SEDL / STP / STP969-EB / STP33111S

Near-Threshold Crack Growth in Nickel-Base Superalloys

Van Stone, HR
Senior engineer and lead engineer, General Electric Co., Cincinnati, OH

Krueger, DD
Senior engineer and lead engineer, General Electric Co., Cincinnati, OH

Pages: 24    Published: Jan 1988

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Abstract

The influence of stress ratio on the elevated-temperature crack growth behavior of the nickel-base superalloys René 95 and Alloy 718 is discussed. The near-threshold and Region II crack growth behavior of a wide variety of microstructures for each alloy are explained using phenomenological descriptions of closure, that is, K-dependent or K-independent closure. K-dependent closure is the type of closure observed for nickel-base superalloys where increasing the R-ratio causes a uniform lateral shift of the entire fatigue crack growth curve. K-independent closure is the type commonly reported for steels and titanium alloys where increasing the R-ratio has a larger influence on near-threshold crack growth than on Region II crack growth. The existence of K-dependent closure in nickel-base superalloys results in microstructurally sensitive crack growth rates, even at high R-ratios. This permits the use of K-based crack growth prediction techniques and simplifies alloy and process development for applications having complex loading spectra.