ELEVATED-TEMPERATURE STABILITY OF MECHANICALLY ALLOYED CU-NB POWDERS

When two-phase mixtures of ductile metals are mechanically alloyed, th ey often assume a convoluted lamellar structure. Since these powders a re consolidated at elevated temperatures, their structures (and, there fore, 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 d iffusion, even though the coarsening temperature was high in terms of the homologous temperature of Cu. Reasons for this behavior are sugges ted. Finally, during heat treatment (carried out in H), a Nb layer for ms around the particles. The thickness of this layer (and the correspo nding zone denuded of Nb within the particle) increases with continued elevated-temperature exposure, and at a rate consistent with the proc ess being driven by curvature forces.