In situ scanning and transmission electron microscopy investigation on plastic deformation in a metastable β titanium alloy

Abstract

Martensitic transformation and deformation twinning are investigated in Ti-24Nb-4Zr-8Sn (wt. %) alloy with in situ tensile deformation in scanning and transmission electron microscopes. Deformation-induced β to α″ martensitic transformation occurs at a stress plateau stage of ∼200 MPa and deformation twinning takes place subsequently at later strain hardening stage. In situ transmission electron microscopy observations combined with finite element method calculation reveal that three deformation mechanisms, α″ martensitic transformation, {112}<111>β deformation twinning and kinks of α″ lamellae, are activated with the increase of applied stress/strain. Among them, deformation-induced β to α″ phase transformation dominates the early-stage deformation. As the stress increasing, {112}<111>β deformation twinning is activated and becomes the dominating deformation mechanism afterward. Interactions between dislocation slips and martensite/twin lamellae are likely responsible for the strain hardening. After the deformation over the ultimate tensile strength, slip bands initiate and become the major deformation mechanism during necking until fracture.