METALS AS SINKS AND BARRIERS FOR INTERSTITIAL DIFFUSION WITH EXAMPLES FOR OXYGEN DIFFUSION IN COPPER, NIOBIUM AND TANTALUM

The free energy of solution of interstitial atoms is rather different in metals of different groups of the periodic table and, therefore, the equilibrium concentration ratio of the interstitials in a metal I/metal II couple reaches several orders of magnitude. Thus one of the metals becomes a sink while the other acts as a barrier. Some experimental procedures were discussed which allow determination of diffusion coefficient D and the diffusivity-solubility product Dcs. The methods described were applied to the oxygen systems of Cu, Nb and Ta where the following results were obtained: Nb-O D0 = 1.7(± 0.2) · 10-6 m2/s (T = 600-1100°C) Q = 108(± 1) kJ/mole Ta-O D0 = 3.5(± 0.3) · 10-7 m2/s (T = 600-1600°C) Q = 99.3(± 0.7) kJ/moleCu-O Dcs = 4.2(-1.3+1.7)·10-6exp(- 21,200±200 T)Do = 5.8(-4.3+15.7·10-7m2/s (T = 600-1000°CQ = 57.4(±11.6)kJ/mole. A comparison with internal friction data indicates that oxygen occupies octahedral sites in the b.c.c. lattice of niobium and tantalum. © 1979.