Effect of porosity and alumina content on the high temperature mechanical properties of compocast aluminium alloy-alumina particulate composite

Abstract

The effects of volume fraction of alumina and porosity on the tensile strength of Al-4 wt% Mg-alumina compocast particulate composite tested at various temperatures up to 623 K have0 been investigated with the help of a phenomenological model. The contribution of porosity on reduction of strength of composites at various levels of alumina content has been expressed as a linear function of porosity and the resulting equation contains two experimentally determined parameters, α0, the ultimate tensile strength at zero porosity level and α, a weakening factor. It is observed that α decreases with an increase in volume fraction of alumine in the composite and it becomes more sensitive to alumina content of the composite with a rise in temperature. Ate given alumina contenta increases with a rise in test temperature but this effect is gradually countered by increasing alumine content of the composite. Finally, in a composite having ∼ 10.3 vol % alumina α decreases with an increase in temperature. This may have occurred because the extent of particle-matrix debonding is determined by the plastic soak in the matrix and the fracture strain of a composite increases or decreases with temperature when the alumina content lies below or above ∼ 9.0 vol % of alumina respectively. At any test temperature σ0 of the composite decreases rapidly with an increase in volume fraction of alumina, but the rate of decrease of σ0 reduces at higher alumina levels. However at the elevated temperatures of 473K and 573K a sharp fall in σ0 is observed at alumina contents beyond ∼ 9.0 vol %. At a lower level of alumina content below about 8.98 vol % the fracture strain of the composites increases with an increase in temperature. However, in the case of higher alumina content beyond the level mentioned above the fracture strain of the composites decreases with the rise in temperature. At a given porosity level the fracture strain of a composite having about 9.4 vol % alumina decreases with an increase in temperature. Scanning electron microscopic observations show that the extent of the growth and linkage of voids before fracture become extensive at higher temperature. At ambient temperature the composites fail by a mixed mode of ductile and clevage fracture. At 573K a number of considerably small dimples along with the larger ones are observed in the fractured surface. At this temperature a large number of newly formed fine grains are observed in the matrix.