Enhancing wear resistance of sustainable CuZr SMA by promoting stress-induced martensitic transformation

The effect of microalloying on the microstructure of Cu50Zr50 shape memory alloy (SMA) has been studied through the development of suction-casted Cu50Zr50, Cu49Zr50Co1 and Cu49Zr50Fe1 at. % rods of 3 and 4 mm diameter, i.e., at two different cooling rates. For low cooling rates (4 mm: ∼250 K/s), the microstructure consists of austenite and a large volume fraction of intermetallics, which are brittle in nature and do not exhibit a stress-induced martensitic transformation. However, for the 3 mm samples, the cooling rate is faster and thus promotes retaining austenite upon quenching, as deduced from XRD, while minimises intermetallic phase formation. Among the microalloying elements, Fe and Co are promising to decrease the stacking fault energy of B2 CuZr austenite phase and therefore promoting stress induced martensitic transformation of CuZr, however, due to its low solubility, addition of Fe was observed to promote more the formation of intermetallic phases upon cooling than Co as seen in XRD. For this reason in order to achieve the closest to the desired microstructure, ie., retained austenite, 1 at. % Co can be added. However, Co is known to be a toxic element and therefore, in order to develop more environmentally friendly/sustainable alloys, the concentration of Co added has been minimized. The addition of 0.5 at. % Co, was observed to enhance the wear resistance of CuZr as deduced from the reduction of mass loss, while, at the same time, it provides a more sustainable option.