EARLY STAGES OF PSEUDOELASTIC FATIGUE IN CU-ZN-AL SINGLE-CRYSTALS CYCLED IN THE CRYOGENIC RANGE

Cu-Zn-Al single crystals were pseudoelastically cycled between the bcc austenitic phase (beta) and the 18R martensite, in the cryogenic rang e. The microstructural changes which take place in the bulk were studi ed by different techniques and a consistent description of the defects which are formed is presented. Dislocations and trapped martensite ar e retained in two-dimensional arrays parallel to the basal plane of th e martensite (basal plane defects), and these defects are responsible for the strong temperature effect on the mechanical behaviour of the a lloys. The dislocations form the bulk defects during the first cycling stage, and afterwards internal stresses are high enough to retain mar tensite which shows a 2H structure. Partial recovery takes place at te mperatures close to room temperature, which supports the role that poi nt defects play in the mechanical behaviour of these alloys.