Microstructure evolution during cyclic deformation of the nanocrystalline superelastic Ti-50.9 at.% Ni alloy with different parent phases

This study presents a comparative investigation of the influence of the phase composition of the initial phase (B2 austenite or R martensite) on the evolution of the deformation structure of the nanocrystalline superelastic Ti-50.9 at.% Ni alloy under tension in the load-unload mode for up to 45 cycles. It was found that for B2 austenite, dislocations accumulate during the initial stage of cycling. Around 10–20 cycles, with the achievement of a high stress level in the strengthened B2 structure, a change in the deformation mechanism of B2 austenite is observed from the accumulation of dislocations to reorientation mechanisms in the strengthened structure with the formation of reorientation mesobands. In comparison, the structure of the R phase remains qualitatively unchanged with increasing stress. The presence of the R phase as an parent phase preserve of a low level internal stress prevents the formation of deformation bands and contributes to the increase in the cyclic stability of superelastic behavior. The R martensite structure provides higher resistance to defect accumulation during mechanical cycling than the B2 austenite structure, which can contribute to maintaining relatively stable mechanical properties of the nanocrystalline TiNi alloy and maintaining fatigue strength.