Formation of heterogeneous interfaces in the TiAl/Ti₂AlNb layered composite and its effect on the fracture toughness

TiAl/Ti₂AlNb intermetallic-intermetallic laminated (IIL) composites featuring brittle/ductile heterogeneous interfaces were fabricated through vacuum hot-pack rolling. The microstructures and the phase transformation behaviors of the interfaces of the IIL composites before and after annealing at 900 °C/6 h were investigated. The heterogeneous interfaces are composed of four distinct regions, individually I (β_o+γ+α₂), II (β_o/B2+ω) (brittle part), III (O lath), and IV (equiaxed O) (ductile part) regions from TiAl to Ti₂AlNb side. Notably, after annealing, an equiaxed O band approximately 50 μm wide was observed in region IV of the interface. In addition, a significant microhardness variation was observed between regions II and IV of the interface, where region II exhibited higher hardness compared to the TiAl alloy, and region IV displayed lower hardness than the Ti₂AlNb alloy. The enhanced fracture toughness of the IIL composites, three times that of the TiAl base alloy, is attributed to the formation of the brittle/ductile heterogeneous interfaces and the layered design incorporating the Ti₂AlNb alloy. The corresponding toughening mechanism was further discussed. The brittle II region plays a role in increasing crack branching, while the ductile IV region inhibits the propagation of microcracks and prevents the formation of main cracks. This work highlights the crucial role of the brittle/ductile heterogeneous interface in the toughening of laminated composites. Furthermore, the discovery of the O band provides novel insights into the design of TiAl/Ti₂AlNb heterostructures.