Effects of Y dopant on mechanical properties and electronic structures of M7C3 carbide in Fe-Cr-C hardfacing coating

Highlights: • Y-doped M₇C₃ carbide can exist stably with Fe₃Cr₃YC₃ crystal structure. • Y dopant can reduce the anisotropy of elastic moduli of M₇C₃ carbides. • Fracture properties of weak crystal faces of M₇C₃ can be improved by Y. • Nano indentation prove the ductility and toughness improvement of Y-doped M₇C₃ carbide. • Ductility and toughness improvement mechanism is electron redistribution caused by Y atoms. In order to reduce cracking and spalling problems of M₇C₃ carbides in hypereutectic Fe-Cr-C hardfacing coating, the effects of Y doping on anisotropy, mechanical properties and electronic structures of M₇C₃ carbide were elucidated by first principles method. Nano indentation experiments were performed to compare with partial calculation results. From first principles calculation, we find that the formation enthalpy of Fe₃Cr₃YC₃ (Y-doped M₇C₃) carbide is lower than that of Fe₃Cr₄C₃ (M₇C₃) carbide, which shows that Fe₃Cr₃YC₃ is more stable and it can form preferentially. No imaginary frequency appears in the phonon dispersion spectrums, which indicates that Fe₃Cr₄C₃ and Fe₃Cr₃YC₃ crystal models are theoretically stable. The Young's modulus is decreased from 450.62 GPa to 228.01 GPa and shear modulus is decreased from 178.44 GPa to 86.22 GPa, after Y atoms doped in Fe₃Cr₄C₃. The intrinsic hardness H_v is decreased from 18.83 GPa to 7.69 GPa. In addition, the anisotropy of elastic moduli of Fe₃Cr₃YC₃ carbide is also reduced significantly. Compared with Fe₃Cr₄C₃ carbide, the fracture elongation of Fe₃Cr₃YC₃ ($\text{2}\overline{\text{1}}\overline{\text{1}}\text{0}$) crystal face is obviously increased from 7% to 14%. The indentation experiments also prove that the ductility and toughness of Y-doped M₇C₃ carbides are improved. Its mechanism is the electron redistribution and more stable chemical bonds formation caused by Y atoms. This can provide new ideas for cracking and spalling improvement of M₇C₃ carbides and design of hardfacing coatings with preferable mechanical properties.