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Conventional alloy design based on a single primary element has reached its limits in terms of performance optimization. An alloy design strategy with multi-principal elements has recently been uncovered to overcome this bottleneck. Multicomponent alloys, generally referred to as high-entropy alloys (HEAs), exhibit many promising properties, especially outstanding mechanical performance at cryogenic, ambient, and elevated temperatures. In this article, we focus on precipitation-hardened HEAs, which are potential candidates for next-generation structural materials, especially at high temperatures. The key issues involved include precipitation behaviors, phase stability, and phase control, all of which provide useful guidelines for further development of high-temperature materials with superior performance. In particular, we address the formation of cellular γ′ precipitates at grain boundaries, which is closely related to the embrittlement of HEAs at intermediate temperatures. Critical issues and design strategies in developing HEAs for high-temperature applications are also discussed.
Understanding changes in chemistry, microstructure, and physical properties during synthesis, processing, testing, and even service is vital for materials design and performance. Compared to traditional postmortem material characterization tools, in situ crystallographic characterization can provide considerable data and information on evolution of chemistry, dislocations, twinning, texture, and strains when a material is under external stimuli. Neutrons especially are able to probe material bulk properties and behaviors in extreme environments, thanks to their deep penetrating power and unique sensitivity to differentiate elements from lightweight to transition-metal atoms. In this article, we introduce and describe a diffractometer named VULCAN, which is located at Oak Ridge National Laboratory. This represents a powerful tool to understand materials properties and behaviors under complex environments, in particular, at high temperatures.
Metallic glasses have attractive properties, but since the glassy state is inherently metastable, they are not normally considered for applications at elevated temperatures. Yet, studies have shown that multicomponent and pseudo high-entropy (PHE) compositions can confer useful heat resistance. The formation, thermal stability, and mechanical and chemical properties of multicomponent Fe-(Cr, Mo)- and Zr-based bulk metallic glasses (BMGs) are reviewed to assess their potential as heat-resistant structural materials. The composition Fe43Cr16Mo16C15B10 is castable and fully glassy with rod diameters up to 2.7 mm. Glassy coatings of this material with low porosity, good mechanical properties, and good corrosion resistance can be produced by high-velocity spray coating. The compositions Zr55–65Al7.5–10(TM1,TM2)27.5–35 (TM1 = Fe, Co, Ni, TM2 = Cu, Pd, Ag, Au) yield PHE BMGs, in which a stable cluster-like glassy phase without crystalline precipitates is formed by annealing at temperatures well above the first calorimetric transformation. It is suggested that proliferation of alloy components is an effective method to synthesize metastable metallic materials that retain high strength at elevated temperatures.
Tougher, lighter, and more formable and machinable metals for broader ranges of applications at higher temperatures are needed now more than ever. High-performance computing, high-resolution microscopy, and advanced spectroscopy methods, including neutrons and synchrotron x-rays, together with advances in metallurgy and metal mixology, reveal the potential of multicomponent advanced metals, such as multicomponent bulk metallic glasses and advanced high-entropy alloys. The development of new experimental approaches relates bulk properties and voxel-associated optimized properties throughout structures with high resolution. The correlations from in situ measurements greatly improve crystal plasticity-based models. This issue of MRS Bulletin overviews recent progress in the field, and this article highlights the importance of these new perspectives. The latest progress and directions in the science and technology for prospective high-temperature metals for structural applications are reported.