Austenite grain size effect on recovery stress and recovery strain of Fe-Mn-Si-Cr-Ni-0.01C alloy severely plastically deformed by differential speed rolling

Abstract

The grain-size effect on the shape memory properties of a low-carbon Fe-18.9Mn-4.3Si-8.1Cr-4.2Ni-0.01C alloy severely plastically deformed by high-ratio differential rolling (HRDSR) was investigated after annealing the deformed samples at temperatures in range of 773–1173 K. The grain sizes of the HRDSR-processed samples obtained after annealing were in the range between 1.9 and 13.8 μm. Above the annealing temperature of 973 K, full recrystallization occurred. The volume fraction of stress induced ε-martensite tended to decrease as the grain size increased, whereas the amount of thermally activated ε-martensite increased. The highest recovery stress was obtained at the grain size of 3.8 μm (at annealing at 973 K), while the largest recovery strain was obtained at the larger grain size of 13.8 μm (at annealing at 1173 K), indicating that optimal grain sizes for the high recovery stress and large recovery strain are different. This result proposes that different combinations of recovery stress and recovery strain can be achieved by controlling grain size. The recovery stress and strains increased when the accumulated plastic strain by cyclic deformation was small due to the training effect, but they decreased when the accumulated plastic strain by repeated cycles was large. Compared with the present Fe-SMA, the HRDSR-processed higher-carbon (0.15%C) Fe-SMA studied in a previous work shows a higher recovery stress. This may because the 0.15C Fe-SMA has a smaller grain size due to the presence of higher density of carbides.