Reduction of Mechanical Anisotropy in Nitinol Thin Sheets

Nitinol sheets develop various crystallographic textures during the rolling process and strong {111} <110> texture was observed in the cold-rolled and hot-flattened thin sheets. The crystallographic texture results in orientation-dependent mechanical response variability, with higher plateau stress at the transverse direction than that along the rolling direction. This anisotropic mechanical behavior shall be considered when manufacturing the material and designing medical implants. The current offering of Nitinol sheets can accommodate to the needs of the medical industry, yet there are demanding applications calling for more uniform material to ensure more reliable and consistent Nitinol implants. This work is to exploit different rolling processes and heat treatments with the aim to reduce the directional dependency of mechanical properties in superelastic Nitinol sheets.

A standard grade of Nitinol alloy and an ultra-clean Enduro material of the same nominal composition Ni50.8Ti49.2 (in atomic percentage) were investigated in this study. The vacuum melted ingots were hot forged into slabs and subsequently hot rolled to 1.6 mm in thickness. They were further cold rolled to thin sheets to a nominal thickness of 0.5 mm by employing the conventional unidirectional rolling process or the newly developed cross-rolling routes. The final heat treatments were conducted in a hot press to achieve good flatness and proper mechanical properties. The tensile specimens were cut and tested at multiple orientations to evaluate the directional variations. The microstructure and texture were characterized with an attempt to correlate processing parameters, microstructure and anisotropy of mechanical behavior in the Nitinol alloy.