Study of precipitation kinetics in a super purity Al-0.8 Pct Mg-0.9 Pct Si alloy using differential scanning calorimetry
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with a programmable recorder. For each condition, two runs were carried out under identical conditions.[9] In the first run, pure aluminum discs were placed in both the reference and the sample pans, while in the second run, the aluminum disc in the sample pan was replaced by the specimen. From these two runs, it is possible to determine the heat effects that occurred during heating the alloy in the DSC cell. The rate of heat evolution in the alloy per unit mass, Q, can be expressed as follows:[9] z E Q5 M
$
%
z z z z z (q2 2q1) 2 (q2 2q1)Q50
[1]
z z where M is the mass of the specimen; q1 and q2 are the heat flows to the reference relative to the sample in runs 1 and
A.K. GUPTA, Research Scientist, and D.J. LLOYD, Principal Scientist, are with the Kingston Research and Development Centre, Alcan International Limited, Kingston, ON, Canada K7L 5L9. Manuscript submitted July 21, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
Fig. 1—DSC curve of a superpurity Al-0.8 pct Mg-0.9 pct Si alloy immediately after quenching from solutionizing temperature (at 560 7C). Heating rate: 10 7C/mm
2, respectively; and E is the calibration constant of the calz z z orimeter. The term (q2 2 q1)Q50 is a small correction factor, which arises due to the differences in the heat capacities of z the aluminum and the specimen. To find Q, the term within the brackets was experimentally determined for each run. The value of the constant E was determined by carrying out calibration runs using an indium standard and was found to be 1.05. The quantity within the brackets of Eq. [1] was experimentally determined for each run and plotted against temperature, as shown in Figure 1. The DSC thermograms were reproducible within an error of 2 7C. Figure 1 shows a DSC thermogram of the alloy obtained at a heating rate of 10 7C/min immediately after solutionizing at 560 7C for 1 minute and quenching in cold water. The overall shape of the DSC curve is similar to those published in the literature on similar alloys.[3–5,10–17] It can be seen that the precipitation sequence develops in a complicated manner during heating at 10 7C/min. Several reactions occur simultaneously and cause overlapping peaks and troughs in the thermogram. The distinct features of the thermogram in Figure 1 are marked by the letters A through G, while the total heat effects and temperature range are summarized in Table I, along with the proposed reactions associated with each peak based on previous work.[3–5,12–17] There are disagreements in the literature with regard to the specific reaction-peak correspondence, which are a result of the following. (1) There is disagreement regarding the number, structure, and composition of the metastable phases formed during aging. (2) There is ambiguity in the nomenclature of different phases. For example, it is not clear whether preclustering reactions are independent reactions or are the initial stages of GP(I) zone formation. In addition, sometimes GP(I) zones and GP(II) are also called Mg-Si clusters and b " phases, respectively, while
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