FLOW-STRESS DEPENDENCE ON THE DISTRIBUTION OF GRAIN-SIZE IN POLYCRYSTALS

A series of grain-size-engineered polycrystals have been produced by powder metallurgy. The polycrystals differed in the mean size of the grains and the grain size distribution function. Hardness measurements showed that the differences in the distribution function resulted in a systematic deviation from the Hall-Petch equation in the form H = H-0 + Kd-1/2. The specimens characterized by higher diversity of grain sizes, as measured for example by a grain area coefficient of variation, CV(A), exhibited lower hardness than predicted by the Hall-Petch dependence. This effect can be explained by a theory that combines the Hall-Petch approach with an approach typical of composite mechanics and results in equation H = H-0 + f[CV(A)]Kd-1/2. The results predict that grain size uniformity can be used to improve the properties of polycrystalline materials.