THE TRANSFORMATION BEHAVIOR OF THE RAPIDLY SOLIDIFIED SOLID-SOLUTION OF YTTRIUM IN MAGNESIUM

The evolution of the microstructure of the solid solution of 1.7-8.3 at.% Y in magnesium made by rapid solidification has been analysed by differential scanning calorimetry (DSC), transmission electron microscopy and X-ray diffraction using the Guinier technique. 11 partial reactions were identified to contribute to three exothermic and to two endothermic thermal effects monitored by isochronal DSC analysis. The extended solid solution of 6 at.% Y in magnesium transforms at about 420 K by forming a base-centred orthorhombic β″ phase of lattice parameter a = 6.47 A ̌, b = 22.42 A ̌ and c = 5.23 A ̊. The activation energy for microstructural evolution in this temperature regime drops drastically at concentrations of more than 5.5 at.% Y. The formation of the flake-like β phase is triggered by β″ agglomerates at a cell boundary network consisting of the equilibrium β phase Mg25Y4 and is completed after 10 h at 563 K. The formation of the equilibrium phases according to isochronal DSC analysis occurs at temperatures of 600 K or higher including crystallization of β from partially amorphous Mg91.7Y8.3 at temperatures 30 K higher than for precipitation of β from the corresponding metastable crystalline phase. © 1990.