engleski

Peculiar diffraction effects in Al-Zn Alloys

Al-Zn alloys, submitted to different thermal treatments, show peculiardiffraction effects. Solid solutions of Zn in Al, obtained by rapid quenching, show spotty diffraction lines ; the spottiness increases with the initial Zn content. On the contrary, a slowly cooled alloys has continuous diffraction lines. The decomposition rate of the solid solution increases with the initial Zn content. The indication of GP zones as first precipitates are broad humps at the high angle side of the solid solution diffractionlines, the latter being little broadening due to strained crystal lattice around GP zones. This is proved by lines of no contrast through GP zones in TEM photographs. A further decomposition proceeds in precipitation of the beta(Zn)-phase, i.e. in a gradual transition of the solid solution (rich in Zn, having GP zones) to the alpha-phase (depleted in Zn, having beta(Zn) grains). Diffraction lines of the alpha-phase are very broad due to the strains around beta(Zn) grains, but become sharper during ageing, at room or at elevated temperature.The temperature dependence of the beta(Zn) diffraction line intensities tends to saturation upon a prolonged ageing, but shows an enhanced increase at short ageing times. This is evidence that big GP zones transform directly to beta(Zn) grains. The phase beta(Zn) exhibits an anisotropy in thermal expansion and in broadening of its diffraction lines upon heating, indicating a shape change of beta(Zn) grains. The temperature dependence of the unit-cell parameters of the alpha and beta(Zn) phases of the quenched alloys differs from that of the slowly cooled alloys. An increased solubility of Zn in Al with the increase of temperature causes a decrease of the unit-cell parameter of the alpha-phase. Due to recrystallization and disorder processes a strange variation of diffraction intensities of the phases present at elevated temperatures takes place. The obtained results indicate different behavior of the Al-Zn alloys, having similar initial Al and Zn content, in relation to that which is predicted by the phase diagram accepted in the literature.

Al-Zn Alloys; GP zones; thermal expansion; line broadening; XRD

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