Grain refinement by Al–Ti–B alloys in aluminium melts: a study of the mechanisms of poisoning by zirconium

Abstract

Conventional grain refining tests confirm that the presence of zirconium in commercial purity aluminium melts dramatically reduces the effectiveness of Al–Ti–B grain refiners. Quantitative comparisons suggest that this poisoning action cannot be attributed solely to reduction in growth restriction arising from changed solute contents in the melt. Microscopic analysis of TiB₂ particles in an aluminium based, zirconium containing metallic glass shows the potential for substitution of zirconium for titanium. Analysis of a conventional Al–5 wt-%Ti–1 wt-%B refiner doped with zirconium shows the potential for similar modification of Al₃ Ti. Thus zirconium can affect the nucleation stage of grain refinement by substituting for titanium in the aluminide and/or the boride phase, consistent with earlier work suggesting that the key to the nucleation potency is the existence of a layer of Al₃ Ti on TiB₂ particles.