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have also been observed to occur during retrogression, although to a much more limited extent, because in the T6 temper, the density of particles suitable to undergo those processes is much lower than that of f'me particles.t2] The aluminum alloys were cast in a centrifugal furnace under argon atmosphere in 40-mm O • 5-mm discs. Two different compositions (A and B) with and without Cu were prepared. Atomic absorption gave the following composition (in weight percent) for alloy A: Zn 5.1, Mg 2.35, Cu 0.14, Zr 0.12, Cr 0.18, Mn 0.3, Ti 0.039, Fe 0.24, and Si 0.08; the content of Cu in alloy B was 2.0, the concentrations of the other additions being those of alloy A. The two alloys have compositions within the ranges of the commercial alloys AA 7017 (alloy A) and AA 7075 (alloy B). The discs were homogenized (10 hours at 480 ~ and cold rolled down to 1.0 mm. To avoid effects related to sample preparation, t~'j DSC samples (6-mm Q • 1-mm discs) were directly punched from the rolled sheet prior to the solutioning heat treatment. All samples were solution heat-treated at 465 ~ for 30 minutes and quenched in water at room temperature (around 25 ~ The dissolution of particles was investigated on T6 material, whereas the formation of particles was studied on samples naturally aged for 2 days. The T6 temper consisted of 2 days at 25 ~ plus 24 hours at 120 ~ All heat treatments were carried out in an air furnace, the temperature being controlled within +-3 ~ The DSC measurements were taken in a PERKINELMER* DSC-2C apparatus. High-purity aluminum was *PERKIN-ELMER is a trademark of Perkin-Elmer Physical Electronics, Eden Prairie, MN.

used as reference. Runs were carried out at the heating rates of 10 ~ min-', 15 ~ min-', 20 ~ min -~, 25 ~ min -~, 40 ~ m i n - ' , and 50 ~ min-' from 25 ~ to 500 ~ under dynamic argon atmosphere (1 L h-'). The kinetics of both dissolution and formation of particles were described by means of the Johnson-MehlAvrami (JMA) equation. [12,~3jAlthough the experimental data could also be fitted by means of other equations, we have found that this law gives reasonably good resuits. The JMA equation may be written as

da - - = n(1 - a) ( - I n (1 - a))"-l/"g(T) [1] dt where a is the fraction reacted, n is an exponent which accounts for nucleation and growth morphology, t stands for time, and the rate function, g(T), was assumed to be the Arrhenius law, g(T) = v exp ( - A E / R T ) [2] where T is the absolute temperature and R the gas constant. The three kinetic parameters characteristic of the law described in Eqs. [1 ] and [2] are the frequency factor (v), the activation energy (AE), and the exponent n. The activation energy and the frequency factor were calculated by means of a peak temperature method; t'4j in this method, the experimental results for the variation of the peak temperature (Tp) with heating rate (h) are fitted by the equation h In Te2 -

AE vR RTp + In--AE

[3]

VOLUME 21A. AUGUST 1990- - 2277

from which AE and l, can be obtained. On the other hand, the exponent n was obtained from a