Influence of heating and cooling routes on the isothermal β → ω transition in Ti–22Nb–6Zr alloy

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

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