Investigation of Hydrogen Behavior and Hydrogen Embrittlement Mechanisms of FCC-BCC Dual-phase High-Entropy Alloys with Controlled Phase Fraction and Stability

Hydrogen embrittlement (HE) has been considered a serious issue as it deteriorates the strength and ductility of metallic materials. Among various kinds of alloys, the face-centered cubic (FCC) + body-centered cubic (BCC) dual-phase alloys are known to be susceptible to hydrogen environments due to the inherent characteristic of the BCC structure stemming from a high diffusivity of hydrogen. Thus, it is reasonable to believe that reducing the fraction of BCC would improve the resistance to HE in the FCC-BCC dual-phase alloys. However, most of these alloys accompany the transformation-induced plasticity (TRIP), indicating that the FCC phase also contributes to the HE susceptibility through undergoing martensitic transformation. This suggests that it is hard to readily estimate the HE susceptibility of the FCC-BCC dual-phase alloys as both the phase fraction and stability have critical roles on the HE susceptibility. Therefore, it is necessary to reveal each effect of FCC and BCC phases on HE phenomenon and to conduct a systematic study by controlling phase fraction and stability. In this study, four model VCrFeCoNi high-entropy alloys are designed to form FCC-BCC martensite whose phase fraction and stability are largely controlled. Among them, two alloys possess a single FCC phase, while the others consist of FCC and as-quenched BCC martensite. As the Ni content decreases, the thermal and mechanical stabilities of FCC reduce, resulting in the increased BCC fraction and sensitivity to deformation-induced martensitic transformation. For the four model alloys, the HE susceptibility is evaluated by slow strain rate tests (SSRTs) for the specimens H pre-charged via electrochemical cathodic methods. These four alloys showing different HE susceptibility are subjected to detailed analysis of hydrogen diffusion, trap behavior, hydrogen-affected brittle zone, and hydrogen-induced cracking. The initial as-quenched martensite provides diffusion paths for hydrogen, while the deformation-induced martensite possesses an inherently high concentration of hydrogen, leading to dominant roles in crack propagation behaviors. This work suggests the guidance for microstructural designs to enhance the resistance to HE for the metastable FCC-BCC dual-phase alloys.