MICROSCOPIC OBSERVATION OF GAMMA-PHASE AND EPSILON-MARTENSITE AND ALPHA'-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, gamma-, epsilon-martensite and alpha'-martensite can be easily distinguished and quantified. Two types of transformations, gamma --> epsilon and gamma --> epsilon --> alpha', occur in the studied alloys. The gamma --> epsilon transformation is observed in the specimen after annealing at high temperatures followed by air cooling to room temperature. Increasing annealing temperature enhanced the gamma --> epsilon transformation during cooling. Both gamma --> epsilon and gamma --> epsilon --> alpha' transformations are found after applying a certain amount of deformation strain. Increasing the amount of applied strain induced the gamma --> epsilon --> alpha' transformation more abundantly. The growth and intersection of alpha'-martensite is enhanced when the applied strain is large, During reverse heating, alpha'-martensite directly transformed back to gamma without reverting through epsilon-martensite.