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Grain refinement is critical for fabricating high-quality Al-Si casting components in the application of automobile and aerospace industries,while the well-known Si-poisoning effect makes it difficult.Nbbased refiners offer an effective method to refine Al-Si casting alloys,but their anti Si-poisoning capability is far from being understood.In this work,the grain refining mechanism and the anti Si-poisoning effect in the Al-10Si/Al-5Nb-B system were systematically investigated by combining transmission electron microscope,first-principles calculations,and thermodynamic calculations.It is revealed that NbB2 provides the main nucleation site in the Al-10Si ingot inoculated by 0.1 wt.％Nb Al-5Nb-B refiner.The exposed Nb atoms on the (0001) NbB2 and (1-100) NbB2 surface can be substituted by Al to form (Al,Nb)B2 intermedia layers.In addition,a layer of NbAl3-like compound (NbAl3') can cover the surface of NbB2 with the orientation relation of(1-100)[11-20]NbB2//(110)[110]NbAl3'.Both of the (Al,Nb)B2 and NbAl3'intermedia layers contribute to enhancing the nucleation potency of NbB2 particles.These discoveries provide fundamental insight to the grain refining mechanism of the Nb-B based refiners for Al-Si casting alloys and are expected to guide the future development of stronger refiners for Al-Si casting alloys.