Microsegregation in Al-Cu alloys
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I. INTRODUCTION
EARLIER work on microsegregation was carried out by the authors in Al-Cu alloys using heat evolution in a sensitive single-pan calorimeter.[1,2] It was pointed out, in the articles, that it is not possible to measure directly the fraction solid as a function of temperature and, thus, the amount of microsegregation using calorimetry, unless it is assumed that the latent heat is constant and the heat evolved does not depend on the composition changes that occur during solidification. However, it was possible to compare the heat evolved using a model of microsegregation and a thermophysical software program like MTDATA.[3] When this comparison was made, the experimental results compared well with the predictions, except at the highest Cu concentrations.[1] Despite the reasonable agreement, it was recognized that calorimetry was a relatively insensitive method for investigating the progress of microsegregation, because there was only a small change in the amount of heat evolved for the two extremes of equilibrium freezing and there was no diffusion in the solid. For this reason, additional experiments were carried out, investigating the amount of microsegregation using the more traditional method of measuring composition as a function of sample fraction.[4] It was concluded that, although the amount of eutectic at the eutectic temperature measured using the calorimeter was as predicted, the final ordered compositionfraction plot did not fit well, particularly at high Cu concentrations. This type of behavior has been found by other authors.[5,6] It was noticed that small changes in the phase diagram (in particular, the solidus and solvus) and the temperature dependence of the solid-diffusion coefficient affected the predicted results. For this reason, parts of the phase diagram and the solid-state diffusion coefficient were remeasured as a check on the input data. The results of these measurements and a comparison with a new series of Al-Cu alloys, varying from 1 to 8 wt pct Cu, are reported in this article. E.C. KURUM recently completed her Ph.D. in Department of Materials, University of Oxford. Contact e-mail: [email protected] H.B. DONG, Lecturer, is with the Department of Engineering, University of Leicester, Leicester, LE1 7RH, United Kingdom. J.D. HUNT, Emeritus Professor, is with the Department of Materials, University of Oxford, Oxford, OX1 3PH, United Kingdom. Manuscript submitted October 29, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
II. EXPERIMENTAL PROCEDURE A. Measurement of the Phase Diagram Superpure Al (99.9999 wt pct) and pure Cu (99.999 wt pct) were used to make up the alloys. The measured alloy compositions are shown in Table I. The liquidus was measured using the single-pan calorimeter. The thermocouples were calibrated with pure Al, and the liquidus temperature was taken to be the break as the last solid melts. The results are shown in Table II and Figure 1. The results do not differ significantly from previous work. The solidus and solvus were measured using electronprobe
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