Precipitation processes of Al-1.7 at% Cu and the influence of Au micro-alloying studied by dilatometry

Micro-alloying represents an important pathway in the design of modern Al-based alloys. It affects the precipitation kinetics during heat treatment which is crucial for optimization of the microstructure. In this study, difference dilatometry is applied to study precipitation processes in Al-1.7 at% Cu alloys with varying Au-content. A quantitative analysis is given for the base Al-Cu alloy on the formation of the metastable -precipitate phase (Al2Cu) and its subsequent dissolution and transformation to the stable θ-precipitate phase (Al2Cu) upon time-linear heating starting from the state of supersaturated solid solution. The dilatometric analysis is enabled by the subtle balance of the relative length changes that arises, on the one hand, from the specific volume increase of the supersaturated Al matrix upon precipitation of the excess alloy component (Cu) and, on the other hand, from the specific volume variation associated with the formation of the precipitates. The relation between the observed relative changes and the amount of precipitates is supported by complementary simulation studies of the precipitation processes using MatCalc software. Micro-alloying with Au is found to promote the -formation by an increasing nucleation and growth rate. In addition to the quantitative length change analysis, information on the kinetics of the -formation is deduced from Kissinger analysis of the maximum rates of length change with temperature for different heating rates.