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University of Birmingham The influence of stress on the oxidation of a ni-based superalloy

Double edge notched specimens of the Ni-based superalloy RR1000 have been subjected to 1-1-1-1 fatigue tests with R=0 at 750 o C in air for 111 hours at a peak elastic stress of 900 MPa. Extensive measurements of the thickness of the chromia layer formed during this exposure period show no influence of the applied stress. In contrast, the depth of the intergranular alumina penetrations underlying the chromia layer were approximately a factor of 2 larger in the most stressed regions (base of the notches) than in unstressed regions adjacent to the notches. This difference was highly significant and is considered to demonstrate Stress Aided Grain Boundary Oxidation (SAGBO). A model for this process has been developed in which the tensile stress at the tip of the alumina intrusion increased the anion vacancy concentration there and the vacancy flux along the intrusion. A consequence was that the intrusion growth rate increased above that in the absence of stress. The observed increase of a factor of 2 in intrusion length is estimated to be achievable at a tensile stress of ~1 GPa within the intrusion at its tip. This is considered to be plausible in such a high-strength alloy.