ROLE OF AL2O3 PARTICULATE REINFORCEMENTS ON PRECIPITATION IN 2014 AL-MATRIX COMPOSITES

Precipitation in commercial aluminum alloy 2014, without and with alumina particulate reinforcements, was studied using microhardness, electrical resistivity, differential scanning calorimetry (DSC), and transmission electron microscopy. The precipitation sequence in 2014 Al was confirmed to be alpha(ss) --> alpha + GPZ --> alpha + lambda' --> alpha + lambda' + theta' --> alpha + lambda (AlCuMgSi) + theta (CuAl2). Reinforcement addition decreased the time to peak hardness, but also reduced the peak matrix microhardness. This was traced to a decrease in the amount of lambda' formed in the composites. Further, it was observed that while Guinier-Preston (GP) zone and theta' formations are accelerated in the composites, lambda' precipitation is decelerated. The acceleration is attributable primarily to enhanced nucleation resulting from an increase in the matrix dislocation density due to coefficient of thermal expansion (CTE) mismatch between the matrix and the reinforcements, whereas the deceleration is associated with a decrease of low-temperature solute diffusivity due to absorption of vacancies at dislocations and interfaces. It was also observed that the degree of overall acceleration in hardening and the reduction in peak matrix microhardness with reinforcement addition decreased with decreasing aging temperatures. The causal relationships of these observations with the associated mechanisms are discussed.