Diffusion of titanium and niobium in bcc Ti-Nb alloys

The diffusion coefficients for titanium and niobium radioactive tracers were simultaneously measured in Ti, ${\mathrm{Ti}}_{94.6}$${\mathrm{Nb}}_{5.4}$, ${\mathrm{Ti}}_{80.4}$${\mathrm{Nb}}_{19.6}$, and ${\mathrm{Ti}}_{64.3}$${\mathrm{Nb}}_{35.7}$ over the temperature range from 950°C to 1511°C using standard lathe-sectioning techniques. The samples were initially heat treated by annealing above the $\alpha -\beta$ phase-transition temperature and then either cooling slowly to room-temperature or quenching. The room-temperature crystal morphology was then examined using x-ray diffraction. Alloy concentrations were chosen both to suppress the $\beta -\alpha$ transition and to obtain the metastable $\omega$ phase. The data may be described by Ti: $D_{\mathrm{Ti}}=4.54\mathrm{}\times {10}^{-4\mathrm{}}{e}^{-\frac{31300}{\mathrm{RT}}}$ ${\mathrm{cm}}^2$/sec; $D_{\mathrm{Nb}}=2.91\mathrm{}\times {10}^{-4\mathrm{}}{e}^{-\frac{31030}{\mathrm{RT}}}$ ${\mathrm{cm}}^2$/sec; ${\mathrm{Ti}}_{94.6}$${\mathrm{Nb}}_{5.4}$: $D_{\mathrm{Ti}}=1.27\mathrm{}\times {10}^{-3\mathrm{}}{e}^{-\frac{35610}{\mathrm{RT}}}$ ${\mathrm{cm}}^2$/sec; $D_{\mathrm{Nb}}=1.79\mathrm{}\times {10}^{-3\mathrm{}}{e}^{-\frac{38220}{\mathrm{RT}}}$ ${\mathrm{cm}}^2$sec; ${\mathrm{Ti}}_{80.4}$${\mathrm{Nb}}_{19.6}$: $D_{\mathrm{Ti}}=3.15\mathrm{}\times {10}^{-3\mathrm{}}{e}^{-\frac{41960}{\mathrm{RT}}}$ ${\mathrm{cm}}^2$/sec; $D_{\mathrm{Nb}}=1.18\mathrm{}\times {10}^{-2\mathrm{}}{e}^{-\frac{47330}{\mathrm{RT}}}$ ${\mathrm{cm}}^2$/sec; ${\mathrm{Ti}}_{64.3}$${\mathrm{Nb}}_{35.7}$: $D_{\mathrm{Ti}}=2.51\mathrm{}\times {10}^{-1\mathrm{}}{e}^{-\frac{59020}{\mathrm{RT}}}$ ${\mathrm{cm}}^2$/sec; $D_{\mathrm{Nb}}=2.98\mathrm{}\times {10}^{-1\mathrm{}}{e}^{-\frac{61730}{\mathrm{RT}}}$ ${\mathrm{cm}}^2$/sec. The hypothesized enhancement of diffusion by $\omega$-phase fluctuations or effects of the hcp to bcc phase transformation cannot be ruled out by this work, but are certainly not supported directly by the present results.