New studies of nucleation mechanisms in aluminium alloys: Implications for grain refinement practice

Grain refinement in commercial aluminium alloys can be achieved by addition of Al–Ti–B master alloys, containing α aluminium, TiB₂, and Al₃ Ti. The final grain size depends on the kinetics of both nucleation and growth of solid in the liquid. Conventional solidification studies permit measurement of growth, but only indirect investigation of nucleation. By embedding grain refining particles in a glassy matrix of Al₈₅Y₈Ni₅ Co₂, novel microstructural studies of nucleation become possible. Such studies show that nucleation of α aluminium occurs on the basal faces of TiB₂ particles coated with Al₃Ti. Well defined crystallographic orientation relationships are observed, together with evidence for strong chemical interactions. The nucleation in the amorphous alloy can be related to phenomena common in conventional casting using grain refiners, such as contact time, fading, influence of titanium content, and poisoning. This increased understanding of the fundamental nucleation mechanism permits further improvements in refining practice.