Formation of Magnesium Silicide in bulk diffusion couples

Abstract

Formation of Magnesium Silicide in binary Mg/Si diffusion couples and in reactions of Mg with Silicon Dioxide was investigated. The position of the Kirkendall marker plane inside the reaction zone of the bulk diffusion couples revealed that at 500 °C, Mg is virtually the only mobile species in the Mg2Si intermetallic compound. Such diffusion behaviour was rationalized using crystallographic information about anti-fluorite structure of the Mg2Si phase. Using general relations between integrated diffusion coefficient for a stoichiometric compound, tracer diffusivities of the components and data on thermodynamic stability of the phases involved in the interaction, the tracer diffusion coefficient DMg⁎ in the Mg2Si was derived.During diffusion-controlled interaction of Mg with Silicon Oxide, the Mg2Si intermetallic is formed within the transition zone microstructure as a continuous phase with the periodically arranged bands of MgO-particles embedded in the matrix phase. In the case of bulk Mg/quartz-SiO2 couples annealed at 500 °C, MgSiO3 and Mg2SiO4 silicates are also found in the reaction zone next to the SiO2-substrate. This can be explained with the help of the equilibrium Mg–Si–O phase diagram. However, no formation of Magnesium Silicates was observed in diffusion-controlled reactions between Mg and fused (amorphous) Silica.At the early stages of the interaction in bulk Mg/SiO2 couples, the situation was found to be different. No quasi-binary compounds MgSiO3 and Mg2SiO4 were detected in the diffusion zone after reaction of Mg with monocrystalline SiO2 as well as fused Silica at 550 °C. In both cases, the layer adjoining the SiO2-substrate is Magnesium Oxide.The formation of periodic layered morphology is considered to be a manifestation of the Kirkendall effect accompanying reactive phase formation in the solid state. The suggested mechanism is operative when the components have distinctly different mobilities in adjacent reaction product layers.