Solutal configuration during coupled two-phase [α (Al) + θ-Al2Cu] planar univariant eutetic growth in Al-Cu-(Ag, Si) ternary eutetic alloys

The solutal configuration, both in the solid as in the liquid, during coupled two-phase planar growth as it can be obtained during unidirectional solidification of ternary Al-Cu-Ag and Al-Cu-Si alloys with a composition close to the univariant eutectic reaction L → α (Al) + θ−Al2Cu has been investigated during preparatory ground experiments. During solidification, both Al and Cu can be redistributed in the liquid by cross-diffusion parallel to the planar solid/liquid interface. However the third element, i.e. Ag and Si, must be segregated over a much longer distance. In the case of Ag, a diffusive stable solute boundary layer has been observed. Si however is found to be solutal unstable and gravity driven solutal convection breaks up the boundary layer. In both cases, however, the interface is prone to a Mullins-Sekerka instability giving rise to two-phase cellular growth at a critical value of the growth velocity. In the solid, the α(Al) composition, determined by the solidus line, changes with changing growth velocity.