Microstructure and Mechanical Properties of Al/SiC Composites with Multimodal Size Distribution of Reinforcements

Abstract

The effect of particle size distribution and particle size ratio of SiC in SiC/SiO2 preforms on the microstructure, microhardness of SiC reinforcements, modulus of rupture and superficial hardness of Al/SiC composites produced by pressureless infiltration has been investigated. SiC/SiO2 preforms in the form of plates (4cm x 3cm x 0.5 cm) have been pressureless infiltrated by the alloy Al-15.52 Mg-13.62 Si (wt%) at 1100°C fro 60 min under inert atmosphere. SiC powders with average particle size of 10, 68 an 140 micras are mixed withy SiO2 powders an preforms of 40% porosity with unimodal, bimodal and trimodal size distribuitions are prepared by uniaxial compactation. The bimodal (small:large) and trimodal (small:medium:large) preforms are prepared with differents particles sizes ratiosin the following levels: 1:1, 3:1.1:3. 2:2:2, 3:2:1, 3:1:2. Results from characterization by XRD, SEM and energy dispersive X-ray spectrometry show that the typical microstructure of the composites contains the MgAl2O4 (spinel), AlN and MgO phases formed during processing as well as particlly reacted silica, SiC, SI and Al. It is found that the density, reinforcements microhardness, modulus of rupture and superficial hardness of the composites increase all with winder particle size distribution. However, whilst the modulus of rupture is mainly affected on going from unimodal and bimodal to trimodal distribution, superficial hardness and microhardness are mostly influenced on going from unimodal to bimmodal adn trimodal distribution.