热暴露对Ti-44Al-4Nb-4Zr-0.2Si-1B合金显微结构及力学性能的影响

Effect of thermal exposure on microstructure and mechanical properties of Ti-44Al-4Nb-4Zr-0.2Si-1B alloy

采用扫描电镜、透射电镜以及拉伸和高周疲劳等试验手段研究10 000 h、700℃大气热暴露过程对Ti-44Al-4Nb-4Zr-0.2Si-1B合金显微结构和力学性能的影响。结果表明:复合含Nb-Zr的TiAl合金的α2+γ层片组织显示出较高的热力学稳定性,合金的α2+γ层片晶团在热暴露过程中,α2→γ和α2+γ→B2(ω)相变并不普遍。合金中B2(ω)晶粒由若干微米胞构成,长期的高温热暴露导致胞壁处具有较高有序度的D88相结构的ω颗粒明显长大,且胞内也大量析出这种脆性的D88相结构的ω颗粒;同时,在B2(ω)晶内还析出微米级别的γ相,这是该合金10 000 h热暴露过程后脆性明显增加的主要原因。此外,该合金拉伸强度和条件屈服强度在热暴露过程中变化不大,室温疲劳极限甚至还有所提高。

The effect of thermal exposure at 700 ℃ in air for 10 000 h on the microstructure and mechanical properties of Ti-44AI-4Nb-4Zr-0.2Si-1B alloy was irivestigated using scanning electron microscopy and transmission electron microscopy and tensile and high cycle fatigue test. The results show that the decomposition of a2 lamellae through a2→y and the formation of B2（w） through a2＋7→B2（w） are not prevail inside the a2＋y colonies. This tends to suggest that the a2＋y lamellae are at a high level of thermodynamic stability. B2（w） grains consist of many micron-sized cells, in which w particles with D88 highly ordered structure in cell-wall regions grow significantly during the long-term exposure. Such D88-w particles also form widely inside the cells. Moreover, the micron-sized equiaxed y phase also precipitates from B2（w） grains. The tensile ductility in the exposed alloy reduces remarkably. While the long-term exposure does not cause detrimental effect on tensile strength and high cycle fatigue limit. The proof stress is still at level of larger than 600 MPa, while fatigue limit increases noticeably after 10 000 h exposure.