Bounds on the strength of a model nanocomposite

Criteria for thermally stable, high-strength metallic nanocomposites in the context of Hall-Petch behavior of the constituent phases are examined. It is argued that both upper and lower bounds exist on the strength of such nanocomposites as a function of volume fraction and particle size of the component phases. Using a two-phase topological model, assuming Hall-Fetch behaviors for the pure constituent phases, and accounting for thermally stable grain sizes in the context of the Zener drag model, we predict that an optimal combination of nanostructured particles in a metal matrix would consist of nanoscale particles with a volume fraction of about 25%, dispersed in a matrix of a metal with a high Hall-Fetch coefficient, and the two phases being immiscible. The matrix phase grains would necessarily be up to ten times larger than the dispersed phase particle size, as a result of the trade-off of grain size stabilization versus Hall-Fetch strengthening. (C) 1997 Acta Metallurgica Inc.