Fourth-generation Single-crystal Superalloy Research Articles

Effect of temperature on the tensile deformation mechanisms of a fourth nickel-based single crystal superalloys

This work conducted tensile experiments from room temperature to 1100 °C on a new fourth-generation single crystal superalloys along [001]. The effect of temperature on the tensile properties and deformation mechanism of the alloy were investigated. Results showed that the yield strength of the alloy reached its peak value (1017 MPa) at 850 °C. From room temperature to 760 °C, the occurrence of stacking faults (SFs) was observed in the γ matrix channel. From the room temperature to 850 °C, the occurrence of stacking faults was observed in the γʹ phase. The interface dislocation networks appeared at 980 °C and above. At room temperature, stacking faults sheared γʹ phase, in which three types of stacking faults occurred, which controlled deformation process. The main deformation mechanism from 650 °C to 850 °C was the shearing of γʹ phase by stacking faults and super dislocations, and at 850 °C, stacking faults in different directions of propagation corresponded to the formation of Lomer-Cottrell locks (L-C locks), which greatly increased the yield strength. At 980 °C and 1100 °C, dislocation climbing and bypassing and anti-phase boundary (APB) coupled dislocation pair shear γʹ phase became the main deformation mechanism.

Effect of coatings on oxidation behavior of a fourth generation Ni-based single crystal superalloys at ultra high temperature

PtAl and MCrAlY coating were prepared on the fourth generation single crystal superalloy. The isothermal oxidation tests of superalloys with different coatings were performed at 1150 °C and 1200 °C in a muffle furnace with static air for 500 h. The isothermal oxidation behavior and mechanism of two coatings at 1150 °C and 1200 °C was investigated. MCrAlY coating improved oxidation resistance by forming adhesive oxide scales with pegs, and PtAl coating formed dense and coherent oxide scales on surface to hinder the diffusion of oxygen. Compared with two coatings, a smaller oxidize increasing weight of PtAl coating (0.54 mg/cm2) than MCrAlY coating (2.48 mg/cm2) indicated that PtAl coating presented a better oxidation resistance during oxidation test at ultra high temperature, although a significant interdiffusion behavior occurred between PtAl coating and substrate. Besides, the interactive behavior between Ru and coatings was also investigated. Ru in fourth generation single crystal superalloy could diffuse into β-NiAl phase of coatings, maintaining the integrity of coatings. The major difference between MCrAlY coating and PtAl coating came in the amounts of β-NiAl phase, which leading to the difference on the oxidation resistance between MCrAlY and PtAl. The mechanism about influence of temperature on the oxidation property on bare alloy, MCrAlY-DD91 and PtAl-DD91 was also discussed. Ultra high temperature would promote more elements to participate in the reaction with oxygen, change the compositions of oxide scale and phases, resulting to the spallation of oxide scale and decrease of oxidation resistance.

Dependence of deformation mechanisms on micro-pores in the fourth-generation single crystal superalloy during out-of-phase thermal-mechanical fatigue

The high-temperature and long-term solution heat treatments of fourth-generation single crystal (SX) superalloys have sparked an increasing number of homogenization pores in the alloy. Out-of-phase (OP) thermal–mechanical fatigue (TMF) experiments were conducted to elucidate the effect of pores on the fatigue behaviors and deformation mechanisms. The results showed that the plastic deformation was mainly concentrated upon γ matrix in regions away from pores, more specifically, intrinsic stacking fault (ISF) and extrinsic stacking fault (ESF) as well as dislocation cross-slipping were detected. In areas near micro-pores, however, the γ/γ′-structure was subjected to severe twinning shearing. High-resolution observation illustrated that the amount of ESF and a/6 〈112〉 twinning dislocations had remarkably increased. Besides, the partial dislocations were more prone to move in different slipping systems at these regions with considerable stress concentration. These factors collectively enabled the formation of deformation twins from the edge of micro-pores. Moreover, the twinning nucleation could occur near both H-pores and S-pores, while the increasing pore size would enhance the nucleation and propagation of pore-induced twins by a large margin. These findings reported have provided important guidance for the future application of the fourth-generation SX superalloys.

Effect of Ru in the underlying superalloy on isothermal oxidation performance of Pt-Al coating

Pt modified aluminide (Pt-Al) coatings were prepared on the third- and fourth-generation single crystal superalloys by electroplating Pt and successive aluminization. The isothermal oxidation behaviors of both coatings were investigated at 1100 ℃ for 350 h. The results showed the Pt-Al coating on the fourth-generation single crystal superalloy had superior oxidation resistance. Ru in the fourth-generation single crystal superalloy diffused into Pt-Al coating during isothermal oxidation and increased spallation resistance of oxide scale. The effects of Ru on the anti-spallation property of oxide scale and the interdiffusion between coating and substrate were discussed. The diffusion process of Ru was investigated.

Study on the relationship between TCP phase and AlN phase in a fourth-generation single-crystal superalloy

Study on the relationship between TCP phase and AlN phase in a fourth-generation single-crystal superalloy

Stress rupture anisotropy of a Ru-containing fourth-generation single crystal superalloy at 760 °C and 1100 °C

The stress rupture anisotropy of a Ru-containing fourth generation single crystal superalloy was studied at two distinct conditions of 760 °C/810 MPa and 1100 °C/165 MPa. The ruptured microstructure and deformation mechanisms of [011]- and [111]-oriented specimens are systematically investigated, and the determinants of anisotropic performance, including crystal orientation and work-hardening effect, are comprehensively examined. The results indicate that this experimental alloy exhibits strong anisotropy at the intermediate temperature and high stress because of the high dependence of deformation on Schmid factors. The low critical shearing stress for <112>{111} slip systems and limited stacking faults interaction propel the formation of continuous stacking faults in [011]-oriented specimens, thereby resulting in its worse stress rupture life at 760 °C/810 MPa. While the high critical stress for <112>{111} slip systems and abundant stacking faults interaction in γ channels, which consequently contributes to the absence of superlattice stacking faults in γ′ phase, could be used to explain the superior stress rupture life of the [111]-oriented specimens at 760 °C/810 MPa. At high temperature and low stress, the anisotropy disparity of the experimental alloy is decreased as the deformation mechanism transforms into <110>{111} slipping and climbing. Moreover, the effects of Ru on anisotropic properties are identified by comparing it with other Ru-free SX superalloys. That is, the addition of Ru could lower stacking faults energy and encourage the formation of stacking faults in γ channels, which would reduce the stress rupture anisotropy, especially at intermediate temperature and high stress.

Oxidation Performance and Interdiffusion Behaviour of two MCrAlY Coatings on a Fourth-Generation Single-Crystal Superalloy

Oxidation Performance and Interdiffusion Behaviour of two MCrAlY Coatings on a Fourth-Generation Single-Crystal Superalloy

Co content dependence on the microstructure characteristic and creep performance at elevated temperature in Ru-containing single crystal superalloys

The composition design and optimization of Ru-containing fourth generation single crystal superalloys, which are currently considered as the most advanced and practical materials for manufacturing high-pressure turbine blades of aero-engines, have been in increasing and urgent demands. In this work, significant adjustments to the pivotal alloying element Co were made and the synthetic effect of Co on the microstructure as well as high-temperature creep property in Ru-containing alloys were systematically evaluated. The results illustrated that the size of γ′ phase was inversely proportional to the Co content but the volume fraction of γ′ precipitates stayed the same in different alloys. The TCP phases were more likely to precipitate in alloys with lower or extremely high Co addition during long-term aging, while the morphology of these particles transformed from long needle-like to short rod-like or blocky as Co content increased. It was noted that the element diffusion as well as the resultant coarsening and rafting of γ/γ′ structures were favorably facilitated by the increasing Co addition. The alloy with 12 wt% Co exhibited the longest stress rupture life at 1140 °C/137 MPa, regardless of the moderate precipitation of blocky TCP particles during creep. Moreover, the TCP phases in different alloys were analyzed to be typical μ phase but the specific element distributions varied with Co addition. Moreover, plenty of 2nd γ′ precipitates were observed after creep rupture, while their nucleation and precipitation could be affected by the Co addition in alloys. Ultimately, the comprehensive Co-effect was discussed and the optimum Co content was determined, in order to provide further guidance in the design and development of Ru-containing single crystal superalloys.

Oxidation behaviour of NiCoCrAlYHfZr coating on a fourth generation single crystal superalloy

The NiCoCrAlY coating and NiCoCrAlYHfZr coating were prepared on a fourth generation single crystal superalloy by arc ion plating. The cyclic and isothermal oxidation behaviours of the two coatings were both investigated in static laboratory air. Numerous large voids were formed in the oxide scale of NiCoCrAlY coating and they were responsible for the scale spallation. While the NiCoCrAlYHfZr coating showed better oxidation performance due to the enhanced scale adhesion. The advantage of Hf and Zr addition in the coating were discussed as well as the mutual diffusion behaviour between the coating and the substrate.

Investigation on solidification path of Ni-based single crystal superalloys with different Ru contents

The solidification path of Ni-based single crystal superalloy with different Ru contents (0–4wt%) was systematically investigated under planar and dendrite solid-liquid interface solidification conditions, aiming to reveal the evolution rule of solidification microstructure and selection mechanism of phases of the fourth generation single crystal superalloys. The results indicated that under plane solidification interface, the fine γ/γ′ eutectic structure started to form after the solidification of γ phase and then further developed the coarse γ/γ′ structure. The solidification finally terminated with the formation of β-NiAl phase enveloped by coarse γ′ phase for Ru-free superalloy. However, when Ru was added in superalloy, the β-NiAl phase was firstly precipitated after the solidification of γ phase, and no fine γ/γ′ structure connecting the γ phase with β-NiAl phase was found. Under the dendrite solid-liquid interface condition, the solidification path was consistent with that in the plane one. The evolution mechanism of solidification path with different Ru contents was discussed, which was helpful to optimize the alloy composition design.

Optimization of the homogenization and hot isostatic pressing heat treatments of a fourth generation single crystal superalloy

Various solution heat treatment sequences including a hot isostatic pressing step were tested on the MC-NG fourth generation single crystal superalloy with the aim to minimize the total duration of the treatment while obtaining a satisfactory level of microstructural homogeneity and low amount and size of pores. A procedure including the HIP step after full solutioning of the material allows to avoid reopening of the pores but needs a short post-HIP heat treatment followed by air quenching to get the desired γ′ precipitate size. The total duration of the solution heat treatment has been reduced from 18 to 6 hours as compared with the previous standard procedure.