Powder metallurgy was used to produce compacts with fine (1 to 10µm) grains that were pinned by sub-micron, to micron-sized, gas-filled voids having volume fractions ranging from 0.05 to 0.2. The effects of subsequent heat treatment depended strongly upon whether the powder was consolidated using cold-pressing and sintering, or hot-pressed. Pressed and sintered compacts densified further, whereas hot-pressed compacts exhibited swelling during subsequent thermal exposure. The materials were tested in compression and tension at room temperature, and high yield strengths were noted which were attributed to grain-size strengthening. A tensile ductility of above 20% was also obtained due to unusual features, such as upper and lower yield points and a low work-hardening rate. Approximate calculations which examined the interaction of dislocations with a void pair, an assembly of voids and the special case in which all voids were located at grain boundaries, indicated that direct strengthening due to voids was not the main cause of the high strength. It was instead the decrease in grain size that was responsible for the observed levels of yield strength.

Microporous Fine-Grained Copper - Structure and Properties. K.S.Kumar, M.S.Duesbery, N.P.Louat, V.Provenzano, M.S.Dipietro: Philosophical Magazine A, 2001, 81[5], 1023-40