Microstructures evolution and phase transformation behaviors of Ni-rich TiNi shape memory alloys after equal channel angular extrusion

Ni-rich TiNi shape memory alloys were subjected to the effect of equal channel angular extrusion (ECAE) processes at 773 K by Bc path. The effects of ECAE processes on microstructures evolution and phase transformation behaviors were investigated. The initial 60–80 μm equiaxed coarse grains of samples were elongated along the shearing force direction of ECAE and refined to 300–400 nm after eight passes ECAE. The R phase transformation of Ni-rich TiNi shape memory alloys was stimulated by ECAE processes within a larger temperature range. The martensite transformation peak temperature (Mp) dropped in previous 1–3 ECAE treatments, but the dropped Mp increased gradually with the increase of ECAE processes. Ti3Ni4 phase was observed in the regions with high density of dislocations after ECAE treatment. Reasons for microstructures evolution and phase transformation changes were also discussed.

Research highlightsNi-rich TiNi shape memory alloys have widely industrial and medical applications due to the remarkable shape memory effect (SME), pseudoelasticity (PE). The unique SME and PE originate from the thermoelastic martensitic transformation. However, microstructures evolution and phase transformation behaviors of Ni-enriched Ti–50.9 at.% Ni alloy after ECAE treatment are seldom studied systematically. This paper focused on the microstructures evolution and the phase transformation behaviors of Ni-rich TiNi alloys after multiple ECAE treatments at lower temperature 773 K by Bc path. This investigation will be helpful for better understanding the phase transformation behaviors of TiNi SMAs, and further designing of other shape memory alloys.