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 FexB 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 FexB 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 FexB layer could significantly suppress the local magnetic moment of Fe atom at TiB2/Fe interface due the formation of strong covalent Fe-B bond.