SHAPE-MEMORY EFFECT AND RELATED PHENOMENA IN A MICROALLOYED FE-MN-SI ALLOY

An Fe-32Mn-6Si-0.05Nb-0.04C alloy aged at different temperatures was employed to study the epsilon (hcp) martensite transformation, the mechanism of the shape memory effect (SME) associated with the epsilon martensite transformation, the effect of precipitation strengthening, and the effect of thermomechanical treatment and deformation temperature on the SME. The results of transformation temperature studies show that the M(s) temperature for various heat-treated specimens is in the range of 75-100-degrees-C, the A(s) temperature is 112-125-degrees-C, and the Neel temperature of this material, T(N), is 8-degrees-C. The SME decreases with increasing plastic deformation and increases with decreasing deformation temperature and increasing thermomechanical training. The critical strength of aged specimens is higher than that of quenched ones due to the precipitation strengthening of NbC, which improves the SME for plastic deformation amounts less than 2% at room temperature. The microstructure observation by transmission electron microscopy shows that stacking faults play an important role in the formation of the epsilon martensite. Surface relief examination at different temperatures and the electrical resistance versus temperature relationship reveal that the epsilon martensite transformation takes place continuously at temperatures below the T(N) temperature. An SME mechanism for Fe-Mn-Si based alloys is proposed.