A mechanism for the poisoning effect of silicon on the grain refinement of Al–Si alloys

The poisoning effect of excess Si solute on the grain-refining potency of Al–Ti–B grain refiners in Al–Si casting alloys has been studied in a crystallographic investigation. The edge-to-edge matching model was used for investigating and comparing the possible poisoning effects of several binary and ternary intermetallic compounds containing Si and Ti. The results show that the poisoning effect is probably due to the formation of a Ti5Si3 coating on the surface of TiAl3, because the Ti5Si3 phase has a much better crystallographic matching with TiAl3 than it does with the Al matrix. However, TiB2 particles appear to survive because an excessively large misfit prevents the Ti5Si3 phase from forming on the surface of TiB2. The implications of this proposed mechanism are discussed in the light of current practical casting solutions and the continuing debate on the grain refinement mechanism.