Microstructural evolution in the pseudoelastic cycling of Cu–Zn–Al single crystals: behavior at a transition stage

Cu–Zn–Al single crystals have been pseudoelastically cycled through the bcc-18R martensitic transformation. The test temperatures varied from low temperatures (liquid nitrogen) to above room temperature. The microstructure evolution during cycling has been analyzed by optical and transmission electron microscopy. A description of defects formed after cycling both in the bulk and on the surface is presented. A transition in the type of formed defects is found corresponding to an increase in the test temperature. Dislocation tangles parallel to the basal plane of the martensite, with trapped martensite inside, give way to dislocation arrays parallel to the habit plane of the transformation, as the test temperature increases. A correlation between this transition behavior and the mechanical evolution was found and the mechanisms of generation of bulk and surface defects are discussed. A correlation between dislocation arrays parallel to the habit plane and intrusion–extrusion defects on the surface is also presented. Both type of defects appear at temperatures higher than 173 K. The effect of point defects introduced during cycling and their change in mobility with test temperature are considered.