Multiphase diffusion in the CoNb and NiNb systems: Part II. Interdiffusion

Abstract

Interdiffusion coefficients for the intermetallic compounds Co 2Nb, CoNb, NiNb, Ni 3Nb and for the Co-rich primary solution of the CoNb system were determined using the Boltzmann-Matano method. The interdiffusion coefficient of the cubic Laves phase Co 2Nb shows a maximum near 73 at.% Co. The activation enthalpies vary from 173 to 157 kJ mol −1 for 70–73 at.% Co and the pre-exponential factor is 4 × 10 −8 m 2s −1. Diffusion very likely occurs via thermal vacancies. Diffusion in the Co-rich primary solution of CoNb is described by a pre-exponential factor of 8· × 10 −6 m 2 s −1 and an activation enthalpy of 252 kJ mol −1. Interdiffusion in NiNb is independent of composition and is described by an activation enthalpy of 271 kJ mol −1 and a pre-exponential factor of 1·9 × 10 −4 m 2 s −1. Coefficients of interdiffusion for the line compound Co 7Nb 2 have been deduced using Wagner's method.