Al-SiC and hybrid Al-SiC-graphite MMCs with up to 70 vol% SiC and 10 vol% graphite were fabricated using spark plasma sintering. Detailed studies of the microstructure, Vickers hardness, longitudinal elastic constant C11, as well as thermal shock resistance of the MMC samples were carried out. Non-destructive ultrasound phase spectroscopy was used to measure the C11 elastic constant of the MMCs, and the measured values were compared with different micromechanical models for particle reinforced composites. For both two- and three-component MMCs, the Hashin Shtrikman lower bound fitted best to the experimental results in the absence of any porosity. Thermal shock resistance of the MMCs was carried out by heating in air to 500 degrees C and subsequently quenching in water for ten times. The extent of thermal shock-induced structural alteration was determined via a systematic study of the change in density, hardness, and C11 of the samples. It is observed that the thermal shock resistance of the MMCs depends upon multiple factors such as SiC content, graphite content, and the amount of residual porosity. While the reduction in mechanical properties of the Al-SiC MMCs due to graphite addition is according to the expected line, the present study shows that graphite addition also reduces the thermal shock resistance of the MMC.

The aim of this work is to compare the stiffness of a graded composite and equivalent monolithic particulate reinforced composite experimentally. Hybrid Al-SiC-graphite composites were fabricated using spark plasma sintering. Longitudinal elastic constants were measured using non-destructive ultrasound phase spectroscopy. It is seen that for the graded composites, the stiffness depends solely on the reinforcement content, while the gradient structure has no influence. Comparison with analytical micromechanical models show that the Hashin Shtrikman lower bound predicts the elastic constants most accurately.

A systematic methodology for developing hybrid Al–SiC-graphite functionally graded composite materials with the highest feasible SiC-content in the top-most layer, without resulting in residual porosities, has been established. The distribution of ceramic reinforcements in individual layers is very homogeneous, and the interlayer regions are free from any defect. Monolithic composites, having composition identical to the overall average compositions of the FGMs, were also fabricated, and their mechanical properties were compared with the corresponding FGM properties. Due to the locally high SiC-content, the top-layer hardness of the FGM is significantly higher than the corresponding monolithic composite. A systematic study of the flexural stress-strain behavior of individual mono-layers and the overall FGM was carried out at different orientations. For identical mono-layer compositions, the orientation of the FGM has a strong influence on its flexural stress-strain behavior. The failure stress is significantly higher when the layer containing high SiC-content is at the compressively loaded side during the 4-point bend test.