Tailoring the anisotropic mechanical properties of hexagonal M7X3 (M=Fe, Cr, W, Mo; X=C, B) by multialloying

As the main strengthening phases in high-chromium cast irons (HCCIs), the elastic and ductile-brittle properties of M7C3 carbides are critical for the wear-resistance and application of HCCIs. The M7C3 carbides are characterized to be Cr3.87Fe3.04C3.09 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 M7C3 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 Cr4Fe3C3, and Mo + B and Mo + W + B doping can improve the hardness of Cr4Fe3C3 in HCCIs with finite decrease of ductility, which are all effective strategy to balance the ductility and strength of Cr4Fe3C3 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 x 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/angstrom(3) range of the effective density of valence electrons, while B/G and Poisson's ratio (sigma) are smallest. Considering that M7C3 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 M7C3 multicomponent carbides weaker than C-Fe and C-Cr bonds in Cr4Fe3C3. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.