Microporous fine-grained copper: structure and properties

Powder metallurgy processing has been used to produce copper compacts with fine grain sizes (1-10 mum) that are pinned by submicron-size to micron-size gas-filled voids in the volume fraction range 0.05-0.2. The effect of subsequent heat treatment on the grain size and void size and shape was quantified. These changes strongly depended on whether the powder was consolidated using a cold-pressing-and-sintering route, or hot pressing; thus the pressed and sintered compacts densified further whereas the hot-pressed compacts exhibited swelling during subsequent thermal exposure. Such materials were mechanically tested in compression and tension at room temperature, and high yield strength, attributed to grain-size strengthening, was recognized. Tensile ductility in excess of 20% was simultaneously obtained although some unusual features, atypical of fcc metals, including upper and lower yield points and a low work-hardening rate were noted. Approximate calculations examining the interaction of dislocations with a void pair, an assembly of voids and the particular case of all voids being located at grain boundaries indicate that direct strengthening due to the voids is not the principal contributor to the high strength; rather it is the refinement in grain size that is responsible for the observed yield strength level.