Grain Refinement of Ni-Cr-W Based Superalloy by Near Liquidus Casting

The grain refinement of Ni-Cr-W based superalloy was obtained by near liquidus casting. The melt was heated to the predetermined temperature near liquidus by induction heating and solidified directly in the crucible by furnace cooling. A fine-grained microstructure was obtained under a certain crucible nucleation undercooling and the related mechanisms are discussed. The solidification process of the undercooled melt is divided into two parts, a nonequilibrium process and a near-equilibrium process. The critical radius Rc for disturbance developing of spherical grains decreases greatly in the nonequilibrium process compared with the near-equilibrium process. Grains grow in dentritic manner in the nonequilibrium process and a crystal multiplication occurs in this process by remelting dentrite. Grain density is determined in the nonequilibrium process. In a near-equilibrium process, the disturbance of the spherical grains will develop with a larger Rc. In situation of high nuclei density, decided by the nonequilibrium process, the larger Rc and the solidification terminates before disturbance developing. Thus, granular grains are obtained.