Anomalous hot deformation behavior and microstructure evolution of as-cast martensitic NiTi alloy during hot compression

The martensite phase with low crystal symmetry in NiTi alloy consists of internal twins and nano-layered compounds. The martensite phase has various inelastic deformation modes, such as twinning, reorientation, and nanocrystallization, during the deformation process, which leads to the further understanding of this deformation mechanism is unclear. Therefore, this research performed hot compression tests on the as-cast martensitic NiTi alloy, focusing on the analysis of the hot deformation behavior of the flow curves with abnormally increased stress value under high strain. The results showed that the peak stress, peak strain and softening stage of the flow curve decrease with the increases in deformation temperature, and the stress value of the flow curve increases at the high strain stage. According to the microstructure analysis of the alloy, the Ti2Ni phase is extruded and broken into small particles with the compression progress. Further spheroidization of the Ti2Ni phase with the compression temperature increases. In the process of hot compression, the martensite variants in the B′19 phase reorient, and the deformation twins and nanocrystalline structures are formed. The hindering effect of the broken Ti2Ni particles on dislocation movement and the strengthening role of nanocrystalline grains are the main reasons for the abnormal increase in stress value at high strain.