Particularities of phase transitions in thermomechanically processed Cu-Al-Mn shape memory alloys

The thermally and stress induced phase transitions occurring in a Cu-Al-Mn Shape Memory Alloy (SMA) have been investigated by means of thermal analysis techniques, tensile testing and microscopic observations. On heating a hot rolled solution annealed Cu-Al-Mn SMA, up to 873 K, two phase transformations, related to equilibrium phase precipitation and to 2-step order-disorder transition, respectively, were revealed by Differential Scanning Calorimetry (DSC). During tensile testing, the Cu-Al-Mn SMA under study experienced almost complete superelasticity, after five mechanical training cycles, as well as good ductility and tensile resistance. On trained specimens the formation of stress induced martensite was revealed by optical microscopy (OM) observations. The reversion to austenite of stress induced martensite was accompanied by relatively large increases of elastic modulus and internal friction, determined on Dynamic Mechanical Analyzer (DMA) and by marked variations of relative elongation, thermal expansion coefficient and elongation rate, determined by dilatometry. Since it is a two step transition, it was associated with the two morphologies of stress induced martensite observed by Scanning Electron Microscopy (SEM).