The Fatigue Behavior And Damage Tolerance Of A Thermally Exposed W, Nb-Containing γ-Tial Based Alloy

Single edge through thickness notch of different depth (0-800 μm) was introduced to the surface of high strength fine-grain alloy Ti-44.5Al-5Nb-0.85W-0.85B(at.%, referred to as alloy 5Nb-1W-1B) and moderate strength coarse-grain alloy Ti-46Al-5Nb-1W(at.%, referred to as alloy 5Nb-1W). The microstructure, fatigue strength and surface defect tolerance capability before and after thermal exposure were studied by using step-increasing fatigue load method. The exposure-induced fatigue strengthening, the fatigue threshold of long cracks and the surface defect tolerance under thermal exposure were quantitatively analysed and discussed in the form of Kitagawa-Takahashi diagram.It is found that the thermal exposure at 700℃ for 10000 h has a contradictory effect both on microstructure and fatigue behavior for the two alloys. The long time soak in the warm air environment could produce a steady state effect similar to the tempering, which induce the positive outcome of "thermal exposure enhancement". On the other hand, the decomposition of coarse α2 laths and the precipitation and coarsening of ω phase during thermal exposure could lead to a negative effect called "thermal exposure microstructural embrittlement". For fine-grain alloy 5Nb-1W-1B, the "thermal exposure microstructural embrittlement" effect is greater than coarse-grain alloy 5Nb-1W because of containing more α2 laths and β3(B2+ω) grain boundary segregation. Consequently, the negative effects outweigh the positive effects. So the fatigue strength reduced when thermal exposure time is less than 5000 hours; However, the fatigue strength increased when thermal exposure time is 10000 hours because the positive tempering effect outweighs the negative embrittlement effect with the prolonged exposure.It is also found that the harmless defect size in both alloys were smaller than 10 μm before and after the thermal exposure. The size range occurring unexpected reduction in fatigue strength is 4-300 μm and 7-500 μm for the fine-grain alloy and 4-200 μm and 4-250 μm for the coarse-grain alloy before and after the thermal exposure. The fatigue behavior of the notched alloys can neither be predicted by smooth sample S-N fatigue nor by long crack threshold based on linear elastic fracture mechanics. Instead, the notched alloys fail below the fatigue limit predicted both by smooth samples and by the linear elastic fracture mechanics. Such behavior is named "the short notch effect". For the sake of safety, the effective fatigue threshold △Keff.th and the transitional notch size ao.eff should be determined by construction of a K-T diagram following the tests from notched and smooth samples.The study shows that the short notch effect size is significantly increased after long time thermal exposure because of a higher degree of exposure-induced microstructure embrittlement(including oxygen release embrittlement and phase change embrittlement), which makes the TiAl alloy more sensitive to the short notches; The short notch effect is more significant in the fine grained alloy than in the coarse grained alloy after the same thermal exposure.