Microstructure and mechanical properties of heterogeneous double fibers reinforced TiAl composites

Nb fibers with excellent plastic, toughness and similar coefficient of thermal expansion with TiAl alloy have extremely advantageous in toughening TiAl alloy, which could be a potential structural material to satisfy the service requirements in space industry. But the unstable interfacial reaction between Nb fiber/TiAl matrix, and limited bearing capacity of reinforced fiber hinder a significant improvement in performance of Nb_f/TiAl composite. Therefore, in this paper, the ceramic coating was used to prevent the interfacial reaction between Nb fibers and TiAl matrix, and the high-strength Al₂O₃ fibers were introduced to offset the insufficient carrying capacity for further strengthening and toughening of Nb_f/TiAl composite. By combining the experiments and the finite element simulation, the formation mechanism of interfacial microstructure and collaborative toughening mechanism of (Nb + Al₂O₃)_f/TiAl composite were elucidated. The results show that the ceramic coating on the Nb fiber prevents the interfacial reaction, and repairs the interfacial defects between Nb fiber and TiAl matrix. The combination between Al₂O₃ fiber and TiAl matrix is strong, with no defects at the interface. The fracture toughness of the (coated-Nb + Al₂O₃)_f/TiAl composite reaches 20.87 MPa·m, which increased by 55 % than Nb_f/TiAl composite, and the improvement can be attributed to two reasons. For one thing, the coating on Nb fiber increases the interfacial bonding strength between Nb fiber and TiAl matrix, which effectively improves interfacial loading transfer capability, as well as promotes the crack deflection. The stress on coated-Nb fiber has enhanced by 75 % than before, and crack propagation length has obvious increase. The second reason is resulted from the Al₂O₃ fibers, which are the main bearing structure, and the stress is 1.8 times of the TiAl matrix.