Coarsening and Microsegregation during Solidification of Ni-AI-Cr Dendritic Monocrystals

Abstract

Dendritic monocrystals of nickel-rich Ni-AI-Cr alloys were directionally solidified at rates ranging from 0.05 to 2.00 m per hour, under gradients of 8 × 10³ and 20 × 10³ K per meter. It was found that if growth is cellular dendritic, the cellular dendritic spacing exhibits the same dependence on growth rate or local cooling rate as does the secondary dendrite arm spacing in columnar dendrites. A study of the isothermal coarsening kinetics of the dendritic solid indicated that an increase in chromium or aluminum concentrations slowed down coarsening, yielding finer cast microstructures. At equal atomic percental increase in concentration the effect of chromium was more significant than that of aluminum in refining the dendritic structure. With increasing local cooling rate the maximum solute concentration remained practically constant, the minimum solute concentration slightly decreased, the segregation ratio slightly increased, and the volume fraction of nonequilibrium interdendritic γ′ phase increased substantially. This phase dissolved during crystal pulling faster in crystals that were grown at a higher rate. The homogenization kinetics of aluminum and chromium were established both analytically and experimentally. A longer time was necessary for chromium than for aluminum in order to achieve the same index or residual segregation.