CALPHAD-linked analysis of grain boundary segregation and phase-field simulation of solute-drag effect in multicomponent magnesium alloys

The application of Mg alloy sheets is currently limited owing to their poor room-temperature formability. The solute-drag effect is a potential mechanism via which the texture intensity of Mg alloys can be lowered to improve their formability. However, detailed analyses of the effects of solute-solute interactions on the solute drag in multicomponent systems are lacking. Herein, a CALPHAD-linked phase-field model was developed to evaluate the grain boundary segregation and solute-drag effects in a multicomponent system. The model was used to calculate the grain boundary segregation and solute drag in several Mg alloys, and the effect of solute-solute interactions on these phenomena was evaluated. The results showed that Zn and Ca mutually enhanced the grain boundary segregation in Mg alloys, leading to greater solute drag. However, the combination of Al and Zn mutually suppressed the grain boundary segregation and consequently, reduced the solute drag. These results support a previously proposed theory that Mg-Zn-Ca alloys exhibit strong solute drag owing to the co-addition of Zn and Ca, which reduces the texture intensity and improves the room-temperature formability. Furthermore, a new index for describing co-segregation and competitive segregation at grain boundaries, based on Hillert's grain boundary phase model, was introduced. The value of the index was calculated for various combinations of alloying elements in Mg alloys. The results showed that many ternary Mg alloys with a low texture intensity tended to co-segregate alloying elements.