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Classical alloy design strategies often aim to benefit from metastability. Examples are numerous: metastable transformation- and twinning-induced plasticity steels, cobalt or titanium based alloys, age hardenable aluminum alloys, and severe plastic deformed nanostructured copper. In each of these cases, superior engineering property combinations are achieved by exploring limits of stability. For the case of high-entropy alloys (HEAs), on the other hand, majority of present research efforts focus on exploring compositions that would yield stable single-phase structures. HEA metastability and its effects on microstructure and property development constitute only a relatively small fraction of ongoing work. To help motivate and guide a corresponding shift in HEA research efforts, here in this paper, we provide an overview of the research activities on metastability in HEAs. To this end, we categorize the past research on the topic into two groups based on their focus, namely, compositional and structural stability, and discuss the most relevant and exciting findings.

Metallic alloys containing multiple principal alloying elements have created a growing interest in exploring the property limits of metals and understanding the underlying physical mechanisms. Refractory high-entropy alloys have drawn particular attention due to their high melting points and excellent softening resistance, which are the two key requirements for high-temperature applications. Their compositional space is immense even after considering cost and recyclability restrictions, providing abundant design opportunities. However, refractory high-entropy alloys often exhibit apparent brittleness and oxidation susceptibility, which remain important challenges for their processing and application. Here, utilizing natural-mixing characteristics among refractory elements, we designed a Ti 38 V 15 Nb 23 Hf 24 refractory high-entropy alloy that exhibits >20% tensile ductility in the as-cast state, and physicochemical stability at high temperatures. Exploring the underlying deformation mechanisms across multiple length scales, we observe that a rare β′-phase plays an intriguing role in the mechanical response of this alloy. These results reveal the effectiveness of natural-mixing tendencies in expediting high-entropy alloy discovery. A refractory high-entropy alloy was designed with the composition chosen based on the natural-mixing characteristics among refractory elements; this alloy demonstrates good tensile ductility in the as-cast state and physicochemical stability at high temperatures.

Recent research in multi-principal element alloys (MPEAs) has increasingly focused on the role of short-range order (SRO) on material performance. However, the mechanisms of SRO formation and its precise control remain elusive, limiting the progress of SRO engineering. Here, leveraging advanced additive manufacturing techniques that produce samples with a wide range of cooling rates (up to 10 7  K s −1 ) and an enhanced semi-quantitative electron microscopy method, we characterize SRO in three CoCrNi-based face-centered-cubic (FCC) MPEAs. Surprisingly, irrespective of the processing and thermal treatment history, all samples exhibit similar levels of SRO. Atomistic simulations reveal that during solidification, prevalent local chemical order arises in the liquid-solid interface (solidification front) even under the extreme cooling rate of 10 11  K s −1 . This phenomenon stems from the swift atomic diffusion in the supercooled liquid, which matches or even surpasses the rate of solidification. Therefore, SRO is an inherent characteristic of most FCC MPEAs, insensitive to variations in cooling rates and even annealing treatments typically available in experiments. Chemical short-range order (CSRO) has been explored as a new design parameter for tuning the performance of multi-principal element alloys (MPEAs), yet its formation kinetics remains unclear. Here the authors demonstrate that significant CSRO forms during solidification, making it ubiquitous in many MPEAs.

Starting with graphite, TiB2, and Ti-6Al-4V powders, the present work demonstrated that hybrid (TiC+TiB) network-strengthened Ti-6Al-4V—based composites can be fabricated via an integrated low-energy ball-milling and reaction hot-pressing-sintering technique. With the aid of phase equilibrium and powder densification kinetic calculations, the corresponding sintering parameters were optimized and tunable network microstructures were subsequently achieved. Tensile properties for these composites were examined at elevated temperatures of 500 °C, 550 °C, 600 °C, and 650 °C, the results of which indicated that the 50-μm network configuration with 5 vol pct reinforcer content led to the most enhanced tensile strength compared to both Ti-6Al-4V alloys and solely TiB-reinforced Ti-6Al-4V composites. The underlying strengthening mechanisms were mainly ascribed to carbon interstitial dissolution, reinforcer-assisted grain refinement, and extensive dispersoids. It was recognized from fractographic analyses that the matrix/reinforce interface contributed to the major crack propagation source at temperatures below 550 °C, leading to brittlelike fracture along the network boundary; however, once testing temperatures rose above 600 °C, matrix tearing and reinforcer cut-through mechanisms took place, giving rise to ductile fracture. Based on the experimental observations and theoretical calculations, future perspectives regarding the processing and microstructural manipulation for advanced high-temperature titanium matrix composites were also discussed.

By investigating a metastable high-entropy alloy, we report a latent strengthening mechanism that is associated with the thermally-induced epsilon-martensite-to-austenite reverse transformation. We show this reversion-assisted hardening effect can be achieved in the same time-scale and temperature range as conventional bake-hardening treatment, but leads to both improved strength and cumulative ductility. Key mechanisms are discussed considering transformation kinetics, kinematics, strengthening and ductilization modules.

Enabling a hydrogen economy requires the development of materials resistant to hydrogen embrittlement (HE). More than 100 years of research have led to several mechanisms and models describing how hydrogen interacts with lattice defects and leads to mechanical property degradation. However, solutions to protect materials from hydrogen are still scarce. Here, we investigate the role of interstitial solutes in protecting critical crystalline defects sensitive to hydrogen. Ab initio calculations show that boron and carbon in solid solutions at grain boundaries can efficiently prevent hydrogen segregation. We then realized this interface protection concept on martensitic steel, a material strongly prone to HE, by doping the most sensitive interfaces with different concentrations of boron and carbon. These segregations, in addition to stress relaxations, critically reduce the hydrogen ingress by half, leading to an unprecedented resistance against HE. This tailored interstitial segregation strategy can be extended to other metallic materials susceptible to hydrogen-induced interfacial failure. This study reveals the role of boron and carbon in protecting critical interfaces sensitive to hydrogen: the tailored segregation critically reduces the hydrogen ingress, leading to an unprecedented resistance against hydrogen embrittlement.

Metallurgists have designed an extraordinary titanium alloy that is light, strong and flexible, and which recovers its original shape after large amounts of deformation — even when as cold as liquid helium or hotter than boiling water. A lightweight but strong alloy that recovers its shape after deformation.

Metallurgical production traditionally involves three steps: extracting metals from ores, mixing them into alloys by liquid processing and thermomechanical processing to achieve the desired microstructures 1 , 2 . This sequential approach, practised since the Bronze Age, reaches its limit today because of the urgent demand for a sustainable economy 2 – 5 : almost 10% of all greenhouse gas emissions are because of the use of fossil reductants and high-temperature metallurgical processing. Here we present a H 2 -based redox synthesis and compaction approach that reforms traditional alloy-making by merging metal extraction, alloying and thermomechanical processing into one single solid-state operation. We propose a thermodynamically informed guideline and a general kinetic conception to dissolve the classical boundaries between extractive and physical metallurgy, unlocking tremendous sustainable bulk alloy design opportunities. We exemplify this approach for the case of Fe–Ni invar bulk alloys 6 , 7 , one of the most appealing ferrous materials but the dirtiest to produce: invar shows uniquely low thermal expansion 6 , 8 , 9 , enabling key applications spanning from precision instruments to cryogenic components 10 – 13 . Yet, it is notoriously eco-unfriendly, with Ni causing more than 10 times higher CO 2 emission than Fe per kilogram production 2 , 14 , qualifying this alloy class as a perfect demonstrator case. Our sustainable method turns oxides directly into green alloys in bulk forms, with application-worthy properties, all obtained at temperatures far below the bulk melting point, while maintaining a zero CO 2 footprint. A one-step hydrogen-based redox process turns oxides directly into green alloys in bulk forms, with application-worthy properties.