Influence of Convection on Eutectic Microstructure

Abstract

The composition field in front of a growing lamellar eutectic was determined numerically, both with and without a lateral convective flow. By minimization of the interfacial undercooling it was found that the lamellar spacing λ is a function of G_uλ_O ²/D and W_e, where λ_O is the value of λ for G_u = 0, G_u is the transverse velocity gradient at the freezing interface, D is the diffusion coefficient in the melt, and W_e is the eutectic composition. The value of λ increases as stirring increases, especially at low freezing rates.

Experiments were performed on MnBi-Bi eutectic using spinup/spin-down to provide controlled convection. The value of λ was reduced significantly by the stirring. As predicted, λ increased with increasing radial position. Furthermore at low freezing rates, the morphology was dramatically altered by convection. With stirring at low freezing rates the manganese mass fraction increased with increasing radial position, and sometimes MnBi was missing entirely from the center.

This research was supported by NASA under contract NAS834887.