Microscopic observation of γ-phase and ε- and α′-martensite in FeMnSi-based shape memory alloys

The martensitic transformation occurring in FeMnSi-based shape memory alloy has been studied by microstructural characterization and by a color etching method developed to reveal the martensite structure. With this technique, γ-, ε-martensite and αʹ-martensite can be easily distinguished and quantified. Two types of transformations, γ→ε and γ→ε→α′, occur in the studied alloys. The γ→ε transformation is observed in the specimen after annealing at high temperatures followed by air cooling to room temperature. Increasing annealing temperature enhanced the γ→ε transformation during cooling. Both γ→ε and γ→ε→α′ transformations are found after applying a certain amount of deformation strain. Increasing the amount of applied strain induced the γ→ε→α′ transformation more abundantly. The growth and intersection of α′-martensite is enhanced when the applied strain is large. During reverse heating, α′-martensite directly transformed back to γ without reverting through ε-martensite.