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

摘要

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 Ti_3Ni_4 particles (d ～ 600 nm) were precipitated at first step of ageing (850 K, 1 h) and nanosized Ti_3Ni_4 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 Ti_3Ni_4 particles during ageing (both at one-step and two-step ageing) in nickel-rich B2-matrix (C_(Ni) = 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 Ti_3Ni_4 particles and its distribution and nanosized Ti_3Ni_4 particles. The effect of the nanosized Ti_3Ni_4 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.