Generation Single Crystal Superalloy Research Articles

New insights of the microcrack initiation induced by Mo addition in Ni-based single crystal superalloys during creep at 900 °C

Mo has received much attention in Ni-based single crystal superalloys as a main strengthening element at elevated temperatures above 1000 °C. However, the effects of Mo addition in new generation single crystal superalloys at medium temperature conditions are still not clear. This paper investigated the creep property at 900 °C and 392 MPa in a 4th generation Ni-based single crystal superalloy with different Mo contents, indicating that Mo addition decreased the creep life and induced large amounts of microcracks. Higher Mo content could weaken the interface strength by inducing more stacking faults, causing crack initiation along the interface. The elemental segregation along the stacking faults triggered the nucleation of other precipitates, which could further lead to cracks. The precipitation of topologically close-packed (TCP) phases introduced by more Mo addition caused crack initiation along the interfaces between the TCP phases and γ′ phase. By investigating the microcrack formation induced by Mo during creep at 900 °C, we provided a new perspective for the alloy design.

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.

Microstructures Evolution of a Second Generation Single Crystal Superalloy after Long Term Aging at 980°C

The microstructure evolution of the second-generation single crystal superalloy DD412 aged at 980°C for 1000 h were investigated. The results show that needle-shaped topologically close packed (TCP) phases rich in Re and W were firstly found in the dendritic core region after aging at 980°C for 400h, while the size and amount of TCP phases increased gradually with aging time, which is due to the fact that high melt point elements Re and W segregated to the dendritic core region. With the increment of aging time, the size of γ′ phase increased and volume fraction of γ′ phase decreased. The γ′ phase in dendrite core and inter-dendrite region remained cuboidal basically, and the γ′ phase in second dendritic arm showed directional coarsening after aging for 800 h. The MC carbides in inter-dendritic region decomposed gradually and transformed to the M6C carbides partially during long term aging, while few fine blocky M6C carbides precipitated in dendritic core after aging for 400h. The different microstructure evolution in dendritic region is due to the dendrite segregation of alloying elements and elements diffusion during aging.

Study on Creep Anisotropy at 760°C of a Single Crystal Superalloy

In this paper, the creep properties of a second generation single crystal superalloy DD412 with [001], [011] and [111] orientation at 760°C were studied. The experimental results show that the creep properties of DD412 alloy with different orientations have significant anisotropy under the stress of 580MPa-620MPa at 760°C. The [001] and [111] oriented alloys have remarkably better creep performance than [011] oriented alloy. And the creep performance of [111] oriented alloy is superior to that of [001] oriented alloy under the condition of 760°C/620MPa, while the tendency is inverse under the condition of 760°C/580MPa. The dislocation configurations of the alloy with different orientations during creep process are significantly different, resulting in creep anisotropy. For [001] and [111] oriented alloy, there are large number of dislocations in the γ channel that intersect and form local high-density dislocation aggregation, which has a strong blocking effect on the movement of dislocations. For [011] oriented alloy, the <110>{111} dislocations intersect each other to form <112>{111} partial dislocations and stacking faults, cutting into the γ′ phase, which lead to rapid growth of creep deformation and the poor creep performance of [011] oriented alloy.

Application of Ni-based superalloy in aero turbine blade: A review

A superalloy is a high-performance alloy used for achieving stability and strength at elevated temperatures. Generally, a temperature of about 800°C is developed at the first stage of the aero turbine blade. So it is a challenge to choose a material that can withstand such high temperatures. Nowadays advanced generation single crystal Ni-based superalloys are being used for the fulfillment of high-temperature requirements in aero turbine components. But the continuous effort is being carried out to manufacture new materials that can withstand more and more temperatures generated at aero turbine because the high-temperature operation of the turbine is directly proportional to the efficiency of a turbine engine. The requirement of high-temperature application upgraded the Ni-based superalloy and different generations of alloys were prepared from time to time those were discussed in the paper. Recent developments of superalloys in the paper mainly include fifth and sixth generations of superalloys those incorporate Hf, Re, and Ru-like elements in significant amounts. A brief extension discussion has been placed on the types, processing, and hot deformation characteristics of Ni-based superalloys. VIM melting, single crystal growth, directional solidification, and powder metallurgy techniques for manufacturing Ni-based superalloys are discussed in detail. The strengthening mechanism and machinability behavior depict the characteristics of superalloys in a mechanical direction. Heat treatment and hot deformation behavior of Ni-based superalloy was precisely discussed in the context so as to correlate the strength of the material with the temperature change. Overall, the paper describes the origin, processing and characteristics of Nickel based superalloy in detail.

Process Optimization Method for Inhibiting TCP Precipitation in A Nickel-Based Single Crystal Superalloy with High Refractory Element Content

In order to study the optimization method of microstructure stability of high generation single crystal superalloys, the influence of manufacturing process on the microstructure stability of a single crystal superalloy containing high content of refractory elements was studied, and the corresponding process optimization method was obtained. The microstructure characteristics of the alloy at different stages were observed and the corresponding element distribution were tested. The microstructure and element distribution of the alloy at different stages of preparation were analyzed with the assistance of kinetic calculation. Results show that the element dendrite segregation is an important reason to reduce microstructural stability. Reducing the dendrite spacing in directional solidification and increasing element diffusion in solid solution heat treatment can reduce the segregation of refractory elements in the dendrite core. As a result, less TCP precipitates in the dendrite core, and thus improve microstructure stability. Based on analyzed results, the manufacturing process were optimized and proved to be effective.

Effect of coatings on microstructure and oxidation behavior of the Ni-based single crystal superalloys containing different Ru contents

The element Ru in single crystal superalloy usually plays an important role in inhibiting the mutual diffusion between coating and substate. The MCrAlY coating and PtAl coating were prepared on the fourth and third generation single crystal superalloy, and the isothermal oxidation behaviors of the two superalloys and the two coatings were investigated in static laboratary air at 1200 °C. The oxidation kinetic curves showed that the mass gain of the coated samples was less than the alloys without coatings, which meant the coatings showed more excellent oxidation resistance at high temperature, and the oxidation resistance of the two coatings was slightly enhanced by the addition of Ru during the oxidation tests. Meanwhile, the interdiffusion of elements, such as Al in coating and refractory elements in substrate, was hindered by the addition of ruthenium. The size of γ' phase was also inhibited. As a result, the formation of interdiffusion zone and the precipitation of topological-close packed phase were suppressed, resulting that the DD91 showed more stable microstructure at elevated temperature than the DD90 during the oxidation process. The advantage of Ru in the alloys with coatings will be discussed in detail as well as the protective performance of coatings.

The role of Mo on the creep deformation of a 4th generation Ni-based single crystal superalloy at 1100 °C

The effects of Mo content on the creep property of a 4th generation Ni-based single crystal superalloy were investigated systematically at high temperature and low stress. Under the condition of 1100 °C and 137 MPa, the performance of SC containing 2 wt% Mo and 3 wt% Mo was close, revealing that low Mo addition could meet the requirements of creep properties at 1100 °C and 137 MPa. However, the massive precipitation of TCP phases significantly degraded the creep performance when Mo content was more than 3 wt%. Mo addition also accelerated the broadening of the γ channel, decreasing the resistance of the dislocation slip. With regard to the positive impacts, Mo addition increased the magnitude of lattice misfit, leading to dense interfacial dislocation networks with high creep resistance. Mo also retarded the diffusion of alloying elements, thereby suppressing the formation of creep pores and the elemental inhomogeneity during creep. The comprehensive Mo-effect was summarized. Moreover, the optimum Mo content in the 4th generation single crystal superalloys was determined for the guidance of alloy design based on the creep results and comparison with other alloys.

Transverse Stress Rupture Properties of a Nickel-Base Superalloy Bicrystal

Effects of low angle boundaries (LABs) on the stress rupture properties of bicrystals of a nickel-based third generation single crystal superalloy at 1093 °C/158 MPa were investigated. The results show that the effect of LABs on the stress rupture elongation of the alloy is higher than that of the stress rupture life at 1093 °C/158 MPa. As the misorientation angle of the LABs reaches 9.0°, the stress rupture life of the alloy with LABs can still retain nearly 50% of that with LABs of 0° at 1093 °C/158 MPa; while the stress rupture elongation of the alloy with LABs drops obviously when the misorientation angle of the LABs is larger than 6.5°. The fracture surfaces of stress ruptured alloy with LABs of 0°~2.9° are characterized by dimple features, while those with LABs of 6.5°~12.3° all exhibit intergranular fracture features. Apparent dimple features can be observed at the intergranular fracture surface of the alloy with LABs of 6.5° and the elongation of it is high. However, obvious dendrite features can be observed at the intergranular fracture surfaces of the alloy with LABs of 7.6°~12.3° and the elongations of them are relatively low.

Effect of Annealing Treatment on the Recrystallization of a Single Crystal Superalloy

Recrystallization often occurs in the production of aero engine turbine blades of single crystal superalloys. It is believed that annealing treatment can reduce the recrystallization tendency, as some of the casting residual stress is released. To evaluate the annealing effect on recrystallization, tensile test is carried out in present study for as-cast specimens of a second generation single crystal superalloy to induce stress. Two different annealing treatments are applied to the prestrained specimens, followed by the solution treatment at 1300°C for 2h. In order to compare the effect of the annealing treatments, the specimens are investigated by x-ray Laue diffraction technique and metallographic observation. According to the comparison, the more effective annealing treatment is recommended.

Investigation on freckle formation of nickel-based single crystal superalloy specimens with suddenly reduced cross section

The formation and distribution of freckles in the third generation single crystal superalloy DD9 were investigated by a designed stepwise specimen with suddenly reduced cross section in this paper. The microstructural characteristics of freckles were analyzed by optical microscope(OM), scanning electron microscope(SEM) and electron back scattering diffraction(EBSD). Numerical simulation was used to analyze the thermal field and the shape of the mushy zone of the specimens during the directional solidification. The results show that freckles are composed of disorderly orientated dendrite fragments and γ/γ′ eutectic. Freckles only appeared on the surface of the specimens and are prone to occur in the larger parts and edges of the specimens. The morphology of the isotherm has an important influence on the formation of freckles. The presence of lateral temperature gradient increases the instability of segregated liquid in the mushy zone near the shell mould wall and makes the freckles tend to form on the higher position of the isotherm. The reduction of the melt volume in the mushy zone combining with the decrease in the tilt of the isotherm suppress the formation of freckles. Based on the experimental results and simulation results, a modified Rayleigh-number based criterion for freckle formation is proposed. • A stepwise casting was designed to thoroughly study the formation of freckles. • Freckles are prone to occur in the higher position of the isotherms. • Lateral heat dissipation has an important effect on the formation of freckles. • Flatten the isotherm and decrease the melt volume can suppress freckle formation. • A modified Rayleigh-number based criterion for freckle formation was proposed.

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.

Microstructure evolution and interdiffusion of PtAl coated a third generation single crystal superalloy during thermal exposure

Microstructure evolution and elemental interdiffusion of PtAl coated third generation single crystal superalloy at 1000 °C and 1100 °C have been investigated. Interdiffusion coefficients and elemental diffusion fluxes are determined by HitDIC software based on the numerical inverse method. The interdiffusion zone (IDZ) with σ and R topologically close-packed (TCP) phases and secondary reaction zone (SRZ) with σ and P phases are formed after exposure at 1000 °C and 1100 °C. Interdiffusion flux of Al is the largest, followed by Pt, Ni, Co and Ta. However, most of the inward diffused Al is absorbed by IDZ, and the enrichment of Ta in IDZ/SRZ primarily accounts for the formation of SRZ. The increasing outward diffusion of refractory elements, such as Re, W and Ta mainly leads to larger thickness of SRZ at 1100 °C. Moreover increasing interdiffusion of Al and Ni further promotes the growth of SRZ and transformation from β-NiAl to γ′-Ni3Al phases. The content of reserved Al in the β layer is the same after thermal exposure at both temperatures, due to the effect of Pt.

In Situ Investigation of TCP Phase Formation, Stress Relaxation and γ/γ′ Lattice Misfit Evolution in Fourth Generation Single Crystal Ni-Base Superalloys by X-Ray High Temperature Diffraction

In nickel-based superalloys, the lattice misfit between the γ and γ′ phases and the propensity to TCP phase formation at service temperatures critically influence the microstructural evolution that takes place and hence the resultant mechanical properties. In this work, the lattice misfits of a series of highly alloyed Ru-containing 4th generation Ni-base superalloys are investigated by in situ X-ray diffraction (XRD) at high temperature. While the lattice misfit values of all alloys range between − 0.3 and − 1.3 pct at room temperature, they show an atypical temperature dependence, becoming less negative above 900 °C. In situ XRD measurements at 1100 °C reveal that the majority of the internal coherency stresses are already relieved after two hours. This is particularly pronounced for the alloys that have both a lattice misfit larger than |0.6| pct at 1100 °C and are prone to TCP phase formation. However, throughout the relaxation of the internal coherency stresses the lattice misfit remains fairly constant. Due to the similar chemical compositions of the alloys studied, qualitative guidelines for an optimum lattice misfit magnitude are developed by comparing the lattice misfit values with previous creep experiments. Our results indicate that no universal optimal lattice misfit value exists for these alloys and the value strongly depends on the applied creep conditions.Graphical

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.

On the role of topological inversion and dislocation structures during tertiary creep at elevated temperatures for a Ni-based single crystal superalloy

Abstract Creep behaviors of a 3rd generation single crystal superalloy DD33 at 1100 °C/1120 °C/1150 °C and 120 MPa have been characterized. The role of topological inversion and dislocation structures during tertiary creep have been studied. It is found that the tertiary duration is inversely related to the creep temperature. The higher the temperature is, the faster the tertiary creep rate rises and the higher the maximum rate is. The topological inversion level also shows a negative correlation with the temperature and the γ/γʹ inversion cannot account for the rapid increase in the tertiary rate. The formation of γʹ-junctions and the diffusion of γ/γʹ-forming elements result in the phase inversion, which strongly depends on the tertiary duration. Moreover, with the temperature increases from 1100 °C to 1150 °C, the main mobile dislocations are the long straight a superdislocations, the individual screw a superdislocations and the interface-deposited 60° dislocations, respectively. The factors affecting the tertiary creep rate have been analyzed in detail.

Microstructure and mechanical properties of a new nickel-based single crystal superalloy

To obtain higher comprehensive mechanical properties while avoid the adverse effects of Re element on the microstructure and economy of the alloy, a Re-free nickel-based single crystal superalloy is developed based on the Rene N5 single crystal superalloy. The microstructure evolution, compression properties, tensile properties, deformation mechanism and fracture behaviour of the self-designed alloy are investigated. The experimental results show that after the complete heat treatment, cuboidal γ′ precipitates with high degree of regular morphology are formed. The size of γ′ precipitates is about 0.49μm, and the volume fraction is approximately 61%, which are better than those for a conventional second generation single crystal superalloy. The alloy reaches its maximum compressive and tensile strength at 760°C and shows excellent microstructure thermal stability and improved mechanical properties. The alloy has an obvious strain hardening stage when the temperature is lower than 760°C, while the softening stage is not obvious. When the temperature reaches 850°C, the stress–strain curve of the sample mainly shows that the variable stress increases abruptly at the beginning and then decreases immediately. The tensile fracture of a single crystal at medium and low temperature is characterized by a slip fracture with a certain plastic deformation. A high temperature tensile fracture is characterized by typical plastic deformation, and its mechanism is a creep fracture with a uniform deformation.

Transverse stress rupture properties of a third generation single crystal superalloy at medium and elevated temperatures

Transverse stress rupture properties of a third generation single crystal superalloy at medium and elevated temperatures

Creep-rupture behavior of a nickel-based single crystal superalloy

A first generation single crystal superalloy was employed to research the high temperature creep behavior. Scanning electron microscope (SEM), transmission electron microscope (TEM) are used. The main results are summarized below: There are two mechanisms for the steady creep. The first one is superlattice dislocation cutting γ′ precipitates, which is the main mechanism for 1000°C creep behavior. In this case, the creep rate increases when the γ/γ′ microstructure undergoes topological inversion, which leads to a rapid increase in the density of γ′ cutting dislocation. The second mechanism is transverse climb of a/2<110> interfacial dislocation, which is the main mechanism for 1100°C creep behavior. In this case, climb of interfacial dislocations produced most of the steady creep strain. A by-product is the vacancies diffusing along the interfaces and low angle boundary to pores.