The effect of interrupted aging on the yield strength and uniform elongation of precipitation-hardened Al alloys

The yield strength and elongation are both important properties of engineering alloys. In precipitation-hardenable alloys, these properties are directly related to the interaction between the precipitates and mobile dislocations and an inverse correlation is usually observed. It was recently reported that bimodal microstructures containing both shearable and shear-resistant precipitates, generated by interrupted aging schedules, can simultaneously increase both the yield strength and the uniform elongation in selected Al alloys. This is an effect of considerable technological significance but the physical origin is not understood. An explanation for the interrupted aging effect is offered in this contribution. The mechanical response of a model Al–Cu alloy after interrupted aging has been characterized using uniaxial tensile tests and tension–compression Bauschinger tests. The undeformed and deformed microstructures have been characterized using transmission electron microscopy, scanning transmission electron microscopy and differential scanning calorimetry. It is shown that dissolution of the shearable Guinier–Preston (GP) zones occurs during uniaxial deformation, and that the repartition of solute from the GP zones to the matrix can provide a positive contribution to strain hardening that is quantitatively capable of explaining the observations of enhanced elongation. The result depends on the effect of the repartitioned solute on the plasticity of the Al matrix and, using a series of Al–X (X = Cu, Mg, Si, Zn) binary solid solutions, the effect of solute in solution on the mechanical response is quantified. This information is incorporated into models for the yield strength and strain hardening of precipitate-containing microstructures. It is suggested that simultaneous increases in both strength and elongation due to interrupted aging may be expected in 2xxx series alloys but are less likely to be significant in the 6xxx series alloys and are unlikely in most 7xxx series.