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Thermal Behaviour of Al-Zn and Zn-Al Alloys

Thermal properties of the title alloys, having the Zn atomic fraction, xZn, from 0.03 to 0.62, have been studied in situ by X-ray powder diffraction (XRD) and differential scanning calorimetry (DSC). It has been found that the temperature dependence of microstructure of the alloys, rapidly quenched from the solid-solution temperature, Tss, to room temperature, RT, is quite different from that of the alloys slowly cooled from Tss to RT [1]. The area between two curves showing that dependence during the first heating from RT to Tss and first cooling from Tss to RT is much smaller for the slowly cooled alloys than for the rapidly quenched alloys. That area slightly increases with the increase of the Zn content in the alloys. The temperature dependence of microstructure of the alloys during the second heating from RT to Tss and second cooling from Tss to RT differs little from that during the first cooling from Tss to RT. The ideal equilibrium state cannot be reached either by slow cooling of the alloys from Tss to RT, or by a prolonged ageing at RT of the rapidly quenched alloys. During cooling from Tss to RT, a temperature hysteresis is observed in reversal phase transitions. Several characteristic phenomena were observed during the temperature rise from RT to Tss: a decrease of diffraction line intensities due to enhanced thermal vibrations, anisotropy of thermal expansion, changes in the precipitate shape, partial or complete dissolution of precipitates in the matrix, phase transitions and formation of solid solution. The phase transitions were also followed by the DSC technique, and confirmed results obtained by XRD studies. The observed sequence of phase transitions in alloys during heating from RT to Tss is different from that which could be expected according to the phase diagram of the system Al-Zn accepted in the literature.

Al-Zn alloys; solid solution; XRD; DSC

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