PHASE TRANSITIONS IN Al-Zn ALLOYS. S.Popovic[[opthyphen]](1,2), B.[[opthyphen]]Gr`eta[[opthyphen]](1), H.[[opthyphen]]Löffler[[opthyphen]](3), G.[[opthyphen]]Wendrock (3); (1)[[opthyphen]]Ruder Boskovic Institute, POB[[opthyphen]]1016; (2) Faculty of Science, POB[[opthyphen]]162; 10001 Zagreb, Croatia; (3) Martin-Luther Universität, D-06099 Halle, PSF[[opthyphen]]8, Germany

Phase transitions and the formation of solid solutions in alloys having 24, 40, 48, 54 and 62[[opthyphen]]at%Zn were followed in situ by X-ray powder diffraction. These alloys in the equilibrium state at RT are two-phase systems: [[alpha]]-phase (fcc, with 99at%Al) and [[club]]-phase (hcp, with more than 99.5at%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 [[club]]-phase exhibited an anisotropy in thermal expansion, this being eight times larger along the c-axis than along the a-axis. A partial dissolution of the [[club]]-phase in the a-phase started at~490[[opthyphen]]K. Above 550[[opthyphen]]K the phase transition [[club]]-[[opthyphen]][[Alpha]]' (fcc) took place. The unit cell of the [[alpha]]'-phase was smaller for 0.9% than that of the [[alpha]]-phase at 560[[opthyphen]]K. For the alloys with the Zn content of 24, 40 and 48 at%, the [[club]]-phase was not present above 555[[opthyphen]]K. As the temperature further increased, the [[alpha]]- and [[alpha]]'-phases merged together and the solid solution, [[alpha]](SS)(fcc), was formed (Popovic et al., 1993). For the alloys with the Zn content of 54 and 62[[opthyphen]]at%, the [[alpha]]-phase disappeared at ~650[[opthyphen]]K, the [[alpha]]'-phase remaining in coexistence with the [[club]]-phase. The [[club]]-phase finally disappeared at ~700[[opthyphen]]K, and [[alpha]](SS) 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's took place. The present results indicate that minor corrections are necessary in the phase diagram of the system Al-Zn, as accepted in literature.

S.[[opthyphen]]Popovic, B.[[opthyphen]]Gr`eta, H.[[opthyphen]]Löffler, G.[[opthyphen]]Wendrock, Phys. Stat. Sol. (a) 140(1993)341-352.