Microstructure and mechanical behavior of spray-deposited Al-Cu-Mg(-Ag-Mn) alloys
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I. INTRODUCTION
SINCE the effect of Ag on the precipitation kinetics of Al-Cu-Mg alloys was first discovered by Polmear in 1964,[1] considerable work has been done to more fully understand the structure and precipitation kinetics of the primary strengthening phases, to determine the effects of alloy composition on microstructural stability, and to understand the influence of microstructure on mechanical behavior. When determining the effect of Ag additions on the aging kinetics and mechanical properties of several alloy systems, Polmear discovered that small additions of Ag to Al-Cu-Mg alloys accelerated the second stage of hardening, substantially increased the peak-hardness values, and resulted in a steady increase in the maximum tensile properties with increasing Ag content.[1] Later investigations revealed that the primary strengthening precipitate varies in artificially aged Al-CuMg-Ag alloys, depending on the Cu/Mg ratio. For artificially aged alloys whose compositions place them in the a 1 S region of the Al-Cu-Mg phase diagram, the addition of Ag has been reported to result in the formation of Cu-Mg Guinier–Preston (GP) zones in the initial stage of hardening and a metastable phase at peak hardness.[2,3,4] First thought to be a cubic T phase,[2,4] more recent studies using electron microdiffraction have shown this precipitate to be a hexagonal phase,[3] referred to as X 8 because of its known metastability, which forms on the {111}a planes.[5] However, artificial aging of ternary alloys with Cu/Mg ratios in excess of 5 to 10, such that they lie in the a 1 u 1 S or a 1 u L. DEL CASTILLO, Graduate Research Assistant, and E.J. LAVERNIA, Professor and Chair, are with the Department of Chemical and Biochemical Engineering and Materials Science, University of California at Irvine, Irvine, CA 92697-2575. Manuscript submitted June 30, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
regions of the Al-Cu-Mg phase diagram, normally leads to the formation of phases found in both Al-Cu and Al-CuMg alloys. The increased age hardening kinetics, enhanced high-temperature performance, and improved mechanical properties that exist in these Al-Cu-Mg-Ag alloys have been attributed to fine and uniform precipitation of V , which occurs on aging above 100 8C and forms as large, thin, hexagonal-shaped plates on the {111}a matrix planes.[6] Among the most important properties of V is the reported stability of this phase at temperatures up to 200 8C.[7] Varied results have been obtained from the many detailed studies performed on the structure and composition of V .[8–15] The form of V was proposed to be either the same structure as u or a slightly distorted form. Based on the results of high resolution transmission electron microscopy and both image and selected area electron diffraction simulation studies, Knowles and Stobbs[9] proposed that V has a face-centered orthorhombic structure and discounted the possibility that the structure of V was similar to that of u. Muddle and Polmear,[10] who obtained similar findings to those
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