Microstructure, tensile properties and fracture behavior of high-Nb TiAl composites reinforced by nitrogen solid solution and in-situ boride

TiAl-based composites have broad application prospects in the aerospace and automotive fields due to their excellent high-temperature properties. In this study, the B/N synergistic effect on microstructural evolution, mechanical properties and fracture behavior was investigated by addition of nano-BN particles to Ti-46.8Al-8.0Nb alloy (at%). It was found that the B reacted with the Ti and Nb in the high-Nb TiAl matrix to form needle-shaped borides of TiB, TiB2 and NbB, which in turn reduced the lamellar colony size by inhibiting the movement of grain boundaries during the sintering process, while the N atoms diffused into the octahedral interspaces of the γ and α2 lattice. The lamellar colony size can be greatly reduced from 351 ± 144 μm to 44 ± 10 μm. The as-sintered microstructure can be gradually transferred from fully lamellar (FL) to dual phase (DP) with increment of nano-BN content. Composite containing 0.75 at% nano-BN exhibit the highest ultimate tensile strength at room temperature, reaching 660 ± 12 MPa. At high temperatures, the composite containing 1.50 at% nano-BN exhibits the highest ultimate tensile strength, reaching 517 ± 9 MPa. The mechanism of improved mechanical properties is attributed to grain refinement strengthening, thermal mismatch strengthening, Orowan strengthening, solid-solution strengthening.