Thermo-dynamic analysis on solid-state reduction of CaO particles dispersed in Mg–Al alloy

► We clarified formation mechanism of Al 2Ca compounds via solid-state reaction between CaO and Mg–Al alloy by calculating standard free energy. AZ61B alloy powder composite with Al 2Ca fine dispersoids was developed by using CaO additive particles as raw materials. For synthesis of Al 2Ca in the matrix, CaO particles were elementally and uniformly mixed with AZ61B alloy chips via ECABMA process. The mechanism in formation of Al 2Ca intermetallics was investigated by XRD and SEM–EDS analysis when AZ61B alloy green compacts containing CaO additives were heat treated at 380–625 °C in argon gas atmosphere. A change in a standard free energy in formation of Al 2Ca via reaction between CaO and Mg–Al alloy was calculated by using a standard Gibbs free energy of each element contained in the green compact. Both of the theoretical analysis by thermo-dynamic and experimental investigation clarified that Al 2Ca and MgO were synthesized by employing Mg–Al alloy, not pure Mg as the matrix material. Microstructural analysis indicated that needle-like intermetallic of (Mg,Al) 2Ca or Al 3Ca 4Mg were formed as intermediately created compounds in the solid-state reaction between CaO particles and Mg–Al alloy to synthesize Al 2Ca and MgO dispersoids.