Metals | Issue Cover Collection Published in 2024

The articles below have been selected as the 2024 Issue Cover Articles by the Editorial Office of Metals (ISSN: 2075-4701), covering multiple fields within the scope of the journal. We hope that they provide insights and references for scholars in these fields.

	1. “Deciphering Microstructures and Phases of Gas-Atomised Novel Al-Fe-Si-Cr-Ni Alloys” by Bhaskaranand Bhatt, Alessandra Martucci, Enrico Virgillito, Federico Gobber, Federica Bondioli, Diego Manfredi, Mariangela Lombardi and Paolo Fino Metals 2024, 14(1), 17; https://doi.org/10.3390/met14010017 Available online: https://www.mdpi.com/2075-4701/14/1/17 Cover story: This groundbreaking research investigated the effects of high cooling rates in the inert gas atomization (IGA) process on the amount of Fe, Cr, and Ni as alloying elements in Al-Si-based alloys. To emphasize the importance of sustainability in developing novel alloys, the widespread AlSi10Mg and AISI 304L were used as starting materials for IGA. The selected AlFe9Si8Cr2Ni and AlFe18Si8Cr5Ni2 compositions were extensively analyzed from compositional and microstructural points of view, focusing on the alloying element content and particle size. The results revealed unique microstructural features, distinct phases, increased melting temperatures than typical Al alloys, and promising potential for additive manufacturing.
	2. “A First-Time Investigation into Ecofriendly and Biocompatible Mg-Se Binary System for a Greener Earth” by Michael Johanes, Vasuudhaa Sonawane and Manoj Gupta Metals 2024, 14(2), 163; https://doi.org/10.3390/met14020163 Available online: https://www.mdpi.com/2075-4701/14/2/163 Cover story: In the 21st century and beyond, researchers must select materials that do not contaminate planet earth (especially water bodies, soil, and air) to make it a sustainable place to live. To ensure this, we have explored a new Mg-Se binary alloy targeting both engineering and biomedical applications. In the Mg-Se system, magnesium is a nutritional element and selenium has medicinal characteristics. Fundamental thermal, mechanical, and electrochemical characterizations revealed promising results paving the way for materials scientists to develop many more Mg-Se-based alloys (ternary and quaternary or even multicomponent alloys) for a wide spectrum of applications.
	3. “Microstructure Evolution and Numerical Modeling of TC4 Titanium Alloy during Ultrasonic Shot Peening Process” by Yuxuan Yi, Fei Yin, Jiajun Zhai and Yanxiong Liu Metals 2024, 14(3), 275; https://doi.org/10.3390/met14030275 Available online: https://www.mdpi.com/2075-4701/14/3/275 Cover story: A gradient nanostructured surface layer was successfully fabricated on the TC4 Titanium alloy via USP technology. The surface microhardness was elevated from 330 HV to 438 HV with a penetrating depth of around 900 μm after USP. EBSD characterization results confirmed the presence of high-density grain boundaries and dislocation density within the gradient structure within the region of 0–200 μm. TEM characterization indicated a substantial amount of nanograin with an average size of 74.58 nm. The surface strengthening effect was predicted. The calculated maximum residual stress reached 973 MPa after multi-ball impact. The impact behavior of the shots was studied.
	4. “The Integrated Preparation of Porous Tungsten Gradient Materials with a Wide Porosity Range” by Ke Zhu, Hao Jia, Jin Huang and Jian Zhang Metals 2024, 14(4), 427; https://doi.org/10.3390/met14040427 Available online: https://www.mdpi.com/2075-4701/14/4/427 Cover story: Porous tungsten gradient materials with ordered gradient variations in pore size are valuable in the field of vacuum electronic devices. This work combines tape casting and dealloying methods to achieve the integrated preparation of porous tungsten gradient materials with a wide range of controllable porosity. This study focuses on the phase composition and microstructure evolution during the preparation of porous tungsten gradient materials. This work provides a design concept for the integrated preparation of porous metal gradient materials.
	5. “Influence of Various Processing Routes in Additive Manufacturing on Microstructure and Monotonic Properties of Pure Iron—A Review-like Study” by Christof J. J. Torrent, Seyed Vahid Sajadifar, Gregory Gerstein, Julia Richter and Thomas Niendorf Metals 2024, 14(5), 557; https://doi.org/10.3390/met14050557 Available online: https://www.mdpi.com/2075-4701/14/5/557 Cover story: The properties of pure iron processed via hot rolling and electron and laser powder bed fusion, using different raw materials and process conditions, are compared. The manufacturing of the specimens led to five distinct microstructures, each with unique features and mechanical responses. Using optical, electron and transmission electron microscopy, the material was explored in various strain states, allowing for an in-depth examination of the material in the course of quasistatic loading. Grain size and substructures within the grains define the homogeneity of strain distribution and localization. Macro-to-micro insight is given to evaluate the behavior of characteristic microstructures.
	6. “Compositional Design and Thermal Processing of a Novel Lead-Free Cu–Zn–Al–Sn Medium Entropy Brass Alloy” by Spyridon Chaskis, Stavroula Maritsa, Paul Stavroulakis, Sofia Papadopoulou, Russell Goodall and Spyros Papaefthymiou Metals 2024, 14(6), 620; https://doi.org/10.3390/met14060620 Available online: https://www.mdpi.com/2075-4701/14/6/620 Cover story: The rapid development in the field of multicomponent and high-entropy alloys provides an appealing alternative to the challenging issue of lead encountered in commercial brass alloys. Engineering challenges arising in attempts to reduce lead could be successfully tackled by applying the high-entropy alloy (HEA) design concept to these otherwise conventional alloys, resulting in the creation of novel high-entropy brasses (HEBs). Our research deals with the compositional design and thermal processing of a novel lead-free Cu–Al–Zn–Sn medium-entropy brass alloy. This alloy has an elevated specific strength, surpassing many commercial brass alloys, while being entirely lead-free. Applying this approach to commercial alloys provides a feasible solution for modern engineering challenges.
	7. “Effect of Ag and Cu Content on the Properties of Zn-Ag-Cu-0.05Mg Alloys” by Gloria Jara-Chávez, Adrián Amaro-Villeda, Bernardo Campillo-Illanes, Marco Ramírez-Argaez and Carlos González-Rivera Metals 2024, 14(7), 740; https://doi.org/10.3390/met14070740 Available online: https://www.mdpi.com/2075-4701/14/7/740 Cover Story: Zn-Ag-Cu base alloys are reported to more than meet the mechanical property requirements for biomedical alloys by showing YS values > 300 MPa, UTS close to 400 MPa, and elongations between 18 and 26%. The joint effect of increasing amounts of the second phase (Ag, Cu)Zn4 and a micro-alloy with 0.05% Mg is explored as a measure to improve mechanical properties and meet the requirements for biomedical applications. The manufacture of a Zn-based heteromaterial is reported, whose microstructure and mechanical behavior achieve a synergy of strength and ductility that allows these alloys to achieve the properties required for biomedical applications.
	8. “Nodular Graphite Dissolution and Nucleus Observation: High-Temperature Dynamics of Ductile Iron Recycling” by I. Adhiwiguna, N. Nobakht and R. Deike Metals 2024, 14(8), 915; https://doi.org/10.3390/met14080915 Available online: https://www.mdpi.com/2075-4701/14/8/915 Cover story: A significant advancement in ductile cast iron research may have been achieved with the detailed description of the dissolution sequence of nodular graphite structures. This study introduces an alternative approach that enhances the observation of non-metallic inclusion cores within nodular graphite, offering substantial potential for improving material analysis and performance. This work marks a critical step forward in metallurgy, providing insights that could drive future innovations in cast iron technology.
	9. “The Interplay between Nucleation and the Rates of Chemical Reduction in the Synthesis of Bimetallic Nanoparticles in Microemulsions: A Computer Study” by Concha Tojo Metals 2024, 14(9), 987; https://doi.org/10.3390/met14090987 Available online: https://www.mdpi.com/2075-4701/14/9/987 Cover story: The interaction between nucleation and the rates of chemical reduction in Au/Ag, Au/Pt, and Au/Pd nanoparticles prepared in microemulsions were studied via simulations. The impact of nucleation on the final nanostructure depends on the critical nucleus size. At high critical nucleus sizes, nucleation is the main factor determining the final nanostructure, even with very large differences in reduction rates between metals. At small critical nucleus sizes, the difference in reduction rates determines the final nanostructure. The relevance of heteroatomic nucleation on the mechanism of nanoparticle formation depends on the difference between the reduction rates of the two metals selected.
	10. “Microstructure and Fatigue Behavior of PM-HIPed Ni-Based Superalloys and Martensitic Tool Steels: A Review” by Faezeh Javadzadeh Kalahroudi, Fengxiang Lin, Pavel Krakhmalev and Mikael Grehk Metals 2024, 14(10), 1159; https://doi.org/10.3390/met14101159 Available online: https://www.mdpi.com/2075-4701/14/10/1159 Cover story: This review investigates the effects of powder metallurgy and hot isostatic pressing (PM-HIP) on the microstructure and fatigue behavior of Ni-based superalloys and martensitic tool steels. The influences of several PM-HIP parameters, e.g., powder size distribution and oxygen content, on the properties of these high-performance materials are discussed. By focusing on grain size refinement, oxide and precipitate formation and distribution, and other microstructural features, this paper demonstrates how these factors influence mechanical properties and fatigue performance. In addition, it provides insights into fatigue crack initiation mechanisms. These findings could highlight the potential applications of PM-HIPed components.
	11. “Electron Beam Additive Manufacturing of SS316L with a Stochastic Scan Strategy: Microstructure, Texture Evolution, and Mechanical Properties” by K. N. Chaithanya Kumar, Shashank Sharma, Madhavan Radhakrishnan, Rohit Randhavan, Krishna Kamlesh Verma, Shelden Dowden, Zane Weldon Hughes, Rajarshi Banerjee and Narendra B. Dahotre Metals 2024, 14(11), 1278; https://doi.org/10.3390/met14111278 Available online: https://www.mdpi.com/2075-4701/14/11/1278 Cover story: Structural and textural evolution in electron beam powder bed fusion (EB-PBF) is governed by the complex interplay of thermo-kinetic and thermo-mechanical phenomena during melting and solidification. This study from the Center for Agile and Adaptive Additive Manufacturing at the University of North Texas explores how stochastic spot melting (FreeMelt®) and preheat temperature affect the evolution of microstructure and texture of SS316L. A 3D thermo-kinetic/mechanical model revealed compressive stress-driven <110> texture formation, preferred {110}<001> and {110}<111> orientations influenced by solidification kinetics and spatial spot arrangement and triangular-shaped grains orthogonal to the build direction. This work provides fundamental insights for optimizing EB-PBF texture and microstructure.
	12. “Effect of Stable and Transient Cavitation on Ultrasonic Degassing of Al Alloy” by Youngki Lee, Jongmin Kim, Taekyu Ha, Byungil Kang and Youngjig Kim Metals 2024, 14(12), 1372; https://doi.org/10.3390/met14121372 Available online: https://www.mdpi.com/2075-4701/14/12/1372 Cover story: This study provides a novel perspective on the ultrasonic degassing of the A356 alloy by elucidating the distinct roles of stable and transient cavitation. In the initial stages of degassing, the number of bubbles available to rapidly and effectively remove dissolved gases from the melts is critical. Transient cavitation promotes bubble collapses, increasing the generation of new tiny bubbles, which facilitates hydrogen diffusion and gas release. Consequently, this study identifies transient cavitation as the dominant factor in the initial degassing stage and clearly demonstrates that optimizing frequency to enhance transient cavitation is an effective approach to improving degassing efficiency.