An in situ synchrotron study of the localized B2↔B19' phase transformation in an Ni-Ti alloy subjected to uniaxial cyclic loading-unloading with incremental strains

High-resolution in situ synchrotron X-ray diffraction was applied to study a cold-drawn and solution-treated 56Ni-44Tiwt% alloy subjected to uniaxial cyclic loading-unloading with incremental strains. The micro-mechanical behaviour associated with the partial and repeated B2↔B19' phase transformation at the centre of the sample gauge length was studied with respect to the macroscopic stress-strain response. The lattice strains of the (110)B2 and different B19' grain families are affected by (i) the transformation strain, the load-bearing capacity of both phases and the strain continuity maintained at/near the B2-B19' interfaces at the centre of the gauge length, and (ii) the extent of transformation along the gauge length. With cycling and incremental strains (i) the elastic lattice strain and plastic strain in the remnant (110)B2 grain family gradually saturate at early cycles, whereas the plastic strain in the B19' phase continues to increase. This contributes to accumulation of residual strains (degradation in superelasticity), greater non-linearity and change in the shape of the macroscopic stress-strain curve from plateau type to curvilinear elastic. (ii) The initial 111B2 fibre texture transforms to [120]B19', [130]B19', [150]B19' and [010]B19' orientations. Further increase in the applied strain with cycling results in the development of [130]B19', [102]B19', [102]B19', [100]B19' and [100]B19' orientations.