Precipitation kinetics in solutionized aluminum alloy 2124: II. Effect of prior Guinier-Preston zone formation

Part I of this study described a calorimetric investigation of kinetics and energetics of Guinier-Preston (GP) zone formation and precipitation in sol utionized (SOL) aluminum alloy 2124. The present paper discusses results of experiments designed to determine the effect of prior GP zone formation on precipitation in the same alloy. The samples studied (designated SOL/GP sa mples) had each been held at temperatures less than or equal to 100 degrees C until GP zone formation was determined to be complete, after which diffe rential scanning calorimetry (DSC) and differential isothermal calorimetry (DIC) precipitation measurements were carried out. The DSC data (for scan r ates from 1.2 to 20 degrees C/min) were analyzed by the Kissinger method to yield activation energies and time constants. From the DIC experiments, we obtained precipitation kinetics information using analysis techniques deve loped in this laboratory. Precipitation activation energies and time consta nts for SOL/GP samples derived from DSC agree rather well with those from D IC. As was the case for the SOL alloy, two processes with slightly differen t temperature ranges are involved in precipitation in SOL/GP 2124. Although prior GP zone formation seems to have little effect on the first precipita tion process (associated with S' (CuMgAl2)), it does reduce the activation energy of the second one (assigned to theta' (CuMg2)) by more than 10%. The exothermic heat of precipitation, Delta Q (measured isothermally) is also relatively unaffected by previous GP zone formation. (C) 1998 Published by Elsevier Science B.V. All rights reserved.