The formation of rhombohedral phase during the slow cooling of the Al-Zn solid solutions

The decomposition of Al-Zn solid solutions (from 20 to 60 wt. % of zinc) during slow cooling was studied by differential thermal analysis. The X-ray diffraction and electron microscopic investigation of structural changes were carried out on samples heat-treated simultaneously with DTA-samples. The pronounced peaks on DTA-curves were explained by the formation of metastable rhombohedral α′ _R -phase. This decomposition of supercooled Al-Zn solid solution starts at temperatures corresponding to the nose of the C-shaped T-T-T curve for the beginning of transformation.