Oxygen-induced intergranular fracture of the nickel-base alloy IN718 during mechanical loading at high temperatures

Abstract

There is a transition in the mechanical-failure behavior of nickel-base superalloys from ductile transgranular crack propagation to time-dependent intergranular fracture when the temperature exceeds about 600 °C. This transition is due to oxygen diffusion into the stress field ahead of the crack tip sufficient to cause brittle decohesion of the grain boundaries. Since very high cracking rates were observed during fixed-displacement loading of IN718, it is not very likely that grain boundary oxidation governs the grain-boundary-separation process, as has been proposed in several studies on the fatigue-damage behavior of the nickel-base superalloy IN718. Further studies on bicrystal and thermomechanically processed specimens of IN718 have shown that this kind of brittle fracture, which has been termed "dynamic embrittlement", depends strongly on the structure of the grain boundaries.

Microstructure and Cyclic Deformation Behavior of a Friction-Stir-Welded 7075 Al Alloy

“…The preliminary result would suggest that the intergranular cracking in the NZ was caused by the oxygen-related issues during FSW, while the intergranular decohesion was normally observed to occur in the stress-corrosion cracking in 7xxx-series aluminum alloys. [64] Krupp et al [65] also reported the oxygen-induced intergranular fracture in a nickel-base alloy IN718 during mechanical loading at high temperatures. Further studies of this aspect are needed.…”

Prediction of crack growth in a nickel-based superalloy under fatigue-oxidation conditions

“…Crack growth could be turned off and on in less than 10s by alternately pumping oxygen from the chamber and re-admitting it. Given the fact that oxidation kinetics for nickel alloys at 650C is extremely slow [25], there is insufficient time for oxide formation to happen ahead of the crack tip within the order of seconds. Direct observations of internal oxidation and associated damage were only available on smooth specimens, which have been exposed to oxidizing environment for a considerably long time, mostly greater than 20 hours [2,12,13,26].…”

“…Secondly, it was found that the cracking process is extremely sensitive to rapid changes in oxygen pressure, where fracture could be turned on and off in less than 10s by alternately pumping oxygen from the chamber and re-admitting it. Oxidation kinetics for nickel alloys at 650°C is extremely slow [25,26], and oxidation reaction does not have sufficient time to occur within the order of seconds. Dynamic embrittlement was introduced to define a time-dependent intergranular brittle cracking process in which the supply of embrittling elements is due to grain boundary diffusion into an area of increasing tensile stress ahead of a crack tip [19][20][21][22][23][24].…”

Environmental and Dwell Effects on the Damage Tolerance Properties of ATI 718Plus® Alloy

“…It has been reported that the main environmental effects of timedependent fatigue crack propagation in Ni-base superalloys is dynamic embrittlement and oxidation of grain boundaries and other microconstituents such as γ′ and grain boundary carbides, the latter two of which are stress-assisted mechanisms [12,13,14]. The dynamic embrittlement process, which is believed to be the governing mechanism for intergranular failure of Ni-base superalloys, involves short-range atomic oxygen diffusion (or other gaseous species) into the grain boundaries, which lead to grain boundary decohesion [14].…”

“…The dynamic embrittlement process, which is believed to be the governing mechanism for intergranular failure of Ni-base superalloys, involves short-range atomic oxygen diffusion (or other gaseous species) into the grain boundaries, which lead to grain boundary decohesion [14].…”