Effect of temperature and strain rate on the compressive flow of aluminum composites containing submicron alumina partic
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INTRODUCTION
A dispersion of submicron oxide particles in a metal enhances both its low-temperature and high-temperature mechanical properties.[1] The volume fraction of submicron dispersoids in metals is usually less than 10 vol pct, limited by the powder-metallurgy processing techniques used in their production. Consequently, continuum strengthening in the form of load sharing between the matrix and the dispersoids is small enough to be neglected, and the effect of the particles on the mechanical properties of dispersion-strengthened metals can be understood by consideration of dislocationparticle interactions.[2] In recent years, novel processing techniques have led to the production of aluminum containing concentrated dispersions of submicron alumina particles. In particular, liquid metal infiltration can produce dispersion-strengthened cast aluminum (DSC-Al*) containing 20 to 60 vol pct submi*DSC-Al is a trademark of Chesapeake Composites Corp., New Castle, DE.
cron oxide particles.[3,4] The combination of large volume fractions and small size of the particles in these metal-ceramic composites results in both continuum composite strengthening and strong dislocation-particle interactions, making these materials interesting hybrids between metal-matrix composites and dispersion-strengthened metals. The creep behavior of DSC-Al with 25 vol pct alumina particles has been studied both experimentally and theoretically.[5,6] For strain rates between 10⫺9 and 10⫺3 s⫺1, the superior creep resistance of DSC-Al, as compared to precipitation-strengthened aluminum alloys, can be attributed to the existence of a threshold stress below which creep by
M. KOUZELI, Postdoctoral Fellow, and D.C. DUNAND, Professor, are with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208. Contact e-mail: [email protected] Manuscript submitted March 10, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A
dislocation climb is negligible. This threshold stress was shown to be a result of the interaction between dislocations and alumina particles acting as dispersoids, and was theoretically calculated after consideration of the stress exerted on a dislocation in the process of detachment from a dispersoid by dislocation pileups at the same and other dispersoids. In these studies, the possible enhancement of creep resistance due to load partitioning between the aluminum matrix and the alumina particles was not considered, because the elasto-plastic mismatch necessary for load transfer was assumed relaxed by local diffusional processes around the dispersoids. Increased creep resistance due to load partitioning, however, has been reported in metal-matrix composites with similar volume fractions of larger ceramic particles tested at similar strain rates,[7,8,9] for which relaxation is slower because of larger diffusion distances. Furthermore, in a recent study on DSC-Al tested in compression at 10⫺3 s⫺1,[10] it was shown that significant enhancements in the elasto-plastic properties of these materials
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