Revealing the novel fracture mechanism of the interfaces of TiB₂/Fe composite from a first principles investigation

We investigate the atomic structures, chemical bonding, stability and fracture mechanism of B- and Ti-terminated incoherent TiB₂ (0001)/Fe (111) and semi-coherent TiB₂ (0001)/Fe (100) interfaces using first-principles calculations. It is found that all Ti-terminated interfaces (Ti-HCP, Ti-MT and Ti-OT) as well as B-HCP type TiB₂ (0001)/Fe (100) interface are non-diffusive type. Meanwhile, B-HCP, B-MT and B-OT configurations of TiB₂ (0001)/Fe (111) interface are diffusive type due to the formation of additional Fe_xB intermetallic compound at the original Fe/TiB₂ interface. The calculated works of adhesion and interfacial energies indicate that Ti-HCP and B-HCP are the most stable structures for both incoherent and semi-coherent interfaces. We find that the magnitude of interfacial elastic energy is comparable to that of the chemical energy for the semi-coherent TiB₂ (0001)/Fe (100) interface. The electronic structures of TiB₂/Fe interfaces reveal the formation of Fe-B, Ti-B and Fe-Ti bonds at or next to the interface. Using Griffith's theory, it is predicted that the mechanical failure of TiB₂/Fe composite would initiate at the interface between TiB₂ and Fe. The first principles tensile experiment performed on all Ti-HCP interfaces agrees with the prediction. In the case of B-HCP interfaces, due to the formation of either Fe_xB diffusive layer or strong covalent Fe-B bonds, the mechanical failure eventually occurs in the Fe slab rather than that predicted by Griffith's theory. We also find the formation of diffusive Fe_xB layer could significantly suppress the local magnetic moment of Fe atom at TiB₂/Fe interface due the formation of strong covalent Fe-B bond.