Effect of product form and heat treatment on the crystallographic texture of austenitic Nitinol

Abstract

The superelastic material Nitinol, a nearly equiatomic alloy of nickel and titanium, is rapidly becoming one of the most important metallic implant materials in the biomedical industry, especially for the fabrication of endovascular stents. The manufacture of these stents, and countless other Nitinol products, originates from various forms of raw material such as tube, sheet or rod. However, depending upon which product form is used, the crystallographic texture in Nitinol can be significantly different, which can lead to marked changes in its mechanical properties. In this paper, we present a study to show the characteristic texture in various Nitinol product forms (tube, sheet, and rod), before and after annealing heat treatments, with specific quantification of the major texture components. We further present predictions of the mechanical response based upon such texture, and provide experimental verification with uniaxial tensile tests. Results show that the form of the starting material has a profound influence on characteristic texture and predicted mechanical response. Furthermore, annealing heat treatments, rather than reducing the texture, are found to increase the strength of this texture.