PHASE FIELD SIMULATION OF EFFECTS OF PORES ON B2-R PHASE TRANSFORMATION IN NiTi SHAPE MEMORY ALLOY

Recently, porous NiTi shape memory alloys (SMAs) have drawn great interest in vari- ous engineering fields, in particular for biomedical applications as one of the promising biomaterials for hard-tissue replacements and orthopedic implants. It is well known that the porous NiTi SMAs exhibit three transformations, B2-B19 � , B2-R and R-B19 � . Among these phase transformations B2-B19 � and R-B19 � involve high lattice distortion and large transformation hysteresis. Consequently, these distortion and transformations usually introduce structural defects which may result in degradation of mechanical stability for the functional application. On the contrary, B2-R transformation is governed by small lattice distortion which indicates less damage to the microstructure and lower sensitivity to the defects, and thus generates higher reversibility and mechanical stability. Due to these unique virtues, it is of great importance to study the R-phase transformation behavior in porous NiTi SMAs, since the pore has a significant influence on B19 � martensitic transformation as well as the R-phase transforma- tion. In this paper, a three-dimensional phase field model aiming at accounting for the pore effect on phase transformation in NiTi SMAs was developed to study the B2-R phase transformation behavior in porous NiTi SMAs. The model was applied to characterize the microstructure evolution of B2-R phase