The effect of high-temperature and high-stress martensite aging on martensitic transformation and microstructure of Ti–51.5 at % Ni single crystals

In this work, the thermomechanical stability of heterophase [001]-oriented Ti–51.5 at % Ni single crystals was investigated. Experiments include a high-temperature and high-stress martensite ageing and also loading/unloading cycling at superelasticity (SE) during forward and reverse B2-B19′ martensitic transformations (MT). It was found that Ti–51.5 at % Ni single crystals containing large Ti₃Ni₄ particles (d ∼ 600 nm after ageing at 850 K for 1 h) is not resistant to stress-assisted martensite ageing and cycling. Stress-assisted martensite ageing at 423 K, 1.7–2 GPa, for 2–4 h leads to the deformation of large Ti₃Ni₄ particles, the appearance of dislocations at the particle-matrix boundaries, an increase in MT temperatures (by 10 K), and a strong diffusion of the MT (the intervals of forward and reverse MT $Δ1σ$ and $Δ2σ$ increase in 2 times). During loading/unloading cycling at 423 K and 1.0–2.0 GPa single crystals with large particles were destroyed after 20–25 cycles. It is possible to increase the thermomechanical stability due to the additional precipitation of nanosized Ti₃Ni₄ particles (d<30 nm by ageing at 673 K, 1 h), which appear in the B2 matrix between large particles. In such crystals with a bimodal structure no changes in the microstructure and functional properties were observed after stress-assisted martensite ageing and loading/unloading cycling.