Effect of Ternary Addition of Cobalt on Shape Memory Characteristics of Ni–Ti Alloys

Abstract

Shape memory alloys (SMAs) are a class of metallic materials that exhibit two unique characteristics: shape memory effect and superelasticity. These alloys are capable of responding to stimuli like heat when they are subjected to thermo-mechanical treatment. Therefore, these alloys have proven their utility in many areas and are very common in applications such as sensors and actuators, various life-saving medical devices and stents. Ni–Ti alloys are in the forefront with respect to shape memory materials and have been employed in many practical applications. In recent times, there have been attempts to improve their shape memory characteristics, mechanical properties and corrosion resistance through alloying addition, thermo-mechanical processing, severe plastic deformation, etc. Among these methods, ternary alloying addition seems to have a greater effect in comparison with other methods. In this work, cobalt was added to a binary Ni–Ti alloy and its influence on transformation temperatures and microstructure was studied. Ternary NiTiCo alloys with varying cobalt contents (1, 2, 3 at.%) were prepared by vacuum induction melting, and the alloys were then hot-rolled and homogenized. Various characterization techniques such as differential scanning calorimetry, transmission electron microscopy and X-ray diffraction were employed to study the transformation temperatures, the microstructure and the phase structure of the ternary SMAs. It was seen that cobalt addition decreased the phase transformation temperatures and influenced the functional properties of the binary Ni–Ti alloy. Elaborate results have been discussed in this paper.