Influence of Cobalt on the Hot Deformation Characteristics of an NiTi Shape Memory Alloy

Abstract

Among shape memory alloys (SMAs), Ni–Ti alloys are more widely used in multifarious sectors because of their functional properties of shape memory effect and superelastic effect. With an emphasis on miniaturization and higher performance of actuators and biomedical devices these days, there is a necessity to develop SMAs with high stiffness, i.e., high upper/lower plateau stress, and high modulus. In recent times, there have been attempts to improve their shape memory characteristics, mechanical properties and corrosion resistance through alloying, thermal treatment, etc. In this context, addition of Co has been made to binary Ni–Ti alloys. But till date, there has been a dearth of literature on hot deformation characteristics of NiTiCo SMAs. This work therefore investigates the role of cobalt on hot deformation characteristics of an Ni–Ti SMA. The addition of cobalt results in raising the yield strength and reducing the Ms temperature of the alloys. In this study, elemental forms of high-purity nickel, titanium and cobalt were melted together in a vacuum induction melting furnace to produce cylindrical rods. These rods were homogenized at 900 °C for 3 h and quenched to room temperature in water and subsequently machined so as to obtain a length-to-diameter ratio of 1.5, which was ideal to carry out hot deformation studies on a Gleeble thermomechanical simulator. The samples were deformed at different strain rates (0.01, 0.1, 1 and 10 s−1) at temperatures of 800, 900, 1000 and 1100 °C, respectively. Elaborate results on the deformation characteristics of an NiTiCo SMA have been discussed in this paper.