Influence of Cu additions on the morphology of GeSi precipitates in an Al-Ge-Si alloy
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f the precipitation processes in binary Al-Ge and Al-Si alloys is the wide variety of shapes and habit planes of Ge and Si precipitates.[1,2,3] This is the direct consequence of the dissimilarities in the crystal structures and of the large differences in the atomic volumes of the parent and product phases. If the average atomic volume of an atom in the matrix and in the precipitate is different, then elastic strains are produced in the matrix and the precipitate.[4,5] It is these strains that govern the shape of the precipitate and the plane on which the precipitate lies. Furthermore, the elastic strains in and around a growing precipitate may be relieved by the annihilation of planes of atoms in the matrix or precipitate by the aggregation of vacancies.[6] As a result, in the absence of quenched-in excess vacancies that fulfill both structural- and volume-accommodation roles in the transformations, the Ge and Si precipitates cannot either nucleate or grow readily in the Al matrix. Numerous studies using binary Al-Ge and Al-Si alloys have indicated that the morphology of Ge and Si precipitates varied from grain to grain as well as locally within the grains. These observations are attributed to the variations in local vacancy supersaturation.[1,2,3] Using an Al-1.8 at. pct Ge alloy, for example, it has been shown that variations in quench-bath temperatures and preaging times at room temperature prior to final aging (in the temperature range of 200 ⬚C to 300 ⬚C) develop a variety of morphologies of the Ge precipitates.[7] This was attributed to the formation A.K. MUKHOPADHYAY, Scientist, is with the Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad - 500 058, India. Manuscript submitted November 27, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
of different types of nuclei with different orientation relationships under different vacancy concentrations. Similar interpretations were also provided to explain changes in the morphology of the Ge precipitates in an Al-1.3 wt pct Ge alloy, in the sequence of {111}Al triangular plates → 具100典Al laths → {100}Al plates, as functions of aging temperatures ranging from 150 ⬚C to 250 ⬚C.[2] These results are indicative of direct connections between heat treatment and microstructure and demonstrate the strong dependence of the nucleation of diamond cubic Ge and Si precipitates on the quenchedin excess vacancies. The present work is concerned with the morphology of GeSi precipitates (i.e., precipitates containing both Ge and Si) formed in Al-Ge-Si base alloys.[8,9] As with the constituent binary systems, the nucleation of precipitates in ternary Al-Ge-Si alloys also depends critically on the quenched-in excess vacancies.[10] However, there are differences between the morphology of the precipitates formed in the binary alloys and those formed in the ternary alloys. In ternary alloys, the precipitates are commonly present in three forms, i.e., equiaxed, 具100典Al lath, and triangular plate; the equiaxed form is the dominant one of the three.[9,10,11] On the other han
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