SUPPRESSION MECHANISM OF FATIGUE CRACK INITIATION OF NICKEL-BASE SURERALLOY AT ELEVATED TEPERATURE

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In the present study, the initiation and early propagation behavior of the small fatigue crack for Ni-base superalloy. Inconel 718, is investigated at the elevated temperature. The influence of temperature on the fatigue strength is examined and the results are discussed from the viewpoints of the softening of matrix and the surface oxidation of specimen at elevated temperature. It is concluded that the initiation and propagation mechanism of a small fatigue crack and the significance of the fatigue limit of a plain specimen are not influenced by the temperature. However, the initiation and propagation of the small fatigue crack is promoted at the elevated temperature owing to the softening of matrix and suppressed at the same time owing to the formation of oxide films on the surface of specimen, which effect is dominant will depend on the temperature and the stress level. As result, the fatigue limit of plain specimen is much higher at elevated temperature than at room temperature because the growth of small cracks in the length of 20-30μm, which occupies about 90% of the fatigue life of plain specimen, is greatly suppressed at elevated temperature.The effect of grinding on the fatigue strength is examined from the viewpoints of residual stress, surface roughness and surface hardening. The surface layer of CBN ground specimen is hardened through CBN grinding and a great compressive residual stress is generated in the layer. As the result, the fatigue strength is increased by CBN grinding.The fatigue strength of notched specimen and the influence of temperature on the notch sensitivity are investigated by using the 60 degree V-grooved specimen. The notch sensitivity of Inconel 718 is relative low when considering its high static strength, but it becomes a little notch sensitive at higher temperature.