The influence of deformation on the microstructure and transformation temperatures of Fe-Mn-Si-Cr-Ni shape memory alloys

The characteristics of the martensitic transformation in a Fe-Mn-Si-Cr-Ni alloy and in a similar alloy containing also Co were investigated in terms of the amount and type of deformation: cold rolling, with 19 and 33% reduction of area, and wire drawing, with 20% reduction of area in the last pass. The temperatures of start and finish of the reverse transformations were measured by dilatometry. The constitution and microstructure of the alloys were characterized by X-ray diffraction, scanning electron microscopy, atomic force microscopy and magnetic force microscopy. The cold forming processes applied to the alloys induced the formation of ε and the α' martensites, even when the start temperature of thermal martensite was well below room temperature. An increase of up to 500 K in the characteristic temperatures of the reverse transformation, when compared to thermal martensite, was also observed. (orig.)