Modeling particle fracture during the extrusion of aluminum/alumina composites
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INTRODUCTION
PARTICLE fracture during the extrusion of metal matrix composites (MMCs) is a well documented but little investigated phenomenon.[1,2] Much research (see, for example, Reference 1) on extruded MMC product, usually directed at investigating other problems, provides anecdotal evidence on the change in particle size and size distribution as a result of extrusion. However, most research on particle fracture has been conducted at low temperatures, characteristic of the service conditions experienced by the materials.[3,4,5] The neglect of high-temperature behavior is somewhat surprising in light of the evidence that much of the in-service mechanical properties of an MMC are determined by the spatial parameters associated with the particles,[6] which are in turn determined by the deformation conditions during production. The ability to control these parameters and allied features such as void density would seem to be desirable for improved properties and higher productivity. Our aim was to model changes in the particle size distribution in order to gain a more informed view of the effect of extrusion on particle fracture. At low temperatures, particle fracture is characterized as ‘‘damage’’ to the material,[7] most notably with concomitant modulus degradation.[3,8] This is because fracture results in void formation. However, the imposition of a hydrostatic pressure extends ductility by forcing matrix material into the void created by a particle fracture event.[9,10] One would C.H.J. DAVIES, formerly Postdoctoral Fellow, The Centre for Metallurgical Process Engineering, is Lecturer, the Department of Materials Engineering, Monash University, Clayton, Victoria 3168, Australia. W.-C. CHEN, formerly Graduate Student, The Centre for Metallurgical Process Engineering, is Research Engineer, Dynamic Systems Inc., Poestenkill, NY 12140. D.J. LLOYD, Principal Scientist, is with Alcan International, Kingston Research and Development Centre, Kingston, ON, Canada K7L 5L9. E.B. HAWBOLT and I.V. SAMARASEKERA, Professors, and J.K. BRIMACOMBE, Professor and Alcan Chair in Materials Process Engineering, are with the Centre for Metallurgical Process Engineering, University of British Columbia, Vancouver, BC, Canada V6T 1Z4. Manuscript submitted September 25, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
imagine that at high temperatures such ‘‘void healing’’ should be easier because of the low flow stress of the matrix material. At low temperatures, particle fracture is correlated with strain, particle size, aspect ratio,[5,10] and particle volume fraction.[8] The fracture of particles is only one of a number of phenomena that are observed to occur on the extrusion of MMCs. Particles are known to align themselves in the direction of extrusion, if they have any significant aspect ratio;[11] banding of particles is commonly observed in extruded product,[12] although the overall homogeneity (as measured by clustering of particles) is improved by extrusion.[12] This article shows the changes in particle distributions result
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