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

The role of matrix microstructure on the fracture of Al-alloy composites wi th 60 vol% alumina particulates was studied. The matrix composition and mic rostructure were systematically varied by changing the infiltration tempera ture and heat treatment. Characterization was carried out by a combination of metallography, hardness measurements, and fracture studies conducted on compact tension specimens to study the fracture toughness and crack growth in the composites. The composites showed a rise in crack resistance with cr ack extension (R curves) due to bridges of intact matrix ligaments formed i n 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 predomina ntly by particle fracture: extensive deformation, and void nucleation in th e matrix. Void nucleation occurred in the matrix in the as-solutionized and peak-aged conditions and preferentially near the interface in the underage d and overaged conditions. Micromechanical models based on crack bridging b y intact ductile ligaments were modified by a plastic constraint factor fro m estimates of the plastic zone formed under indentations, and are shown to be adequate in predicting the steady-state toughness of the composite.