Solute redistribution during and after solidification of Ni-Al-Ta dendritic monocrystals

Abstract

Dendritic Monocrystals of Ni-Al-Ta alloys were grown at 0.05, 0.25, and 2.00 m/h and in some cases at other intermediate rates, under thermal gradients of 8 × 10³ and 18 × 10³ K/m. The growth of such monocrystals provides a rapid and easy way for:a) establishing the distribution of solute during and after solidification, as well as its dependence on local cooling rate; b) determining the effect of dendritic coarsening on this distribution and; c) studying the solution kinetics of the nonequilibrium interdendritic γ′ phase. Back-diffusion in the solid rather than dendritic coarsening was found to control the evolution of the solute distribution profile across the dendritic structure during solidification. With increasing local cooling rate the maximum solute concentration,C _M, remained practically unchanged, the minimum solute concentration,C _m, slightly decreased, the segregation ratio,S = C _M/C_m, increased and so did the volume fraction of nonequilibrium interdendritic γ′ phase. This phase dissolved during crystal pulling much faster at higher crystal growth rates. Solution kinetics were found to depend on the dimensionless parameterDθ/L ², whereD is diffusivity of solute at a given temperature at which a given transverse cross-section of the crystal remains for a timeθ andL is half the primary dendrite arm spacing.