Influence of TiB2 particles on growth of eutectic silicon in aluminium–silicon alloys

Abstract

Large amounts of an Al–5 wt-%Ti–1 wt-%B grain refiner master alloy have been added to an Al–12 wt-%Si alloy. It has been found that high concentrations of TiB2 particles influence the growth of the Si crystals in the Al–Si eutectic. Not only are the sizes of the particles reduced, but the morphology is also changed towards a more rounded or fibrous shape, similar to Al–Si eutectics that have been modified by Sr or Na. Addition rates between 0·5 and 10 wt-% of master alloy have been used in the present investigation, in contrast to normal addition rates which are on the order of 0.1–0.5 wt-%. The microstructure, and particularly the eutectic growth morphology was investigated by optical microscopy, scanning electron microscopy and electron backscattering diffraction. In addition to the eutectic silicon modification effect, which was highly reproducible, observations were made about the settling, agglomeration and distribution of TiB2 particles and the association between TiB2 particles and areas of modified eutectic growth. Different hypotheses about the mechanism of Si refinement, like physical blocking, nucleation of the Si or eutectic Si colonies by TiB2 were investigated. In addition, some experiments were preformed with Ti addition; TiAl3 crystals were found to nucleate the Si crystals.