Microstructural evolution and thermal stability in rapidly solidified high-chromium-containing copper alloys

Rapidly solidified samples of CuCr alloys containing 2 wt.% Ag were prepared by high pressure gas atomization and by melt spinning. Both quenching techniques yielded microstructures which showed evidence of the formation of two immiscible liquids prior to solidification. To evaluate the effect of high temperatures on the stability of the as-solidified microstructures, samples prepared by hot isostatic pressing (HIP) consolidation of the atomized powder and by heat treatment of the melt-spun ribbons were examined in the scanning electron microscope and transmission electron microscope. Results showed that the shape of the chromium phase in all cases was spheroidal and the spheroid size depended upon the solidification rate and alloy composition. The chromium spheroids in the melt-spun ribbon readily coarsened at higher temperatures, but the coarser chromium spheroids in the atomized powders ripened less quickly when hot isostatically pressed at comparable temperatures. Although capillary-driven diffusional coarsening predominated, diffusion-controlled growth from supersaturation also played a part in increasing the spheroid size. Coarsening in the annealed melt-spun ribbons resulted in a loss of microhardness, while HIP consolidation had little effect on the microhardness of the atomized powder.