MULTIPHASE DIFFUSION IN THE CO-NB AND NI-NB SYSTEMS .2. INTERDIFFUSION

Interdiffusion coefficients for the intermetallic compounds Co2Nb, CoN b, NiNb, Ni3Nb and for the Co-rich primary solution of the Co-Nb syste m were determined using the Boltzmann-Matano method. The interdiffusio n coefficient of the cubic Laves phase Co2Nb shows a maximum near 73 a t.% Co. The activation enthalpies vary from 173 to 157 kJ mol 1 for 70 -73 at.% Co and the pre-exponential factor is 4 X 10(-8) m2 s-1. Diffu sion very likely occurs via thermal vacancies. Diffusion in the Co-ric h primary solution of Co-Nb is described by a pre-exponential factor o f 8.7 X 10(-6) m2 s-1 and an activation enthalpy of 252 kJ mol-1. Inte rdiffusion in NiNb is independent of composition and is described by a n activation enthalpy of 271 kJ mol-1 and a pre-exponential factor of 1.9 X 10(-4) m2 s-1. Coefficients of interdiffusion for the line compo und Co7Nb2 have been deduced using Wagner's method.