Oxidation and the Effects of High Temperature Exposures on Notched Fatigue Life of an Advanced Powder Metallurgy Disk Superalloy
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 TiO2-Cr2O3 external oxide formed with a branched Al2O3 internal subscale that extended into a recrystallized - dissolution layer. These surface changes can potentially impact disk durability, making layer growth rates important. Growth of the external scales and dissolution layers followed a cubic rate law, while Al2O3 subscales followed a parabolic rate law. Cr- rich M23C6 carbides at the grain boundaries dissolved to help sustain Cr2O3 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 m 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 M23C6 carbide dissolution are partially responsible for the intergranular cracking. Removing the scale and subscale while leaving a layer where M23C6 carbides were dissolved did not lead to a significant fatigue life improvement, however, also removing the M23C6 carbide dissolution layer led to nearly full recovery of life, with a transgranular initiation typical to that observed in unexposed specimens.
Early Trials • Mechanical fatigue of at high temperatures coated specimens sometimes promoted enhanced cracking of the coating : did thermal cycling play a role in this ?
Powder metallurgy disk superalloys have been designed for higher engine operating temperatures through improvement of their strength and creep resistance. Yet, increasing disk application temperatures to 704 C and higher could enhance oxidation and activate hot corrosion in harmful environments. Protective coatings could be necessary to mitigate such attack. Cylindrical coated specimens of disk superalloys LSHR and ME3 were subjected to thermal cycling to produce cyclic oxidation in air at a maximum temperature of 760 C. The effects of substrate roughness and coating thickness on coating integrity after cyclic oxidation were considered. Selected coated samples that had cyclic oxidation were then subjected to accelerated hot corrosion tests. This cyclic oxidation did not impair the coating’s resistance to subsequent hot corrosion pitting attack.
Oxidation of high-temperature alloys (superalloys) at elevated temperatures in air. II
In continuation of previous work, the oxidation behavior of three more alloys, namely UHBA 25L, Sanicro 28, and Inconel 690, was studied at high temperatures (600 to 1200°C). The oxidation kinetics of UHBA 25L and Sanicro 28 followed the parabolic-rate law at 800 and 1000°C. At 600°C oxidation rates were very low, while at 1200° C the parabolic-rate law was initially observed, followed by “breakaway.” Both alloys suffered extensive spalling at all temperatures, despite their high Cr contents of about 25% and 27%, respectively. This is atrributed to significant amounts of Mn present. Sanicro 28, with higher Cr, suffered an early breakaway at 1200°C because of the presence of Mo, which forms a low-melting oxide. Inconel 690 showed mixed behavior at 600 and 800°C, parabolic at 1000°C and cubic at 1200°C. Inconel 690 and Incoloy 800H gave the best overall performance for the range of temperature and exposure time under study. This may be ascribed to the absence of Mo, the presence of small amounts of Ti and Al, and relatively small amount of Mn in these alloys.