Comparative study of the Columnar-Equiaxed Transition in microgravity and on ground during directional solidification of a refined Al - 3.5 wt% Ni alloy

Abstract
A comparative analysis of directional solidification experiments performed on a refined Al - 3.5 wt% Ni alloy in microgravity (facility AGHF, STS95 mission, Nov. 98 ; facility TITUS, MIR-PERSEUS mission, May-June 99) and on ground is presented here. The objective is to separate the respective influence of growth rate and natural convection on the formation of dendritic microstructures in the columnar/equiaxed transition range. Local segregations observed both in µg and on ground in transient regions along the solidification direction are discussed. Ground reference experiments, which were performed vertically upwards i.e. in a solutally stabilizing configuration, show increasing radial macrosegregation at decreasing low growth rates, resulting from a local thermosolutal flow pattern, which is characteristic of this configuration. In microgravity, equiaxed microstructures are observed at all the investigated rates, a specific dendritic morphology being obtained at the lowest rate where the equiaxed grain size is the largest. By contrast, the ground experiments present transitions from purely columnar to mixed columnarequiaxed and purely equiaxed structures for increasing rates.