Open AccessArticle
The Effect of Gadolinium on the Microstructures and Charpy Impact Properties of Super Duplex Stainless Steels
Metals 2018, 8(7), 474; https://doi.org/10.3390/met8070474 -
Abstract
Super duplex stainless steels (SDSSs), exhibiting excellent strength and corrosion resistance, serve as the attractive materials in a variety of industries. However, improvements in their ductility and impact-toughness are required in extreme environments. In this study, the effects of gadolinium on the microstructures
[...] Read more.
Super duplex stainless steels (SDSSs), exhibiting excellent strength and corrosion resistance, serve as the attractive materials in a variety of industries. However, improvements in their ductility and impact-toughness are required in extreme environments. In this study, the effects of gadolinium on the microstructures and Charpy impact properties of super duplex stainless steels were investigated. A base super duplex stainless steel (BDSS) and a gadolinium-added super duplex stainless steel (GDSS) were successfully fabricated using an air casting method. The oxygen content and grain size of SDSSs were found to decrease because of high reactivity of gadolinium with oxygen. Moreover, the average inclusion size and area of GDSS also decreased even with a slight decrease in the average distance between inclusions. Both the BDSS and GDSS exhibited typical Charpy impact transition behavior from −196 °C to 200 °C. Moreover, the GDSS impact energies using Charpy test were higher than those of BDSS over the entire temperature range. Moreover, the ductile-to-brittle transition temperature (DBTT) of SDSSs calculated from the fracture appearance transition temperature (FATT) significantly decreased by over 20 °C with the addition of gadolinium. Full article
Figures

Figure 1

Open AccessArticle
Electrochemical Properties of Nb-Substituted Zr-Ti-Ni Hydrogen Storage Alloy Negative Electrodes for Nickel-Metal Hydride Batteries
Metals 2018, 8(7), 473; https://doi.org/10.3390/met8070473 -
Abstract
Crystal structure, pressure-composition isotherms and electrochemical properties of the Zr0.6−xTi0.4NbxNi (x = 0.01, 0.02, and 0.05) alloys were investigated. Their X-ray diffraction profiles demonstrated that all the Zr0.6−xTi0.4NbxNi
[...] Read more.
Crystal structure, pressure-composition isotherms and electrochemical properties of the Zr0.6−xTi0.4NbxNi (x = 0.01, 0.02, and 0.05) alloys were investigated. Their X-ray diffraction profiles demonstrated that all the Zr0.6−xTi0.4NbxNi alloys consisted of the primary phase with the B33-type orthorhombic structure and the secondary phase with the B2-type Ti0.6Zr0.4Ni cubic structure. Rietveld refinement demonstrated that the atomic fraction of the secondary phase increased with the Nb content. The Zr0.6−xTi0.4NbxNi alloys were lower in hydrogen storage capacity than the Nb-free Zr0.6Ti0.4Ni alloy due to an increase in the abundance of the secondary phase. In the charge-discharge tests with the Zr0.6−xTi0.4NbxNi alloy negative electrodes, all the initial discharge curves had two potential plateaus due to the electrochemical hydrogen desorption of trihydride to monohydride and monohydride to alloy of the primary phase. The total discharge capacities at 333 and 303 K for the Zr0.58Ti0.4Nb0.02Ni alloy negative electrode were 384 and 335 mAh g−1, respectively, which were higher than those of the other Zr0.6−xTi0.4NbxNi and Zr0.6Ti0.4Ni alloy negative electrodes. Full article
Figures

Figure 1

Open AccessArticle
Machinability Study of Developed Composite AA6061-ZrO2 and Analysis of Influence of MQL
Metals 2018, 8(7), 472; https://doi.org/10.3390/met8070472 -
Abstract
Aluminium metal matrix replaces high melting point and high density conventional materials, thus minimizing the usage of energy and supporting the environment. This work develops a low-weight, high-strength composite material with the help of AA 6061 and ZrO2 through a stir casting
[...] Read more.
Aluminium metal matrix replaces high melting point and high density conventional materials, thus minimizing the usage of energy and supporting the environment. This work develops a low-weight, high-strength composite material with the help of AA 6061 and ZrO2 through a stir casting route incorporated with a squeeze casting setup. Machining and machining tools create impacts on clean environments, as they deal with lubricants and power consumption. Having taken this issue into consideration, this research studies the effect of machining parameters on surface roughness, tool wear, and cutting force, while turning the developed metal matrix composite in dry and minimum quantity lubrication conditions. The turning experiment was performed by designing parameters using an L27 orthogonal array. The turning condition was dry and with minimum quantity lubrication (MQL). The responses obtained in the turning process were analysed using the analysis of variance (ANOVA) technique to find the most influential factor and its percentage contribution. Optimal machining parameters were investigated and tabulated with the help of main effect plots and S/N ratio graphs. Studies prove that there is a linear relationship between MQL versus surface roughness and tool wear, and there was no substantial effect on cutting force. Full article
Figures

Figure 1a

Open AccessArticle
Research on Selective Laser Melting of Ti6Al4V: Surface Morphologies, Optimized Processing Zone, and Ductility Improvement Mechanism
Metals 2018, 8(7), 471; https://doi.org/10.3390/met8070471 -
Abstract
The quality and mechanical properties of titanium alloy fabricated using selective laser melting (SLM) are critical to the adoption of the process which has long been impeded by the lack of uniformity in SLM-fabrication parameter optimization. In order to address this problem, laser
[...] Read more.
The quality and mechanical properties of titanium alloy fabricated using selective laser melting (SLM) are critical to the adoption of the process which has long been impeded by the lack of uniformity in SLM-fabrication parameter optimization. In order to address this problem, laser power and scanning speed were combined into linear energy density as an independent variable, while surface morphology was defined as a metric. Based on full-factor experiments, the surface quality of SLM-fabricated titanium alloy was classified into five zones: severe over-melting zone, high-energy density nodulizing zone, smooth forming zone, low-energy density nodulizing zone, and sintering zone. The mechanism resulting in the creation of each zone was analyzed. Parameter uniformity was achieved by establishing a parameter window for each zone, and it also revealed that under smooth forming conditions, the relationship of linear energy density to the quality of the formed surface is not linear. It was also found that fabrication efficiency could be improved in the condition of the formation of a smooth surface by increasing laser power and scanning speed. In addition, maximum elongation of the SLM-fabricated titanium alloy increased when the densified parts were processed using an appropriate heat treatment, from a low value of 5.79% to 10.28%. The mechanisms of change in ductility of the alloy were thoroughly analyzed from the perspectives of surface microstructure and fracture morphology. Results indicate that after heat treatment, the microcosmic structure of the alloy was converted from acicular martensite α’ phase to a layered α+β double-phase structure, the fracture type also changed from quasi-cleavage to ductile fracture. Full article
Figures

Graphical abstract

Open AccessArticle
Microstructural Characterization of Surface Softening Behavior for Cu-Bearing Martensitic Steels after Laser Surface Heat Treatment
Metals 2018, 8(6), 470; https://doi.org/10.3390/met8060470 -
Abstract
The surface hardening and softening behavior of two types of medium carbon martensitic steel (AISI P20-improved and AISI P21) after laser-assisted heat treatment was quantitatively compared. The laser-assisted heat treatment was performed using a high-power diode laser with in situ temperature and laser
[...] Read more.
The surface hardening and softening behavior of two types of medium carbon martensitic steel (AISI P20-improved and AISI P21) after laser-assisted heat treatment was quantitatively compared. The laser-assisted heat treatment was performed using a high-power diode laser with in situ temperature and laser power control (two-color pyrometer system). For AISI P20-improved steel, the peak hardness value within the hardening zone was approximately 640 HV after laser-assisted heat treatment at a temperature of 1473 K. In other words, the hardness increased by 120% from the base metal level (290 HV). However, for AISI P21 steel, the hardness within the heat-treated zone did not change from that of the base metal (410 HV), despite being accompanied by martensite transformation. Moreover, it was clearly observed that the hardness dropped below the level of the base metal at the boundary between the heat-treated zone and the base metal region, forming a softening zone. This softening behavior was strongly related to coarsening and a looser distribution of Cu precipitates compared with that of the base metal region, despite the same matrix phase (i.e., tempered martensite) existing in the softening zone and in the base metal region. Full article
Figures

Figure 1

Open AccessFeature PaperArticle
Techno-Economic Analysis of High-Pressure Metal Hydride Compression Systems
Metals 2018, 8(6), 469; https://doi.org/10.3390/met8060469 -
Abstract
Traditional high-pressure mechanical compressors account for over half of the car station’s cost, have insufficient reliability, and are not feasible for a large-scale fuel cell market. An alternative technology, employing a two-stage, hybrid system based on electrochemical and metal hydride compression technologies, represents
[...] Read more.
Traditional high-pressure mechanical compressors account for over half of the car station’s cost, have insufficient reliability, and are not feasible for a large-scale fuel cell market. An alternative technology, employing a two-stage, hybrid system based on electrochemical and metal hydride compression technologies, represents an excellent alternative to conventional compressors. The high-pressure stage, operating at 100–875 bar, is based on a metal hydride thermal system. A techno-economic analysis of the metal hydride system is presented and discussed. A model of the metal hydride system was developed, integrating a lumped parameter mass and energy balance model with an economic model. A novel metal hydride heat exchanger configuration is also presented, based on minichannel heat transfer systems, allowing for effective high-pressure compression. Several metal hydrides were analyzed and screened, demonstrating that one selected material, namely (Ti0.97Zr0.03)1.1Cr1.6Mn0.4, is likely the best candidate material to be employed for high-pressure compressors under the specific conditions. System efficiency and costs were assessed based on the properties of currently available materials at industrial levels. Results show that the system can reach pressures on the order of 875 bar with thermal power provided at approximately 150 °C. The system cost is comparable with the current mechanical compressors and can be reduced in several ways as discussed in the paper. Full article
Figures

Figure 1

Open AccessArticle
Ag2O Nanoparticles-Doped Manganese Immobilized on Graphene Nanocomposites for Aerial Oxidation of Secondary Alcohols
Metals 2018, 8(6), 468; https://doi.org/10.3390/met8060468 -
Abstract
Ag2O nanoparticles-doped MnO2 decorated on different percentages of highly reduced graphene oxide (HRG) nanocomposites, i.e., (X%)HRG/MnO2–(1%)Ag2O (where X = 0–7), were fabricated through straight-forward precipitation procedure, and 400 °C calcination, while upon calcination at 300 °C
[...] Read more.
Ag2O nanoparticles-doped MnO2 decorated on different percentages of highly reduced graphene oxide (HRG) nanocomposites, i.e., (X%)HRG/MnO2–(1%)Ag2O (where X = 0–7), were fabricated through straight-forward precipitation procedure, and 400 °C calcination, while upon calcination at 300 °C and 500 °C temperatures, it yielded MnCO3 and manganic trioxide (Mn2O3) composites, i.e., [(X%)HRG/MnCO3–(1%)Ag2O] and [(X%)HRG/Mn2O3–(1%)Ag2O], respectively. These nanocomposites have been found to be efficient and very effective heterogeneous catalysts for selective oxidation of secondary alcohols into their respective ketones using O2 as a sole oxidant without adding surfactants or nitrogenous bases. Moreover, a comparative catalytic study was carried out to investigate the catalytic efficiency of the synthesized nanocomposites for the aerobic oxidation of 1-phenylethanol to acetophenone as a substrate reaction. Effects of several factors were systematically studied. The as-prepared nanocomposites were characterized by TGA, XRD, SEM, EDX, HRTEM, BET, Raman, and FTIR. The catalyst with structure (5%)HRG/MnO2–(1%)Ag2O showed outstanding specific activity (16.0 mmol/g·h) with complete conversion of 1-phenylethanol and >99% acetophenone selectivity within short period (25 min). It is found that the effectiveness of the catalyst has been greatly improved after using graphene support. A broad range of alcohols have selectively transformed to desired products with 100% convertibility and no over-oxidation products have been detected. The recycling test of (5%)HRG/MnO2–(1%)Ag2O catalyst for oxidation of 1-phenylethanol suggested no obvious decrease in its performance and selectivity even after five subsequent runs. Full article
Figures

Figure 1

Open AccessArticle
Relationship between Flow Behavior and Microstructure Evolution during Isothermal Compression of near β Titanium Alloy Ti-55531 with Acicular Starting Microstructure
Metals 2018, 8(6), 467; https://doi.org/10.3390/met8060467 -
Abstract
Near β titanium alloy Ti-55531 with an acicular starting microstructure was isothermally compressed at 750–825 °C and 10−3−1 s−1. The microstructure evolution and its influence on the flow behavior of yielding and softening were investigated. Discontinuous or continuous yielding
[...] Read more.
Near β titanium alloy Ti-55531 with an acicular starting microstructure was isothermally compressed at 750–825 °C and 10−3−1 s−1. The microstructure evolution and its influence on the flow behavior of yielding and softening were investigated. Discontinuous or continuous yielding depends on the hindrance to the dislocation motion coming from the β grain boundary or α phase. At higher temperatures, the hindrance mainly comes from the β grain boundary. Its discontinuous action, including the piling-up and subsequent loosening of dislocations at the β grain boundary, leads to discontinuous yielding. At lower temperatures, the continuous hindrance to the dislocation motion, which is exerted by the β grain boundary and acicular α, causes continuous yielding. Sequentially, the substructures in acicular α are evolved from high-density dislocations or local shear bands, which depend on the orientation relationship between β and α. Then, the β matrix edges into the acicular α along substructure boundaries. The higher strain rate decreases the deformation time to carry out the fragmentation of acicular α, while the higher temperature decreases the dislocation density due to the recovery of β, which does not benefit the substructure formation and subsequent fragmentation of acicular α. Therefore, the retardation of acicular fragmentation and the as-resulted decreased flow softening rate are observed. Full article
Figures

Figure 1

Open AccessArticle
Annealing-Induced High Ordering and Coercivity in Novel L10 CoPt-Based Nanocomposite Magnets
Metals 2018, 8(6), 466; https://doi.org/10.3390/met8060466 -
Abstract
A novel class of quaternary intermetallic alloys based on CoPt is investigated in view of their interesting magnetic properties induced by the presence of hard magnetic L10 phase. A Co48Pt28Ag6B18 alloy has been prepared by
[...] Read more.
A novel class of quaternary intermetallic alloys based on CoPt is investigated in view of their interesting magnetic properties induced by the presence of hard magnetic L10 phase. A Co48Pt28Ag6B18 alloy has been prepared by rapid solidification from the melt and subjected to various isothermal annealing procedures. The structure and magnetism of both as-cast and annealed samples as well as the phase evolution with temperature are investigated by means of thermal analysis, X-ray, and selected area electron diffraction, scanning and high-resolution electron microscopy, and magnetic measurements. The X-ray diffraction (XRD) analysis shows that both the as-cast alloy and the sample annealed at 400 °C (673 K) have a nanocrystalline structure where fcc CoPt phase predominates. Annealing at 473 °C promotes the formation of L10 phase triggered by the disorder-order phase transformation as documented in the differential scanning calorimetry results. The sample annealed at 670 °C (943 K) shows full formation of L10 CoPt as revealed by XRD. Magnetic measurements showed coercivity values ten times increased compared to the as-cast state. This confirms the full formation of L10 CoPt in the annealed samples. Moreover, detailed atomic resolution HREM images and SAED patterns show the occurrence of the rarely seen (003) superlattice peaks, which translated into a high ordering of the L10 CoPt superlattice. Such results spur more interest in finding novel classes of nanocomposite magnets based on L10 phase. Full article
Figures

Figure 1

Open AccessFeature PaperArticle
Understanding the Recovery of Rare-Earth Elements by Ammonium Salts
Metals 2018, 8(6), 465; https://doi.org/10.3390/met8060465 -
Abstract
While the recovery of rare earth elements (REEs) from aqueous solution by ionic liquids (ILs) has been well documented, the metal compounds that are formed in the organic phase remain poorly characterized. Using spectroscopic, analytical, and computational techniques, we provide detailed chemical analysis
[...] Read more.
While the recovery of rare earth elements (REEs) from aqueous solution by ionic liquids (ILs) has been well documented, the metal compounds that are formed in the organic phase remain poorly characterized. Using spectroscopic, analytical, and computational techniques, we provide detailed chemical analysis of the compounds formed in the organic phase during the solvent extraction of REEs by [(n-octyl)3NMe][NO3] (IL). These experiments show that REE recovery using IL is a rapid process and that IL is highly durable. Karl-Fischer measurements signify that the mode of action is unlikely to be micellar, while ions of the general formula REE(NO3)4(IL)2 are seen by negative ion electrospray ionization mass spectrometry. Additionally, variable temperature 139La nuclear magnetic resonance spectroscopy suggests the presence of multiple, low symmetry nitrato species. Classical molecular dynamics simulations show aggregation of multiple ILs around a microhydrated La3+ cation with four nitrates completing the inner coordination sphere. This increased understanding is now being exploited to develop stronger and more selective, functionalized ILs for REE recovery. Full article
Figures

Graphical abstract

Open AccessArticle
Machining Distortion of Titanium Alloys Aero Engine Case Based on the Energy Principles
Metals 2018, 8(6), 464; https://doi.org/10.3390/met8060464 -
Abstract
The simulation of a complete manufacturing process to produce an aero engine case, including forging, rolling, and machining processes, is analyzed via finite element software. The deformation of the turning and drilling processes is quantitatively studied using the energy principles. Firstly, simulations of
[...] Read more.
The simulation of a complete manufacturing process to produce an aero engine case, including forging, rolling, and machining processes, is analyzed via finite element software. The deformation of the turning and drilling processes is quantitatively studied using the energy principles. Firstly, simulations of multi-stage forging of aero engine case and machining-induced residual stress are conducted and verified via the residual stresses test in order to provide the initial elastic strain energy condition prior to machining processes. The effects of blank forging-induced residual stress and machining-induced residual stress on the deformation of titanium alloys aero engine case are investigated. Secondly, a potential energy expression for the machining processes is developed. The predicted results of turning and drilling simulations indicate that there is an optimal process in which the deformation and potential energy decline rapidly compared with the other processes and finally, gradually stabilize at the end of the process. Full article
Figures

Figure 1

Open AccessArticle
Connected Process Design for Hot Working of a Creep-Resistant Mg–4Al–2Ba–2Ca Alloy (ABaX422)
Metals 2018, 8(6), 463; https://doi.org/10.3390/met8060463 -
Abstract
With a view to design connected processing steps for the manufacturing of components, the hot working behavior of the ABaX422 alloy has been characterized for the as-cast and extruded conditions. In the as-cast condition, the alloy has a limited workability, due to the
[...] Read more.
With a view to design connected processing steps for the manufacturing of components, the hot working behavior of the ABaX422 alloy has been characterized for the as-cast and extruded conditions. In the as-cast condition, the alloy has a limited workability, due to the presence of a large volume of intermetallic phases at the grain boundaries, and is not suitable to process at high speeds. A connected processing step has been designed on the basis of the results of the processing map for the as-cast alloy, and this step involves the extrusion of the cast billet to obtain a 12 mm diameter rod product at a billet temperature of 390 °C and at a ram speed of 1 mm s−1. The microstructure of the extruded rod has a finer grain size, with redistributed fine particles of the intermetallic phases. The processing map of the extruded rod exhibited two new domains, and the one in the temperature range 360–420 °C and strain rate range 0.2–10 s−1 is useful for manufacturing at high speeds, while the lower temperature develops a finer grain size in the product to improve the room temperature strength and ductility. The area of the flow instability is also reduced by the extrusion step, widening the workability window. Full article
Figures

Figure 1

Open AccessArticle
Assessment of Metal Flow Balance in Multi-Output Porthole Hot Extrusion of AA6060 Thin-Walled Profile
Metals 2018, 8(6), 462; https://doi.org/10.3390/met8060462 -
Abstract
For the porthole hot extrusion of a thin-walled tube based on metal flow, the role of the die’s structure should be focused on to achieve precision formation, especially for multi-output extrusion and/or complex cross-sectional profiles. In order to obtain a better metal flow
[...] Read more.
For the porthole hot extrusion of a thin-walled tube based on metal flow, the role of the die’s structure should be focused on to achieve precision formation, especially for multi-output extrusion and/or complex cross-sectional profiles. In order to obtain a better metal flow balance, a multi-output porthole extrusion die was developed, including some novel features such as a circular pattern of the portholes with a dart-shaped inlet bridge, a buckle angle in the inlet side of the upper die, a two-step welding chamber, and a non-uniform bearing length distribution. Through the use of thermo-mechanical modeling combined with the Taguchi method, the underlying effects of key die features were investigated, such as the billet buckle angle, the porthole bevel angle, the depth of the welding chamber, and the type of bridge on the metal flow balance. The experimental validation showed that the developed numerical model for the multi-output porthole extrusion process had high prediction accuracy, and was acceptable for use in an industrial extrusion with a complex section. Full article
Figures

Figure 1

Open AccessArticle
Synthesis of Core-Shell Carbon Encapsulated Fe2O3 Composite through a Facile Hydrothermal Approach and Their Application as Anode Materials for Sodium-Ion Batteries
Metals 2018, 8(6), 461; https://doi.org/10.3390/met8060461 -
Abstract
Carbon encapsulated Fe2O3 nanoparticles (C@Fe2O3) were successfully synthesized via a facile and environmentally friendly hydrothermal method and prototyped in anode materials for sodium ion batteries (SIBs). High-resolution transmission and scanning electronic microscopy observations exhibited the formation
[...] Read more.
Carbon encapsulated Fe2O3 nanoparticles (C@Fe2O3) were successfully synthesized via a facile and environmentally friendly hydrothermal method and prototyped in anode materials for sodium ion batteries (SIBs). High-resolution transmission and scanning electronic microscopy observations exhibited the formation of a highly core-shelled C@Fe2O3 composite consisting of carbon layers coated onto uniform Fe2O3 nanoparticles with a median diameter of 46.1 nm. This core-shell structure can repress the aggregation of Fe2O3 nanoparticles, preventing the harsh volume change of the electrode, enhancing the electric conductivity of the active materials, and promoting Na-ion transformation during cycling. The electrochemical performances of the C@Fe2O3 composite, as anodes for SIBs, retained a reversible capacity of 305 mAh g−1 after 100 cycles at 50 mA g−1 and exhibited an excellent cyclability at various current densities due to the synergistic effect between the carbon layers and Fe2O3. These results suggest that C@Fe2O3 composites present much potential as anode materials for rechargeable SIBs. Full article
Figures

Figure 1

Open AccessArticle
Microstructure, Mechanical Properties and Wear Behavior of the Rheoformed 2024 Aluminum Matrix Composite Component Reinforced by Al2O3 Nanoparticles
Metals 2018, 8(6), 460; https://doi.org/10.3390/met8060460 -
Abstract
The 2024 nanocomposite reinforced with Al2O3 nanoparticles was fabricated by the ultrasonic assisted semisolid stirring (UASS) method and rheoformed into a cylinder component. Microstructure, mechanical properties, and wear behavior of the rheoformed composite components were investigated. The results showed that
[...] Read more.
The 2024 nanocomposite reinforced with Al2O3 nanoparticles was fabricated by the ultrasonic assisted semisolid stirring (UASS) method and rheoformed into a cylinder component. Microstructure, mechanical properties, and wear behavior of the rheoformed composite components were investigated. The results showed that the composite components with complete filling status and a good surface were rheoformed successfully. The deformation of semisolid slurries was mainly dominated by flow of liquid incorporating solid grains (FLS), sliding between solid grains (SSG), and plastic deformation of solid grains (PDS). Mechanical properties of the rheoformed composite components were influenced by stirring temperature, stirring time, and volume fraction of Al2O3 nanoparticles. The optimal ultimate tensile strength (UTS) of 358 MPa and YS of 245 MPa were obtained at the bottom of the rheoformed composite components after a 25-min stirring of composite semisolid slurry with 5% Al2O3 nanoparticles at 620 °C. Enhancement of mechanical properties was attributed to high density dislocations and dislocation tangles and uniform dispersed Al2O3 nanoparticles in the aluminum matrix. Natural ageing led to the occurrence of needle-like Al2CuMg phase and short-rod-like Al2Cu phase. UTS of 417 MPa and YS of 328 MPa of the rheoformed composite components were achieved after T6 heat treatment. Improvement of mechanical properties is due to the more precipitated needle-like Al2CuMg phase and short-rod-like Al2Cu phase. Wear resistance of the rheoformed composite components was higher than that of the rheoformed matrix component. Wear resistance of the rheoformed composite component increased with an increase in Al2O3 nanoparticles from 1% to 7%. A slight decrease in wear rate resulted from 10% Al2O3 nanoparticles due to greater agglomeration of Al2O3 nanoparticles. A combination mechanism of adhesion and delamination was determined according to worn surface morphology. Full article
Figures

Graphical abstract

Open AccessArticle
Study of Plasma Electrolytic Oxidation Coatings on Aluminum Composites
Metals 2018, 8(6), 459; https://doi.org/10.3390/met8060459 -
Abstract
Coatings, with a thickness of up to 75 µm, were formed by plasma electrolytic oxidation (PEO) under the alternating current electrical mode in a silicate-alkaline electrolyte on aluminum composites without additives and alloyed with copper (1–4.5%). The coatings’ structure was analyzed by scanning
[...] Read more.
Coatings, with a thickness of up to 75 µm, were formed by plasma electrolytic oxidation (PEO) under the alternating current electrical mode in a silicate-alkaline electrolyte on aluminum composites without additives and alloyed with copper (1–4.5%). The coatings’ structure was analyzed by scanning electron microscopy, X-ray microanalysis, X-ray photoelectron spectroscopy, nuclear backscattering spectrometry, and XRD analysis. The coatings formed for 60 min were characterized by excessive aluminum content and the presence of low-temperature modifications of alumina γ-Al2O3 and η-Al2O3. The coatings formed for 180 min additionally contained high-temperature corundum α-Al2O3, and aluminum inclusions were absent. The electrochemical behavior of coated composites and uncoated ones in 3% NaCl was studied. Alloyage of aluminum composites with copper increased the corrosion current density. Plasma electrolytic oxidation reduced it several times. Full article
Figures

Figure 1

Open AccessArticle
Effects of Vanadium on Microstructure and Wear Resistance of High Chromium Cast Iron Hardfacing Layer by Electroslag Surfacing
Metals 2018, 8(6), 458; https://doi.org/10.3390/met8060458 -
Abstract
The 3.6C-20Cr-Fe-(0–2.32)V high chromium cast iron (HCCI) hardfacing layers were deposited on low alloy steel by electroslag surfacing. The microstructure of hardfacing layers were observed and the carbide types, size and area fraction were measured. In addition, the hardness and wear resistance were
[...] Read more.
The 3.6C-20Cr-Fe-(0–2.32)V high chromium cast iron (HCCI) hardfacing layers were deposited on low alloy steel by electroslag surfacing. The microstructure of hardfacing layers were observed and the carbide types, size and area fraction were measured. In addition, the hardness and wear resistance were tested. Results show that the interface between hardfacing layer and low alloy steel is defect free. 3.6C-20Cr-Fe hardfacing layer contains primary carbides and eutectic. Increasing V wt % in the hardfacing layer, primary carbides are decreasing by increasing eutectic along with martensite formation. For 1.50 wt % of V, the microstructure contains a lot of eutectic and a little of martensite. For 2.32 wt % of V, primary austenite formed, the microstructure is primary austenite, eutectic and a little of martensite. In the V alloyed hardfacing layers, V has strong affinity with carbon than chromium, hence V can replace a part of Cr in M7C3 and (Cr4.4–4.7Fe2.1–2.3V0.2–0.5)C3 type carbides are formed. When the V is 2.32 wt %, (Cr0.23V0.77)C carbides are formed in the hardfacing layer. The hardness and wear resistance are improved by increasing V from 0 to 1.50 wt %. However, when the V is 2.32 wt %, the primary austenite has reduced the hardness and wear resistance of hardfacing layer. Full article
Figures

Figure 1

Open AccessArticle
Tensile Creep Characterization and Prediction of Zr-Based Metallic Glass at High Temperatures
Metals 2018, 8(6), 457; https://doi.org/10.3390/met8060457 -
Abstract
The high temperature creep behaviors of a Zr-based bulk metallic glass (BMG) are studied by uniaxial tensile creep experiments under applied stresses of 50–180 MPa at temperatures of 660–700 K. The microstructural observations of the BMG samples after creep tests show that crystalline
[...] Read more.
The high temperature creep behaviors of a Zr-based bulk metallic glass (BMG) are studied by uniaxial tensile creep experiments under applied stresses of 50–180 MPa at temperatures of 660–700 K. The microstructural observations of the BMG samples after creep tests show that crystalline phases can be detected under high temperature or high applied stress. Constitutive models for predicting the high temperature creep behaviors of the studied Zr-based BMG are established based on the θ projection method. The creep activation energy and stress exponent are also calculated to establish the creep model. The parameters of the established models are found to be closely associated with the applied stress and temperature. The results show an excellent agreement between the measured and predicted results, confirming the validity of the established model to accurately estimate the high temperature creep curves for the Zr-based BMG. Moreover, based on the classical diffusion creep theory, a schematic model is proposed to describe the creep behaviors of BMGs from the framework of free volume theory. Full article
Figures

Figure 1

Open AccessArticle
A New Cumulative Fatigue Damage Rule Based on Dynamic Residual S-N Curve and Material Memory Concept
Metals 2018, 8(6), 456; https://doi.org/10.3390/met8060456 -
Abstract
This paper introduces a new phenomenological cumulative damage rule to predict damage and fatigue life under variable amplitude loading. The rule combines a residual S-N curve approach and a material memory concept to describe the damage accumulation behavior. The residual S-N curve slope
[...] Read more.
This paper introduces a new phenomenological cumulative damage rule to predict damage and fatigue life under variable amplitude loading. The rule combines a residual S-N curve approach and a material memory concept to describe the damage accumulation behavior. The residual S-N curve slope is regarded as a variable with respect to the loading history. The change in slope is then used as a damage measure and quantified by a material memory degeneration parameter. This model improves the traditional linear damage rule by taking the load-level dependence and loading sequence effect into account, which still preserves its superiority. A series of non-uniform fatigue loading protocols are used to demonstrate the effectiveness of the proposed model. The prediction results using the proposed model are more accurate than those using three popular damage models. Moreover, several common characteristics and fundamental properties of the chosen fatigue models are extracted and discussed. Full article
Figures

Figure 1

Open AccessArticle
Theoretical Study of Particle Dissolution during Homogenization in Cu–Fe–P Alloy
Metals 2018, 8(6), 455; https://doi.org/10.3390/met8060455 -
Abstract
The effect of temperature, soaking time and particle size on the dissolution of particles (Fe3P and Fe) during homogenization was simulated employing Thermocalc® and DICTRA software. The initial precipitate size was determined through metallographic evaluation on industrial as-cast Cu–Fe–P alloy.
[...] Read more.
The effect of temperature, soaking time and particle size on the dissolution of particles (Fe3P and Fe) during homogenization was simulated employing Thermocalc® and DICTRA software. The initial precipitate size was determined through metallographic evaluation on industrial as-cast Cu–Fe–P alloy. The particle sizes vary from submicron (<1 μm) up to 10 μm before the heat treatment. As homogenization temperature rises, the dissolution rate increases as well, but only on temperatures above 1273 K (1000 °C) is the rate capable of completely dissolving particles effectively. At temperatures above 1273 K (1000 °C), precipitates with sizes below 5 μm dissolve completely into the Cu matrix, while larger particles only slightly decrease their size. Particles at enriched copper areas remain undissolved and slightly increase their size which is attributed to micro segregation and the local change of equilibrium conditions. The simulation results are in agreement with homogenization trials at lab scale. Full article
Figures

Figure 1