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

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Abstract
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.