Grain Boundary Segregation and Grain Boundary Diffusion of Carbon in Niobium. Comparison of Data for Substitution and Interstitial Solid Solutions

摘要

Bulk and grain boundary (GB) diffusion of ~(14)C in Nb has been studied by radiotracer serial sectioning technique in B (800-950 K) and C (973-1173 K) kinetics regimes. The values of P = sδD_(gb), D_(gb) and s follow Arrhenius dependencies: P = 5.15-10~(-15) exp [-(83.1 kJ/mol)/RT] m~3/s, D_(gb) = 2.3-10~(-6) exp [-(133.0 kJ/mol)/RT] m~2/s, s = 4.7 exp [(49.9 kJ/mol)/RT]. The increase of GB diffusion comparing with self-diffusion is very large despite the retardation due to the strong segregation. The results for GB diffusion of C in Nb and also for other interstitial solutes (P, S) in bcc transition metals (α-Fe, Mo) are discussed in a framework of the transition state theory. It is supposed that GB segregation decreases the energy of ground state whereas the change of diffusion mechanism from vacancy to interstitial leads to the strong decrease of transition state energy and consequently to the fast GB diffusion of interstitial solutes in spite of the large tendency to segregate to GBs.