Hardness and modulus of Fe2B, Fe3(C,B), and Fe23(C,B)6 borides and carboborides in the Fe-C-B system

Highlights: • Hardness and modulus of the Fe₂B, Fe₃(C,B), Fe₂₃(C,B)₆ and M₇C₃ phases are analyzed. • Hardness and modulus of the Fe₃(C,B) phase are increasing with its B content. • Size effects are apparent for hardness of Fe₂B, Fe₃(C,B), Fe₂₃(C,B)₆ and M₇C₃. • The model of Nix and Gao was used to evaluate the load-independent hardness H₀. • Hardness ranking is Fe₂B > M₇C₃ > Fe₃(C,B) > Fe₂₃(C,B)₆ - Abstract: This work provides a comparative and comprehensive study of the indentation hardness and indentation modulus of iron-rich borides and carboborides of types Fe₂B, Fe₃(C,B), and Fe₂₃(C,B)₆. In addition, the hardness and elastic modulus of Cr-rich M₇C are investigated for comparative purposes. We investigated the impact of increasing B content and indentation size effect (ISE). The phases of interest were stabilized in cast Fe-C-B alloys that varied with respect to the B / (B + C) ratio and heat treatment. The resulting microstructures were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and wavelength X-ray spectroscopy (WDS). Dynamic in-situ nanoindentation experiments based on the method of continuous stiffness measurement (CSM) were coupled to SEM and EBSD investigations to determine the mechanical properties of the individual borides and carboborides as a function of the indentation depth. The results were compared to values obtained for the Cr-rich M₇C₃ carbide. It was found that the hardness of the B-rich Fe₃(C_,B) phase is considerably higher than pure Fe₃C and increases with increasing B content. The ISE was present in all investigated phases, and the hardness decreased as a function of indentation depth. The hardness at infinite indentation depth H₀ was estimated according to the model of Nix and Gao. The Fe₂B phase was found to be the hardest phase (H₀ = 19.04 GPa), followed by M₇C₃ (H₀ = 16.43 GPa), Fe₃(C,B) (H₀ = 11.18 to 12.24 GPa), and Fe₂₃(C,B)₆ (H₀ = 10.39 GPa).