Fracture toughness and fracture mechanisms of Al-Al2O3 composites at cryogenic and elevated temperatures

The fracture toughness of two Al alloys (2014 and 6061) reinforced with 15 vol.% Al2O3 particulates was measured in the temperature range from -150 degrees C to 300 degrees C. Both composites maintained acceptable levels of toughness (over 20 MPa m(1/2)) up to 200 degrees C, but the fracture toughness dropped very quickly above this temperature. Fracture always took place by a ductile mechanism involving the nucleation of voids from the ceramic particulates, the progressive growth of these voids, and the final coalescence through the matrix. Quantitative microscopy analyses demonstrated that voids were initiated from reinforcement fracture at low temperatures and by failure at the interface or in the matrix near the interface at elevated temperatures. Regardless of these differences, the changes in toughness could be explained by the deterioration in the mechanical properties with temperature rather than by the microstructural changes induced by the exposure to elevated temperatures.