ELEVATED-TEMPERATURE STABILITY OF MECHANICALLY ALLOYED CU-NB POWDERS

When two-phase mixtures of ductile metals are mechanically alloyed, they often assume a convoluted lamellar structure. Since these powders are consolidated at elevated temperatures, their structures (and, therefore, properties) are likely to be altered by consolidation processing. We have investigated microstructural changes that take place on heat-treating mechanically alloyed Cu-20 vol pet Nb alloys. The transition from a ''platelike'' to a Spherical microstructure is described, and the kinetics of this process appear controlled by a type of boundary diffusion, even though the coarsening temperature was high in terms of the homologous temperature of Cu. Reasons for this behavior are suggested. Finally, during heat treatment (carried out in H), a Nb layer forms around the particles. The thickness of this layer (and the corresponding zone denuded of Nb within the particle) increases with continued elevated-temperature exposure, and at a rate consistent with the process being driven by curvature forces.