The Phase Equilibria Of The Zn-Al-Zr Ternary System

7×××(Al-Zn-Mg-Cu) aluminum alloy is a kind of structural material with high strength and hardness, good hot workability and weldability, good corrosion resistance and high toughness,which were widely used in aerospace, automotive, machinery manufacturing. The alloy composition, heat treatment technology and preparation technology have a significant effect on the properties of the Al-Zn-Mg-Cu series aluminum alloys,especially the effects of the trace elements(Zr, Sc) addition on the microstructure and mechanical properties. Therefore, for a further study of the ultra high strength aluminum alloys, understanding the mechanism and phase relationships between the alloying elements is extremely important. The information of phase equilibrium in the Zn-Al-Zr ternary system was investigated in the present study in order to provide the theoretic foundation for the new materials research.In the present work, the 450°C and 800°C isothermal sections of Zn-Al-Zr ternary system have been determined experimentally by means of equilibrated alloys method and diffusion couple method. Five techniques were utilized to determine the phase diagrams. They included optical microscopy, scanning electron microscopy equipped with energy dispersive X-ray spectroscopy, X-ray powder diffraction and Electron probe microanalysis(EPMA).For the Zn-Al-Zr ternary system, a new ternary compound named T has been found. T phase was fund on analysis to be the Simple crystal structure cubic, belongs to the cell type of cubic, and its lattice parameter is 0.5756 nm. Sixteen three-phase regions have been determined experimentally in the isothermal section at 450°C, i.e., Zn14 Zr + T + L-Zn, Zn14 Zr + T + Zn6 Zr, Zn6 Zr + T + Zn3 Zr, α-Al + T + L-Zn, α-Al + T + ZrAl3, ZnZr + Zn2 Zr + Zn3 Zr, α-Zr + ZnZr + Zr3 Al, Zn Zr + Zr3 Al + Zn3 Zr, Zr3 Al + Zr2 Al + Zn3 Zr, Zr2 Al + Zr3Al2 + Zn3 Zr, Zr3Al2+ Zr4Al3 + T, Zr4Al3 + ZrAl + T, Zr Al + Zr2Al3 + T, Zr2Al3 + ZrAl2+ T, ZrAl2 + T + ZrAl3. The Sixteenth-phase regions has been inferred according to the rule of phase-regions’ contact. It is Zr3Al2 + T + Zn3 Zr. The T phase can be equilibria with L-Zn, α-Al, Zr Al3, ZrAl2, Zr2Al3, ZrAl, Zr4Al3, Zr3Al2, Zn14 Zr, Zn6 Zr, Zn3 Zr at 450°C, and it has a very large composition range of Zn from 11.9 to 58.5 at.% and Zr from 25.8 to 27.1 at.%.For the 800°C isothermal sections of Zn-Al-Zr ternary system, the T phase still existed, and it has a small composition change compared with that at 450℃. The T phase can be equilibria with L, ZrAl3, ZrAl2, Zr2Al3, ZrAl, Zr4Al3, Zr2 Al, Zr3Al2, Zn3 Zr and ZnZr, and the solubility of Zn, Zr in T is 26.4 to 28.55 at.% Zr,17.2 to 55.1 at.% Zn, respectively. nine three-phase regions have been determined experimentally in the isothermal section at 800°C, i.e., ZrAl3 + T + L, Zn2 Zr + Zn3 Zr + T, Zn3 Zr + T + L, Zr2 Al + Zr3Al2 + T, Zr3Al2+ Zr4Al3 + T, Zr2Al3 + ZrAl2 + T, ZrAl2 + T + ZrAl3, Zr2Al3 + ZrAl + T and Zr Al + Zr3Al2 + T.