Nature of precipitates and constituent particles present in a ternary Al-Ge-Si alloy
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GHOSAL, K.S. PRASAD, and V.V. RAMA RAO, Scientists, are with the Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad-500 058, India. Manuscript submitted July 23, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
through the use of suitable trace alloy additions (such as Sn and In) having strong affinity to bind vacancies in Al:[16] the homogeneity of the precipitates was greatly disturbed by the presence of trace amounts of either Sn or In in the alloy.[16] The conclusions of these studies[16] were later reconfirmed by Røyset and Ryum.[17] Despite the large volume of work carried out on the nucleation and the homogeneity of the precipitates in Al-Ge-Si alloys,[10,11,16,17] the lattice parameter of the precipitates has not yet been reported. Evaluation of the lattice parameter of the precipitates may throw light on their chemistry. Furthermore, studies have shown that with increasing Si 1 Ge contents (on equiatomic proportions) beyond 1 at. pct, and especially with increasing Si content in the alloy, it becomes difficult to homogenize such alloys using practical homogenization and solution treatment schedules.[11,17] No work has so far been carried out to examine the nature of the coarse constituent particles that survive the practical homogenizing annealing treatments. Examination of such particles may provide important information regarding the nature of the phase precipitates that form in these alloys during artificial aging.[18,19] In this article, using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), TEM, electron diffraction, and X-ray diffraction, results on the chemistry and the crystallographic nature of the constituent particles and the lattice parameter of the diamond cubic precipitates formed during artificial aging are presented. The mass composition of the Al-Ge-Si alloy used in this investigation is Al-2.6Ge-1.04Si. The elements Ge and Si are present in equiatomic proportion, i.e., 1 at. pct each in the alloy. The high-purity alloy, in the form of sheet of 3mm thickness, was prepared in the Department of Materials Science and Engineering, University of Virginia (Charlottesville, VA). The reason for the selection of the particular alloy composition used in this investigation is that both the fine precipitates (formed during aging) and the constituent phase particles (that survive the practical homogenizing annealing treatments) would be present in the alloy in sufficient amounts to be detected by X-ray diffraction. The alloy was solution treated at 500 8C as well as at 540 8C for times ranging from 1 to 40 hours and artificially aged at 160 8C for 10 hours. Unless otherwise stated, the standard solution treatment given to the alloy was for 1 hour at 500 8C. The solution treatment at the higher temperature and for longer times was given to the alloy in an attempt to dissolve the coarse constituent phase particles that survive the practical homogenization treatment. On the other hand, the reason for the selection of the particular aging time and temperature is that t
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