The effects of experimental casting parameters on crystalline and amorphous phase balance during casting of Mg-Zn-Ca alloys

With a high enough cooling rate from above the liquidus, certain Mg-Zn-Ca alloys form Bulk Metallic Glasses. Here, we study the cooling rate-driven amorphous/crystalline phase transformations in Mg-Zn-Ca alloys with limited glass-forming ability. We investigated the microstructural evolution of bioresorbable Mg-Zn-Ca alloys initially melted using induction heating in an inert atmosphere. Rods of Mg 66 Zn 30 Ca 4 (at. %) of different lengths and diameters were fabricated using suction casting into a cylindrical water-cooled copper mould, in which the cooling rate varies along the length. In parallel, a computer simulation was conducted using finite element-based software to estimate the cooling rate in the alloy, based on the melt pouring and mould temperatures measured during the experiment. Microstructural and compositional characterisations were carried out to reveal the effects of phase transformation kinetics on the degree of crystallinity in the resulting rods. These analyses were conducted using X-ray diffraction and scanning electron microscopy with energy-dispersive X-ray spectroscopy. The Ø3 mm rod can be cast to create an amorphous structure at all axial locations, indicating the critical cooling rate has been achieved throughout, but the microstructure of Ø5 mm rod is highly dependent on the cooling rate, characterised by location in the axial direction. Melt flow discontinuity and compositional inconsistencies are also possible factors contributing to the structural inhomogeneity. This finding will be useful in determining the required setup for producing Mg 66 Zn 30 Ca 4 bulk metallic glass and composites for biomedical applications.