Nucleation behaviour of TiB₂ particles in pure AI and effect of elemental additions

Abstract

Particles of TiB₂ have been introduced into pure AI, AI-Cu, AI-Si, and AI-Ti alloys and their ability to encourage grain refinement has been studied. There was little or no reduction in the grain size in pure Al but the addition of 0.025 wt-%Ti, 1.5 wt-%Cu, or 0.8 wt-%Si enables the TiB₂ particles to become potent grain nuclei. When the effect of these alloying additions on the restriction of the crystal growth rate is assessed, it is found that the above Ti, Cu, and Si additions are roughly equivalent. It is concluded that hypoperitectic Ti additions enable TiB₂ particles to become effective grain nuclei through crystal growth rate restriction rather than through the formation of a TiAI₃ layer on the particle surfaces.