Dependence of shape memory effect on austenitic grain sizes in thermo-mechanical treated Fe-Mn-Si-Cr-Ni shape memory alloys

Abstract

It is a controversial point that the effect of austenitic grain sizes on the shape memory effect of processed Fe-Mn-Si-based shape memory alloys so far. To settle this controversy, the effect of austenitic grain sizes on the stress-induced ε-martensite transformation and shape memory effect of a solution-treated Fe-21.63Mn-5.6Si-9.32Cr-5.38Ni alloy before and after thermo-mechanical treatment was studied. For the solution-treated alloy, the generation of stress-induced ε-martensite was suppressed owing to the constraining effect from austenitic grain boundaries and annealing twins. Therefore, the shape memory effect and the amount of stress-induced ε-martensite increased with increasing the austenitic grain sizes ranging from 51.5 μm to 148.5 μm, accompanied by decreasing the densities of austenitic grain boundaries and annealing twins. After thermo-mechanical treatment, a high density of geometrically necessary dislocations was preferentially introduced near the austenitic grain boundaries and the original annealing twins. These dislocations were beneficial for the stress-induced ε-martensite transformation and thus weakened the constraining effect of austenitic grain boundaries and annealing twins to a relatively low level. As a result, the shape memory effect and the amount of stress-induced ε-martensite changed negligibly with the austenitic grain sizes ranging from 57.5 μm to 185.1 μm in the case of thermo-mechanical treated Fe-21.63Mn-5.6Si-9.32Cr-5.38Ni alloy.