On the creep strengthening of SiC particulates in SiC-Al composites

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On the Creep Strengthening of SiC Particulates in SiC-Al Composites FARGHALLI A. MOHAMED Discontinuous silicon carbide reinforced aluminum (SiCAl) alloy composites, in which SiC particulates (SiCp) or whiskers (SiCw) are incorporated in an Al alloy matrix, have received considerable attention, partly due to low raw materials costs and partly due to ease of fabrication using conventional metal processes. In addition, these composites are suitable for general engineering applications since they exhibit near-isotropic properties. A systematic comparison between the creep behavior of discontinuous SiC-Al composites and that of their unreinforced matrix alloys is of practical and scientific significance. From a practical point of view, such a comparison is essential to the process of assessing the potential of these composites for use as structural materials for high-temperature applications. From a scientific point of view, information obtained from such a comparison is critical not only to the characterization of creep in SiC-Al composites in terms of deformation mechanisms but also to the identification of various processes that may limit the creep strengthening imparted by SiC particulates or whiskers. In this note, the results of a comparison between the steady-state creep rates of 10 vol pct SiCp-2124 Al composite and those of the unreinforced matrix alloy, 2124 Al, are presented and analyzed. The creep behavior of both 10 vol pct SiCp-2124 Al composite and the unreinforced matrix alloy, 2124 Al, was investigated under similar conditions.[1,2] These conditions are related to the method of preparing the two materials (powder metallurgy processing), the configuration of test specimens[3] (double shear), the temperature range for creep tests (648 and 678 K), and the strain rate range covered by creep data (seven orders of magnitude: 1028 to 1022 s21). Creep tests were conducted at 648 and 678 K on either a creep-testing machine operated at constant load or an Instron machine operated at a constant rate of crosshead displacement. The two test temperatures, 648 and 678 K, were selected for the following reasons. First, these test temperatures fall within the temperature range (505 to 723 K) that was previously used by several investigators[4] in evaluating the mechanical properties of discontinuous SiC-Al composites under creep conditions. Second, earlier creep data were reported for powder metallurgy (PM) 6061 Al[5] and PM 30 vol pct SiCp6061 Al[6] at 648 and 678 K. Accordingly, it is possible to

FARGHALLI A. MOHAMED, Professor, is with the Department of Chemical and Biochemical Engineering and Materials Science, University of California, Irvine, CA 92697. Manuscript submitted April 17, 1997.

2780—VOLUME 28A, DECEMBER 1997

provide a direct comparison between the creep strengthening of SiC particulates in 10 vol pct SiCp-2124 Al composite and that of the particulates in PM 30 vol pct SiCp-6061 Al by using the same test temperatures. Figure 1(a), in which the creep data of both PM 10 vol pct SiCp-2124 Al composite an

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