The variant selection of Fe-Mn-Si based shape memory alloy by the Schmid's factor

Abstract

The shape memory effect (SME) in Fe-Mn-Si based alloy is associated with γ ⇄ ε martensitic transformation which is caused by motion of the Shockley partial dislocation. The variants in Fe-Mn-Si based alloy could appear the four orientations according to Shouji-Nishiyama (S-N) relation in the transformation from the austenite phase (A-phase) to the martensite phase (M-phase). It was reported that a single variant formed under the stress induced transformation in the SME and that is the advantage for the shape recovery in the variant selection behaviors. In this study, the variant selection of Fe-Mn-Si based alloy under stress applied is investigated on the basis of crystallography in the SME with Electron Backscatter Diffraction (EBSD). The amount of shape recovery was quantified by means of bending tests and the best recovery ratio was run into 83% for the specimen annealed 873K. The SME is improved by not only the increase of A-phase but also the promotion of the single variant under stress induced transformation. The single variant orientation is selected by the degree of Schmid's factor in the behaviors. In the specimen annealed at 873K, the increase of M-phase concentrates at 0.475 of the Schmid's factor, the single variant orientation, and that suggests the single variant formation.

Keywords

• shape memory alloy
• fcc-hcp transformation
• schmid's factor
• single variant
• variant selection