MICROSTRUCTURE AND HOMOGENEITY OF NANOCRYSTALLINE CO-CU SUPERSATURATED SOLID-SOLUTIONS PREPARED BY MECHANICAL ALLOYING

Mechanical alloying (MA) has been performed in the Co,Cu-x((100-x)) (x = 10, 25, 50, 60, 75, and 90) system. High resolution electron micros copy (HREM) and field emission gun transmission electron microscopy (F EG TEM) were used to characterize the microstructure and homogeneity o f the nanocrystalline Co25Cu75 solid solution. After 20 h of MA, all t he mixtures show an entirely face-centered cubic (fee) phase. HREM sho ws that the ultrafine-grained (UFG) materials prepared by MA contain a high density of defects. Two kinds of typical defects in UFG Co25Cu75 are deformation twins and dislocations. The dislocations are mostly 6 0 degrees type, and in many cases they dissociate into 30 degrees and 90 degrees partials. The grain boundaries are ordered in structure, cu rved, and slightly strained, which is similar to that observed in NC-P d. Nanoscale energy dispersive x-ray spectroscopy (EDXS) shows that th e Co concentration in both the interior of grains and the GB's is clos e to the global composition, which proves that supersaturated solid so lutions are indeed formed. In the meantime EDXS revealed that the mixi ng of Co and Cu in the solid solutions is homogeneous at nanometer sca le. MA in the Co-Cu system is suggested to be a diffusion-controlled p rocess, and stress-stimulated diffusion is proposed to be the reason f or the formation of supersaturated solid solutions in this immiscible system.