Abstract: Continuous alumina fibers are widely used in fields including aerospace, national defense, military industry and energy resources, due to their exceptional chemical stability, oxidation resistance, structural stability and excellent high-temperature mechanical properties, making them promising engineering fibers with significant research value and market prospects. The long-term service temperature of alumina fibers varies depending on their composition, structure and morphology. In this work, phase composition, microstructure and mechanical property evolution of two commercial fibers (i.e. Nextel^TM 440 and NITIVY ALF^TM), and one lab-synthesized fiber (i.e. SLF72) were investigated after long-term treatment at elevated temperatures. Characterization was performed using different methods. The influence of phase composition and microstructure on mechanical properties was discussed, and the underlying mechanisms were elucidated. Furthermore, the mechanical properties of the fibers after heat treatment at different temperatures were compared. The results indicate that all three fibers undergo transform to mullite, accompanied by a general decline in single-filament tensile strength with increasing temperature. The formation and growth of mullite phase are identified as the primary reasons for the strength degradation after heat treatment. Compared with Nextel^TM 440 and NITIVY ALF^TM, the lab-synthesized fiber SLF72 demonstrates superior mechanical properties after long-term exposure at elevated temperatures. And the long-term service temperature limits of the fibers follow the order: SLF72>NITIVY ALF^TM>Nextel^TM 440.

Key words: alumina fiber, long-term heat treatment, microstructure, single filament mechanical properties