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INTRODUCTION

FOR the past several years, attempts have been made to produce dispersion-strengthened aluminum alloys with desirable combinations of mechanical properties and microstructural stability with respect to elevated temperature exposure.[1–5] These attempts have been partially successful in that materials with good elevated temperature stability have been produced. Dispersion-strengthened alloys must have relatively high volume fractions of dispersoid in order to develop good strength levels. The required dispersoid loading ensures that the grain size in these alloys will be small, generally submicron. This fine grain size results from pinning of boundaries by the numerous, very stable dispersoids; conversely, the majority of the dispersoid will be associated with grain boundaries.[6,7] It has been previously observed that many fine grain, dispersion-strengthened aluminum alloys have very limited uniform elongation in tension and deform at constant or declining true stress in compression after relatively low strains.[8,9,10] However, the deformation mechanisms responsible for these behaviors have not been established. The authors contend that the tensile and compression behaviors result primarily from the submicron grain size, which is characteristic of this class of alloys. Support for this contention has been sought by examination of the tensile and compressive behavior in monotonic loading and by examination of the Bauschinger effect in fully reversed loading at strains below the uniform strain observed in tensile tests. II.

BACKGROUND

A. Material Alloy 8009 is a planar flow cast, powder metallurgy (PM) material, formerly produced by Allied Signal (Mor-

A.P. REYNOLDS, Assistant Professor, and J.S. LYONS, Associate Professor, are with the Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208. Manuscript submitted August 13, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A

ristown, NJ), with a nominal composition (wt pct) of Al8.31Fe-1.48V-1.81Si. The microstructure of 8009 has been characterized previously: the alloy has nearly equiaxed, submicrometer size grains (average about 0.5 mm) and approximately 25 vol pct of 50 to 100-nm dispersoids.[6,11–13] The microstructure of 8009 is relatively insensitive to thermomechanical treatment, as the dispersoids are formed in the early stage of processing and stabilize the grain structure.[14,15] Alloy 8009 exhibits excellent microstructural stability and retention of room-temperature properties after long-term exposures at temperatures as high as 316 7C.[16] However, alloy 8009 also exhibits monotonically declining fracture toughness with increasing test temperature, a ductility trough at intermediate temperature, and low uniform elongations at all test temperatures.[8,13,17,18] In addition, the fine grain size of alloy 8009 (and many other powder alloys) results in very smooth fatigue fracture surfaces and high fatigue crack growth rates relative to most ingot metallurgy aluminum alloys (presumably due to low levels of crack