The bulk and grain-boundary diffusion of 14C was studied by using the radio-tracer and serial sectioning technique. The B and C kinetic regimes were operative for grain-boundary diffusion at 800 to 1173K. It was found that the values of P (= sδDgb), Dgb and s obeyed the Arrhenius dependences:

973–1173K:     P (m3/s) = 5.15 x 10-15 exp[-83.1(kJ/mol)/RT]

800–950K:     Dgb (m2/s) = 2.3 x 10-6 exp[-133.0(kJ/mol)/RT]

s = 4.7 x 100 exp[-49.9(kJ/mol)/RT]

The increase in grain-boundary diffusion, as compared with self-diffusion, was very large; in spite of a probable retardation effect due to strong segregation. The results for the grain-boundary diffusion of C were considered within the framework of transition state theory. It was assumed that grain-boundary segregation decreased the energy of the ground state, whereas a change in diffusion mechanism (such as from vacancy to interstitial) led to a marked decrease in the transition state energy. This change in the diffusion mechanism resulted in the rapid grain-boundary diffusion of interstitial solutes; in spite of their marked tendency to segregate to grain-boundaries.

B.Bokstein, I.Razumovskii: Interface Science, 2003, 11[1], 41-9

Table 75

Grain Boundary Diffusivity of Co in Nb