On the impact toughness of Al-15 vol.% B4C metal matrix composites

The present work was performed on ten metal matrix composites (MMCs) produced using the new powder injection technique. These MMCs were divided into two series in which pure aluminum was the matrix for one series, while an experimental 6063 alloy was the matrix for the second series. Small amounts of Ti, Zr and Sc were added to those composites, either individually or combined. In all cases the volume fraction of the reinforced B4C particles was in the range 12–15 vol. %. The molten metal was cast in an L-shaped metallic mold preheated at 350°C. Unnotched rectangular impact samples (1 cm × 1 cm × 5 cm) were prepared from these castings and heat treated. Samples were tested using instrumental impact testing machine. Microstructure and fracture surface were examined using Hitachi SU-8000 FESEM. The results show that the presence of Ti improves the wettability of the B4C particles and their adherence to the matrix. Repeated remelting at 730 °C applying vigorous mechanical stirring could lead to fragmentation of some of the B4C particles. Aluminum based composites exhibited better toughness compared to those obtained from 6063 based composites in all the studied conditions. The composite impact toughness was controlled by the precipitation and coarsening of hardening phase particles namely Mg2Si, Al3Zr and/or Al3Sc. Cracks in the fracture surface were observed to be initiated at the particle/matrix interfaces and propagate either through the B4C particles or through the protective layers. No complete debonding was reported due the presence of Zr/Ti/Sc rich layers which improved the particle/matrix adhesion.