Intergranular crack tip oxidation in a Ni-base superalloy

Abstract

High temperature intergranular crack tip oxidation under a single 600 s long sustained tensile load at 700 °C was studied for the Ni-base superalloy Allvac 718Plus. High-resolution analytical techniques showed oxidation to take place at and immediately ahead of the tip of an open crack, forming a closed but layered oxide structure about the prior (now oxidized) grain boundary. Near the prior grain boundary the oxide is Ni-rich with a Co-enriched layer furthest away from the metal and a Feenriched region below this. A Cr-rich oxide is present below the outer Ni-rich oxides throughout the crack, also in the direction of crack growth. This is believed to have a hindering effect on oxidation ahead of the crack. Ni 3 (Nb,Al) γ' precipitates close to the grain boundaries were found to oxidize and form regions of near-stoichiometric NiO within the oxide layers. Remaining constituents of γ' (e.g. Al and Nb) were found to be enriched in the surrounding oxidized matrix and also to produce thin oxide layers near the interface between unoxidized metal and the Cr-rich oxide. The formation of the crack tip oxides is discussed with regard to thermodynamics, kinetics and the influence of applied mechanical load.

Mechanisms of dwell fatigue crack growth in an advanced nickel disc alloy RR1000

“…3). This finding is consistent with other researches' observations using more accurate characterisation techniques, like Atom Probe Tomography (APT) and Transmission Electron Microscope (TEM) [2,11], which also showed no further damage, such as oxide, oxygen or micro cracks, far ahead of the crack tips. These results prove that the depth of oxides/oxygen damage is very limited ahead of open crack front, and obviously much smaller than the damage zone measured by d.c.p.d.…”

“…In addition intergranular fracture surface morphology was observed [5,9]. However, only very limited oxide intrusions ahead of the dwell fatigue crack tip (<10 microns) are observed on 2D metallographic sections [2,11], which is contradictory to the long range diffusion argument. Although long range oxygen diffusion was suggested [5] the measurement technique adopted could not capture the uneven advance of intergranular cracks.…”

“…This long range oxygen/oxides damage therefore increased the baseline fatigue crack growth rate and changed the baseline fatigue crack growth mechanism from transgranular to intergranular. However, the long range diffusion theory has not been verified because no evidence of oxygen segregation far ahead of the crack tip has been observed using ATP [2] or TEM [11]. Instead, only very limited oxide intrusions were found ahead of the crack tip in these deliberately designed experiments, which suggests that long range oxygen diffusion and consequent grain boundary embrittlement does not exist.…”

“…Increased time dependent dwell crack growth can occur in oxidizing environments at temperatures in excess of 500 • C as a result of grain boundary oxidation and subsequent embrittlement during the prolonged dwell period at the peak load. This leads to a change in crack growth mechanism from transgranular to intergranular and an increase in crack growth rates [1][2][3][4][5].…”

Influence of overloads on dwell time fatigue crack growth in Inconel 718

“…Previous studies [20,21] have shown that complex oxides of Ni, Cr, and Fe, as well as oxides formed from niobium carbides, can be formed at the crack tip in Inconel 718. This conforms with our observations where it can be seen that the crack growth rate at 550 • C increased by a factor of 100 when a dwell time of 2160 s was applied at the maximum load, see Fig.…”