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

Abstract : {112}<111>(beta) twins are observed in a superelastic beta 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 {112}<111>(beta) twinning system of bcc structures. These are then {112}<111>(beta) antitwins. Moreover, a full stress-induced martensitic (SIM) transformation of beta phase into a`` 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, {112}<111>(beta) antitwins are shown to be passively formed from the reversion, during the reverse SIM transformation when the stress is released, of {110}<110>(alpha '') twins actually formed in alpha '' 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 {112}<111>(beta) twins can never be activated in b titanium alloys involving SIM transformation, while the non-classical {112}<111>(beta) twinning system in the antitwinning sense is always favored.