No abstract

No abstract

The morphological characteristics of so called duplex titanium aluminide (TiAI 3 ) particles in several AI-5Ti-IB (wt-%) master alloys in the form of rods have been investigated using a wide range of microscopical techniques. It has been found that both rough blocky andfaceted blocky aluminides are present in such materials and that hexagonal platelets are entrained within these blocky aluminides. Auger spectroscopy and transmission electron diffraction have confirmed that the platelets are diborides. In previous work, it has been suggested that duplex aluminides are particularly effective in promoting grain refinement during solidification. To investigate this, a master alloy containing duplex blocky aluminides was briefly remelted and recast from a range of temperatures. This resulted in the formation of significantly different aluminide types. When the original and remelted alloys were tested in commercial purity AI, using standard procedures, it wasfound that the presence of duplex aluminides does not, in itself, seem to be of great importance in influencing grain refiner efficiency. The experimental findings are discussed in the context of previously published experimental and theoretical work.MSTj 1650.

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.