TEM Studies of In Situ Formation of MgO and Al₄C₃ During Thixomolding of AZ91 Magnesium Alloy Conducted in CO₂

MgO and Al₄C₃ compounds were produced in situ by reaction of CO₂ with AZ91 alloy at semi-solid temperature range. Using modified magnesium injection molding, reactive carbon dioxide was introduced to hot zone of cylinder to conduct controlled oxidation reaction with partially melted Mg alloy. In result, nano-scale native MgO (30–50 nm) and small amount of Al₄C₃ carbide within eutectic mixture consisted of α(Mg) and β-Mg₁₇Al₁₂ were formed. Apart from eutectic obtained directly from liquid state, proeutectic magnesium solid solution α(Mg)_p is crystallized. Homogeneously distributed α(Mg) globular grains (not melted during the process) with size 20–50 μm and volume 8–12% were also visible in the microstructure. Electron beam was aligned along [011] zone axis of MgO and [01-10] of αMg matrix. (11-1) planes of MgO were parallel to (0002) planes of the matrix. Orientation relationship was [011] MgO ║ [01-10] αMg and (11-1) MgO ║ (0002) αMg.