Thermomechanical cyclic response of an ultrafine-grained NiTi shape memory alloy

We focus on the effects of ultrafine grains on the thermomechanical cyclic stability of martensitic phase transformation in Ni_49.7Ti_50.3 shape memory alloy fabricated using equal-channel angular extrusion (ECAE). The samples were ECAE-processed between 400 and 450 °C resulting in average grain sizes of 100–300 nm. Tensile failure experiments demonstrated that the strength differential between the onset of transformation and the macroscopic plastic yielding increases after ECAE. Such increase led to a notable improvement in the thermal cyclic stability under relatively high stresses. The experimental observations are attributed to the increase in critical stress level for dislocation slip due to grain refinement, change in transformation twinning mode in submicron grains, the presence of R-phase, and multi-martensite variants or a small fraction of untransforming grains due to grain boundary constraints. The effects of these microstructural factors on the transformation behavior are discussed in the light of transformation thermodynamics.