Fracture and R-curves in high volume fraction Al2O3/Al composites

摘要

Fracture toughness and fracture mechanisms in Al2O3/Al composites have been described. The unique flexibility offered by pressureless infiltration of molten Al-alloys into porous alumina preforms has been utilised to investigate the effect of microstructural scale and matrix properties on the fracture toughness and the shape of the crack resistance curves ( R - curves). The results indicate that the observed increment in toughness is due to crack bridging by intact matrix ligaments behind the crack tip. The deformation behaviour of the matrix, which is shown to be dependent on the microstructural constraints is the key parameter that influences both the steady state toughness and the shape of the R-curves. Previously proposed models based on crack bridging by intact ductile particles in a ceramic matrix have been modified by the inclusion of an experimentally determined plastic constraint factor (P) that determines the deformation of the ductile phase and are shown to be adequate in predicting the toughness increment in the composites. Micromechanical models to predict the crack tip profile and the bridge lengths (L) correlate well with the observed behaviour and indicate that the composites can be classified as i) short range toughened and ii) long range toughened based on their microstructural characteristics.