High temperature fatigue of nickel-base superalloys - A review with special emphasis on deformation modes and oxidation

The cyclic stress-strain response and the low cycle fatigue life of conventionally heat treated Inconel 718 were studied. Fully reversed strain-controlled tests were performed at room temperature and at 823 K. Optical and electron microscopy were used to study the development of deformation and cracking during cycling. A power-law relationship between life-time and plastic strain amplitude was obtained. A substantial decrease in fatigue life occurred as the temperature was increased from 298 to 823 K and as the cycling frequency was lowered from 3 cyclesJmin to 0.3 cyclesJmin at 823 K. At 298 K, for all the strain amplitudes investigated, an initial rapid hardening was followed by softening, while at 823 K only softening occurred. Electron microscopy showed that the precipitates were sheared in the course of cyclic straining and that plastic deformation proceeded by the propagation of planar bands. These bands were identified as twins. Twinning was found to be more abundant at elevated temperatures than at room temperature, especially at lower frequencies. Cracking was generally initiated along the interfaces between these twin bands and the matrix but, at elevated temperatures and low strain rates, intercrystalline cracking took place, as well. The influence of particles shearing and twinning on the cyclic stress-strain response of the material are discussed. The importance of planar deformation and twinning on intergranular cracking is emphasized.

Low Cycle Fatigue Behavior of Inconel 718 at 298 K and 823 K

The ductility dip cracking susceptibility of two Ni-base filler metals, Filler Metal 52 (59%Ni–29%Cr–9%Fe–1%Mn–1%Al) and Filler Metal 82 (72%Ni–20%Cr–1%Fe–3%Mn–3%Nb) was evaluated using the strain-to-fracture test The analysis of these results and detailed weld metal microstructural characterization (SEM and TEM) was presented in Part I Part II of this work provides new insight into the creep-like, grain boundary sliding mechanism that leads to elevated temperature intergranular cracking in these weld deposits The effect of precipitation on grain boundary tortuosity and sliding, and its influence on ductility-dip cracking resistance is discussed in the context of current theories of high temperature creep Finally, the effect of composition, including both precipitate forming elements and impurity elements, on ductility-dip cracking is discussed

High temperature behavior of Ni-base weld metal: Part II – Insight into the mechanism for ductility dip cracking

Dislocation-precipitate interaction and cyclic stress-strain behavior of a γ′ strengthened superalloy

Microstructural processes of plastic instabilities in strengthened metals

The low cycle fatigue behavior of three wrought nickel-base alloys, alloy 901, Waspaloy and alloy 718, has been examined at both room temperature and 811 K (1000°F). The fatigue properties can be rationalized in terms of the Manson-Coffin equation relating total strain range with the number of cycles to failure. For a given total strain range the fatigue life of the alloys at 811 K (1000°F) is substantially less than at room temperature. Microstructural examination revealed that crack nucleation and initial propagation was of classical stage I type. The cracks develop at the surface of test specimens and propagate along slip planes. Evidence is presented to relate the observed fatigue softening with shear and dissolution of γ precipitates.

H. F. Merrick

Papers

The low cycle fatigue of three wrought nickel-base alloys