Multiphase diffusion in the CoNb and NiNb systems: Part I. Solid-solid phase equilibria and growth of intermetallic phases

Abstract

Diffusion couples of cobalt or nickel and niobium were employed to study solid-state diffusion reaction (multiphase diffusion). The formation and growth of intermetallic phases was investigated by means of optical metallography and electron probe microanalysis. Parabolic layer growth of Co 7Nb 2 and Co 2N b— the dominant phases in the diffusion zone of CoNb pairs—was observed. The growth constants were determined at various temperatures. Slow growth of the CoNb compound was also observed, but the layer thickness never exceeded a few μm. In the diffusion zones of NiNb pairs the two expected intermetallic phases NiNb and Ni 3Nb were present. The phase diagrams of CoNb and NiNb have been carefully redetermined for the temperature regions of solid-solid equilibrium.