Effect of Grain Size on the Shape Recovery in a Melt-Spun Fe-24%Mn-4%Si-5%Cr-5%Co SMA Ribbon

Abstract

Effect of grain size on microstructure, transformation characteristics and shape recovery has been studied in Fe-24%Mn-4%Si-5%Cr-5%Co shape memory ribbons fabricated by melt spinning. Mean grain size of the ribbons could be controlled by wheel speed; in case of ribbon manufactured in the wheel speed of 10 m/sec, mean grain size was more or less 20 μm, while the mean grain size in 50 m/sec was about 1 μm. Thermal ε-martensite in the shape of plate could be observed in austenite matrix and the volume fraction of that decreased in smaller grains because of grain constraint. As a grain size decreased, one-way and two-way shape recoveries were increased. A change in shape recovery of ribbons was closely related to deformation mode i.e., transformation-induced plasticity or slip according to grain size; it could be confirmed that reversible ε-martensite was induced in preference to irreversible slip in the ribbon with smaller grains during deformation by X-ray diffractometry. Not like in bulk specimen, ε-martensite being very thin in the width of 10 ~ 20 nm and lots of stacking faults being inferred from streaks of SAD patterns, were revealed in melt-spun ribbons manufactured in the wheel speed of 50 m/sec.