Characterization of submicron particles in Al5Ti1B grain refining master alloys

Abstract

Al5%Ti1%B master alloys are frequently used in the grain refinement of aluminium alloys to induce heterogeneous nucleation during solidification and to promote an equiaxed grain structure. Boride particles are often observed in these alloys and could act as nucleation sites. However there are many more particles than nucleated grains in the samples and hence the precise role of the borides and other particles is not clear. We have therefore been undertaking a systematic TEM study of submicron particles extracted from master alloys using carbon extraction replication of fractured samples, with a view to investigating the nucleation mechanisms.Many of the particles analysed adopted a well defined hexagonal prism morphology (Fig.1) and varied in size from 5μm to 100 nm across the hexagonal face. A combination of EDX and EELS analysis plus electron diffraction was used to identify isolated particles as TiB2 containing little if any aluminium or potassium. These other elements are often detected in agglomerations of particles.