On the solidification microstructure of Mg–30Zn–2.5Y metal–intermetallic alloy

The solidification microstructures of the Mg–30Zn–2.5Y alloy have been studied in detail using both optical and analytical electron microscopy. CALPHAD simulation has been carried out for in-depth understanding of the solidification process. The work clarifies phase equilibrium in the vicinity of the Mg corner of the Mg–Zn–Y ternary system, showing that a ternary icosahedral (I) quasicrystal phase is in equilibrium with the (Mg) solid solution phase. The suppression of the Z-phase during solidification is attributed to low thermodynamic driving force and sluggish transformation kinetics for ordering icosahedral clusters into the crystalline structure. The current work lays the foundation for developing (Mg)–matrix composites with a ternary icosahedral phase as well as other intermetallic compounds. The work presents a case study of solidification process to demonstrate the feasibility to cast (Mg)–matrix composite containing the I-phase from the L+IL+I semi-solid phase region, as long as the equilibrium volume fraction of the I-phase is not too high to guarantee practical slurry fluidity before casting. This would be useful for making use of primary I-phase particles as effective nucleation sites to improve structural refinement and thus maximize the benefit from the I-phase reinforcement.