In-situ Observation of Partial Dislocation Motion during γ→ε Transformation in a Fe–Mn–Si Shape Memory Alloy

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Abstract
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)[1\bar10]+(a⁄2)[011]+(a⁄2)[0\bar11]→(2a⁄3)[1\bar11]+(a⁄6)[\bar112] where the first term in the right hand side is the pole dislocation proposed by Seeger. Another aspect of the present study concerns the ε→γ reverse transformation governed by partial dislocation motion upon heating. It is shown that the reverse transformation temperature depends strongly on the structure of ε-martensites interacting with other ε-martensites. The simpler martensites reverse transform at a lower temperature. It is also demonstrated that small α-martensites are formed at intersection of two ε-martensites under certain experimental conditions and that they reverse transform at 773 K.