Deformation and fracture of a particle-reinforced aluminum alloy composite: Part I. Experiments
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INTRODUCTION
DISCONTINUOUSLY reinforced aluminum (DRA) alloys possess superior stiffness, specific strength, and wear resistance compared to unreinforced aluminum alloys.[1,2] However, DRAs typically have lower ductility and fracture toughness than the unreinforced alloys. This limitation poses a major constraint in structural applications. Consequently, efforts are underway to improve the fracture resistance of DRA materials through an understanding of the deformation and failure mechanisms.[3–6] Microstructural observations have revealed two major damage modes, i.e., particle fracture and matrix/particle interface decohesion, depending on the matrix composition, type of reinforcement, and heat-treatment and processing conditions, and attempts have been made to correlate the ductility and/or the fracture toughness to a particular damage mode. In one study on 7XXX/SiCp composites, it was shown that the toughness in the overaged (OA) condition was almost half that in the underaged (UA) condition.[7] This difference in toughness was ascribed to different dominant damage modes in the material: (1) SiC particle fracture in the UA condition and (2) matrix/particle interface failure in the OA condition.[7] Suggested reasons for the latter were precipitation of a MgZn2 phase at the interface, as well as A.B. PANDEY and B.S. MAJUMDAR, Senior Scientists, are with UES, Inc., Dayton, OH 45432. D.B. MIRACLE, Group Leader, is with Metal Matrix Composites, Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson Air Force Base, OH 45433. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
a precipitate-free zone (PFZ) in the same region.[8] However, it is unclear what mechanism was responsible for the toughness reduction, i.e., whether the precipitates enhanced interface debonding, or whether the weaker solute-free region (the PFZ) next to those precipitates accelerated void nucleation and growth in the reasonably high triaxial tensile-stress region around the SiC particles. We suspect that it is the latter, since the fracture surface showed evidence of nearinterface dimple failure rather than clean interface failure. The problem with the explanation is that void nucleation and growth involves a significant expenditure of energy, so that it is difficult to ascribe low toughness to a low-strength but highly ductile region (the PFZ) next to the particles. In a study on Al-2618/SiCp DRA, significant differences in ductility and fracture toughness were observed for the naturally aged and peak aged (PA) conditions.[9] However, unlike as in Reference 7, SiC particle fracture was reported to be the dominant damage mode for both the heat treatments.[9] The lower ductility and toughness of the composite in the PA condition was related to the larger number of
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