Stabilization and Two Way Shape Memory Effect in Cu-Al-Mn Alloys Transforming at Elevated Temperatures

Stabilization of martensite and two way shape memory effect (TWSME) was studied in Cu-Ai-Mn alloys containing 9.7-11.8 wt% Al, and 3.0-9.25 wt% Mn, which were quenched to various temperatures. The transforming temperatures were in the range -70 to +220°C. The majority of alloys quenched directly to martensite did not show reverse transformation on heating due to stabilization and further tempering of martensite. However, when quenched to temperatures several degrees above Af, reversible transformation was observed in all the investigated alloys. The TWSME training was performed by a repetitive cooling under external stress and heating after unloading. In alloys transforming below 150°C saturation of TWSME occurred between 50 and 80 training cycles, while in alloys transforming at higher temperatures it was earlier, i.e. after 20-40 training cycles. The latter alloys have shown TWSME elongation of about 0.4%, while alloys transforming at lower temperatures attained up to'l .4% higher TWSME. DSC studies revealed a reduction in transformation heat by about 30% after training, which can be related to the presence of residual martensite detected by optical microscopy. This martensite could not be removed after flash or slow heating in the calorimeter for any of the investigated alloys, apparently prone to martensite stabilization. Transmission electron microscope studies revealed the presence of dislocation bands in low temperature transforming alloys and almost homogeneous distribution of dislocations in alloys transforming at higher temperatures. This fact explains the poor training ability of alloys transforming at higher temperatures, since the thermal mobility of dislocations prevents formation of stacks favouring nucleation and progress of trained variants. However, the role of residual martensite in the training process cannot be neglected.