The microstructure and mechanical properties of Al-Si-B4C metal matrix composites

B4C particles have been added to molten Al- 7wt% Si- 0.3 wt% Mg alloys, at levels of 5 and 10 wt%, using a propriatory K-Al-Ti-F flux. The resulting composites were examined metallographically and mechanically tested in the as-manufactured condition and after heat treatment for 48 hours at 500°C and 700°C. During incorporation into the melt, a complex Ti-B-C reaction layer was formed on the particle surfaces. The reaction layer remained intact during heat treatment and the stable, protective nature of this layer gave rise to a significantly reduced rate of particle degradation compared to other Al-B4C composites. Significant increases in stiffness were observed; modulus increases per volume percent of particles added were similar to those for the Al-TiC system where strong interfacial bonding occurs. Improved adhesion between the solidified matrix and the B4C reinforcement was encouraged by the enhanced metallic character of the reaction layer. Solid state reaction at 500°C produced little change in mechanical properties. Heat treatment and reaction at 700°C resulted in an increase in the volume fraction of stiff, brittle reinforcing phases, leading to an increase in stiffness and a decrease in ductility.