Revealing the Mode and Strain of Reversible Twinning in B19' Martensite by in situ Synchrotron X-Ray Diffraction

The challenges in the identification of reversible twinning modes and in the measurement of reversible twinning strain impede the thorough understanding of twinning induced elasticity (TIE). In this report, we exploited the mode and strain of reversible twinning in B19' martensite in a Nb-nanowire/NiTiFe-matrix alloy. TIE strain up to 5.1% was achieved by pre-deforming the alloy up to an applied strain of 55.7%. In situ synchrotron X-ray diffraction (XRD) results show that B19' ( ) and ( ) twins were induced by pre-deformation. More importantly, a large portion of these twins were found reversible for the first time. It is suggested that the reversibility is likely due to the pinning effects of high density dislocations and nanosized martensite variants or twins. Furthermore, the reversible twinning strain was measured using XRD based on a 'lattice strain matching' concept such that the contribution of reversible twinning to TIE was clarified. The measured twinning strain was compared with the calculated strain based on twinning crystallography.