A new model of competitive grain growth dominated by the solute field of the Nickel-based superalloys during directional solidification

Abstract

To accurately predict the microstructure evolution of competitive columnar grain growth, the influence of the potential dominant factors of competition growth such as thermal field, solute field and flow field on the overgrowth behavior of trinary-crystal samples during directional solidification was systematically investigated, with the preferred orientation of the experimental grains orienting parallel and at a limited misorientation angle with respect to the temperature gradient direction. It was found that the grain overgrowth rate was weakly dependent on the temperature gradient, which was inconsistent with the classical theoretical assumption that the grain overgrowth rate was determined by the difference of the tip undercooling between the competing grains. In contrast, the grain overgrowth rate was sensitive to the solute field around the dendrite tips. Additionally, with increasing the natural convection, the grain overgrowth rate tended to be promoted at low withdraw speed. These phenomena were attributed to the mechanisms of solute interaction in the converging case and sidebranching events in the diverging case, while the solute field was the dominant factor to govern the overgrowth behavior of the competing grains. Moreover, a new model of competitive grain growth based on the solute field was proposed to predict the microstructure evolution of the Nickel-based superalloys during the directional solidification process. In this new model, the primary spacing of the well-oriented grain in the converging case was smaller than that in the diverging case due to the sidebranching events and dendrite lateral motion, which highlighted a new mechanism of primary spacing evolution in the directional columnar solidification structure.