Evolution of microstructure and hardness in NiTi shape memory alloys processed by high-pressure torsion

Experiments were conducted on Ni-50.2 at.% Ti and Ni-50 at.% Ti alloys in order to examine the evolution of hardness and microstructure after processing by high-pressure torsion at room temperature. Disks were pressed through different numbers of revolutions up to a maximum of 40 using an applied pressure of 2.0 GPa. It is shown that there is a gradual evolution in both the hardness and the microstructure with increasing numbers of turns but even after 40 turns there is not full homogeneity. There is evidence that after 10 turns the edges of the disks achieve a well-defined saturation hardness and by further processing to 40 turns the hardness in the centers of the disks increases. The results show that a martensite-to-austenite transformation occurs during processing. The austenitic transformation around the edge of the disks achieves saturation after 5 and 10 turns in the Ni-50 at.% Ti and Ni-50.2 at.% Ti alloys, respectively.