Rapid solidification of highly undercooled bulk liquid superalloy: Recent developments, future directions

The status of rapid solidification of highly undercooled bulk liquid superalloys is critically evaluated and future directions of research in this technologically important area are proposed. First, consideration is given to purification mechanisms and preparation of a novel SiO₂-ZrO ₂-B₂O₃ (i.e. Si-Zr-B) compound mould coating, for multi-component superalloy melts, with an emphasis on DD3 alloy. On this basis, thermodynamics of the undercooled multi-component melt and the nucleation inhibiting effect of the Si-Zr-B coating are discussed. Consideration is also given to evolution of microstructure and sub-structure within a wide undercooling regime, in particular, the mechanism of dendrite growth and the characteristics of rapid solidification. Growth mechanisms for the two types of grain morphology, at low and high undercoolings, are reviewed, as is precipitation of the strengthening phase, γ′ (Ni₃Al(Ti)), in the as solidified superalloy subjected to different melt undercoolings. Finally, directional rapid solidification of the undercooled superalloy melt in Si-Zr-B coated moulds is considered, which leads on to discussion of the mechanical properties of the ultra-fine directional dendritic structures produced at different melt undercoolings.