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Microstructure of Al-Zn and Zn-Al alloys

The microstructure of Al-Zn alloys, having the Zn atomic fraction, xZn, from 0.03 to 0.62, was studied by XRD in dependence on the composition, temperature and previous thermal treatment. Each alloy was subjected to two thermal treatments: (i) rapid quenching from the solid-solution temperature, Tss, in water at RT (quenching rate >104 K/s, samples WQ), (ii) slow cooling from Tss to RT (samples SC). The samples WQ were solid solutions immediately after quenching, at least up to xZn  0.40. These samples were transformed to a quasi-equilibrium state by a prolonged ageing at RT or at elevated temperature, and the precipitation processes were followed in detail [1]. Further studies showed that the samples SC were closer to the equilibrium state than the aged WQ samples ; the microstructure in the latter depended on residual strains, quenched-in vacancies and a non-uniform distribution of precipitates. Both SC and aged WQ samples were slowly heated from RT to Tss and cooled back to RT, and dissolution and precipitation processes along with associated phase transitions were followed in situ. Several characteristic phenomena were observed during the temperature rise from RT to Tss: a decrease of diffraction line intensities due to enhanced thermal vibrations, anisotropy of thermal expansion, changes in the precipitate shape, partial or complete dissolution of precipitates in the matrix, phase transitions and formation of solid solution. In the cooling run, the alloys exhibited a temperature hysteresis in reversal phase transitions in relation to the heating run. The phase transitions were also followed by the DSC technique. It was found that the temperature dependence of microstructure for the SC samples was quite different from that of the aged WQ samples. The ?ideal? equilibrium state was not reached for any thermal treatment. Instead of phase transitions predicted by the phase diagram, accepted in literature, the following sequences were found: (1) for the alloys with xZn = 0.44, and 0.48 (in which Al is dominant), , (2) for the alloys with xZn = 0.54 and 0.62 (in which Zn is dominant), , where  is an fcc phase rich in Al,  (Zn) is a hexagonal phase rich in Zn,  ' is an fcc phase containing a smaller fraction of Zn than  (Zn) and  ss is an fcc solid solution [2, 3, 4]. References: [1] S. Popović and B. Gržeta, Croat. Chem. Acta 72 (1999) 621-643 ; [2] Ž. Skoko and S. Popović, Fizika A (Zagreb) 15 (2006) 1, 61-72 ; [3] Ž. Skoko and S. Popović, Fizika A (Zagreb) 10 (2001) 4, 191-202 ; [4] S. Popović and Ž. Skoko, 22nd European Crystallographic Meeting, Budapest, 2004.

Al-Zn alloys; XRD; solid solution; precipitation

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