Dissecting the phase transformation mechanism of Ti hydride at atomic scale

Abstract

Revealing the hydride transformation behavior in Titanium (Ti) alloys is crucial for understanding hydrogen absorption and embrittlement mechanisms. However, dissecting the atomic-scale phase transformation of hydrides in Ti alloys, including phase nucleation, transformation pathway, and associated atomic movements, remains a significant challenge. The current work integrates advanced atomic-scale characterization techniques with deep learning-based molecular dynamics simulations to explore the phase transformation processes of hydrides in pure Ti under hydrogen charging. Atomic-scale observations reveal distinct interface structures and corresponding orientation relationships (ORs) between the hydrides and the Ti matrix. A customized deep potential model is developed to accurately predict the energetics of various Ti hydrides. It is demonstrated that deformed α-Ti with H atoms occupying tetrahedral interstitial sites exhibits the highest stability, promoting hydride formation by adjusting the interlayer distance of the {0001}_HCP planes to align with {111}_FCT planes. The basal-type (B-type) OR transformation from HCP to FCT occurs via successive basal slip, facilitated by a reduced slip barrier in hydrogenated α-Ti. Furthermore, a novel polymorphic transformation pathway featuring HCP→BCC→FCC→FCT is identified, following a pyramidal-type (P-type) OR, with BCC and FCC hydrides acting as intermediate phases. This polymorphic mechanism minimizes the atomic displacement by decomposing the transformation into two intermediate pathways. These findings provide valuable insights into the complex phase transformations during hydride precipitation and enhance the understanding of hydrogenation mechanisms in Ti alloys.