Investigation of interfacial thermal stabilities of Nb_f/TiAl composites with and without Al₂O₃ coating in interface

Continuous Nb fiber-reinforced TiAl-matrix composites have tremendous potential for replacing brittle TiAl alloy. However, at elevated temperature, interfacial reaction between fiber and matrix is aggravated, causing instability of the interfacial microstructure and mechanical property of Nb_f/TiAl composite. Fortunately, the problem could be solved by introducing fiber coating. Consequently, in this study, Al₂O₃ coating was prepared on Nb fiber by cathodic plasma electrolytic deposition (CPED) technique. Concurrently, interfacial microstructure changes of the composites were explored during thermal treatment at 1100 and 1200 °C. The results show that, during heat treatment at 1100 °C for up to 100 h, for the composite without Al₂O₃ coating, the interfacial microstructure changes are as follows: Nb/σ/α₂/TiAl→Nb/σ/B2/α₂/TiAl→Nb/σ/TiAl, which leads to serious thermal residual stress and cracks in interface. Inversely, the introduction of Al₂O₃ coating can prevent the formation of brittle σ phase during preparation and the 1100 °C heat treatment. However, under 1200 °C heat treatment, the Al₂O₃ coating gradually fads away and be replaced by α₂-Ti₃Al phase. Finally, the interfacial phases including σ and α₂ are formed. The existence of the Al₂O₃ coating can delay the formations of the brittle phases and avoid cracks for a long time, which greatly improves interfacial thermal stability of the Nb_f/TiAl composite.