On the origin of grain boundary α phase mediated strain localization and crack initiation in metastable β titanium alloy

Abstract

Grain boundary α (α_GB) phase and adjacent precipitate-free zones (PFZs) in metastable β titanium alloys are critical microstructural features that govern strain localization and crack initiation, yet their synergistic roles in mechanical degradation remain inadequately understood. In this study in-situ mechanical testing coupled with microstructural strain mapping by high-resolution digital image correlation (HRDIC) were employed to elucidate the interplay between PFZ-mediated strain localization and plasticity of α_GB in an additive-manufactured metastable β titanium alloy. Results from in-situ tensile tests revealed that premature cracking of α_GB occurs at the incipient plasticity, and PFZs act as preferential pathways for rapid crack propagation, which dramatically reduces the ductility of the alloy. Hydrogen content analysis and lattice strain measurement demonstrated that the adsorption of surface hydrogen during sample preparation in a hydrous environment embrittles the α_GB. By substituting the preparation method with hydrogen-free ion milling, α_GB cracking is eliminated; however, strain localization in PFZs persists, enabling crack initiation at a later stage of plasticity. Crystallographic analyses demonstrated that slip systems with high Schmid factors in β phase regions adjacent to α_GB lamellae drive strain localization magnitudes up to two orders higher than far-field strains. These findings reveal a dual detrimental role of α_GB lamellae: promoting PFZ formation and strain localization, and serving as hydrogen-sensitive crack nucleation sites. This work further indicates that hydrogen intake during processing and service could largely alter the character of α_GB under loading and affect the performance of metastable titanium alloys in demanding environments.