Strengthening of an Al–Cu–Sn alloy by deformation-resistant precipitate plates

A variation of the Orowan equation is developed for aluminium alloys strengthened by dispersed, deformation-resistant {100}α precipitate plates, and the quantitative effects of precipitate volume fraction, number density and aspect ratio on increments in Orowan strengthening are examined. The validity of the model is examined by a combination of mechanical property measurements and rigorous quantitative stereology of the size and distribution of {100}α plates of θ′ phase in an Al–4Cu–0.05Sn (wt.%) alloy aged isothermally at 200°C. For the ageing conditions selected, the predictions of the model for Orowan strengthening increments are in good quantitative agreements with those observed experimentally. The model predicts that, for a given volume fraction and number density of precipitate plates, an increase in plate aspect ratio can lead to a substantial increase in strength. In addition, it is not necessary to invoke a transition from precipitate deformation to Orowan strengthening to account for the form of the precipitation-strengthening response of the alloy. The maximum strengthening increment is associated with the minimum effective inter-particle spacing, rather than a critical thickness of the precipitate plates.