NEW STUDIES OF NUCLEATION MECHANISMS IN ALUMINUM-ALLOYS - IMPLICATIONS FOR GRAIN-REFINEMENT PRACTICE

Grain refinement in commercial aluminium alloys can be achieved by add ition of Al-Ti-B master alloys, containing a aluminium, TiB2, and Al-3 Ti. The final grain size depends on the kinetics of both nucleation a nd growth of solid irt the liquid. Conventional solidification studies permit measurement of growth, but only indirect investigation of nucl eation. By embedding grain refining particles in a glassy matrix of Al 85Y8Ni5Co2, novel microstructural studies of nucleation become possibl e. Such studies show that nucleation of 2 aluminium occurs on the basa l faces of TiB2 particles coated with Al3Ti. Well defined crystallogra phic orientation relationships are observed, together with evidence fo r strong chemical interactions. Th nucleation in the amorphous alloy c an 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 nuclea tion mechanism permits further improvements in refining practice. (C) 1998 The Institute of Materials.