The Ti-44Al-4Nb-4Hf-lB alloy was exposed at 700 ℃ in air for 10 000 h to assess the thermal stability of TiAl alloy containing Nb-Hf. The changes in microstructure were characterized using transmission electron microscopy and scanning electron microscopy. The corresponding changes in mechanical properties were examined by tensile and fatigue tests. The results show that both the decomposition of a2 lamellae through a2→a2+y and the formation of B2(w) through a2+y→B2(co) occur inside the a2+y lamellar colonies. After 10 000 h exposure, the average thickness of a2 lamellae is roughly halved while the area fraction of B2(w) in micron and submicron size range reaches 8.4%. As a result, the tensile ductility at room temperature reduces by one third after 10 000 h exposure at 700 ℃. The outcome indicates that TiAl alloy containing Nb-Hf demonstrates a higher thermal stability than its counterparts containing Nb due to reduced influence of the "oxygen-release induced embrittlement" and "B2+w-formation induced embrittlement". The long-term exposure does not cause detrimental effect on tensile strength and high cycle fatigue limit. After 10 000 h exposure, the proof stress is still at a level of 600 Mpa, while the fatigue limit increases noticeably.%对合金Ti-44Al-4Nb-4Hf-1B在700℃大气气氛中开展了长达10 000 h的热暴露处理,系统地探索和分析含Nb-Hf的TiAl合金的高温热稳定性,采用透射电镜和扫描电镜观察合金的显微组织变化并测试相应的力学性能.研究发现:长期大气高温热暴露导致合金中α2+γ层片晶团内的a2层片发生了一定程度的α2→γ相变:部分α2层片转变成为细小的γ层片,到10 000 h时,α2层片的原始厚度减少了约一半.长期大气高温热暴露也导致合金中α2+γ层片条束上发生了α2+γ→B2(ω)相变:条束上部分α2+γ消失,代之以微米及亚微米尺度的B2(ω)块状相.在10 000 h时,其面积分数达到8.4％,随着高温热暴露的逐渐进行,合金的室温塑性伸长率逐渐降低.在10 000h时,合金的塑性约为热暴露前的2/3,表明在复合含Nb+Hf的TiAl合金中,热暴露所导致的“释氧脆化”和“B2+ω生成脆化”的影响有限.复合含Nb+Hf的TiAl合金具有优于单纯含Nb的TiAl合金的高温热稳定性.长期高温热暴露对合金的断裂强度和条件屈服强度没有明显的有害影响.在10 000 h时,其条件屈服强度总体上仍保持在600MPa级别,而合金的室温疲劳极限还有所提高.
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