Hydrogen embrittlement and failure mechanisms of multi-principal element alloys: A review

• Hydrogen embrittlement (HE) of multi-principal element alloys is reviewed. • HE behavior between traditional alloys and high-entropy alloys is compared. • Further research directions on HE of high-entropy alloys are identified. • Strategy for the improvement of strength and HE-resistance are suggested. Multi-principal element alloys exhibit excellent physical, chemical and mechanical properties, and they are used as novel structural materials for potential applications in nuclear energy, hydrogen energy, and petrochemical fields. However, exposing components made of the alloys to service conditions related to the mentioned applications may induce hydrogen embrittlement (HE) as one of the typical failure mechanisms. In this review, we report and summarize the progress in understanding HE in multi-principal element alloys, with a particular focus on high-entropy alloys (HEAs). The review focuses on four aspects: (1) hydrogen migration behavior (hydrogen dissolution, hydrogen diffusion, and hydrogen traps); (2) factors affecting HE (hydrogen concentration, alloy elements and microstructure); (3) tensile mechanical properties in the presence of hydrogen and micro-damage HE mechanisms; (4) the design concept for preventing hydrogen-induced mechanical degradation. The differences in the HE behavior and failure mechanisms between HEAs and traditional alloys are compared and discussed. Moreover, specific research directions for further investigation of fundamental HE issues and a strategy for a simultaneous improvement in strength and HE resistance are identified.