The formation of randomly textured magnesium alloy sheet through rapid solidification

The rapid solidification of three magnesium-based alloys has been carried out on copper substrates with four tailored surface morphologies. Increasing surface roughness improved the wetting characteristics of the substrates, as did coating with carbon. Of the alloys tested, AZ31 showed inferior wetting behaviour due to the formation of alumina oxides at the interface between the melt and the copper substrate. This behaviour was obviated by the addition of calcium to the melt. The pure magnesium castings all produced a strongly columnar region and showed a weak crystallographic texture. The alloy castings produced weakly columnar regions with an essentially random texture. The alloys cast onto the carbon-coated substrates produced a fully equiaxed microstructure due to the inhibition of heterogeneous nucleation on the substrate surface. It is demonstrated that a randomly textured microstructure with equiaxed grains can be produced through rapid solidification and the implications of this microstructure for the ductility of magnesium sheet is discussed.