On the mechanism of two way shape memory effect obtained by stabilised stress induced martensite

Abstract

Two way shape memory effect (TWME) can be obtained by suitable thermomechanical processing which involve repetitive training routines. J. Perkins found that TWME take place as a result of a macroscopic non-uniform residual stress field, concluding that plastic deformation was necessary to get TWME. K. Enami et al found that complex dislocations arrays are generated by thermomechanical cycling during the training procedure. TWME results obtained by the above training methods are not as good as would be expected, because during the thermal cycles, above and below A[sub f] and M[sub f] respectively, new dislocations are generated which interact with the dislocations that control TWME so giving a loss of TWME. A different training method has been developed by J.M. Guilemany et al based on the stabilization of stress induced martensite variants (SSIM). This method has been derived from the observation made by J. Perkins and R.O. Sponholz who found that retained (not stabilized) martensite acts as a nucleation site of thermal martensite. Thus, during cooling the stabilized martensite would grow or influence the nucleation and growth of the thermal martensite giving TWME. The effect of training temperature, time and stress on TWME obtained by SSIM has been studied.