TRANSFORMATION REVERSIBILITY IN FE-MN-SI SHAPE-MEMORY ALLOY

The change of the surface relief associated with stress-induced epsilo n martensite before and after the reverse transformation in an Fe-33%M n-6%Si alloy (Ms=264K) has been investigated. The permanent strain of a 2.7% tensile-strained specimen was 1.1% after the reverse transforma tion. In this specimen, a large part of surface relief still remained after heating to 623K (above Af). When five training cycles of 2.5% st raining at room temperature and heating at 623K were applied, the perm anent strain became negligible and the surface relief vanished well. T EM observations showed that usual permanent slip deformation hardly oc curred in the interior of an austenite grain in a 3.8% tensile-straine d specimen where the permanent strain after the reversion was 2%. Thes e results indicate that the permanent strain in the shape memory behav ior is mainly attributed to the lack of transformation reversibility, i.e., the lack of reversible movement of Shockley partial dislocations . The conditions for the reversibility were discussed and the importan ce of the back stress formed by the forward transformation was pointed out. The effects of traning were also briefly discussed.