Inhomogeneous Plastic Deformation Mechanisms of NiTiFe Shape Memory Alloy Subjected to Local Canning Compression

Abstract

Mechanism for inhomogeneous plastic deformation of NiTiFe shape memory alloy (SMA) is revealed under local canning compression at room temperature. Microstructures of the compressed NiTiFe sample are very inhomogeneous due to the occurrence of the different plastic strains corresponding to the different deformation zones. B2 austenite, B19′ martensite, nanocrytalline grains, amorphous phase and the retained coarse grains are observed in the deformed NiTiFe sample. B19′ martensite phase is induced by means of mechanical loading and it is stabilized by large plastic strain. In addition, B19′ martensite phase is formed after plastic deformation for dislocation slip has occurred. The phenomenon demonstrates that at the initial stage of deformation, the critical resolved shear stress (CRSS) for dislocation slip of NiTiFe SMA is lower than the critical stress for inducing B19′ martensite. Formation of B19′ martensite is dependent on the higher plastic strain where the CRSS for dislocation slip of NiTiFe SMA is increased substantially enough to be higher than the critical stress for inducing B19′ martensite. With the progression of plastic deformation, a lot of dislocations are accumulated substantially and they provide the driving force for formation of nanocrystalline grains and amorphous phase.