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

Precipitation in commercial aluminum alloy 2014, without and with alum ina particulate reinforcements, was studied using microhardness, elect rical resistivity, differential scanning calorimetry (DSC), and transm ission electron microscopy. The precipitation sequence in 2014 Al was confirmed to be alpha(ss) --> alpha + GPZ --> alpha + lambda' --> alph a + lambda' + theta' --> alpha + lambda (AlCuMgSi) + theta (CuAl2). Re inforcement addition decreased the time to peak hardness, but also red uced the peak matrix microhardness. This was traced to a decrease in t he amount of lambda' formed in the composites. Further, it was observe d that while Guinier-Preston (GP) zone and theta' formations are accel erated in the composites, lambda' precipitation is decelerated. The ac celeration 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 reinfor cements, whereas the deceleration is associated with a decrease of low -temperature solute diffusivity due to absorption of vacancies at disl ocations and interfaces. It was also observed that the degree of overa ll acceleration in hardening and the reduction in peak matrix microhar dness with reinforcement addition decreased with decreasing aging temp eratures. The causal relationships of these observations with the asso ciated mechanisms are discussed.