Kinetic features of the isothermal ω-phase formation in superelastic Ti-Nb-Zr alloys

Dependence of the isothermal omega(iso)-phase formation on heating, holding and cooling conditions during the ageing of Ti-Nb-Zr shape memory alloys remains insufficiently studied. The crystal structure and the omega(is)(o)-phase morphology of a thermomechanically treated Ti-22Nb-6Zr (at%) alloy during ageing in the 250-375 degrees C temperature range under different heating, holding and cooling conditions is studied using X-ray diffraction and transmission electron microscopy. It is shown that beta -> omega(iso) transformation during ageing of this alloy manifests a C-shaped kinetics with a maximum at 300 degrees C, and the omega-phase formation and growth are predominantly controlled by the shear-related mechanisms and only marginally, by the diffusion-related mechanisms.

Automated summary

The dependence of isothermal omega(iso)-phase formation on heating, holding, and cooling conditions during the ageing of Ti-Nb-Zr shape memory alloys is insufficiently understood. This study investigates the crystal structure and morphology of the omega(is)(o)-phase in a thermomechanically treated Ti-22Nb-6Zr (at%) alloy during ageing under different temperature conditions using X-ray diffraction and transmission electron microscopy. The results show that the beta -> omega(iso) transformation exhibits C-shaped kinetics with a maximum at 300 degrees C, and the formation and growth of the omega-phase are mainly controlled by shear-related mechanisms and to a lesser extent, diffusion-related mechanisms.