Mechanisms for influence of post-deformation annealing on microstructure of NiTiFe shape memory alloy processed by local canning compression

Abstract

Mechanisms for the influence of post-deformation annealing on the microstructure of NiTiFe shape memory alloy (SMA) processed by local canning compression were investigated by means of experiment. The microstructure composition of the NiTiFe SMA processed by post-deformation annealing at lower temperature is different from that of binary NiTi SMA, where coarse grains are found in addition to the amorphized structure (AMS) and the nanocrystalline structure (NCS). The main reason for this phenomenon lies in the fact that the value of the deformation temperature relative to the martensite start temperature Ms of the NiTiFe SMA is much larger than that of binary NiTi SMA and some coarse grains are retained after the alloy is processed by severe plastic deformation (SPD). The mechanisms of structure changes in the NCS areas during post-deformation annealing of NiTiFe SMA were found to resemble those of binary NiTi SMA and the mechanisms of structure changes in AMS area of the alloy during post-deformation annealing were revealed for the first time. Texture analysis on the NiTiFe SMA processed by SPD and post-deformation annealing below recrystallization temperature reveals that the strong <111> texture is formed on the condition of post-deformation annealing at 600 ℃. The <111> texture may result from two approaches, where the first one is the <111> -oriented NCS or nano-sized crystalline inclusions (NCI) formed by SPD and the second one is the <111> preferred orientation of the grains nanocrystallized from the amorphous phase. {112} B2 austenite twin is a kind of deformation twin which is usually formed during the deformation above the critical temperature of stress-induced martensitic (SIM) transformation Md. However, this kind of twin was found for the first time in the NiTiFe SMA processed by local canning compression at room temperature and post-deformation annealing at 600 ℃. These {112} B2 austenite twins may be the reverse phase transformation product of the {113} martensite twins induced by the SPD and they grow up under the higher annealing temperature.