Influence of Heating and Cooling on the Isothermal β → ω Transition in Ti–22Nb–6Zr Alloy

The influence of heating and cooling routes prior to aging of the Ti–22Nb–6Zr shape memory alloy (at %) on the intensity of formation of the isothermal ω_iso phase in the temperature range from 250 to 350°C for 1 and 3 h is studied by X-ray diffraction. It is shown that, for intense formation of the ω_iso phase, the most efficient scheme for entering the aging interval includes rapid water cooling water to room temperature from an annealing temperature of 600°C and subsequent rapid heating to an aging temperature of 300°C. All other routes used for entering in the aging interval, including slow cooling and/or heating, do not lead to the formation of an X-ray identifiable amount of the ω_iso phase, whereas, the β → ω_iso transformation in the temperature range from 250 to 350°C has a pronounced C-shaped kinetics with a maximum at 300°C. Aging in the entire range of 250–350°C leads to the dispersion hardening and an increase in the hardness of the alloy compared to the initial state. Moreover, the hardness gradually increases with an increase in the aging temperature from 250 to 300°C and remains constant in the temperature range of 300–350°C. The β-phase lattice parameter of Ti–22Nb–6Zr alloy remains unchanged over the entire aging temperature range of 250–350°C, which indicates the absence of noticeable diffusional redistribution of elements in the solid solution during the formation of the ω_iso phase. The w_iso phase formed during the aging of the Ti–22Nb–6Zr alloy over the entire temperature range of 250–350°C has the ratio c_ω/a_ω = 0.613 ± 0.002, which is similar to the ratio c_ω/a_ω for the shear-type athermal ω_ath phase, which in turn further emphasizes the identity of these two phase varieties.