Development of Al–Nb–B master alloy with high Nb/B ratio for grain refinement of hypoeutectic Al–Si cast alloys

Abstract

Al–Nb–B master alloy has been regarded as a promising grain refiner that can reduce grain size of hypoeutectic Al–Si casting alloys. However, its grain refinement performance remains to be improved. In this work, the grain refinement efficacy of Al–Nb–B master alloy is significantly enhanced by modifying the Nb/B ratio through thermodynamic calculation. An Al–Nb–B master alloy with optimum Nb/B ratio of ~ 10:1 provides a fully equiaxed structure across the sections of the Al–10Si and commercial Al–9Si–0.08Ti alloys with an average grain size below 220 μm. The phenomenon is attributed to the existence of NbAl₃ and the higher number density of NbB₂ at the Nb/B ratio of ~ 10:1, which offers sufficient active nucleating sites to promote the formation of smaller grains. Moreover, the segregation behavior of Si atoms and interfacial energies after doping Si are investigated by first-principles calculations, and the results reveal that Si tends to segregate to the NbAl₃/α-Al interface, whereas grain refining potency of NbAl₃ for Al remains unchanged. This study has implications for strategic design of Al–Si cast alloy with fine and equiaxed grain structure inoculated by grain refiner.