Lüders elongation in AI-Mg alloy AA5182

Abstract

Aluminium-magnesium alloys such as AA5182 exhibit a yield point elongation, or Lüders strain, when deformed in the annealed O temper, and the extent of the Lüders strain is a function of processing history and the deformation conditions. The yield point effect is usually attributed to Mg solute pinning, but is has been appreciated for some time that factors such as grain size, and cooling rate from the annealing temperature, affect the Lüders strain, and it has not been clear how this behaviour is consistent with solute pinning. In this paper it is shown that the presence, or absence of Lüders effect (Lüdering) is related to the grain boundary structure, and processing which results in a high concentration of grain boundary dislocations removes the yield point effect, whereas fabrication routes which produce defect free bondaries result in a Lüders strain. The extent of the Lüders strain, and its dependence on grain size and deformation conditions, is reasonably consistent with some recent models of propagative plastic instabilities.