Intergranular crack tip oxidation in a Ni-base superalloy

• High-temperature intergranular crack tip oxidation under a single 600 s long sustained tensile load at 700 degrees 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 Fe-enriched 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) gamma' precipitates close to the grain boundaries were found to oxidize and form regions of near-stoichiometric NiO within the oxide layers. Remaining constituents of gamma' (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 the 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.

Subject headings

TEKNIK OCH TEKNOLOGIER  -- Materialteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Materials Engineering (hsv//eng)

Keyword

microstructure

growth-behavior

Secondary ion mass

Oxygen

alloy 718

fatigue

propagation

mechanisms

atom-probe tomography

high-temperature

embrittlement

Diffusion

Transmission electron microscopy

Atom probe tomography

nickel-based superalloy

Publication and Content Type

art (subject category)

ref (subject category)