Pre-precipitate clustering in an AlCuMgAg alloy

Abstract

Small additions of silver can modify age hardening in aluminum alloys that contain magnesium. Thus, although silver has no significant effect on artificial aging on Al-Cu alloys, it causes a complete change to precipitation processes that normally occur in aged Al-Cu-Mg alloys. In this regard, special interest has been shown in those alloys with high Cu:Mg ratios in which silver promotes formation of a new precipitate, designated [Omega]. [Omega] forms as a uniform dispersion of thin, hexagonal-shaped plates that precipitate on the (111)[sub [alpha]] planes of these Al-Cu-Mg-Ag alloys when they are aged in the medium temperature range. The thinness of the [Omega] plates is attributed to the fact that they are coherent with the matrix along the (111)[sub [alpha]] planes whereas there is a misfit of approximately 9% around the edges. The aim of the present work was to gain an understanding of the actual mechanism by which nucleation of [Omega] occurs through a study of the decomposition of supersaturated Al-Cu-Mg-Ag alloys immediately after quenching. Special use has been made of atom probe field ion microscopy (APFlM) supplemented by transmission electron microscopy (TEM) to study alloys in the as-quenched condition and after aging for very short times at 180more » C.« less