不同表面状态和热暴露对γ-TiAl合金疲劳性能的影响

Effects of varied surface conditions and thermal exposure on the fatigue properties of a γ-based titanium aluminide alloy

摘要:

本文研究了中等强度粗晶粒γ-TiAl合金Ti-46Al-5Nb-1W(at.%)热暴露前、后不同表面加工状态下的疲劳性能。研究发现，热暴露前虽然绝大多数样品在σmax<σ0.2的条件下进行疲劳，但是由于层片晶粒尺寸变化幅度大，容易在粗大的层片晶粒和软位向上，出现σmax>σ0.2的局部异常情况，导致早期屈服并诱发裂纹萌生。因此，表面加工质量的改善对S-N疲劳强度的有益影响明显小于细晶粒合金。研究还发现，热暴露中由于释氧和有序相相变引起的脆化效应并不明显，合金反而因样品在热浴中长期浸泡，导致残余应力释放和缺陷钝化而出现了疲劳性能增强的趋势。其增强的幅度和表面质量改善的程度相关。此外，该粗晶粒合金显示出较低的缺口敏感系数，在引入V型缺口后，其疲劳缺口敏感系数在热暴露前为0.27，热暴露后为0.32。

Abstract:

An intermediate strength coarse grain γ-TiAl based alloy Ti-46Al-5Nb-1W(at.%) has been studied in this paper to assess the fatigue properties under different surface conditions both before and after thermal exposure. It has been found that although the majority of specimens were fatigued under condition of σmax <σ0.2 before thermal exposure, pre-yield crack initiation can still occur in coarse lamellar grains, especially in soft orientation. Improvement of the surface quality in the coarse grain alloy for better fatigue performance is found to be less effective in the coarse grain alloys than in the fine grain alloys. It has been also found that the embrittlement effects owing to oxygen release and ordered phase formation caused by thermal exposure were not particularly considerable. On the contrary, the fatigue performance was enhanced because of a long-term immersion of these specimens within warm air, within which residual stresses were releases and defects become less harmful. The degree to which the fatigue performance was enhanced is controlled by the surface quality. The coarse alloy exhibits a relatively low fatigue notch sensitivity factor, which is only 0.27 before exposure and 0.32 after exposure for a V-notch.