Ageing effects after heat treatment in Cu–Al–Mn shape memory alloys

Different types of homogenization heat treatments, with homogenization temperatures varying from 800 to 920 °C for 1 h and subsequently water quenching, were performed on Cu–Al–Mn shape memory alloys with Al content 11.4–12.3 wt.% and Mn-content 5.0–6.9 wt.%. All homogenization heat treatments of the investigated alloys lead to an increase of isothermal damping with increasing temperature. The microstructure and phase fractions of the investigated samples depend on the homogenization temperature and cooling rate. The transition temperatures increased after every homogenization heat treatment. Moreover, narrowing and increasing of the transformation peaks were observed by acoustic emission measurements. Different martensitic transformations could be detected by acoustic emission measurements depending on the heat treatment. Strong and fast aging effects, decreasing the very high damping capacities directly after all homogenization heat treatments were found in room temperature amplitude dependent damping measurements. At least, there are three different possible explanations for the found ageing effects in Cu–Al–Mn shape memory alloys: (1) pinning of twin and phase boundaries by point defects diffusing to the boundaries during ageing, (2) partial phase transformation during ageing, (3) decrease of the transformation temperatures.