Origin of {112} < 111 > antitwinning in a Ti-24Nb-4Zr-8Sn superelastic single crystal

Abstract

$${\left\{112\right\}\langle {111}\rangle }_{\beta }$$ twins are observed in a superelastic β Ti-24Nb-4Zr-8Sn (wt.%) single crystal after tensile test. A careful Schmid factor analysis shows that these twins are formed in the antitwinning sense regarding the classical $${\left\{112\right\}\langle {111}\rangle }_{\beta }$$ twinning system of bcc structures. These are then $${\left\{112\right\}\langle {111}\rangle }_{\beta }$$ antitwins. Moreover, a full stress-induced martensitic (SIM) transformation of β phase into α" martensite is evidenced from in situ synchrotron X-ray diffraction. This transformation is fully accomplished before the onset of plastic deformation and, in turn, the formation of twins. From crystallographic reconstruction, $${\left\{112\right\}\langle {111}\rangle }_{\beta }$$ antitwins are shown to be passively formed from the reversion, during the reverse SIM transformation when the stress is released, of $$ \left\{ {110} \right\}\left\langle {110} \right\rangle _{{\alpha ^{{\prime \prime }} }}$$ twins actually formed in α" martensite. The martensitic transformation occurring before twinning plays a key role in the activation of antitwinning systems by reducing both shear and shuffle magnitudes of twinning. Variant selection of stress-induced martensite and Schmid factor analysis show that the classical $${\left\{112\right\}\langle {111}\rangle }_{\beta }$$ twins can never be activated in β titanium alloys involving SIM transformation, while the non-classical $${\left\{112\right\}\langle {111}\rangle }_{\beta }$$ twinning system in the antitwinning sense is always favored.