Study Of Surface Microcrack Initiation And Fatigue Strength For A Thermal-exposed γ-TiAl Based Alloy Ti-46Al-5Nb-1W

Fatigue specimens with four types of surfaces were assessed under three exposure conditions(no exposure, block exposure, individual exposure-oxidation at 700 ℃ for 10000 hours) to quality the effects of stress concentration, surface defects, oxidation and inner microstructural embrittlement on fatigue strength of an intermediate strength coarse grain y-TiAl based alloy Ti-46Al-5Nb-1W(at.%). At the same time, the oxidation behavior of this alloy subjected to an air exposure for up to 10000 hours at 700℃ has been investigated and analysed.It has been found that the loading condition of σmax>σ0.2 can occur locally in coarse lamellar grains, especially in soft orientations, which results in local plastic deformation and microcrack initiation even under a global loading condition of σmax<σ0.2. Improvement of the surface quality in the intermediate strength coarse grain alloy for better fatigue performance is found to be relatively limited and less effective than in the fine grain alloys.Long-term block exposure caused both positive thermal exposure enhancement and negative inner microstructural embrittlement for the coarse grain alloy. It has been found that the embrittlement effects owing to oxygen release and ordered phase formation caused by thermal exposure were not particularly considerable for the coarse grain alloy Ti-46Al-5Nb-1W. As a result, the positive exposure enhancement, coming from the relaxation of inner stress concentration, alleviation of segregation and passivation of inner defects and microcracks, outweighs the negative microstructural embrittlement.Besides the two contradictory effects of exposure, individual exposure-oxidation had a negative impact of oxidation on fatigue resistance. However, extra beneficial effect due to relaxation of surface tensile residual stresses can sustain the fatigue resistance for EDM surfaces, while extra detrimental effect duo to relaxation of surface compressive stresses reduces the fatigue resistance for shot peening surfaces. In addition, electropolishing rather than shot peening is more effective in retaining oxidation resistance when subjected to long-term exposure-oxidation.The notch root were always conducted under a loading condition of σmax>σ0.2, where plastic deformation may occur and lead to yield enhancement, due to the stress was higher than the yield strength, thus the notch sensitivity reduced.The loose mixed oxidation layer is mainly composed of TiO2 and Al2O3 formed during the exposure at 700 ℃. The oxidation layer coarsened gradually from granular to stick or long needle. The existence of oxidation layer can greatly weaken the fatigue strength of the improved surfaces, especially the shot peening specimens. The improvement of surface roughness through electropolishing is able to enhance the oxidation resistant effectively.