Revealing the heterogeneous nucleation mechanism of Mg₁₇Al₁₂ on impurity Mg₂Si particles in commercial AZ31 alloy

Silicon (Si) is an inevitable impurity element in the AZ31 alloy. In this study, the Si impurity was detected mainly as fine Mg₂Si particles dispersed widely within the central region of the Mg₁₇Al₁₂ phase. During the solidification process, the Mg₂Si particle precipitates at about 565 ℃, before the Mg₁₇Al₁₂ phase of 186 ℃, potentially acting as the heterogeneous nucleation core for the Mg₁₇Al₁₂ phase. The orientation relationship between Mg₂Si and Mg₁₇Al₁₂ was investigated using the Edge-to-Edge matching model (E2EM) calculations, which showed a misfit of only 0.1%. This low misfit suggests that Mg₂Si can serve as a heterogeneous nucleation site for Mg₁₇Al₁₂. The surface and interface structures of Mg₂Si (220) and Mg₁₇Al₁₂ (332) were constructed, and then investigated through the first-principles calculation. The theoretical results indicate that Mg and Al are easily adsorbed on the surface of Mg₂Si, with Al showing higher adsorption energy than Mg. Furthermore, the interface between Mg₂Si and Mg₁₇Al₁₂ exhibits favorable thermodynamic stability. Combined with experiments and theoretical calculations, it is confirmed that the Mg₂Si particles, formed due to the Si impurity, provide effective heterogeneous nucleation sites for the Mg₁₇Al₁₂ phase.