Effect of broaching on high-temperature fatigue behavior in notched specimens of INCONEL 718
Notches were machined in specimens of INCONEL 718 by a broaching process, where differing broaching runs led to differing extents of subsurface deformation and surface roughness. Fatigue tests were carried out at 600 °C with a trapezoidal loading waveform at 0.25 Hz. The broaching process that led to the more severe subsurface deformation (but lower surface roughness) showed the worst fatigue performance. Analysis of total strain amplitude in the notch root with the aid of an elastoplastic finite-element (FE) model showed that the work hardening related to the subsurface deformation caused by the different broaching can account for the difference in fatigue lives. Differences in initiation and growth behavior were seen for the two broached finishes as well as for broached and subsequently polished samples. These differences are discussed in terms of a change in crack growth initiation and growth mechanisms due to the presence of the work-hardened layer.
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
Abstract Low cycle fatigue tests on cast cobalt superalloy MAR-M 509 were conducted at various temperatures from 20 to 1100 °C in laboratory air using sawtooth cycling under plastic strain control at a constant total strain rate. Below about 650°C the fatigue life was found to be independent of temperature for a given strain amplitude, whereas above 650°C the fatigue life was reduced drastically. The microstructure, the initiation and the propagation of fatigue cracks were studied using optical and electron microscopy. The change in fatigue life behaviour with increasing temperature is associated with a change in crack initiation mechanisms from crystallographic and MC carbide cracking at low temperatures to cracking along preferentially oxidized carbides at high temperatures. The influence of temperature on low cycle fatigue behaviour is discussed. Oxidation is thought to be the main cause of life reduction at high temperatures.
AN INVESTIGATION ON THE STRENGTHENING OF INCONEL 718 SUPERALLOY BY THERMAL MECHANICAL TREATMENTS
ABSTRACT The Ni-Fe base disk superalloy Inconel 718 under normal heat treatment and thermal mechanical treatment conditions has been studied in this investigation. It was found that the segregation of alloying elements and the detrimental effect of δ-Ni 3 Nb phase could be reduced notably after the present thermal mechanical treatments. The experimental results indicated that the tensile strength and fatigue properties as well as the ductility of the alloy were increased substantially with its stress rupture lives comparable to that under normal heat treatment condition. The improvement in these mechanical properties of the alloy after thermal mechanical treatment can be ascribed to the refinement of microstructures and the strengthening of grain boundary regions by dispersed granular δ-Ni 3 Nb particals, which, in turn, alternate the fracture process of the alloy at elevated temperature, i.e. a change from an intergranular cleavage rupture to a ductile “dimple” fracture.