Compositional characterization of Cu-rich phase particles present in as-cast Al-Cu-Mg(-Li) alloys containing Ag
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I. INTRODUCTION
THE as-cast microstructure of most age-hardenable Al alloys consists of cored dendrites of Al solid solution with a variety of constituents (/phases) being present at the grain boundaries or interdendritic spaces.[1,2,3] Depending on the alloy compositions and aging conditions, many of such phases reprecipitate during artificial aging following solution treatment and quenching.[2,3,4] The chemistry (in terms of the amounts of alloying elements) and the lattice parameters of the phases present in the as-cast alloy may not always be similar to those of the same type of phases (i.e., phases having the same compound formulae) when present in the heat-treated alloy.[2,3,4] The phase particles present in the ascast alloy could be, on the other hand, utilized effectively (due to their coarse nature) to obtain firsthand information regarding their possible association with a specific alloying element. Of further interest, the as-cast alloy may often contain metastable phases, and studies involving such phases may provide a basis for further understanding of the phase transformations that occur in the alloy. The present article is concerned with the influence of small additions of Ag on the nature of the Cu-rich phases of commercial interest, i.e., u, S, and T1, present in appropriate as-cast alloys based on Al-Cu, Al-Cu-Mg, and Al-Cu-Li-Mg. It is well known that trace additions of Ag to Al-Cu-Mg alloys, having compositions lying within the u-containing phase fields (i.e., a 1 u and a 1 u 1 S) of the ternary Al-Cu-Mg phase diagram,[5] greatly increase the nucleation A.K. MUKHOPADHYAY, Scientist, is with the Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad-500 058, India. Manuscript submitted August 27, 1998 METALLURGICAL AND MATERIALS TRANSACTIONS A
frequency of the V phase during artificial aging.[6–17] The V phase, precipitated in the form of thin plates on {111}Al matrix planes, is regarded as the chemically modified coherent form of the equilibrium u-Al2Cu phase.[13–16] Using quaternary Al-Cu-Mg-Ag alloys, it has been demonstrated through several carefully designed experiments that V can nucleate readily upon aging at temperatures as high as 470 8C,[16] while prolonged aging even at 200 8C causes this phase to be replaced by the u phase.[14,17] These results apparently demonstrate that, between u and V , the formation of the latter phase is definitely easier, that V is replaced by u and not vice-versa, and that a possibility of V or similar structures being stabilized by Ag in the as-cast microstructure of these alloys exists. No work has, so far, been carried out to examine the influence of Ag on the nature of the phases present in the as-cast Al-Cu-Mg alloy. Small additions of Ag and Mg to Al-Cu-Li alloys are also known to enhance the nucleation of the plate-shaped T1 (Al2CuLi) phase during artificial aging.[18,19] Once again, there is no record in the literature of the influence of Ag on the nature of the phases present in the as-cast microstructure of these alloys. One of the major a
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