On the macrosegregation of silicon in niobium silicide based alloys

• Actual compositions used to calculate material parameters and partitioning of solutes.
• Macrosegregation of Si linked to parameters based on Tm and ΔHm of sd and sp elements.
• Macrosegregation of Si linked with solute partitioning.
• “Alloy entropy of fusion”, “alloy enthalpy of melting” explain macrosegregation of Si.
• Report how typical alloying additions influence the macrosegregation of Si.

Sixty nine Nb–Si based alloys with Al, B, Cr, Fe, Ge, Hf, Mo, Sn, Ta, Ti, V, W, Zr additions that were produced using cold hearth based solidification processing were used to study the macrosegregation of Si. The study used chemical analysis data for the compositions of the cast alloys and phases present in their microstructures to calculate various material parameters and the partitioning of solutes. The material parameters were based on the melting temperatures and enthalpies of melting of sd and sp electronic configuration alloying elements in Nb. The key material parameters controlling the macrosegregation of Si were the alloy melting temperature Tm, the melting temperature of the sd elements of the alloy Tmsd, the enthalpy of melting of the sd elements ΔHmsd, the ratio of the melting temperatures of the sd and sp elements in the alloys Tmsd/Tmsp and the “alloy entropy of fusion” ΔHm/Tm. The macrosegregation of Si was explained using the concepts of “alloy entropy of fusion”, and “alloy enthalpy of melting” ΔHm and Tm and by considering (i) the stability of S/L interfaces in terms of constitutional supercooling theory and the growth of S/L interfaces in terms of their structure and (ii) the partitioning of solutes between the Nbss and the intermetallics present in the microstructures of the alloys. High Si macrosegregation was observed in alloys that had high ΔHm/Tm and Tmsp values and low Tmsd/Tmsp and Tm.