Synthesis, consolidation, and mechanical testing of nanophase metals

Nanophase materials possess a grain size on the order of nanometers. Due to the large volume fraction of grain boundaries in nanophase materials, it is expected that the mechanical properties of nanophase materials will be different from their conventional microcrystalline counterparts. For this study, nanocrystalline copper and palladium powders were produced by vapor condensation, and titanium aluminide powder was produced by high energy ball milling. The powders produced by both techniques were characterized, and then the powders were consolidated to provide specimens for mechanical testing.;Sinterforging experiments were performed on low relative density Ti-48Al specimens to observe creep behavior and to densify green compacts. An activation energy of 328kJ/mol and an average stress exponent of 4.0 were determined from these experiments for a temperature range of 600-650;Indentation creep experiments were performed on nanocrystalline copper and palladium specimens and also on sinterforged Ti-48Al specimens. The copper and palladium specimens showed a higher creep rate than their microcrystalline counterparts, exhibiting creep even at room temperature. The activation energies for both specimens are lower than activation energy for grain boundary diffusion in microcrystalline materials. A model is proposed to explain the higher creep rate. It assumes that the specimens produced from vapor condensed powders have a higher grain boundary energy due to a higher disorder in their grain boundary structure. Contrary to the pure metal specimens, the sinterforged Ti-48Al specimens exhibited an activation energy close to that for lattice diffusion in the microcrystalline alloy and also the activation energy determined from the sinterforging experiments.