Evolution of Microstructure and Anisotropy of Mechanical Behavior in Nitinol Thin Sheet

Nitinol thin sheet has gained increasing traction in medical applications due to the unique mechanical property and its two-dimensional shape which provide additional design opportunity and the versatility in components processing. The superelastic and shape memory properties of Nitinol sheet are orientation dependent and evolve along with microstructure during thermo-mechanical processing. The objectives of this study were to investigate the evolution of microstructure, the development of texture and the impact of anisotropy on the mechanical behavior of superelastic Nitinol sheet.

A NiTi alloy of nominal composition Ni50.8Ti49.2 (in atomic percentage) was manufactured through double melting consisting of Vacuum Induction Melting (VIM) and Vacuum Arc Re-melting (VIM-VAR). It was hot forged into slabs which were subsequently hot rolled from 90 mm to 1.6 mm in thickness and eventually cold rolled to thin sheet with a nominal thickness of 0.5 mm. The final heat treatment was conducted in a hot press to ensure good flatness and proper mechanical properties. The tensile specimens were taken from hot rolled sheet, cold rolled sheet and the sheet with final heat treatment along three different directions including 0^o, 45^o and 90^o to the rolling direction. The evolution of microstructure and texture was studied by the means of microstructural observations and X-ray texture analysis. An attempt was made to correlate processing parameters, microstructure and anisotropy of mechanical behavior in this Nitinol alloy.