THERMAL-STABILITY OF ULTRAFINE-GRAINED METALS AND ALLOYS

Grain growth in ultrafine-grained elemental metals (Cu, Ag, Pd) and alloys has been studied by differential scanning calorimetry and transmission electron microscopy. The samples were prepared via the inert gas condensation technique, followed by uniaxial high pressure compaction. Abnormal grain growth is observed in all the pure elemental samples. The onset of this secondary recrystallization is at or slightly below the recrystallization temperature known from the respective conventionally cold-worked metals. The activation energies for grain growth were determined by the Kissinger method and correspond to the typical values for grain boundary diffusion. For samples measured directly after preparation, a two-step process was found. The low temperature reaction can be attributed to relaxation processes, because no microstructural changes can be observed. The reaction at higher temperatures is due to abnormal grain growth. Principally the same behaviour was found in samples with increased residual porosity (up to 15 vol.%) and concentration of substitutional solutes (up to 5 at.% Au). Gaseous impurities (oxygen) increase the onset temperature for abnormal grain growth. Normal grain growth is observed in duplex microstructures, as demonstrated using AgCu as a model system.