Effects of abnormal grain growth on shape memory characteristics of Cu-Al-Mn alloys

Abstract:
Shape memory alloys (SMAs) such as Ni-Ti- and Cu-based materials are commercially attractive due to the practical utilization of their shape memory properties. Despite low cost and good shape memory properties, the application of Cubased SMAs is limited due to the intergranular cracking. The excellent ductility and high shape memory characteristics of the Cu-Al-Mn-based SMAs depend on the control of microstructural properties, such as grain size and texture. A pronounced texture and large grain size (or low grain boundary density) improve the ability of the phase transformation coordination among different grains and improve their ductility and shape memory properties. In this study, we intended to improve the mechanical characteristics and thermoelastic martensitic transformation of a Cu-8.1Al-9.8Mn-2.1Ni alloy through cyclic heat treatment (CHT). Wire samples produced by hot and cold rolling were subjected to 1 to 3 cycles of heating to 900 °C for 5 min, followed by convective air cooling at room temperature. In the next step, all the samples were quenched in water after heating to 900 °C for 5 min. Finally, the specimens were aged at 200 °C for 15 min, followed by air cooling. The expected results indicate that the abnormal grain growth (AGG) was induced by the CHT. The subgrains formed during the CHT are consumed during grain boundary migration, leading to AGG. During CHT, the growth rate rises with increasing misorientation between the subgrains. The grain growth reduces the fraction of grain boundaries (with high energy), enhancing the superelasticity and the shape memory characteristics of the alloy by increasing the grain size relative to the cross-sectional area of the samples. The cyclic stress-strain curves of the CuAlMnNi alloy samples indicate that after CHT3 the superelastic strain could reach 6 %.