engleski

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 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 cca 490 K. For the alloys with 24, 40 and 48 at% Zn the phase transition beta - alpha' (fcc) took place at cca 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, alphass (fcc), was formed. For the alloys with 54 and 62 at% Zn the shape of beta-phase precipitates changed on heating. Above cca 550 K a partial transition beta - alpha', took place. The alpha-phase disappeared at cca 650 K, the alpha'-phase remaining in coexistence with the beta-phase. The beta-phase finally disappeared at cca 700 K, and alphass 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 K took place. The present results indicate that minor corrections in the phase diagram of the system Al-Zn are necessary.

phase transformation ; solid solution ; XRD ; alloys

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