Segregation of Mg-6Gd alloy under natural convection: From macro solute distribution to micro dendrite growth.

Segregation is a serious defect in alloy ingots which severely deteriorates materials performance. The segregation defect in Mg-6Gd alloy is studied by coupling macro thermal-solutal-convection transport and micro dendrite growth. The macroscopic fluid dynamics and mass transfer equations are resolved to forecast the segregation behavior under conditions of continuous temperature variation during the solidification process. The numerical model is validated by testing double-diffusive natural convection in a closed square cavity. A phase field model is then applied to simulate the micro dendrite growth, using macro undercooling and liquid flow velocity as boundary conditions. Results show that the multiscale segregation behavior, including macro solute distribution and micro dendritic morphology, is strongly dependent on the temperature condition and the liquid convection, which provides guidance for reducing and eliminating the segregation defect. [ABSTRACT FROM AUTHOR]