Search

Density functional theory (DFT) is applied to investigate the stability and mechanical properties of NbxCy compounds. The structures of NbxCy compounds are optimized, and the results are in good agreement with previous work. The calculated results of the cohesive energy and the formation enthalpy of NbxCy show that they are thermodynamically stable structures, except for Pmc21-Nb2C. The mechanical properties such as the bulk modulus, Young's modulus, the shear modulus, and Poisson's ratio are obtained by Voigt—Reuss—Hill approximation. The results show that the Young's modulus and shear modulus of NbC are larger than other NbxCy compounds. The mechanical anisotropy is characterized by calculating several different anisotropic indexes and factors, such as universal anisotropic index (AU), shear anisotropic factors (A1, A2, A3), and percent anisotropy (AB and AG). The surface constructions of bulk and Young's moduli are illustrated to indicate the mechanical anisotropy. The hardness of NbxCy compounds is also discussed in this paper. The estimated hardness for all NbxCy compounds is less than 20 GPa. [ABSTRACT FROM AUTHOR]

Abstract As the main strengthening phases in high-chromium cast irons (HCCIs), the elastic and ductile-brittle properties of M 7 C 3 carbides are critical for the wear-resistance and application of HCCIs. The M 7 C 3 carbides are characterized to be Cr 3.87 Fe 3.04 C 3.09 and hexagonal system (P6 3 mc) in Fe-25.81 wt% Cr-4.45 wt% C alloy. Based on the elemental ratio and distribution, the crystals are built by a non-dilute ordered model. Mulialloying of Fe, Cr, W, Mo and B is adopted to design the mechanical properties of M 7 C 3 carbides. Results from first-principles calculations and nanoindentation show that the W + B and W + Mo doping can increase the ductility but not significantly decrease the mechanical modulus of Cr 4 Fe 3 C 3 , and Mo + B and Mo + W + B doping can improve the hardness of Cr 4 Fe 3 C 3 in HCCIs with finite decrease of ductility, which are all effective strategy to balance the ductility and strength of Cr 4 Fe 3 C 3 and enhance the wear-resistance of HCCIs. The relationship between intrinsic hardness (H V) and Pugh ratio (B / G) are fitted as H V = 29.4 GPa-7.6 GPa × B / G , from which the maximum H V and B / G are 29.4 GPa and 3.87 by multialloying strategy, respectively. The bulk, shear, Young's modulus and hardness are largest during 0.61–0.63 electrons/Å3 range of the effective density of valence electrons, while B / G and Poisson's ratio (σ) are smallest. Considering that M 7 C 3 carbide is rod-like monocrystal with strong orientation in HCCIs, the calculated and experimental Young's modulus from nanoindentation along non-[0001] direction is smaller than other directions, which provides guidance to achieve high wear-resistance of HCCIs by directional solidification. The elastic anisotropy is determined by the different atomic arrangement and chemical bonding along different crystallographic orientation. The decrease of mechanical modulus is attributed to the C-Mo and C-W bonds in M 7 C 3 multicomponent carbides weaker than C-Fe and C-Cr bonds in Cr 4 Fe 3 C 3. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]