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Microstructure of Al-Zn Alloys

A detailed study of microstructure of Al-Zn alloys was done by X-ray powder diffraction and electron microscopy. Alloys with different composition (up to 65 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]. 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 alfa-phase (the matrix, M), having 1 at%Zn, and hcp beta(Zn)-phase (the precipitates), having 0.5 at%Al. This means that the mutual solid solubility of Zn and Al at room temperature is small. The solid solubility of Zn in Al increases with temperature and reaches about 67 at% at about 655 K. 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(alfa) radiation. The samples were exposed to air (105 Pa), but in accordance with previous studies the results did not depend on air pressure (between 10-3 and 105 Pa). TEM and high-resolution transmission electron microscopy (HRTEM) investigations were carried out by using JEOL JEM 2010 200 kV microscope (CS=0.5 mm, point resolution of 0.19 nm) having a beryllium window energy-dispersive (EDS) detector. For the purpose of TEM investigations samples are firstly mechanicaly thinned and then are exposed to ion bombardment in Gatan ion-mill under the Ar atmosphere, until the hole through the sample appeared. This was necesarry to do, in order to satisfy requirements from kinematical approach of imaging theory [3]. 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 unit-cell parameter of M and equilibrium phase beta(Zn), on the GP zones stability, dissolution and associated phase transitions occuring in the system, and also about the size, shape and orientation of the precipitates. The obtained results indicate different behaviour of the Al-Zn alloys, having similar initial Al and Zn contents, in relation to that which is predicted by the phase diagram accepted in the literature.

Al-Zn; microstructure; XRD; TEM; HRTEM

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