ON S(AL2CUMG) PRECIPITATION IN AN AL-CU-MG ALLOY CONTAINING SMALL ADDITIONS OF BERYLLIUM

The effect of a small addition of Be on S(Al2CuMg) precipitation in an Al-Cu-Mg alloy was investigated. Alloys of nominal weight composition Al-0.1Be, Al-2.5Cu-1.2Mg and Al-2.5Cu-1.2Mg-0.1Be were cast, homogeni zed, rolled, solution heat treated, quenched; naturally aged and artif icially aged for various times at temperatures ranging from 160 to 300 degrees C. The microstructures and the age hardening behavior were ch aracterized and the results for these alloys were compared with the av ailable data in the literature on the similarly treated pure Al and te rnary Al-Cu-Mg alloy. It is shown that Be additions to both pure Al an d the Al-Cu-Mg alloy inhibit vacancy condensation during quenching, wh ilst encouraging extensive dislocation loop formation during subsequen t natural aging. Upon artificial aging, the dislocation loop structure s in the quenched binary alloy are replaced by a uniform distribution of very small, spherical and stable Be precipitates throughout the mic rostructure. In the similarly treated Be modified Al-Cu-Mg alloy, the dislocation loop structures provide sites for heterogeneous nucleation of the S phase. Be precipitates, which occur in the binary alloy, als o form in the artificially aged Al-Cu-Mg-Be alloy and are found to pro vide additional sites for heterogeneous nucleation of the S phase. Thi s resulted in refinement of the S precipitate distribution. Copyright (C) 1996 Acta Metallurgica Inc.