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.

Influence of orientation-dependent grain boundary oxidation on fatigue cracking behaviour in an advanced Ni-based superalloy

“…Compared with the studies of isothermal oxidation/cyclic oxidation for the disc alloys, the oxidation under (cyclic) stress exhibits a greater discrepancy between grain boundary and grain interior [12,22]. The grain boundary oxidation is much more significant than that in the grain interior.…”

“…indicates that the GBs without bulged Co-rich oxide complex are decorated with a Ni/Ti/Alrich oxide complex. Additionally, it is found that the content of Al is lower than its original content either at GBs or within grains and the content of Cr is relatively higher than its original content within grains, this is believed to be associated with the formation of an internal alumina layer and an outer layer of chromium oxide as observed in Ni-based superalloys [12,18,22,[27][28][29]. However, it should be noted that point out that the elements in the regions nearby and underneath the GB oxides may also contribute to the composition of the GB oxides due to the relatively large electron beam size used for EDS analysis and the large activation volume.…”

“…thermodynamic unstable oxides such as CoO and NiO at the surface and thermodynamic stable oxides such as Cr 2 O 3 , TiO 2 and Al 2 O 3 at the subsurface. Such layered oxides are usually observed in stressed and unstressed disc alloys RR1000 and ME3 [12,18,20,28], so may also form in the LSHR alloy due to the similarity in alloying composition and the comparable/equivalent test conditions. At the beginning of the fatigue loading process at elevated temperature, non-selective oxidation can occur at the specimen surface, and thereby Ni and Co oxidise rapidly and form a continuous single-phase solid solution scale due to their abundance in the alloy and the same crystal structure of NiO and CoO [18,30].…”

“…These oxide complexes usually possess a layered structure which consists of a thermodynamically unstable central layer of NiO [3,16,18]. Moreover, study of isothermal oxidation and/or cyclic oxidation in disc alloys at elevated temperatures also shows similar oxide layers at surfaces which usually consist of an external Cr 2 O 3 layer and an internal Al 2 O 3 layer [12,22]. Formation of such oxides at/or ahead of the crack tip and/or at specimen surface mostly depends on the composition of the investigated disc alloys and environmental conditions such as O 2 partial pressure.…”

Use of Slow Strain Rate Tensile Testing to Assess the Ability of Several Superalloys to Resist Environmentally- Assisted Intergranular Cracking

“…These alloys were consolidated from powder atomized in argon, then extruded and isothermally forged into disks. Blanks were extracted from the rim of fully heat treated ME3 disks [14] and the bore of a fully heat treated Udimet 720 disk [15], which each had been previously studied. Rectangular blanks of LSHR were extracted and separately heat treated.…”