Grain size effect on the recovery stress and strain of a Fe–Mn–Si shape memory alloy in a wide range of grain sizes

Abstract

The grain size effect on the recovery strain and recovery stress was systematically examined over a very wide grain-size range (between 1.4 μm and 171 μm) in the Fe-15.9Mn-5.1Si-8.1Cr-3.9Ni-0.02C alloy for a better understanding of the grain size effect on shape memory properties of the Fe–Mn–Si based alloys. The recovery strain rapidly increases with grain size up to approximately 16 μm and then gradually increases with a further increase in grain size. A linear relationship between recovery strain and logd (d: grain size) is observed in the whole grain size range. The densities of grain boundaries and annealing twin boundaries also exhibit the linear relationship with logd, but the trend is opposite, suggesting that recovery strain is directly related to the densities of grain boundaries and twin boundaries and the former increases as the latter decreases. The recovery stress behavior during heating and cooling of the samples is different depending on the grain size range. When the grain size is smaller than 6 μm, yielding does not occur during sample cooling. When the grain size is in the range between 16 μm and 33 μm, yielding occurs during cooling. When the grain size is larger than 46 μm, yielding and forward martensitic transformation -occur in sequence during cooling. The highest recovery stress is obtained at a grain size of 6 μm, which is the largest grain size where yielding can be avoided during cooling, and the lowest recovery stress is obtained at a grain size of 171 μm, where both yielding and forward martensitic transformation occur during cooling. Depending on the degree of grain refinement, the quality of improvement in recovery stress and recovery strain is expected to be different. Simultaneous improvement of recovery stress and strain is predicted to be hard to effectively achieve only through grain refinement.