Oxidation and the Effects of High Temperature Exposures on Notched Fatigue Life of an Advanced Powder Metallurgy Disk Superalloy

Abstract

Oxidation and the effects of high temperature exposures on notched fatigue life were considered for a powder metallurgy processed supersolvus heat-treated ME3 disk superalloy. The isothermal static oxidation response at 704 °C, 760 °C, and 815 °C was consistent with other chromia forming nickel-based superalloys: a TiO 2 -Cr 2 O 3 external oxide formed with a branched Al 2 O 3 internal subscale that extended into a recrystallized γ'-dissolution layer. These surface changes can potentially impact disk durability, making layer growth rates important. Growth of the external scales and γ'dissolution layers followed a cubic rate law, while Al 2 O 3 subscales followed a parabolic rate law. Cr-rich M 23 C 6 carbides at the grain boundaries dissolved to help sustain Cr 2 O 3 growth to depths about 12 times thicker than the scale.The effect of prior exposures was examined through notched low cycle fatigue tests performed to failure in air at 704 °C. Prior exposures led to pronounced debits of up to 99 % in fatigue life, where fatigue life decreased inversely with exposure time. Exposures that produced roughly equivalent 1 µm thick external scales at the various isotherms showed statistically equivalent fatigue lives, establishing that surface damage drives fatigue debit, not exposure temperature. Fractographic evaluation indicated the failure mode for the pre-exposed specimens involved surface crack initiations that shifted with exposure from predominately single intergranular initiations with transgranular propagation to multi-initiations from the cracked external oxide with intergranular propagation. Weakened grain boundaries at the surface resulting from the M 23 C 6 carbide dissolution are partially responsible for the intergranular cracking. Removing the scale and subscale while leaving a layer where M 23 C 6 carbides were dissolved did not lead to a significant fatigue life improvement, however, also removing the M 23 C 6 carbide dissolution layer led to nearly full recovery of life, with a transgranular initiation typical to that observed in unexposed specimens.

Oxidation of a Commercial Nickel-Based Superalloy under Static Loading

“…[10][11][12][13][14] Energy-dispersive x-ray analyses showed that the overall oxide morphology from the external scale to internal oxides of Cr 2 O 3 /TiO 2 in the outer scale to Al 2 O 3 internally agrees with the expected thermodynamic behavior; however, and in contrast to the thermodynamic predictions, the alloy formed an outer layer of TiO 2 , believed to be at the expense of its oxidation resistance. 14,15 Cationic diffusion is inferred from the known transport mechanisms for Cr 2 O 3 growth 16,17 and from the fact that titanium must diffuse outwards to form above the outer scale.…”

Effect of prior oxidation on high cycle fatigue performance of RR1000 and role of oxidation in fatigue crack initiation

“…[3][4][5][6] This is typical of Ni based superalloys used in rotor disc applications. 1,7,8,21 The alumina penetrations draw aluminium out of the c9 precipitates (Ni 3 Al) causing a c9 denuded zone to form ahead of the internally oxidised zone, this again penetrates into the alloy in two distinct penetrations (intergranular/intragranular) mirroring the internal oxide. 5,20 Previous studies on Udimet 720, ME3 and RR1000 have found that oxidation under cyclic fatigue or dwell fatigue conditions leads to an enhancement in the oxidation damage but the composition of the oxide was consistent with the un-stressed preexposed condition.…”

“…It could be the case that cracking of the intergranular internal oxide leads to a crack forming in both the external chromia scale and the sub-surface c/c9 alloy. Sudbrack et al 8 found that the carbide dissolution zone was the main cause of the high temperature fatigue life deficit in the Ni based superalloy, ME3, where weakened grain boundaries were thought to be the cause of initiation at high temperature. In the present case, the carbide dissolution zone is present appreciably beyond the crack and internal oxides ( Fig.…”

“…The reduction in fatigue life was found to be proportional to oxidation damage, with the thicker the external scale and deeper the internal damage the more pronounced the reduction in life. 8 The reduction in life was driven by M 23 C 6 carbide dissolution as the removal of the internally oxidised region did not lead to a complete recovery in fatigue life whereas the removal of the carbide dissolution zone did. Another study using similar prior exposures on ME3 and Udimet 720 found that the mean lives of pre-exposed specimens had up to a 70% reduction in life, as well as a change in crack initiation location from sub-surface (as received) to surface (pre-oxidised).…”

“…When these alloys oxidise they form a protective chromia external oxide scale along with significant internal oxidation of aluminium. [1][2][3][4][5][6][7][8][9] The internal oxides often form both intergranularly and intragranularly and are preceded by a weaker region depleted in c9 precipitates. [4][5][6] The role of environmental degradation in the mechanical performance of a component is important, since the formation of internal oxides is undesirable and may have a significant detrimental effect on the alloy.…”