The effect of high temperature and high stress-assisted austenite ageing on martensitic transformations in high-strength Ti - 51.8 at. % Ni single crystals

High-strength materials containing dispersed Ti₃Ni₄ particles ( d ~ 800 nm) and exhibiting superelasticity (SE) in a wide temperature range from 200 to 450 K were design due to ageing at 823 K, 1 h of nickel-rich [001]-oriented Ti-51.8 at. % Ni single crystals. The influence of the stress-assisted austenite ageing with different regimes (a temperature, applied stresses and duration) on the stability of the shape memory effect (SME) and SE was studied. It was shown that, over the entire SE temperature range, single crystals are stable to stress-assisted austenite ageing at T < 523 K under a load of 0.8σ_cr. An increase in the temperature up to 523-573 K for stress-assisted (1400 MPa) and stress-free ageing pending 1-10 h leads to the precipitation of nanosized Ti₃Ni₄ particles and changes of SE and SME. The largest effect was observed after ageing at 573 K for 10 h (0 and 1400 MPa), which consists in increase the M_s temperature by 22-27 K during SME, decrease the critical stress σ_cr during SE (by 10-34 %), and increase the deformation hardening coefficient θ = d σ/ d ε and coefficient of critical stress growth with temperature α = d σ/ dT .

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