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Microstructure and Thermal Properties of Al-44at% Zn and Al-48at% Zn Alloys

Zinc atoms do not form intermetallic phases with Al atoms due to the weak mutual interaction. The difference in atomic radius of the two elements (0.143nm for Al, 0.134 nm for Zn) has an important influence on the microstructure of Al-Zn alloys. The equilibrium state of an Al-Zn alloy can be reached after a prolonged ageing. In this state, the alloy contains two phases, namely the fcc  - phase (the matrix, M), having 1 at % Zn, and hcp  (Zn)-phase (the precipitates), having 0.5 at % Al. The solid solubility of Zn in Al increases with temperature and reaches about 67 at% at about 655 K [3]. A detailed study of microstructure and thermal properties of the Al-Zn alloys was done by transmission electron microscopy (TEM) and X-ray powder diffraction (XRD). Alloys with 44 and 48 at% Zn were quenched rapidly from the solid solution temperature, and aged at room temperature or at elevated temperature. These alloys were supersaturated solid solution which decomposed and slowly reached their equilibrium state [1, 2]. TEM measurements were performed on transmission electron microscope “ JEOL” JEM-200CX with tungsten cathode, operated at 200 kV. For TEM measurements the powder samples were first suspended in chloroform, then treated in an ultrasonic bath, mounted onto the carbon-coated copper grids and finally dried in air. Selected area electron diffraction (SAED) patterns were taken using aperture with a diameter of 20 μ m. The samples were studied by X-ray diffraction using a Philips diffractometer having a high- temperature attachment, a proportional counter and a graphite monochromator, with CuK radiation. The samples were exposed to low air (10-2 Pa), but in accordance with previous studies [4] the results did not depend on air pressure (between 10-3 and 105 Pa). These studies have resulted in much new information, e.g., on the zinc content in the matrix (M) and in precipitates (P), in contact with M, on the strains occurring at the M/P interfaces, on the GP zones stability, on the size, shape and orientation of the precipitates (Fig. 1) ; on dissolution and associated phase transitions occurring in the system (Fig. 2), and also about the unit-cell parameter of M and equilibrium phase  (Zn) (Fig. 3) The obtained results indicate different behavior of the Al-Zn alloys, having similar initial Al and Zn contents, in relation to that predicted by the phase diagram accepted in the literature [1, 2]. References: [1] Ž. Skoko and S. Popović, Dependence of microstructure of Al-44 at % Zn and Al-48 at % Zn alloys on temperature, Fizika A (Zagreb) 10 (2001) 4, 191-202. [2] Ž. Skoko and S. Popović, Temperature dependence of microstructure of (1- x)Al-Zn alloys, x = 0.44, 0.48, 0.54 and 0.62, to be published in Fizika A (Zagreb). [3] Hans Löffler, Structure and Structure Development of Al-Zn Alloys, Akademie Verlag, Berlin (1995). [4] S. Popović and B. Gržeta, Croat. Chem. Acta 72 (1999) 621.

microstructure ; x-ray diffraction ; alloys ; thermal

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