Characterization of residual stress evolved in iron-based shape memory alloys

It is considered that complicated residual stresses may occur in shape memory alloys (SMAs) during deformation, as the matrix phase is transformed to martensitic phase during plastic deformation and the martensitic phase is reversely transformed by heating. Since the final shape of SMAs are influenced by residual stresses, it is important to characterize the residual stresses formed in SMAs during plastic deformation and annealing. The X-ray diffraction method was used to characterize the phase transformation and the residual stress formed in an Fe-Mn-Si-Cr SMA in this study. The samples were tensiledeformed to different strains and subsequentl annealed. The results showed that a part of ?-phase with the face-centered cubic (fcc) structure was found to be transformed to e-phase with the hexagonal close-packed (hcp) structure by room-temperature tensile deformation in the SMA, and the e-phase was reversely transformed by subsequent heating. It has been also shown that the compressive stress occurred in the tensile direction of the ?-phase on tensile deformation and unloading. The compressive stress is believed to result from the formation of the e-phase during stress-induced martensitic transformation. After the deformed samples were annealed to recover their shapes, the residual stress was considerably released. This is considered to be due to the decrease in the amount of the transformed e-phase during annealing. These results indicated that residual stress in the fcc matrix phase is correlated with the shape recovery characteristics of the SMA after martensitic and reverse martensitic transformations.