The superelasticity and shape memory effect in Ni-rich Ti-51.5Ni single crystals after one-step and two-step ageing

Abstract

In this work the microstructure, superelasticity and shape memory effect in [001]-oriented Ti-51.5at.%Ni single crystals after one-step ageing (at 850 K, 1 h) and two-step ageing (at 850 K, 1 h + 673 K, 1 h) were investigated. Large Ti3Ni4 particles (d ~ 600 nm) were precipitated at first step of ageing (850 K, 1 h) and nanosized Ti3Ni4 particles (d < 30 nm) were formed between the large particles at second step of ageing (673 K, 1 h).It is experimentally shown that precipitation of Ti3Ni4 particles during ageing (both at one-step and two-step ageing) in nickel-rich B2-matrix (CNi = 51.5 at.%) made it possible to create ultrahigh-strength [001]-oriented single crystals in which the strength properties of the B2-phase exceeded 2 GPa and the high-temperature superelasticity with a completely reversible strain in temperature range from 200 K to 460 K were observed.The thermal- and stress-induced martensitic transformations in aged crystals were characterized by wide intervals of forward and reverse martensitic transformations within stress and temperature, which were determined by the microstructure: large Ti3Ni4 particles and its distribution and nanosized Ti3Ni4 particles. The effect of the nanosized Ti3Ni4 particles, precipitated during the second step of ageing, is concluded in an additional widening of martensitic transformation intervals, a significant increase in strength properties of the B2-phase (from 2.0 to 2.4 GPa), a modification of the mechanism of accumulation and dissipation of energy during martensitic transformation (increase in reversible elastic energy and reduce of energy dissipation). As a result a more stable microstructure with bimodal particle distribution for high-temperature and high stress cycling was obtained.