PRECIPITATION KINETICS IN AN AIR-COOLED ALUMINUM-ALLOY - A COMPARISONOF SCANNING AND ISOTHERMAL CALORIMETRY MEASUREMENT METHODS

The purpose of this work is to establish for the first time the equiva lence of determinations of both precipitation time constants, tau, and activation energies, E-act, by two calorimetric techniques: different ial scanning calorimetry (DSC) and differential isothermal calorimetry (DIG). To accomplish this, kinetics and energetics of precipitation i n air-cooled (AGO) aluminum alloy 339 were determined by both methods, using Perkin-Elmer instruments. The ACO alloy was chosen as the subje ct of the study because of its calorimetric simplicity: a single preci pitation exotherm dominates each DSC scan. The DSC data were analyzed using a modified Kissinger equation, from which both time constants an d activation energies were derived. From differential isothermal calor imetry experiments, we determined precipitation time constants by fitt ing the almost exponential decay of DIC heat release curves, using ana lyses developed in our laboratory. Arrhenius plots of the time constan ts then yielded values of E-act. Both activation energies and time con stants from the DSC/Kissinger analysis agreed rather well with those f rom DIC provided DSC temperature scan rates were slow compared to the calorimeter's instrumental equilibration time (lag time). Thus, the eq uivalence of the DSC and DIC techniques has been established, at least for this test case. (C) 1998 Elsevier Science B.V.