belongs to the Special Issue Recent Developments in Medium and High Manganese Steels)

13 pages, 5018 KB

Open AccessEditor’s ChoiceArticle

Investigation on Corrosion Resistance Properties of 17-4 PH Bound Metal Deposition As-Sintered Specimens with Different Build-Up Orientations

by Pietro Forcellese, Tommaso Mancia, Michela Simoncini and Tiziano Bellezze

Metals 2022, 12(4), 588; https://doi.org/10.3390/met12040588 - 30 Mar 2022

Cited by 19 | Viewed by 5549

Abstract

Additive manufacturing is a promising and emerging technology that can transform the global manufacturing and logistics by cutting costs and times of production. Localized corrosion resistance properties of 0°, 45°, and 90° build-up orientations of 17-4 PH as-sintered samples, manufactured by means of [...] Read more.

Additive manufacturing is a promising and emerging technology that can transform the global manufacturing and logistics by cutting costs and times of production. Localized corrosion resistance properties of 0°, 45°, and 90° build-up orientations of 17-4 PH as-sintered samples, manufactured by means of Bound Metal Deposition (BMD), have been investigated by electrochemical and morphological investigations. The cyclic potentiodynamic polarization curves and the open circuit potential monitoring, together with potential drop analysis, revealed that the BMD localized corrosion resistance properties were lowered if compared to a wrought 17-4 PH: a characteristic anodic behavior and many drops in potential were recorded for BMD, whilst the wrought specimens presented a typical passive behavior with pitting corrosion. Morphological investigations by scanning electron microscopy and energy-dispersive X-ray analysis revealed the presence of porosities and defects, especially for the 90° build-up orientation, and inclusions of SiO₂. The 45° build-up orientation showed the best corrosion resistance properties among all the BMD specimens, even though defects and porosities were observed, suggesting that their morphology and geometry influenced the overall corrosion behavior. Full article

(This article belongs to the Special Issue Corrosion and Protection of Stainless Steels)

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