Microstructural aspects related to pseudoelastic cycling in ultra fine grained Ni-Ti

Ni-Ti alloys show an important evolution in the stress-strain curve during pseudoelastic cycling. The transformation stress and the hysteresis width are notably reduced. To study the origin of this evolution, a Ni-Ti ribbon (50.9 at.% Ni) obtained by cold rolling and with a final straight annealing treatment was selected. The microstructure before and after pull-pull pseudoelastic cycling was analyzed by transmission electron microscopy. The observed microstructure of the ribbons consisted of a B2 austenite phase with very small grains, 50 nm-2 μm in diameter, a large density of dislocations and Ni4Ti3 precipitates. After cycling, the only observed differences are a slight increase in the dislocation density and regions with the R-phase. This suggests that the decrease in the transformation stress could be attributed to the nucleation process of the particular dislocations necessary to accommodate the martensite in the present configuration of grains and precipitates.