ABSTRACT
This chapter focuses on the nucleation mechanisms of highly potent heterogeneous sites in aluminium castings, separating the potential influences of nucleation and growth restriction. Heterogeneous nucleation in aluminium alloys is still the weak link in the understanding and modelling of grain refinement as a nucleation and growth restriction process. The microstructure of refiner rods has been extensively studied without clarifying a nucleation mechanism. The phases formed on boride particles in contact with liquid aluminium can only be predicted from ternary equilibrium diagrams of aluminium, titanium aluminide and silicon without detailed knowledge of the dilute but complex boride–Al–Ti–Si multi-component systems. For simple one-dimensional temperature gradients, predictive modelling of grain sizes by grain refinement requires detailed information on growth restriction and heterogeneous nucleation undercooling. At higher titanium concentrations boride particles readily act as nucleation centres for titanium aluminide particles and these may grow if the particle is held in the stable two-phase field.
