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Thermal behaviour of microstructure of the Al-44 at% Zn and Al-48 at% Zn alloys

Thermal behaviour of microstructure of the Al-44at%Zn and Al-48at%Zn alloys Ž. Skoko and S. Popović Physics Department, Faculty of Science, University of Zagreb, 10002 Zagreb, P.O.B. 331, Croatia; E-mail spopovic@phy.hr Al-Zn alloys in the equilibrium state, formed after a prolonged annealing, are two-phase systems, consisting of the fcc a-phase (the matrix, M), having 1 at%Zn, and hcp b(Zn)-phase (the precipitates), having 0.5 at%Al. The solid solubility of Zn in Al increases with temperature. The precipitation and dissolution phenomena and associated phase transitions in the Al-rich and in Zn-rich alloys, subjected to different thermal treatments, were studied recently by XRD (S. Popović, B. Gržeta, Croat. Chem. Acta 72(1999)621-643; Mater. Sci. Forum 321-324(2000)635-640; Fizika 8(1999)173-182, and reference therein). In the present work the thermal behaviour of microstructure of the two title alloys was studied in situ by XRD. These alloys were not studied in much detail in the cited papers. Each alloy was subjected to two different thermal treatments: (i) rapid quenching from the solid-solution temperature, TSS, in water at RT and storing at RT for a week (samples WQ); (ii) cooling slowly from TSS to RT over 5 days and storing at RT for a week (samples SC). One may suppose that the microstructure of the two groups of samples was different; the samples SC were closer to the equilibrium state than the samples WQ, the latter having residual strains, quenched-in vacancies and a non-uniform distribution of b(Zn) precipitates. This is the reason why the microstructural parameters of the samples SC depended on temperature in a different way than the ones of the samples WQ, during the slow heating from RT to TSS. The solid solution, aSS, was formed above 700 K for the samples SC, and above 800 K for the samples WQ. The following general features were observed for both alloys. As the temperature increased, a decrease of diffraction line intensities took place, due to increased thermal vibration amplitudes of atoms. Also, a gradual shift of diffraction lines toward smaller Bragg angles was observed due to thermal expansion. The phase b(Zn) exhibited an anisotropy in thermal expansion and a change in the precipitate shape. Above 500 K a partial dissolution of b(Zn) in a(M/b) took place. Above 560 K a partial transition of b(Zn), and probably of a(M/b) into the fcc a´-phase took place, the lattice constant of the latter being smaller for about 0.8% than that of a(M/a´,b). On further increase of temperature the three phases coexisted and finally the solid solution state was reached. During the slow cooling from TSS to RT the samples SC and WQ behaved in a rather similar way. In the cooling run the alloys exhibited a shift in temperatures of phase transitions in relation to the heating run. Instead of the phase transitions expected according to the phase diagram, namely a(M/b) + b(Zn) - a´ + a(M/a´) - aSS, the following sequence was observed for both alloys: a(M/b) + b(Zn) - a´ + b(Zn) + a(M/a´, b) - aSS. A similar thermal behaviour was observed for the Zn-rich alloys, Al-54at%Zn and Al-63at%Zn.

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