Martensite aging phenomena in Cu-based alloys: Effects on structural transformation, mechanical and shape memory properties: A critical review

Cu-based shape memory alloys have been hyped as the ‘heir’ and pragmatic substitute to NiTi alloys for shape memory applications. Considerations from relatively low materials cost, processing ease, and modest shape memory properties, have been advanced as reasons justifying this projection. However, structural transformation induced phase stabilization - referred to as martensite ageing, has been reported to be a huge scourge constraining the thermo-responsiveness of these alloys, and limiting their service reliability. Studies on the mechanisms and effects of martensite ageing in Cu-based shape memory alloys (SMAs) have been reported in bits and patches, or encapsulated in broad ranged topical issues on the system. A comprehensive and exclusive review of martensite ageing in Cu-based SMAs has been lacking – thus the need for the present work. This review covers the general mechanisms of martensite ageing and its effects on the transformation behaviour, mechanical properties, shape memory functionality, and considers the implications on commercial utilization of the Cu-based SMAs. Specifically, Cu-Al-Mn, Cu-Al-Be, Cu-Al-Ni, Cu-Zn-Al, and Cu-Zn-Sn alloys were studied. The observations indicated that factors such as alloy composition, phase and microstructural parameters, and processing conditions, significantly dictate the mechanism and propensity to martensite stabilization, and also the extent to which the mechanical and shape memory characteristics are altered.