Al-Zn-Sn Phase Diagram X-ray diffraction study at various temperatures

Further studies of the ternary phase diagram Al-Zn-Sn have been carried out by X-ray diffraction at various temperatures and by thermodilatometry in order to confirm our earlier experimental results which have been refuted by some recent work based on thermodynamic optimization. These new results, in particular the change with temperature of the crystallized fraction for Al-Zn-Sn mixtures containing up to 31.5 mass% of tin, confirm our previous results on the existence of a significant retrograde miscibility of tin in a solid solution α′ss (in the temperature range 286 to 335°C) which protrudes into the ternary system starting from the Al-Zn binary up to a tin concentration of about 50 mass%. This disagreement between theory and experiment highlights the difficulties of a thermodynamic optimization approach based on solid state solubilities in the binary systems and on ternary liquidus data which disregards the ternary interaction parameters in the solid state. Besides, this experimental study highlights the difficulties in understanding the phenomena which depend on phase stabilities.