Si segregation deters prenucleation at the interfaces between liquid-aluminum and TiB2 substrates, the origin of ‘Si poisoning’

• The outmost Ti atoms of the basal {0001} surface of TiB 2 attract Si atoms being segregated at Al/TiB 2 {0001} interfaces. • Segregated Si ions attract Al atoms to the layer adjacent to the TiB 2 substrate. • Si segregation deters prenucleation at the interfaces and thus, poisons the grain-refiners. The mechanism of ‘Si poisoning’ of Al-Ti-B based grain-refiners in Al-Si alloys has been a topic of intensive study for over half a century. We here investigate prenucleation of Al at the Si segregated interfaces between liquid Al and TiB 2 {0001} substrates using ab initio techniques. Our study reveals that chemical affinity between Ti and Si atoms empowers Si segregation at the Al(l)/substrate interface during solidification. The Si interfacial segregation curbs atomic ordering in the liquid Al adjacent to the substrate. Consequently, prenucleation of Al atoms at the Al(l)/{0001}TiB 2 interface is deteriorated and thus, the subsequent nucleation process adversely affected, which causes the so-called ‘Si poisoning’ effect during the practice of grain refinement via inoculation with addition of TiB 2 particles in Al-Si alloys.