Study Of Surface Defect Tolerance Of High And Low Strength Fully Lamellar ?-TiAl Based Alloys In Fatigue

Three fully lamellar y-TiAl alloys:Ti-44Al-8Nb-1B,Ti-44Al-4Nb-4Hf-0.2Si-1B and Ti-45.6Al-2.25Nb-1Cr-0.7Mn-0.15C-0.1Si (all in at.%, named alloy 8Nb, alloy 4Nb-4Hf and alloy 4)were exposed at 700? for up to 10000 hours. The changes in microstructure were characterized using scanning electron microscopy before and after thermal exposure. The fatigue limit of through-thickness single edge notched specimens with varied notch depth has been determined using a stepwise load increase method. The effect of notches on fatigue behaviour and the tolerance to surface defects have been investigated in the form of the Kitagawa-Takahashi diagram for the three alloys before and after thermal exposure.It has been found that the microstructural changes occurred in varied degree and extent in the three alloys:significantly more ?2 lamellae decomposed and more ?(B2+?) formed in fine-grained alloy 8Nb, prevalent and widespread formation of silicides and ?(B2+?) particles occurred in fine grained alloy 4Nb-4Hf. On the other hand, relatively small amount of ?2 decomposition and formation of ?(B2+?) were observed in coarse grained Al-rich alloy 4.Research shows that there exists a critical short notch size ac for the three alloys (106?m?140?m for alloy 8Nb; 100?m?400?m for alloy 4Nb-4Hf and 300?m?200?m for alloy 4 before and after thermal exposure, respectively). Above the critic notch size ac samples failed at or above a fatigue stress predicted by the long crack threshold based on linear elastic fracture mechanics; while below ac samples failed at a fatigue stress lower both than that determined by smooth samples and predicted by the long crack threshold based on linear elastic fracture mechanics. This phenomenon is called the short notch effect for the TiAl alloys examined. As a result, it is necessary to re-define the effective fatigue notch propagation threshold ?Keff.th and the effective transition notch size ao.eff.It has been found that the short notch effect is significantly increased for fine-grained high strength alloy 8Nb and alloy 4Nb-4Hf. This is because higher degree of exposure-induced embrittlement was produced after 10000 h exposure at 700?, which outweighs the beneficial outcome from long-term annealing in a warm-bath through stress dissipation and defects inactivation. On the other hand, the short notch effect is found to be less considerable in coarse-grained low strength alloy 4 after exposure. This is because the ?2 is relatively less in volume fraction and more uniformly distributed in the Al-rich alloy. The driving force to restore the equilibrium in alloy 4 is therefore less than that in two Al-lean alloys. Consequently, the beneficial effect of long-term annealing in the warm bath outweighs the detriment effect from exposure-induced microstructural embrittlement.