Isothermal precipitation of commercial 3003 Al alloys studied by thermoelectric power

We have used thermoelectric power (TEP) techniques to study the precipitation process in samples of cold-rolled commercial 3003 Al alloys, in which we guarantee the initial microstructure by diverse thermal treatments. Kinetics are obtained, isothermally, at several aging temperatures, proving that the manganese impurity controls the precipitation process. The Nordheim-Gorter rule is used to determine the effect of impurities on the diffusional TEP, permitting us to know the amount of Mn in solid solution for the diverse work conditions. The time-temperature-transformation (TTT) curves, obtained from the isothermal kinetics plots, indicate the existence of different precipitate phases which depend upon the thermal treatment applied to the sample. For homogenized samples, the observed behavior was associated to an equilibrium phase with a nose toward 540 deg C and with an activation energy of (41 plus /minus 2) Kcal/mol. For strained samples, we observe a metastable phase with an activation energy of (19 plus/minus 2) Kcal/mol and also a stable phase with an activation energy similar to the one obtained for the homogenized samples. For as-cast and as-cast-heated samples, we predict the existence of two precipitated phases, the activation energy of the precipitated phase at low temperature being (31 plus/minus 2 Kcal/mol. The energy of the second phase seems to be the same for all microstructural conditions. The results obtained for the stable phase agree with the results reported in the literature.