INSITU OBSERVATION OF PARTIAL DISLOCATION-MOTION DURING GAMMA-S.RA-EPSILON TRANSFORMATION IN A FE-MN-SI SHAPE MEMORY ALLOY

Tensile deformation and heating experiments have been made in a high voltage electron microscope in order to clarify the mechanism controlling partial dislocation motion in a Fe-Mn-Si shape memory alloy. It is found that a small angle boundary composed of three types of perfect dislocations acts as a dislocation source by a pole mechanism. A dislocation reaction generating a pole dislocation is found to be (a/2)[110BAR] + (a/2)[011] + (a/2)[011BAR] --> (2a/3)[111BAR] + (a/6)[112BAR] where the first term in the right hand side is the pole dislocation proposed by Seeger. Another aspect of the present study concerns the epsilon --> gamma reverse transformation governed by partial dislocation motion upon heating. It is shown that the reverse transformation temperature depends strongly on the structure of epsilon --> martensites interacting with other epsilon-martensites. The simpler martensites reverse transform at a lower temperature. It is also demonstrated that small alpha-martensites are formed at intersection of two epsilon-martensites under certain experimental conditions and that they reverse transform at 773 K.