Study Of Surface Microcrack Initiation And Fatigue Behaviour For A Thermal-Exposed Ti-44.5Al-5Nb-1W-1B Alloy

Fatigue specimens with four types of surface treatment process were assessed under three exposure conditions (no exposure, block exposure and individual exposure-oxidation at 700℃ for 10000 h) to quantify the effects of stress concentration, defects, oxidation and inner microstructural embrittlement on fatigue strength of a near lamellar y-TiAl alloy Ti-44.5Al-5Nb-1W-1B (at.%). Besides, the oxidation behavior of this alloy subjected to an air exposure for up to 10000 hours at 700℃ were investigated.It has been found that this alloy is very sensitive to the surface condition. Surface improvement leads to a significant increase in fatigue strength.The fatigue strength of the four surface conditions is in an order of electropolishing> shot peening> EDM> V notch.Long-term block exposure leads to two kinds of effect on the inner microstructure of this alloy. The relaxation of inner stress concentration, alleviation of segregation and passivation of inner defects and microcracks lead to the positive "thermal exposure enhancement" effect. However, the decomposition of coarse α2 laths and the precipitation and coarsening of ω phase lead to the negative "thermal exposure microstructural embrittlement" effect. For both the shot peened and electropolished surfaces, the negative effects outweigh the positive effects, demonstrating a reduced fatigue performance after block exposure in this alloy.Besides the two contradictory effects, individual exposure-oxidation of EDM specimens brings about additional relaxation of residual tensile stress and the passivation of surface defects and microcracks, which can reduce the sensitivity to cracking and therefore improve fatigue strength noticeably. On the other hand, individual exposure-oxidation of the shot peened specimens results in the introduced compressive stresses being released, which is detrimental to the fatigue properties of this alloy. Also found is that electropolishing rather than shot peening is more effective in retaining oxidation resistance when subjected to long-term exposure-oxidation.The oxide layer of this alloy formed during the exposure at 700℃ for 10000 h shows a loose structure and is mainly composed of TiO2 and Al2O3. Besides, the α2 laths at near-surface position dissolve during oxide formation, which causes internal oxidation and is detrimental to the mechanical properties of this alloy. The electropolishing specimens not only have a very smooth surface without defects and stress concentration, but also have passivation layer on the surface which would prevent oxidation reaction inside. Therefore, the electropolishing specimens are capable of resisting the deterioration of the fatigue properties during long-term thermal exposure.