Effect of Re on Dendrite evolution and segregation of nickel-based superalloys studied by phase field method

Nickel-based single-crystal superalloys exhibit outstanding mechanical properties at high temperatures, and the addition of rhenium (Re) can significantly improve the alloy's temperature limit. To explore how the Re affects the dendrite morphology transition and micro-segregation in these alloys, we conducted directional solidification simulations of Ni-9 wt.%Al-x wt.%Re alloys (x ​= ​3, 4, 5, 6). These simulations utilized a phase field model incorporating a fundamental thermodynamics database. By employing PANDAT thermodynamic software, the functional relationships between phase equilibrium composition, element concentrations, and temperature have been established and then integrated into the phase field model for accurate simulations. The simulation results show that the microstructure undergoes a planer-cell-dendrite morphology transition in the initial stage of directional solidification and ultimately maintains the dendrite morphology in the later stage. The Re content influences the speed of morphology transition; as the Re content increases, the solidification parameters, such as dimensionless undercooling (U) and dimensionless supersaturation (Ω), decrease, resulting in a delayed time that reaches the critical point of the morphology transition. During directional solidification, the distribution coefficients of Al and Re fluctuate initially but quickly stabilize, and the stabilized distribution coefficients of both Al and Re decrease with Re content. In addition, Al exhibits distinct enrichment in inter-dendrite regions and Re in dendrite cores. As Re content increases, the segregation degree of Al and Re is respectively intensified and reduced, which is the result of the combined effect of solute distribution coefficient and liquidus-solidus temperature range.