The fracture toughness, K
max, which dose not satisfy the requirement for the plane-strain, of aluminum alloy die castings reinforced with NiAl and Ni
3Al, intermetallic compound, and SiC particles has been studied by 3 point bending tests in the temperature range from 20°C to 300°C. The K
max's were almost comparable in the materials investigated ; the highest in unreinforced alloy, and then SiC-, Ni
3Al- and NiAl-particulate reinforced one in the order. These appear to be attributed to the microscopic characteristics observed in the fracture surfaces and longitudinal cross-sections : (1) debond at matrix/particle interface and fracture of particles, (2) size and aspect ratio of particles, and (3) precipitates in the matrix. The K
max decreased with an increase in particulate volume fraction because of decrease in the amount of plastic deformation in the matrix and damages associated with particles. The K
max's in the reinforced alloys at 20°C were comparable to those at 200°C. The temperature dependencies of K
max appeared to be associated with those of 0.2% proof stress.
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