Influence of matrix characteristics on fracture toughness of high volume fraction Al2O3 / Al-AlN composites

摘要

The role of matrix microstructure on the fracture of Al-alloy composites with 60 % volume of alumina particulates has been studied. The matrix composition and microstructure were systematically varied by changing the infiltration temperature and heat treatment. Characterisation was carried out by a combination of metallography, hardness measurements and fracture studies conducted on compact tension (CT) specimens to study the fracture toughness and crack growth in the composites. The composites show a rise in crack resistance with crack extension (R-curves) due to bridges of intact matrix ligament formed in the crack wake. The steady state or plateau toughness reached upon stable crack growth was observed to be more sensitive to the process temperature rather than to the heat-treatment. Fracture in the composites was predominantly by particle fracture, extensive deformation and void nucleation in the matrix. Void nucleation occurred in the matrix in the as - solutionised (AS) and peak - aged (PA) conditions and preferentially near the interface in the under - aged (UA) and over - aged (UA) conditions. Micromechanical models based on cracking bridging by intact ductile ligaments were modified by a plastic constraint factor from estimates of the plastic zone formed under indentations and are shown to be adequate in predicting the steady state toughness of the composite.