Surface Defect Tolerance Of High, Intermediate And Low Strength γ-TiAl Based Alloys In Fatigue

Three y-TiAl alloys:Ti-44Al-4Nb-4Zr-0.2Si-1B (4Nb-4Zr, high strength), Ti-45A1-2Mn-2Nb-0.8vol%TiB2 (2Mn-2Nb, intermediate strength) and Ti-48Al-2Cr-2Nb-lB (2Cr-2Nb, low strength) were exposed at 700℃ for 10000 hours. The changes in micro structure were characterized using scanning electron microscopy before and after thermal exposure. The fatigue strength of thro ugh-thickness single edge notched specimens with different notch depths has been determined using a stepwise load increase method. The effect of notches on fatigue behaviour and the surface defect tolerance capability of the three alloys have been analyzed quantitatively in the form of the Kitagawa-Takahashi diagram before and after thermal exposure.The results show that some α2 phases which are not in thermodynamic equilibrium in the three y-TiAl alloys have decomposed into α2+γ phases and β(B2+co) phases after exposure at 700℃ for 10000 hours. This phenomenon is more significant in high strength alloy 4Nb-4Zr and least significant in low strength alloy 2Cr-2Nb.It has been found that there exists a critical short notch size ac for the three alloys (120μm,230μm for alloy 4Nb-4Zr; 260μm,420μm for alloy 2Mn-2Nb and 220μm,270μm for alloy 2Cr-2Nb before and after thermal exposure, respectively). Above the critic notch size ac, specimens failed at or above a fatigue stress predicted by the long crack propagation threshold based on linear elastic fracture mechanics; while below ac, specimens failed at a fatigue stress lower than that determined both by smooth specimens and by the long crack threshold. This phenomenon is called the short notch effect for the TiAl alloys. As a result, it is necessary to re-define the effective fatigue notch propagation threshold △Kth.eff and the effective transition notch size ao.eff.The research reveals that the short notch effect has increased variedly after long-term thermal exposure at 700℃ for 10000 hours for the three alloys. This is attributed to microstructure embrittlement occurred in long-term thermal exposure.The increase of notch sensibility caused by thermal exposure is found to be positively related to the yield strength. It is most harmful to the high strength alloy, less to the intermediate alloy and least to the low strength alloy. This is because an increased degree of "oxygen release embrittlement" and "transformation embrittlement" occurred in alloy 4Nb-4Zr containing lower percent Al and higher percent β-stabilizing element (8%), which significantly outweighs the beneficial effect coming from long-term annealing in a warm air bath. However, the increase in short notch effect is less significant in intermediate and low strength alloys which contain relatively less α2 and almost free of B2+co,producing a reduced degree of "oxygen release embrittlement" and "transformation embrittlement" while the same beneficial long-term annealing effect applies.