Texture Development in the Ni47Ti44Nb9 Shape Memory Alloy During Successive Thermomechanical Processing and Its Effect on Shape Memory and Mechanical Properties

Abstract

For improving the shape memory performance and mechanical properties of shape memory alloys (SMAs), crystallographic texture and second phase are generally induced in SMAs by suitable thermomechanical processing. For this purpose, the development of texture in the Ni47Ti44Nb9 SMA during successive processing (e.g., hot forging, hot rolling, cold rolling, and heat treatment) and the effects of texture, grain size, and β-Nb particle precipitation on recoverable strains and tensile properties were studied. In the hot-forged and hot-rolled Ni47Ti44Nb9 alloy rods, intense 〈111〉 fibers are formed, and water quenching from 873 K and 1123 K (600 °C and 850 °C) leads to the decrease in intensity of 〈111〉 fiber in the hot-rolled rod. When the hot-forged rod is hot-rolled into sheet, intense {001} and weak {123} fibers appear, but grain growth leads to the disappearance of {001} fiber and {110}〈001〉 becomes the strongest component. Cold-rolling deformation of the hot-rolled sheet promotes the development of γ-fiber and the convergence of {332} and {123} fibers to {233}〈110〉 and {123}〈121〉 components, respectively, and the intense component is turned into {111}〈110〉; in this case, the recoverable strain (eSRS) and tensile yield strength (σYS) exhibit an anisotropy. When the quenching temperature is 1123 K (850 °C), some weaker components appear, the anisotropy of eSRS disappears, and the difference level in σYS along the rolling direction (RD) and transverse direction (TD) becomes smaller. Therefore, an appropriate heat-treatment temperature should be selected to maintain the deformation texture and also to obtain fine grains for different thermomechanical processing.