The influence of microstructure on work hardening in aluminum

The influence of microstructure on the work hardening behavior of pure aluminum was studied by means of tensile tests at 77K on samples that had been treated to introduce or eliminate subgrains. A recrystallized or well-annealed microstructure, free of subgrains, develops well-defined dislocation cells when deformed, and has a work hardening rate that decreases rapidly with increasing stress. In contrast, when the test sample is recovered, subgrains form which hinder the formation of dislocation cells. As an apparent consequence, a high rate of work hardening is retained at high stress, which leads to an improved combination of strength and elongation. Both the recrystallized and recovered microstructures obey constitutive relations of the Kocks-Mecking form: Theta = Theta(0) - K-sigma, where Theta is the work hardening rate and sigma is the flow stress. However, the values of the initial work hardening rate, Theta(0), and slope, K, depend on the microstructure. The values determined for the recrystallized microstructure are reasonably close to those previously found for aluminum. In comparison, the values of Theta 0 and K for the recovered microstructure are significantly lower, but are, interestingly, compatible with the Kocks-Mecking model if it is assumed that approximately 60% of the total dislocation density is used to maintain geometric compatibility and is unavailable for hardening. This interpretation is at least quantitatively consistent with TEM observations, which show significant localized dislocation activity along the subgrain boundaries.