Cu-based shape memory alloys with enhanced thermal stability and mechanical properties

Cu-Based shape memory alloys were developed in the 1960s. They show excellent thermoelastic martensitic transformation. However, the problems in mechanical properties and thermal instability have inhibited them from becoming promising engineering alloys. A new Cu–Zn–Al–Mn–Zr Cu-based shape memory alloy has been developed. With the addition of Mn and Zr, the martensitic transformation behaviour and the grain size can be better controlled. The new alloy demonstrates good mechanical properties with ultimate tensile strenght and ductility, being 460 MPa and 9%, respectively. Experimental results revealed that the alloy has better thermal stability, i.e. martensite stabilisation is less serious. In ordinary Cu–Zn–Al alloys, martensite stabilisation usually occurs at room temperature. The new alloy shows better thermal stability even at elevated temperature (∼150°C, >Af=80°C). A limited small amount of martensite stabilisation was observed upon ageing of the direct quenched samples as well as the step quenched samples. This implies that the thermal stability of the new alloy is less dependent on the quenching procedure. Furthermore, such minor martensite stabilisation can be removed by subsequent suitable parent phase ageing. The new alloy is ideal for engineering applications because of its better thermal stability and better mechanical properties.