Recrystallization of porous copper

Abstract

The fragmentary information available about recrystallization and grain growth in porous metals and ceramics is surveyed, together with the known facts concerning the migration, merging and swelling of gas-filled pores. In the present experiments, loose-sintered copper and silver components were hot-rolled into rods at several temperatures and then cold-drawn to wire. The recrystallization of the wires on progressive annealing was followed by measurement of mechanical properties and examination of microstructure; further information about the size, scale and concentration of gas-filled pores was obtained from high-temperature swelling experiments combined with micrographic examination of pore configuration in the hot-worked rods. Recrystallization was inhibited when fine, irregularly shaped powder was sintered to give highly porous compacts, hot-worked at a temperature as low as possible consistent with adequate ductility. The hot-working conditions are crucial, and in particular flat-rolling is not effective in trapping gas in pores. The trapped gas probably consisted of nitrogen and water vapour. The pores in the heavily cold-drawn wires are too fine to detect even at the highest magnifications of the electron microscope, but are nevertheless just as effective in controlling recrystallization as are fine distributions of second phase particles. The estimated mean separation of pores in the most highly inhibited wires wasca. 0.3 μm.