Effect of annealing on cold-rolled Ni–Ti alloys

In this paper, we present a study on the effect of annealing a cold-rolled near-equi-atomic Ni–Ti alloy strip by transmission electron microscopy. Transmission electron microscopy of the as-received sample shows the presence of alternating amorphous and crystalline bands. The crystalline bands have widths of the order of a few microns and contain B2 nanograins of around 20 nm diameter, which are preferentially oriented along the [1,−1,−1] direction, normal to the rolling direction. As-received samples were annealed for 30 min at different temperatures up to 800 °C. With the increase in annealing temperature, crystallization starts in the amorphous bands at around 350 °C and finally ends up with the coarsening of the grains in the entire sample. The annealing of the samples at 450 °C entirely transforms the amorphous bands to crystalline bands and as off 600 °C Ni3Ti2 precipitates are formed. At 800 °C the grain size increases to 30–50 μm. Diffraction patterns from such grains reveal the presence of diffuse intensity around reciprocal lattice positions 1/3〈1 1 0〉* indicating the formation of the R-phase.