An Al--Cu--Si--Ge quaternary alloy uses Si--Ge additions to provide a reasonably dense and homogeneous distribution of precipitates. Heterogeneous nucleation on this Si--Ge template is used to enhance both strength and thermal stability. These precipitates are used as a template for heterogeneous precipitation of other hardening phases, particularly the .cndot.' (Al.sub.2Cu) phase. Thus, a Si--Ge addition is used to provide a dense template of heterogeneous nucleation sites for subsequent precipitation of Al--Cu precipitates.