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}(alpha) 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}(alpha) plates of theta' phase in an Al-4Cu-0.05Sn (wt.%) alloy aged isothermally at 200 degrees 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. (c) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.