Hydrogen-induced twin boundary passivation in multi-principal element alloy: a micropillar compression study

The ingress of nascent hydrogen into alloys can significantly alter their mechanical behaviors, leading to the well-known phenomenon of hydrogen embrittlement (HE) and catastrophic failure of structural components in service. As an emerging class of materials, some face-centered cubic multi-principal element alloys (MPEAs) exhibit unique resistance to HE, with the frequent presence of coherent twin boundaries (TBs) widely acknowledged as a contributing factor. However, the underlying mechanisms of TB-enhanced HE resistance remain under debate. Here, we selectively activate orientation-dependent TB-dislocation interactions by compressing [$\overline{1}\overline{1}$2]- and [0$\overline{1}$1]-oriented CoCrFeNi MPEA micropillars containing an individual TB. This approach provides a benchmark for elucidating the hydrogen-induced deformation behaviors. An enhanced yield strength and orientation-dependent strain hardening are observed, attributed to hydrogen-induced TB passivation against slip transmission, with minimal impact on intragranular dislocation activities. Microstructural analysis reveals dislocation impediments at TBs and dislocation entanglements within the grains, confirming the hydrogen-induced TB passivation mechanism. These findings provide critical insights into the role of hydrogen in TB-facilitated plastic deformation and offer guidance for future studies aiming to comprehensively understand the HE resistance of MPEAs.