Ti and Nb alloying effect on the mechanical property of M7C3 carbide in 9Cr18Mo stainless steel: First-principles calculation

The effect of Ti and Nb alloying on the mechanical property of M7C3 carbide has not been thoroughly explored till date. Based on first-principle calculations, several parameters such as binding energy, mechanical properties and electronic structure were investigated and calculated systematically in this work. The results show that the Ti and Nb preferably substitute the Cr sites instead of Fe, due to the smaller formation enthalpy associates with Cr sites. Also, the substitution of Cr with Ti and Nb atoms lead to improve ductility of Fe3Cr4C3 and Cr3Fe4C3 phases. The ductility of the carbide phases could be improved as the G/B ratio decreases by alloying M7C3 carbide with Ti and Nb. The Cauchy pressures involved in substitution of Cr by Nb or Ti are larger than the pressures associated with the substitution of Fe in Cr3Fe4C3 or Fe3Cr4C3 phases. Furthermore, the initial anisotropy degree A increases to 0.9–1.1 after alloying with Ti or Nb. Mulliken population shows the presence of large amount of metallic bonds when the Cr is substituted with Ti or Nb elements in M7C3 carbide. The DOS shows a drop in intensity peaks after Ti or Nb alloying process. Thus the improved ductility associated with the Ti or Nb alloying process results from the interactions between Ti–Cr or Nb–Cr metallic bonds.