Tensile properties of particulate-reinforced metal matrix composites

Room temperature tensile tests have been carried out on MMCs, all based on the Al alloy 7075 and on monolithic material. The particulate reinforcements used were SiC in three nominal sizes, 5, 13 and 60 mu m. Three matrix ageing conditions were studied, peak aged and equivalent underaged and overaged matrix conditions, based on microhardness measurements. The effects of ageing condition on the tensile properties of the composites follow those produced in unreinforced material. Composites containing 5 and 13 mu m particles both had greater 0.2% proof stress and tensile strength values than unreinforced material. However, the composite reinforced with 60 mu m particles had reduced 0.2% proof stress and tensile strength in the underaged and peak aged condition, and a greater 0.2% proof stress in the overaged condition compared to the monolithic alloy. All the composites had lower ductility than the unreinforced material, with the material containing 5 mu m SiC being the most ductile and that reinforced with 60 mu m SiC having very low ductility. Failure appears to occur by the accumulation of internal damage to particles either by particle fracture or interfacial failure. Such damage introduces voids which grow and lead to reduced ductility in these composites. Large 60 mu m particles fracture easily at low applied stresses, leading to reduced 0.2% proof stress and premature failure compared to the other composites. The small 5 and 13 mu m particles damage less easily and so these composites are stronger than the monolithic material and are more ductile than the composite reinforced with 60 mu m particles. Copyright (C) 1996 Elsevier Science Limited