Investigation Of Microstructure, Phase Transformation And Properties In An Fe-Mn-Si-C Shape Memory Alloy

The relationship of shape memory effect (SME) with microstructures under different states and phase transformation in an Fe-18.1Mn-5.5Si-0.32C(mass percent) shape memory alloy was investigated systematically. And the effect of carbon on SME was further studied. The heat-treatment processing has an important effect on SME in Fe-Mn-Si based alloys. In present paper, the effect of quenching temperature on the SME of Fe-Mn-Si-C alloy was investigated. The results show that the optimum SME can be obtained by quenching from 700℃. MS rises with the increasing of quenching temperature in the range of temperature at which austenite grains grow greatly. However, quenching temperature has little effect on the MS in the range of temperature at which austenite grains don't grow markedly. An appropriate amount of the thermally induced martensite is advantageous to the nucleation and growth of stress induced martensite (SIM) and improves SME by strengthening austenite and suppressing the slide of perfect dislocations. The micromechanism of SME improved by quenching was discussed further. Not only SME but also superelasticity is associated with SIM transformation in Fe-Mn-Si based alloys. In present work, the effect of factors including stacking fault energy, deformation amount and deformation temperature on superelasticity and SME were investigated. It is concluded that the specimen with higher stacking fault energy can exhibit larger superelasticity. The superelasticity increases with the increasing of deformation temperature reaching the maximum then decreases when deformation temperature goes on rising in the deformation temperature ranged from 25℃ to 300℃ . At high temperature, the recovery strain caused by FCC to HCP martensitic transformation can fully contribute to the superelasticity resulting in the great degradation, even disappearance of SME. Complete superelasticity is likely to be available under tiny deformation (<0.7%). The atomic force microscopy was applied to observe the variation of surface relief of SIM on the surface of a polished specimen in a consecutive tension procedure. The difference of surface relief of SIM before and after heating recovery was compared. It was found that the growth of SIM was not consecutive under large deformation. The mechanism of reverse transformation of SIM is determined by the back motion of Shockley partial dislocation from γ/ε interface to inner of ε martensite, however, not by the nucleation and growth of FCC stacking faults in ε martensite. The corrosion resistance of Fe-Mn-Si-C alloy in acid solutions can be greatly improved, even higher than the one of Fe-Mn-Si-Cr-Ni alloy in acid solutions, by electrodepositing a nickel coat on the surface of specimen. However, the corrosion resistance of electrodeposited Fe-Mn-Si-C alloy in 3.5% NaCl solution was not enhanced. The SME of Fe-Mn-Si-C alloy, which was coated by nickel, degrades with increasing of the thick of coat. When the thick of coat exceeds 1/3 of the one of specimen, SME decreases sharply.