Natural aging characteristics of aluminium alloy 6061 reinforced with SiC whiskers and particles

The aging response at room temperature of aluminium alloy 6061 was compared with that of its composites containing SiC whiskers or particles. Materials were solutionized for 2 h at two different temperatures (529 and 557°C), quenched in cold water and naturally aged for up to 90 days. The improvement in mechanical characteristics was checked by means of hardness measurements. The microstructural features of aged samples were studied by differential scanning calorimetry. Both aluminium alloy and its composites showed the same aging sequence. Natural aging resulted in the progressive formation of primitive Guinier-Preston zones. Composites, when solutionized at 529°C, showed different aging kinetics from the aluminium alloy used as a matrix. In fact, in the case of composites such a temperature was not sufficient to allow complete solution of the alloying elements. This resulted in a lower aging rate. On the contrary, composites solutionized at 557°C showed the same kinetic behaviour as alloy 6061. Hence the driving force of the natural aging process depends on the concentration of alloying elements in the supersaturated solid solution only. The high density of dislocations close to the interface between the matrix and the ceramic reinforcement did not accelerate the aging response, contrary to what happens during artificial aging of composite materials.