EXPERIMENTAL AND THEORETICAL STUDY OF THE AL-CU-ZN PHASE DIAGRAM

The Al-Cu-Zn ternary system has been investigated intensively in the past because of its technical importance for developing of the light-weight alloys. Our work is focused on doubtful regions of the phase diagram which have not yet been satisfactorily resolved in the scientific literature and on the divergent results from the experimental and theoretical publications [1,2]. Our experimental study of the Al-Cu-Zn binary system was mainly focused on a description of the solubility between γ_AlCu and γ_CuZn binary brass phases and mutual relation between ternary intermetallic phase τ and τ'. The theoretical part was focused on the CALPHAD reassessment, which includes newly described phases and improvement of the sublattice models of some phases with respect to their crystal structure and solubility. Samples were prepared from the pure elements (5N) encapsulated in evacuated quartz ampoules and melted in conventional tube furnace. Samples were remelted several times for homogenization. Long term annealing was performed in evacuated quartz glass ampoule. Annealed samples were quenched in water to the room temperature. A combination of dynamic and static methods was used for the phase diagram investigation. Overall and phase composition of metallographically prepared samples were analyzed by scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM-EDX). X-ray diffraction (XRD) was applied to the structure determination of the intermetallic phases found in studied system. HT-XRD was used for the evaluation of the high temperature phases. Temperatures of phase transitions were measured on high-temperature DTA in evacuated quartz glass DTA ampoules with heating and cooling rate 5 K min'1. Fig. 1 shows the obtained isothermal section at 400° C of the experimental ternary phase diagram. The ternary phase diagram Al-Cu-Zn was reassessed by CALPHAD-type approach by the ThermoCalc software. The assessment was based mainly on published experimental data [1] and on our new experimental results. With respect to the new experimental results the assessment has slightly changed in comparison with recent work published by Liang [2]. The liquid phase was remodeled to obtain better agreement with the experimental results [2]. We used four-sublattice models for y.AlCu and y_CuZn phases which are based on their structure. The x* phase was implemented into the phase diagram with respect to the experimental [2] and our structural results. For the final r