Precipitation kinetics in an air-cooled aluminum alloy: A comparison of scanning and isothermal calorimetry measurement methods

Abstract

The purpose of this work is to establish for the first time the equivalence of determinations of both precipitation time constants, τ, and activation energies, E act, by two calorimetric techniques: differential scanning calorimetry (DSC) and differential isothermal calorimetry (DIC). To accomplish this, kinetics and energetics of precipitation in air-cooled (ACO) aluminum alloy 339 were determined by both methods, using Perkin–Elmer instruments. The ACO alloy was chosen as the subject of the study because of its calorimetric simplicity: a single precipitation exotherm dominates each DSC scan. The DSC data were analyzed using a modified Kissinger equation, from which both time constants and activation energies were derived. From differential isothermal calorimetry experiments, we determined precipitation time constants by fitting the almost exponential decay of DIC heat release curves, using analyses developed in our laboratory. Arrhenius plots of the time constants then yielded values of E act. Both activation energies and time constants from the DSC/Kissinger analysis agreed rather well with those from DIC provided DSC temperature scan rates were slow compared to the calorimeter's instrumental equilibration time (lag time). Thus, the equivalence of the DSC and DIC techniques has been established, at least for this test case.