Affects of applied stresses on the isothermal and cyclic high-temperature oxidation behavior of superalloys

An Evaluation of the Use of X-ray Residual Stress Determination as a Means of Characterizing Oxidation Damage of Nickel-Based, Cr2O3-Forming Superalloys Subjected to Various Oxidizing Conditions

“…However, under applied stresses, the alloys generally exhibited slower oxidation rates, which translated into lower k p -values (Table 1). In some cases, applied stresses have caused or were theorized to cause a reduction in the amount of oxidation due to a modification of diffusion properties, resulting in a faster formation of more protective oxides, and thus, a decrease in the less protective transient oxidation period [15,16,[42][43][44], wherein more rapidly growing and therefore less protective oxides form [29]. The modification of diffusion properties may be due to the applied stresses causing a greater generation of defects in the grains and grain boundaries of the alloys.…”

“…Since the oxidation process is immediately slowed by the formation of a protective oxide layer, the faster formation of the protective Cr-based layers in the stressed specimens would result in less continued oxidation over the relevant period as compared with the unstressed specimens, which would form a greater amount of less-protective, Ni-based oxides under the longer transient-oxidation periods. The effects of applied stresses and thermal cycling on the oxidation behavior of the two alloys are documented in greater detail elsewhere [43,44].…”

“…These images showcase the oxide layers as well as the slivers of oxide stretching downward from the oxide scales that make up the internal oxidation layers. Cross-sections of the two alloys under the other experimental conditions may be viewed in other sources [43,44]. Although an applied stress caused the number of internal oxidation slivers to increase in some cases [43], the average and maximum depth of the internal oxidation slivers usually decreased under an applied stress [43,44].…”

“…The 230-alloy specimens oxidized cyclically at 800°C (stressed vs. unstressed) were the most notable exceptions to these trends [43,44]. A comparison of the 75-alloy specimen oxidized isothermally at 800°C under an applied stress of 13.5 MPa with its 6.7 MPa-stressed counterpart revealed that a reduction in the amount of applied stress caused a decrease in both the average and maximum depth of internal oxidation penetrations [43,44]. The trends were more ambiguous under alloy-to-alloy comparisons, with the 230 alloy having a smaller average and maximum depth of the internal oxidation slivers in some cases, and the 75 alloy in others.…”

“…The trends were more ambiguous under alloy-to-alloy comparisons, with the 230 alloy having a smaller average and maximum depth of the internal oxidation slivers in some cases, and the 75 alloy in others. Internal oxidation penetrations for both alloys were found to be rich in aluminum and oxygen [43,44]. The measured compressive stresses in the substrates are also likely related to the shallow penetration depth of the X-rays employed.…”