Microstructure and properties of magnesium and matrix composites

摘要

The effect of the reinforcing particle distribution in magnesium alloy matrix composites (AS41 reinforced with SiC particles and WE54 reinforced with TiB2 particles) on the resulting materials mechanical properties have been investigated. The distribution of SiC particles in AS41 matrix composites, produced by a range of casting techniques with different solidification rates and the effects of different volume fractions of TiB2 particles in a WE54 matrix alloy were studied.;The particle distributions were quantified using three different methods; namely, nearest neighbour distance, Dirichlet neighbour distance and local volume fraction of the reinforcing particles. Of these techniques, Dirichlet neighbour distance showed better representation in terms of representing the particle distribution in the materials. For the AS41 composites, the particle distribution in the high-pressure die cast composite was more homogeneous than in the sand and chill cast composites. For the WE54 composites, the particle distribution in the composite reinforced with 5% volume fraction of TiB2 was more homogeneous than in the composites reinforced with 1% and 2% volume fraction of TiB2.;Clustering parameters, determined from the ratio of the variance and skewness of the interparticle spacing and nearest neighbour distance distributions relative to the corresponding variance or skewness obtained from a random particle distribution, were used to characterise the inhomogeneity of the particle distributions in the materials. Attempts have been made to relate these parameters to the materials tensile properties and fracture behaviour. High values of the clustering parameter were found to correlate with low ductility. Quantitative measurements of the fracture surface were carried out by using the fractal dimension of the profile and surface roughness. These were also correlated to the ductility of the composite materials. For the AS41 composites, it was found that the higher the surface roughness, the lower the ductility, whereas the WE54 composites showed the reverse behaviour.;The tensile properties of the composites, such as yield and ultimate tensile strength and modulus at different temperatures, were investigated as well as their relationship to microstructural features like the reinforcing particle distribution and eutectic phase distribution. The effect of the degree of the inhomogeneity of the reinforcement's distribution on the fracture behaviour was studied by measuring the particle density (in the AS41 composites) and eutectic phase density (in the WE54 composites) along the fracture line and also by analysing the damage present, such as fractured particles and voids. The effect of the degree of the inhomogeneity of the reinforcement's distribution on the micro-yield behaviour of the composites was also studied by analysing the derivatives of the materials initial stress-strain responses after the conventional and interrupted tensile test.;The results of this work suggested that the tensile properties of the high-pressure die cast composite were better than of the sand cast and chill cast composites because the die cast composites had a random particle distribution and a fine grain size. Whereas sand and chill cast composites exhibited severe particle clustering and other casting defects. The higher particle density along the fracture line of the chill cast and sand cast composites was due to the fracture path following the interdendritic regions of particle clustering and the eutectic phase.