Plastic deformation behavior and microscopic mechanism of metastable Ti-10V-2Fe-3Al alloy single crystal pillars orientated to <011>_β in submicron scales Part II: Phase transformation dependence of size effect and deformation mechanism

Double size effects are uncovered in metastable β Ti-10V-2Fe-3Al alloy single-crystal micropillars subjected to compress along <011>_β in the Part I of this paper. In this Part II, a series of samples at different deformation stages were systematically analyzed by transmission electron microscopy (TEM) to understand the deformation mechanism for double size effects. Two distinct phase transformations were observed to take place at different stages of plastic deformation. In the initial stage, dislocation interaction with ω precipitates and deformation-induced ω variants transformation from one to another are predominant plastic deformation mechanisms when the total strain is less than 10%. As a result, high density of ω precipitates contributes to the weak size effect and the continuous stable strain-hardening behavior in the metastable β Ti-10V-2Fe-3Al alloy. In comparison, the martensitic transformation from β to α″ is induced when the critical strain reaches to 10%. The strain-induced martensitic transformation becomes primary deformation modes, which result in higher strain-hardening exponent and stronger size effect, in the second deformation stage. These results provide a new perspective of designing micro-electromechanical materials with an excellent combination of the enhanced yield strength and stable plasticity.