THERMOELASTIC MARTENSITE AND SHAPE-MEMORY EFFECT IN DUCTILE CU-AL-MN ALLOYS

Ductile shape memory (SM) alloys of the Cu-Al-Mn system have been deve loped by controlling the degree of order in the beta phase. Additions of Mn to the binary Cu-Al alloy stabilize the beta phase and widen the single-phase region to lower temperature and lower Al contents. It is shown that Cu-Al-Mn alloys with low Al contents have either the disor dered A2 structure or the ordered L2(1) structure with a lower degree of order and that they exhibit excellent ductility. The disordered A2 phase martensitically transforms to the disordered A1 phase with a hig h density of twins. The martensite phase formed from the ordered L2, p hase has the 18R structure. The SM effect accompanies both the A2 --> A1 and L2(1) --> 18R martensitic transformations. These alloys exhibit 15 pct strain to failure, 60 to 90 pct rolling reduction without crac king, and 80 to 90 pct recovery from bend test in the martensitic cond ition. Experimental results on the microstructure, crystal structure, mechanical properties, and shape memory behavior in the ductile Cu-Al- Mn alloys are presented and discussed.