Regulation in mechanical properties and structural morphology of M2B-type borides in Fe-B-C alloy

Abstract

Fe₂B in Fe-B-C alloy has the characteristics of inherent brittleness and continuous and coarse morphology. When stress is concentrated, cracks will be easily formed, resulting in failure of the alloy, which limits its application and development. In this paper, theoretical calculations and experiments are combined, Fe₂B is toughened by atomic doping firstly, then the morphology of Fe₂B is regulated by heterogeneous nucleation, and finally the service performance of the alloy is greatly improved. The results of first principle calculation show that compared to Mn, Co and Ni elements, Cr element exhibits the best toughening effect, which is because the addition of Cr weakens the directional covalent bonding of Fe₂B. Compared to the lattice mismatch between TiN, TiC, α-MnS and M₂B, the lattice mismatch between α-MnS and (110)M₂B is low and less than 6 %, and the interface energy of α-MnS(100)//M₂B(002) is greater than 0, indicating that α-MnS is an effective heterogeneous core of M₂B. After adding K₂SO₄ to Fe-B-C alloy, α-MnS is formed in the alloy. During the solidification process, M₂B grows around the effective heterocore α-MnS, forming isolated blocks in the alloy, and the shape factor of M₂B increases from 0.067 to 0.353. Meanwhile, the impact toughness of Fe-B-C alloy increases from 5.9 J cm⁻² to 14.2 J cm⁻², demonstrating the improving mechanical property of Fe-B-C alloy.