On the Mechanisms and Thermocyclic Stability of β → ωiso Transformation in a Superelastic Ti–Nb–Zr Shape Memory Alloy
摘要
The role of diffusion in the isothermal ω-phase formation and the β → ωiso transformation stability (reproducibility) during thermal cycling of a superelastic Ti–Nb–Zr shape memory alloy remain insufficiently studied. The distribution of chemical elements in the vicinity of ωiso-phase particles after aging at 300 and 375 °C and the stability of β → ωiso → β transformation in a heating half-cycle during thermal cycling of Ti–22Nb–6Zr (at. %) alloy were studied using high-resolution scanning transmission electron microscopy with energy dispersive X-ray spectroscopy techniques and X-ray diffractometry. It was shown that the leading mechanism of ωiso-phase formation in the Ti–22Nb–6Zr alloy is a crystal lattice shear, while the diffusional redistribution of elements in the vicinity of ωiso-phase particles plays a secondary role which appears only at higher aging temperatures. The β → ωiso → β transformation features, β- and ω-phases structure parameters are stable and reproducible during thermal cycling up to at least ten cycles.