Precipitation kinetics in solutionized aluminum alloy 2124: Determination by scanning and isothermal calorimetry

Abstract

Kinetics and energetics of precipitation in solutionized (SOL) aluminum alloy 2124 have been determined by differential scanning calorimetry (DSC) and differential isothermal calorimetry (DIC). DSC experiments at several temperature scan rates were analyzed by the Kissinger method to give activation energies and rate constants. From the DIC experiments, we obtained kinetics information using a 2-exponential fit, a rate-averaged time constant, and (for GP formation) an Avrami model. It appears that the 2-exponential fit is applicable when two distinct processes contribute to precipitation, while the rate-averaged time constant is appropriate when one process is dominant. Criteria are established for choosing the proper analysis. Activation energies and time constants from DSC and DIC agree fairly well for both GP zone formation and precipitation. Kinetics results for GP zone dissolution in SOL 2124 were obtainable only from DSC experiments. Both calorimetric methods indicate that, after GP zones have formed and dissolved, two mechanisms are involved in precipitation. The results are compared to DSC studies of other workers for similar alloys. TEM studies indicate that the two precipitation mechanisms in alloy 2124 involve formation of S′ (CuMgAl 2) and θ′ (CuAl 2) phases. Δ Q, the heat evolved during GP zone formation and precipitation, was measured isothermally over the 30–300°C range. At the temperature of maximum GP zone formation rate (∼70°C), Δ Q≈−14.7 J/g; at the precipitation maximum (∼270°C) Δ Q≈−27.2 J/g.