Monitoring of Ti-25Nb and Ti-25Nb-1O shape memory alloys in the initial stage of tensile deformation by combined techniques of acoustic emission and digital image correlation

This work concerns an experimental investigation of the Ti–25Nb and Ti–25Nb–1O (at%) SMAs in the initial stage of tensile deformation using acoustic emission (AE) and digital image correlation (DIC). The stress-strain responses of the considered SMAs are different. The Ti–25Nb SMA exhibits shape memory effect due to the stress–induced martensitic transformation from the cubic β phase to the orthorombic α″ phase. In the case of the Ti–25Nb–1O SMA, the addition of 1 at% of oxygen results in a nonlinear superelastic behavior with small hysteresis and an increased yield stress. The stress-induced phase transformation in the Ti–25Nb–1O SMA is hindered due to the addition of oxygen interstitials. The difference between the deformation mechanisms and the resulting mechanical behaviors of the SMAs was clearly reflected by the recorded AE signals and deformation fields. It was shown that the AE can serve to track the development of the stress–induced phase transformations in the Ti–25Nb and Ti–25Nb–1O SMAs during tension. The AE signals were correlated to the strain fields of the SMAs, which showed a Lüders–type deformation of the Ti–25Nb SMA and a distinct but still inhomogenous deformation of the Ti–25Nb–1O SMA. The results of this study show that DIC and AE techniques are effective tools for monitoring phase transformations of the Ti–25Nb and Ti–25Nb–1O SMAs during tensile loading.