AbstractRoom temperature fracture toughness and tensile tests were carried out on metal matrix composites based on the aluminium alloy 7075, and also on monolithic 7075. The particulate reinforcements used were SiC in three nominal sizes: 5, 13, and 60μm. Three aging conditions were studied: peak aged, underaged, and overaged conditions of equivalent matrix micro hardness values.The addition of 5 and 13μm particles increased the 0·2% proof stress and tensile strength, and reduced the ductility, compared with the monolithic material. Composites containing 60μm particles had lower 0·2% proof stress and tensile strength, and very low tensile ductility. In all cases the toughness of the composites was lower than that of the unreinforced material. However, in contrast to the tensile ductility, the material containing 60μm particles was the toughest of the composites. The failure mechanism is believed to be one of particle fracture and/or decohesion at low applied stress intensities, followed by ductile failure of the matrix material. A model to predict toughness from tensile ductility and nominal interparticle spacing is proposed which is consistent with the observed experimental results.MST/3217