On S(Al2CuMg) precipitation in an Al[sbnd]Cu[sbnd]Mg alloy containing small additions of beryllium

Abstract

The effect of a small addition of Be on S(Al2CuMg) precipitation in an AlCuMg alloy was investigated. Alloys of nominal weight composition Al0.1Be, Al2.5Cu1.2Mg and Al2.5Cu1.2Mg0.1Be were cast, homogenized, rolled, solution heat treated, quenched, naturally aged and artificially aged for various times at temperatures ranging from 160 to 300°C. The microstructures and the age hardening behavior were characterized and the results for these alloys were compared with the available data in the literature on the similarly treated pure Al and ternary AlCuMg alloy. It is shown that Be additions to both pure Al and the AlCuMg alloy inhibit vacancy condensation during quenching, whilst encouraging extensive dislocation loop formation during subsequent natural aging. Upon artificial aging, the dislocation loop structures in the quenched binary alloy are replaced by a uniform distribution of very small, spherical and stable Be precipitates throughout the microstructure. In the similarly treated Be modified AlCuMg alloy, the dislocation loop structures provide sites for heterogeneous nucleation of the S phase. Be precipitates, which occur in the binary alloy, also form in the artificially aged AlCuMgBe alloy and are found to provide additional sites for heterogeneous nucleation of the S phase. This resulted in refinement of the S precipitate distribution.