The full potential of engineering materials to resist fatigue damage and failure is never approached in current high temperature components and structures. Various microstructure and design related limitations contribute to the gap between the intrinsic fatigue potential of materials and the equivalent component behaviour. Uncertainties with respect to the service conditions and our inability to correctly predict service performance on the basis of laboratory test data, force the designer to use safety factors. The safety factors are applied to the lower bound of the scatterband which, by itself, results from the intra- and interheat variability of the microstructure as well as from a lack of tightness in the application of testing standards. The present paper, in dealing with the microstructural limitations of the fatigue resistance, focusses attention on the intrinsic fatigue potential of materials and on ways to optimise it.

Fatigue and Microstructure

Failure of metals II: Fatigue

Effect of carbides on the creep properties of a cast Ni-base superalloy M963 tested at 800 and 900 °C over a broad stress range has been investigated. Correlation between the carbides and creep properties of the alloy is enabled through scanning electron microscopy (SEM) and transmission electron microscopy (TEM). During high temperature creep tests, the primary MC carbide decomposes sluggishly and a large amount of secondary carbides precipitate. The cubic and acicular M 6 C carbide precipitates at the dendritic core region. Extremely fine chromium-rich M 23 C 6 carbide precipitates preferentially at grain boundaries. The M 6 C and M 23 C 6 carbides are found to be beneficial to the creep properties of the alloy. At lower temperature (800 °C), the interface of MC carbide with matrix is one of the principal sites for crack initiation. At higher temperature (900 °C), the oxidation and the precipitation of μ phase are the main factors for significant loss in creep strength of the alloy.

Effect of carbides on the creep properties of a Ni-base superalloy M963

The natural initiation and growth of short cracks in Inconel®718 U-notch specimens has been studied at 600 °C in air. U notches were introduced through broaching, and hardness traces and optical microscopy on cross-sections through the U notch broaching showed that the broaching process had introduced a deformed, work hardened layer. Fatigue tests were conducted under load control using a 1-1-1-1 trapezoidal waveform, on specimens with as-broached and polished U-notches. Multi-site crack initiation occurred in the notch root. Many of the cracks initiated at bulge-like features formed by volume expansion of oxidising (Nb,Ti)C particles. In unstressed samples, oxidation of (Nb,Ti)C particles occurred readily, producing characteristic surface eruptions. Scanning electron microscopy on metallographic sections revealed some sub-surface (Nb,Ti)C oxidation and localised matrix deformation around oxidised particles. A mechanism for crack initiation by carbide expansion during oxidation is discussed. Surface short crack growth rates in the notch root of polished specimens were measured using an acetate replica technique. Observed short-crack growth rates were approximately constant across a wide range of crack lengths. However, there was a transition to rapid, accelerating crack growth once cracks reached several hundred micrometers in length. This rapid propagation in the latter stages of the fatigue life was assisted by crack coalescence. Polishing the U-notch to remove broaching marks resulted in a pronounced increase in fatigue life.

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J. Reuchet

Papers

High temperature low cycle fatigue of MAR-M 509 superalloy I: The influence of temperature on the low cycle fatigue behaviour from 20 to 1100°C

High temperature low cycle fatigue of MAR-M 509 superalloy II: The influence of oxidation at high temperatures