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Phase transitions in the system Al-Zn

Phase transitions and the formation of solid solutions at elevated temperatures in Al-Zn alloys having 8, 24, 40, 48, 54 and 62 at% Zn were followed in situ by X-ray powder diffraction. In the equilibrium state at RT, after quenching from the solid solution temperature and prolonged ageing, these alloys are two-phase systems: alpha -phase (fcc, with 99 at% Al) and beta -phase (hcp, with more than 99.5 at% Zn). As the temperature of the previously annealed alloy increased, a decrease of the peak intensities of diffraction lines took place due to increased thermal vibration amplitudes. Also, a shift of diffraction lines due to thermal expansion was observed. The beta -phase exhibited an anisotropy in thermal expansion, this being several times larger along the c-axis than along the a-axis. A partial dissolution of the beta -phase in the alpha -phase started at approximately=490 K. For the alloys with 24, 40 and 48 at% Zn the phase transition beta to alpha '(fcc) took place at approximately=550 K. The beta -phase was not present above 555 K. At 560 K the unit-cell parameter of the alpha '-phase was smaller for 0.9% than that of the alpha -phase. As the temperature further increased, the alpha - and alpha '-phases merged together and the solid solution, alpha /sub ss/ (fcc), was formed. For the alloys with 54 and 62 at% Zn the shape of beta -phase precipitates changed on heating. Above approximately=550 K a partial transition beta to alpha ', took place. The alpha -phase disappeared at approximately=650 K, the alpha '-phase remaining in coexistence with the beta -phase. The beta -phase finally disappeared at approximately=700 K, and alpha /sub ss/ was formed. In the cooling cycle, a temperature hysteresis in reversal phase transitions was observed. During a repeated heating of the same specimen, a temperature delay in phase transitions for several tens of K's took place. The present results indicate that minor corrections are necessary in the phase diagram of the system Al-Zn.

Phase transition. solid solutions. thermal expansion. XRD

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