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 recrystallize d or well-annealed microstructure, free of subgrains, develops well-de fined dislocation cells when deformed, and has a work hardening rate t hat decreases rapidly with increasing stress. In contrast, when the te st sample is recovered, subgrains form which hinder the formation of d islocation cells. As an apparent consequence, a high rate of work hard ening is retained at high stress, which leads to an improved combinati on of strength and elongation. Both the recrystallized and recovered m icrostructures 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 har dening rate, Theta(0), and slope, K, depend on the microstructure. The values determined for the recrystallized microstructure are reasonabl y close to those previously found for aluminum. In comparison, the val ues of Theta 0 and K for the recovered microstructure are significantl y lower, but are, interestingly, compatible with the Kocks-Mecking mod el if it is assumed that approximately 60% of the total dislocation de nsity is used to maintain geometric compatibility and is unavailable f or hardening. This interpretation is at least quantitatively consisten t with TEM observations, which show significant localized dislocation activity along the subgrain boundaries.