Abnormal high yield strength and strain softening in a metastable β titanium alloy at room temperature

Understanding the relationship between deformation behaviors and mechanisms is significant for the processing and application of metastable β titanium alloys. Here we aim to investigate and evaluate the abnormal yield strength and strain softening of a Ti-15.1Mo-2.77Nb-3.1Al-0.21Si alloy at room temperature. This alloy exhibits a high yield strength of 970 MPa, followed by the continuous stress drop behavior in the entire engineering strains (or true strains of 0.018 ∼ 0.056). Digital image correlation (DIC) reveals that the flow stress drop results from local strain softening associated with a local increase in strain rate, instead of Lüders strain. The pinning between dislocations and Si atoms as well as other interstitial atoms at and near grain boundaries is mainly responsible for the high yield strength. Subsequently, dislocations originating from grain boundaries can easily slip in a planar pattern along the {110} 〈111〉 slip systems, resulting in a continuous stress drop. In addition, both the low density of dislocations within β grains and large grain size also provide favorable conditions for dislocation slip over a long distance. This study reveals the mechanisms of both high yield strength and strain softening in the metastable β Ti alloys.