Effects of reinforcement content and shape on cavitation and failure in metal-matrix composites

Abstract

Cavity development in alumina-reinforced aluminium composites during tensile loading at room temperature has been monitored using microstructural studies and precision density measurements. The materials examined were based on commercial purity aluminium, reinforced with 10 and 20 volume% of short fibres, conventional angular particles or spherical particles, produced by a thermal spraying process. The composites were made by a powder blending and extrusion route, involving no liquidation of the matrix and leaving arrays of fine oxide particles aligned parallel to the loading direction. In all cases, stable voids were found to form well before final failure. Significant void contents were developed earlier in the test for the higher reinforcement content and when fibres were present. However, extensive voiding, corresponding to approximately hemispherical voids being formed at most fibres or particles, occurred in all cases before final failure. Voids tend to form adjacent to the reinforcement, most readily when it is elongated in the direction of applied stress and when it has a relatively flat surface normal to the stress axis. Sharp corners do not themselves appear to be favoured sites for cavity formation. Consistent with this, cavities can form with spherical particles, although their incidence is somewhat less than with angular particles, presumably because of the absence of elongated shapes and surfaces which are actually flat. A simple model is proposed which allows prediction of the failure strain for a given reinforcement volume fraction and aspect ratio. This is based on the constraining effect of the reinforcement on plastic deformation in adjacent regions of matrix and the contribution of cavitation to the observed strain. Fairly good agreement is observed between the predictions of this model and the experimental data.