Abstract
Mechanical properties of polycrystalline copper and Cu-13 at%Al alloy have been investigated as a function of grain size and specimen thickness at room temperature, with the following results: (1) The flow stress of both copper and Cu-13 at%Al alloy decreases as the ratio of specimen thickness to grain size decreases below a critical value. (2) The Hall-Petch relation against the grain size shows a turning point corresponding to the critical value. (3) The Hall-Petch relation is always satisfied in the thick specimens of Cu-13 at%Al alloy. In thick specimens of copper, however, the grain size in the Hall-Petch relation is replaced by the mean free path of dislocations at various strains. (4) The mean free path of dislocations in copper corresponds to the length of slip bands instead of the cell size, because the cell structure does not develop sufficiently.
