Research Progress on the Brittle Behaviour of High-entropy Metal Materials in Hydrogen Environment
Hydrogen embrittlement occurs in a wide range of metal materials, and its invisibility and time lag often lead to catastrophic accidents, restricting the application of metal materials in extreme working conditions. Multi- principal-element alloys or high-entropy alloys have broken through the traditional alloy design concept and become a hot spot in the field of new metal materials. It is found that some high-entropy alloys or multi-principal-element alloys possess excellent properties beyond those of traditional alloys or are comparable to traditional alloys, such as ultra-high strength and ductility matching, heat resistance, corrosion resistance, and hydrogen embrittlement resistance, which are expected to be used in extreme working conditions. The work aims to introduce the theoretical mechanisms of hydrogen embrittlement and review the research progress of hydrogen-embrittlement-resistant multi-principal-element alloys.Starting from the concept of hydrogen embrittlement, the mechanisms of hydrogen embrittlement were introduced, including hydrogen pressure theory, hydrogen-enhanced localized plasticity mechanism, hydrogen-enhanced de-cohesion mechanism, hydrogen-enhanced strain-induced vacancies, nanovoid coalescence, and adsorption-induced dislocation emission. Based on the two theories of hydrogen-enhanced localized plasticity mechanism and hydrogen-enhanced de-cohesion mechanism, the factors affecting the hydrogen embrittlement resistance of multi-principal-element alloys were discussed.The factors affecting the hydrogen embrittlement resistance of multi-principal-element alloys include hydrogen content, alloying elements, microstructure, preparation process, heat treatment processes, and experimental conditions. The effect of hydrogen content on the mechanical properties of multi-principal-element alloys is mainly manifested in two aspects: on the one hand, when the hydrogen content exceeds the critical hydrogen concentration of the material, it is easy to induce hydrogen embrittlement of multi-principal-element alloys, resulting in the decline of the mechanical properties of the material; on the other hand, hydrogen can improve the mechanical properties of multi-principal-element alloys through solid solution strengthening and hydrogen-induced nano-twinning mechanism. The role of alloying elements on the hydrogen embrittlement resistance of multi-principal-element alloys is more complex, and many studies have shown that the addition of appropriate amounts of Cr, Mo, Ti, Nb, and other elements has a positive impact on the hydrogen embrittlement resistance of multi-principal-element alloys. The microstructure is also one of the important factors affecting the hydrogen embrittlement resistance of multi-principal-element alloys, which is mainly manifested in the effect on the solid solution and diffusion of hydrogen, the strength and number of hydrogen traps, dislocation motion, crack initiation, and extension, etc. In addition, the microstructure of multi-principal-element alloys obtained by different preparation processes can be adjusted. Besides, there are differences in the hydrogen embrittlement resistance of multi-principal-element alloys obtained by different preparation processes. In this work, the hydrogen embrittlement susceptibility distributions of different materials under electrochemical hydrogen charging are plotted according to the hydrogen embrittlement susceptibility index, which provides a reference for the design of hydrogen-embrittlement- resistant multi-principal-element alloys.Many studies have shown that some multi-principal-element alloys have better characteristics than traditional metal materials in terms of hydrogen embrittlement resistance, and have great development prospects as materials used in extreme working conditions. However, the design of multi-principal-element alloys that satisfy the mechanical properties and hydrogen embrittlement resistance at the same time is still a major challenge. Considering the broad composition space of multi-principal-element alloys, machine learning will be a strong tool for the design of hydrogen-embrittlement-resistant multi-principal-element alloys.
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