Precipitation in nanocrystalline Al-Ag prepared by inert gas condensation and by mechanical alloying

摘要

Experimental investigations of precipitation from supersaturated solid solutions have mostly been carried out on single crystals or coarse-grained polycrystalline material. By contrast, precipitation reactions in nanocrystalline solids, i.e. polycrystals with a grain size of a few nm, have been studied less extensively. Because of the fine grain size, nanocrystalline solids contain a large number of grain boundaries. In general, nanocrystalline materials also exhibit considerable inhomogeneous lattice strain. It may be expected that these microstructural elements influence the way in which the alloy decomposes. A large number of structural defects constitute potential nucleation sites and provide paths for accelerated diffusion. In addition, the presence of grain boundaries may modify the overall solubility and thereby change the thermodynamic driving force for decomposition (1). Consequently, in nanocrystalline alloys both the driving force for precipitation and the kinetics for nucleation and growth of precipitates may be modified from single crystal or coarse-grained polycrystalline materials. Therefore, it seems of interest to investigate the the mierostructural evolution during the decomposition of a nanocrystalline supersaturated solid solution.