MULTIPHASE DIFFUSION IN THE CO-NB AND NI-NB SYSTEMS .1. SOLID-SOLID PHASE-EQUILIBRIA AND GROWTH OF INTERMETALLIC PHASES

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 Co7Nb2 and Co2Nb-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 mum. In the diffusion zones of Ni-Nb pairs the two expected intermetallic phases NiNb and Ni3Nb were present. The phase diagrams of Co-Nb and Ni-Nb have been carefully redetermined for the temperature regions of solid-solid equilibrium.