High Cycle Fatigue Behavior of Nickel-Base Superalloys at High Temperature

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  • 高温におけるニッケル基超耐熱合金の高サイクル疲れ挙動
  • コウオン ニ オケル ニッケルキ チョウタイネツ ゴウキン ノ コウサイクル ツカレ キョドウ

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High cycle fatigue testing has been conducted on wrought Ni-base superalloys M 252, Waspaloy, U 500 and U 700, which are widely used as turbine blade materials for jet engine and gas turbines. The results obtained are as follows.<BR>(1) At short lives up to 106 cycles, all alloys are the strongest at room temperature, whereas at long lives near 107 cycles, M 252, Waspaloy and U 500, U 700 are the strongest at about 600 and 800°C respectively.<BR>(2) The fractures show a cleavage like feature over a comparatively large range from the initiating site.<BR>(3) From the results of some experimental work, the following conclusions were obtained in regard to such an unusual temperature dependence and fractured surfaces:<BR>(i)\phantomii When the strain rate of flow stress is made to approach that of fatigue testing, the peak temperature of flow stress coincides with that of high cycle fatigue testing in each alloy and the peak temperature is determined primarily by the volume faction of γ′.<BR>(ii)\phantomi During high temperature fatigue testing of U 700, dislocation networks surrounding γ′ are formed partially, which are similar to dislocation substructures attained by thermomechanical processing, therefore they may be considered to take part in improving the high cycle fatigue strength at high temperature.<BR>(iii) Crack propagation rate in the cleavage-like fracture is much slower than that of the succeeding striation area and its mode is considered to be Stage-I fatigue mechanism. As the crack propagation mode and areas are approximately the same irrespective of alloys and temperatures, it is impossible to explain the longer lives at high temperature by the change of the crack propagation mechanism.

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