Successive stress-induced phase transformations with large stress-strain hysteresis in single crystal Cu-Al-Mn shape memory alloys

The successive stress-induced phase transformations in single-crystal Cu-Al-Mn alloys, which exhibit excellent superelasticity and ductility, were investigated using electron microscopy and synchrotron X-ray diffraction. In a Cu-16.8Al-9.6Mn (at.%) alloy, which is in the martensitic phase at room temperature, a stress-induced martensitic transformation from a 6M-type stacking structure to a 2M-type stacking structure was revealed. At the 6M and 2M interface, it was observed that structural changes progress with partial dislocations running every three layers within the 6M structure. A Cu-17Al-11Mn (at.%) alloy, which is in the parent L2 1 phase at room temperature and exhibits superelasticity, undergoes a successive stress-induced phase transformation of L2 1 → 6M → 2M, resulting in a shape recovery exceeding 16 % in strain. The 6M → 2M transformation leads to an expansion of the stress-strain hysteresis loop, thereby enhancing the energy dissipation capability. The microstructure observations suggest that deformation in the 2M phase and the complex form of the reverse transformation contribute to the energy dissipation.