Mechanical properties and microstructure of 6061 aluminum alloy matrix composite reinforced with alumina microspheres

Experiments were conducted on a composite material (Comral-85) fabricated by ingot metallurgy. The composite consisted of a 6061 aluminum alloy matrix and a reinforcement of 20 vol% Al₂O₃-based microspheres having an average diameter of 20 μm. The age hardening behavior, temperature dependence of the yield stress and the creep behavior of the composite after extrusion were investigated and compared with those of 6061 aluminum alloy. In the as-quenched state, the yield stress of the composite is 40∼85MPa higher than that of the 6061 alloy. This difference is attributed to the high density of dislocations within the matrix which are introduced by the difference in thermal expansion coefficients between the matrix and the reinforcement. The difference in yield stress between the composite and the 6061 alloy decreases with aging time and the age hardening curves for both materials have a similar trend. At room temperature, the strain hardening rate of the composite is higher than that of the 6061 alloy, probably because the distribution of reinforcement enhances the dislocation density during deformation. Both the yield stress and the strain hardening rate of the T6-treated composite decrease as the testing temperature increases, and the rate of decrease is faster in the composite than in the 6061 alloy. Under creep conditions, the stress exponents of the T6-treated composite vary from 8.3 at 473 K to 4.8 at 623 K. These exponents are larger than those of the 6061 matrix alloy.