Evolution of fiber fragmentation in a short-fiber-reinforced metal-matrix model composite during tensile creep deformati
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I. INTRODUCTION
MULTIDIRECTIONALLY fiber-reinforced metalmatrix composites exhibit higher strength and a much improved high-temperature creep resistance compared to the unreinforced materials. Since such composites can also be produced cost-efficiently via squeeze casting, they have made their way into a number of high-temperature applications, e.g., as parts of pistons for automotive diesel engines. Such composites, with a relatively low volume fraction (⬍30 vol pct) of randomly oriented short fibers, are, in many cases, more attractive from an application point of view than composites reinforced with a high content of aligned short or continuous fibers, but have received much less scientific attention. Due to the complexity of the microstructure of randomly reinforced materials, the micromechanical processes occurring inside the composite during creep are difficult to grasp, and it is not easy to establish simple but still plausible models of the structure-property relationship. Investigations by Zok et al.[1] and Dragone and Nix[2] have indicated that the accumulation damage in the reinforcement phase plays an important role for the macroscopically observed creep behavior of short-fiber-reinforced composites. A plausible and realistic view of the possible micromechanical scenario during creep has been given by Eggeler and co-workers.[3,4,5] In their work on short-fiber-reinforced aluminum-matrix composites, these authors have proposed that the macroscopic creep behavior of such composites is SVETLANA P. ZWERSCHKE, formerly Postdoctoral Researcher, Institut fu¨r Metallkunde, Universita¨t Stuttgart, is on leave. ALEXANDER WANNER, Group Leader and Permanent Member of Academic Staff, is with the Institut fu¨r Metallkunde, Universita¨t Stuttgart, D-70569 Stuttgart, Germany. EDUARD ARZT, Full Professor, Institut fu¨r Metallkunde, Universita¨t Stuttgart, is Department Director, Max Planck Institute for Metals Research, Stuttgart, Germany D-70569. Contact e-mail: wanner@ mf.mpg.de Manuscript submitted March 5, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
controlled by the interplay of three elementary microstructural processes: (1) loading of fibers through the formation of a work-hardened zone (WHZ), (2) a recovery process which decreases the dislocation density in the WHZ, and (3) fragmentation of the fibers, which steadily increases the intensity of the recovery process by reducing the recovery path. These authors also established a simplified micromechanical model based on these three processes, with which they were able to rationalize experimental creep curves obtained at different temperatures and stresses. However, so far it has not been possible to obtain any quantitative information about the onset and evolution of fiber fragmentation in a such a material. Despite a large experimental effort, mostly by microscopic investigation of creep-deformed composites, so far only a rough, qualitative image of the damage evolution could be obtained. Especially in the early stages of creep, where the volume density of f
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