Micromechanisms of Low-Cycle Fatigue in Nickel-Based Superalloys at Elevated Temperatures

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Abstract
The micromechanisms of high-temperature fatigue crack initiation and crack propagation in solid-solution and precipitation-strengthened nickel-base super-alloys are reviewed. The marked decrease in fatigue strength of a given superalloy with increasing temperature cannot be solely correlated with the temperature dependence of the short-time mechanical properties. The interactions between oxidation rates and fatigue strengths are very complex. The air environment plays a large role in accelerating the initiation and propagation of fatigue cracks at elevated temperatures. Increasing temperature and decreasing frequency lead to a transition from transgranular to intergranular fracture path. The influence of different microstructural factors on the high-temperature low-cycle fatigue (LCF) behavior of a typical superalloy, Astroloy, was investigated in detail. A combination of fine 500 Å matrix γ′ in conjunction with a wavy grain boundary produced by both coarse intergranular M23 C6 carbides and primary γ′ is found to offer the best LCF performance in the range of 0.2 to 0.7 Tm. The fine γ′ retards Stage I cracking, and the wavy grain boundaries retard intergranular cracking. The observations made with Astroloy can be extrapolated to different γ/γ′ nickel-base superalloys.
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