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.

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In this work, fatigue crack growth measurements have been made on center-cracked tension specimens of Inconel 718, where the focus has been to observe the effect of high temperature hold times on the fatigue crack growth behaviour of the material. The material testing has been done at three different temperatures, namely 450 degrees C, 550 degrees C and 650 degrees C. All testing were done in an isothermal LCF context with a standard test method for measuring the fatigue crack growth rates