THERMAL-STABILITY OF ULTRAFINE-GRAINED METALS AND ALLOYS

Grain growth in ultrafine-grained elemental metals (Cu, Ag, Pd) and al loys has been studied by differential scanning calorimetry and transmi ssion 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 conventio nally cold-worked metals. The activation energies for grain growth wer e determined by the Kissinger method and correspond to the typical val ues for grain boundary diffusion. For samples measured directly after preparation, a two-step process was found. The low temperature reactio n can be attributed to relaxation processes, because no microstructura l 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 concentr ation of substitutional solutes (up to 5 at.% Au). Gaseous impurities (oxygen) increase the onset temperature for abnormal grain growth. Nor mal grain growth is observed in duplex microstructures, as demonstrate d using AgCu as a model system.