Fast diffusion of cobalt along stationary and moving grain boundaries in niobium

Abstract

57Co diffusion along grain boundaries (GBs) in high-purity Nb has been studied in the temperature range 823–1471 K by the serial sectioning technique. As it was not possible to fully stabilize the grain size by any pre-diffusion annealing, part of the GBs could migrate during the diffusion anneals. Using the earlier proposed method [ Z. Metallk. 84, 584 (1993)], the product P = sD GB δ [ s being the 7GB-segregation factor, D GB the GB-diffusion (GBD) coefficient, δ the GB width] for stationary GBs and the velocity V of moving GBs were deduced from the diffusion profiles. The P-values follow an Arrhenius dependence P = (4.84 −2.68 5.99) × 10 −13 exp[-(14.5 +- 7.2) kJ mol −1/ RT] m 3 s −1 and are 4–2 orders of magnitude higher in the measured temperature range than the expected P-values for GB self-diffusion in Nb. This observation is well consistent with the known fact of fast lattice diffusion of Co in Nb and provides evidence for a combined vacancy-interstitial GBD mechanism. From the temperature dependence of V an activation enthalpy of GB motion in Nb H m ≅ 182 kJ mol −1 is estimated.