Hydrogenation induced surface bulging at the phase interfaces in AB-type hydrogen storage alloys

Excess hydrogen typically causes bulging and subsequent cracking in metals and alloys, leading to what is commonly recognized as hydrogen damage in conventional metals and alloys. However, similar hydrogen-induced degradation can play a beneficial role in hydrogen storage alloys. Despite the well-established understanding of bulging and cracking in conventional metals, their mechanistic origin in multiphase hydrogen storage alloys remains poorly understood. In this study, we reveal that hydrogen-induced bulging, followed by crack and crater formation, is associated with volume expansion at phase boundaries of a Ce-containing TiFe alloy. Initially, the secondary phase Ce particles exist as CeO 2 . As hydrogenation progresses, some transform into CeH 2 , causing bulging due to local volume expansion. Further hydrogenation leads to crack formation at the phase boundary, and once CeH 2 dominates, the affected area rises and detaches, forming a crater. These observations address a critical gap in understanding the early-stage hydrogenation behavior of multiphase hydrogen storage alloys by directly revealing how hydrogen-driven phase transformations occur within rare-earth-containing secondary phases. Understanding the underlying mechanism of these sequential processes could not only facilitate the design of improved hydrogen storage materials but also provide valuable insights into hydrogen damage in conventional metals and alloys.