[en] Highlights: • Effects of fluxing on Fe₅₀Ni₃₀P₁₃C₇ bulk metallic glasses were investigated. • The incorporation of boron in fluxed Fe₅₀Ni₃₀P₁₃C₇ alloy was found firstly. • Mechanism of more densely packed microstructure of fluxed alloys was revealed. In this work, we report on advanced insights into the effects of fluxing on Fe₅₀Ni₃₀P₁₃C₇ bulk metallic glasses. We find that the employed oxide fluxing agent results in significant incorporation of boron in the alloy by atom probe tomography analysis. It demonstrates that the improved glass-forming ability induced by fluxing attributes to not only the reduction of heterogeneous nucleation, but also boron alloying. In addition, the more densely packed microstructure of the fluxed alloys is also related to amount of boron addition. Such an understanding is of great importance to adequately process glass-forming liquids, in order to achieve optimized bulk metallic glass parts with enhanced thicknesses and improved mechanical properties.