The microstructure evolution, mechanical properties, and tribological properties of high boron cast steel (HBCS) with various Mo concentrations are investigated. The results indicate that Mo addition can significantly modify the microstructure and enhance the comprehensive properties. With the increase of Mo concentration, borides change from the original fish-bone Fe-rich and Cr-rich M₂B to dendritic Fe-rich M₂B, blocky and cluster-like Cr-rich M₂B, and grainy Mo-rich M₂B. The hardness of HBCS increases gradually with the increase of Mo content due to the solid solution strengthening and the refinement of M₂B. It can be found that all the samples exhibit quasi-cleavage, but the impact toughness increases firstly and reaches the maximum value when the concentration of Mo is 2.10 wt.%, which is the result of the dispersive distribution of M₂B rather than the original fish-bone M₂B. Subsequently, the impact toughness begins to decrease as the concentration of Mo further increases because of the extensive formation of grainy Mo-rich M₂B at the grain boundary. Meanwhile, the wear results reveal that the average friction coefficient and wear ratio decrease with the increase of Mo content, and the wear mechanism changes from abrasive wear and adhesive wear to abrasive wear when the concentration of Mo exceeds 2.10 wt.%.

First-principles calculations were performed to study the structural, mechanical, electronic and magnetic properties of Fe_(8−x)Mo_xB₄ (x = 0, 0.25, 0.5, 1, 2, 3, 4, 5, 6, 7, 8). Results show that the Mo atoms would enter the Fe₂B lattice by replacing the Fe atoms and cause a change in the crystal structure. According to the calculated formation enthalpy, all borides are thermodynamically stable; moreover, Mo would reduce the stability of Fe₂B when the Mo atomic number is less than four in the unit cell. On the whole, with the increase of Mo content, the Young’s moduli and shear moduli decrease firstly and then increase, while the bulk moduli get bigger. Although the hardness of Fe₂B would decline with increasing Mo content, Mo is an excellent candidate for improving the ductility of Fe₂B on the basis of the G/B and Poisson’s ratio ν values. The charge density plots indicate that the integral strength of the linear boron chains in [002] orientation is reduced in Mo-alloyed Fe₂B. The toughness improvement of Fe₂B would be attributed to the evolutions of B–B bonds, Fe/Mo–B bonds and Fe/Mo–Fe bonds, which may induce the deflection of the Fe₂B cleavage plane. Besides, the magnetic moments of these borides monotonically decrease with the increase in Mo content.

First-principles calculations were performed to investigate the site preference of alloying elements, and the effect of alloying elements on stability, electronic structure and magnetism of Ni₃P-type Fe₃B. The calculated energies suggested that all studied compounds are thermodynamically stable while it is relatively difficult to synthesize the (Fe_2.875,Cu_0.125)B, (Fe_2.875,W_0.125)B and (Fe_2.875,Nb_0.125)B. The (Fe_2.875,W_0.125)B is the most stable compound from the view of cohesive energy. Mn element prefers to occupy the Fe2 site, whereas the others are more likely to reside in the Fe1 site. It can be found from the electronic structures that the DOSs of both Fe₃B and alloyed Fe₃B are dominated by Fe-d states, and all the compounds mainly contain Fe-B covalent bond, Fe-Fe covalent bond and Fe-Fe metallic bond. Based on the magnetic moments (Ms) results, it can be known that the Fe₃B, (Fe_2.875,Mn_0.125)B, (Fe_2.875,Co_0.125)B, (Fe_2.875,Ni_0.125)B and (Fe_2.875,Cu_0.125)B are ferromagnetic compounds, whereas the others are ferrimagnetic compounds. Only Mn and Co are able to enhance the magnetism of Fe₃B. Moreover, Mn is the most favorable candidate for improving the magnetic properties of Fe₃B among the alloying elements. These results can be used to guide the composition design and performance optimization of magnetic materials containing Fe₃B with Ni₃P-type structure.