Thermomechanical cycling of a NiTi shape memory alloy-macroscopic response and microstructural evolution

Thermomechanical cycling of a Ni49.9Ti50.1 (at.%) shape memory alloy was investigated. Combined ex situ macroscopic experiments and in situ neutron diffraction measurements were performed to relate the macroscopic evolution in behavior (e.g., dimensional instabilities) observed during thermal cycling to the responsible microscopic mechanism(s) through texture, internal strain, peak shape, and phase evolution from the neutron data. Pre-deformation in the austenite or martensite phases affected the macroscopic cyclic behavior (e.g., actuation strain), depending on the level of pre-strain and the associated microstructural changes. However, the pre-deformation did not completely stabilize the cyclic response. Subsequent thermomechanical cycling revealed that the martensite texture changed with continued thermal cycling, while the austenite texture did not. For the conditions investigated, stagnation of the martensite texture occurred around the eighth cycle, consistent with asymptotic saturation of the macroscopic transformation strains. Moreover, diffraction spectra peak shapes (broadening) were found to vary with cycling indicative of the accumulation of lattice defects, consistent with the constant increase in residual strain.