Cavity formation during tensile straining of particulate and short fibre metal matrix composites

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The formation of cavities in commercially pure aluminium composites, made by both powder and casting routes and reinforced with alumina (short fibres, angular particles and spherical particles), has been monitored using periodic density measurements during tensile testing and microstructural examinations. Stable cavities always form well before final failure, usually adjacent to the reinforcement, particularly when it is elongated in the loading direction and has a relatively flat surface normal to the stress axis. Sharp corners are not favoured cavitation sites and cavities can form at spherical particles, although the incidence is somewhat less than for angular particles. Cavitation occurred earlier for higher reinforcement contents and with powder-route, as opposed to cast, material, although the void contents and composite strains at failure were similar. A simple geometrical model is proposed, allowing prediction of the failure strain as a function of the reinforcement content, aspect ratio and strain to failure of the unreinforced matrix. The data presented are in good agreement with predictions from this model.