Phase transformation, kinetics and thermodynamics during the combustion synthesis of Mg₂Al₃ alloy

The Mg₂Al₃ alloy was prepared by the combustion synthesis (CS) process for the first time. The phase transformation during the process was investigated by X-ray diffraction (XRD) analysis of the intermediate products obtained from two pathways. One was synthesized at different temperatures in the CS reactor, and the other was quenched at different temperatures during the differential scanning calorimeter (DSC) test. The results showed that the formation of Mg₂Al₃ alloy from elemental Mg and Al experienced four stages upon heating, accompanied with the appearance and transformation of Mg₁₇Al₁₂ alloy. In the first stage (from room temperature to about 633 K), the Mg₁₇Al₁₂ phase nucleated and grew prior to the Mg₂Al₃ alloy, owing to the relatively higher diffusivity of Al and lower formation energy of Mg₁₇Al₁₂ alloy under this condition. The second stage (from 633 to 713 K) included a eutectic reaction between Mg₁₇Al₁₂ and Mg to form the liquid phase which promoted the alloying process effectively, thus leading to the disappearance of Mg completely. In the third stage (from 713 to 723 K), the peritectoid reaction between residual Mg₁₇Al₁₂ and Al occurred, besides the significant endothermic melting of Mg₂Al₃ alloy. And finally Mg₂Al₃ single phase with high purity was obtained immediately after heating upon 723 K. Based on these experimental results, the mechanism on the combustion synthesis of Mg₂Al₃ alloy was proposed and discussed from both kinetic and thermodynamic perspectives.