The effect of grain size and deformation sub-structure on mechanical properties of polycrystalline aluminum

Abstract

The effect of grain size on the apparent yield and flow stresses is investigated in polycrystalline aluminum of various grain sizes in the temperature range from 77 to 473°K. The apparent yield stress can always be correlated with the grain size by the Hall-Petch relation. In low temperature deformation at 77–293°K. however, the flow stresses can not be expressed by a simple linearrelation, but are subdivided into two groups characterized by different linear functions of the inverse square root of the grain size. Transition between these two groups always occurs at about 80 μ in grain size. On the other hand, these flow stresses can be expressed by a single linear function of the inverse of the cell size determined by the amount of deformation. Even in high temperature deformation at 373–473°K, deviation from the Hall-Petch relation when only the grain is considered becomes prominent when the strain exceeds about 15 per cent. Furthermore, when the specimens include subgrains in their interior, neither the apparent yield nor the flow stresses satisfy the Hall-Petch relation even when both grain sizes and subgrain sizes are considered. However, in this case, the flow stresses at 20 per cent strain are a linear function of the inverse of the cell size. These results are discussed from a view point of the mean free path of mobile dislocations.