Coprecipitation of Ω and σ phases in Al-Cu-Mg-Mn alloys containing Ag and Si

Abstract

The individual influence of small additions of Ag and Si on the nucleation of the Ω (i.e., a chemically modified coherent form of ϑ-Al₂Cu) and σ (Al₅Cu₆Mg₂) phases, respectively, in Al-Cu-Mg alloys has been known. These phases nucleate directly and exhibit reduced rates of coarsening at the commercial aging temperatures. Alloys containing a uniform and fine distribution of these phases may, therefore, be of interest for further investigation for applications at temperatures below 200 °C. In a recent study, using an Al-Cu-Mg-Mn-Ag-Si alloy aged at 180 °C, it was shown that the Ω phase formed as a major precipitate nucleated predominantly upon the Mn-bearing dispersoids, while the σ phase was present as a minor one. This article describes the conditions under which widespread nucleation of σ phase may occur at unidentified sites in the matrix as well as upon the Mn-bearing dispersoids in the alloy. Widespread nucleation of σ phase begins in the alloy following the onset of dissolution of Guinier-Preston-Bagaryatski (G-P-B) zones that form early in the aging cycle as the major precipitate. It is established that nucleation of σ as a major phase precipitate requires a critical minimum supersaturation of Si in the solid solution. This article further points out that several constituent phases (implying those which form as solidification products and survive the homogenization treatment) together with the Mn-bearing dispersoids dissolve Si, thereby considerably reducing the Si supersaturation in the solid solution. The implications of these observations are discussed in view of the available data on the nucleation of σ phase in such alloys.