The outstanding effect and mechanism of non-inert casting atmospheres on glass forming ability of P-containing Fe-based soft magnetic bulk metallic glasses

•O2 can improve GFA of P-containing Fe-based BMGs while fails for FeCoBSiNb alloy.•Surface composition of P-containing Fe-based varied much with casting atmospheres.•P segregation takes place for P-containing Fe-based liquids during melting process.•P segregation leads to the crystallization start in surface rather than in center.•Enhancement of GFA by oxidizing atmosphere was ascribed to elimination of P element.