The sintering of a-alumina reinforced aluminium alloy matrix composites

A series of experiments have been performed to determine the effect of volume fraction of reinforcement and composition on the liquid-phase sintering of particulate α-alumina reinforced aluminium alloy composites. Transmission electron microscopy (TEM) was used to analyse the structure of the reinforcement-matrix interface at high magnification, with an emphasis on the distribution of alloying elements.        

Green samples were prepared by mixing elemental metal powders with α-alumina particles. This mixture was then placed in a die and compressed using a hydraulic press. Samples were sintered at 625°C for 1 hour under a nitrogen atmosphere.            

The results show that a critical volume fraction of reinforcement, V_fcrit, exists as predicted by percolation theory. At reinforcement levels lower than V_fcrit the sintered density of the composites is dependent on the driving force for sintering of the matrix alloy overcoming the adverse effect of the inert reinforcement particles on densification. At reinforcement levels greater than V_fcrit the sintered density is dependent on the composition of the matrix alloy. Densification, in these samples, is dependent on the sintering liquid filling the pores that result from pressing. These pores develop because a self-supporting network of reinforcement particles hinders compaction. The pore filling is dependent on the wetting of the particles by the liquid, which in turn is dependent on the composition of the matrix alloy.        

It was further established that magnesium and tin improve sintering whereas copper and silicon have a deleterious effect. Transmission electron microscopy demonstrated that magnesium is concentrated at the reinforcement-matrix interface. It was concluded that magnesium aids wetting by reducing the solid-liquid interfacial energy, while tin improves wetting by lowering the surface tension of the sintering liquid. Furthermore, the adverse effect of copper and silicon on sintering was attributed to the dilution of magnesium in the liquid, that results from their addition.