CHANGES IN STACKING-FAULT SEQUENCES DURING THE MARTENSITIC PHASE-TRANSFORMATION IN CU-ZN-AL SHAPE-MEMORY ALLOYS

The movement of an A/D-type intervariant twin boundary, of the internally faulted M18R martensite, was studied during deformation. Transmission electron microscopy in situ observations of Cu-Zn-Al single crystals show that changes in the stacking sequence of close-packed planes occur in the converting A- and D-variants. A simple mechanism is proposed which explains the origins of stacking fault changes across the A/D-interface and which accounts for the absence of a pure shear type stacking fault as has been reported in the literature. This mechanism consists of the transformation of one type of random stacking fault into another fault type by the motion of a partial dislocation on the basal plane. Glissile defects on the martensitic basal planes were observed experimentally, when high internal stresses are built up during the retransformation process.