ECAP based regulation mechanism of shape memory properties of NiTiNb alloys

NiTiNb is a kind of wide hysteresis shape memory alloy with great application potential in aerospace. However, the mechanism of improving the shape memory properties of the NiTiNb alloy by tailoring grain size and grain boundary type has not been well identified. Accordingly, taking equal channel angular pressing (ECAP) as the grain refinement method, this paper focused on the regulation mechanism of grain size and grain boundary type on the shape memory properties. First, grain refinement structures with a special distribution of low- and high- angle grain boundaries (LAGBs and HAGBs) were prepared by ECAP with different passes. Then, the mechanism of the grain boundary on the shape memory properties was first revealed. It was found that many LAGBs appeared in NiTiNb alloy after ECAP, and with increasing of ECAP passes, the LAGBs were transformed into the HAGBs, achieving grain refinement. The stress-induced transformation produced (001) compound twins in the dense sub-microcrystalline region surrounded by LAGBs, leading to a non-monotonic variation of transformation strain with decreasing grain size. Meanwhile, with the decrease of grain size (or the increase of grain boundary content), the transformation hysteresis temperature and recovery stress of the NiTiNb alloy increased simultaneously. To obtain better shape memory characteristics, we can tailor the type and distribution of specific grain boundaries through the design of ECAP.