Generation Nickel-based Single Crystal Superalloy Research Articles

Creep properties of a low-cost 3rd generation nickel-based single crystal superalloy with orientation deviated from [001] direction

In this paper, the creep properties and deformation mechanisms of a low-cost 3rd generation nickel-based single crystal superalloy deviating 1.5°, 7.8°, and 14.7° from [001] direction were studied at 800 °C/760 MPa and 1100 °C/137 MPa. The microstructure after full heat treatment and creep rupture of specimens were observed by scanning electron microscopy (SEM). The dislocation configurations of specimens after creep rupture were observed by transmission electron microscopy (TEM). The results indicate that specimens with different orientation deviations exhibit strong anisotropy at 800 °C/760 MPa, and the creep lives of specimens decrease with the increase of θ angle. In specimens A (1.5°) and B (7.8°), the creep deformation is controlled by multiple <112>{111} slip systems, which would produce working hardening. Multi-directional stacking faults are apt to form stacking fault (SF) locks, which inhibit the slipping and cross-slip of dislocations and then increase creep life of the specimen. In specimen C (14.7°), the creep deformation is controlled by single <112>{111} slip system, unidirectional stacking faults cut into γ′ particles, causing rapid failure of the alloy. The creep anisotropy of specimens with different orientation deviations is insignificant at 1100 °C/137 MPa. The same N-type rafted structures and the dislocation networks with similar spacing form in three specimens, which have the same ability to hinder dislocations cutting into γ′ particles.

Study on the mechanism of γ' phase rafting in a 4th generation nickel-based single crystal superalloy during thermal exposure at high temperatures

Non-directional rafting of γ' phases in high generation nickel-based single crystal superalloys happens when the alloys are thermally exposed at high temperatures, but so far the reason remains unclear. The microstructures and internal stress fields of a 4th generation single crystal superalloy during thermal exposure at 1100 ℃ and 950 ℃ are investigated using transmission electron microscopy and finite element analysis. It is found that the non-directional rafting of γ' phases occurs after 1100 ℃/200 h and 950 ℃/500 h thermal exposures. The γ' phases are mainly rodlike shape in the space after thermal exposures and a small quantity of γ' rafts exist as well. The elastic strain energy gradient in γ matrix caused by the inhomogeneity of matrix channel width in the alloy is most likely the driving force for the non-directional rafting. Additionally, dislocations are observed only after the thermal exposure process at 1100 ℃. The effect of dislocations on non-directional rafting during thermal exposure is discussed in detail in the paper.

Influence of crystal orientations on the creep fracture of a nickel-based single crystal superalloy

Due to the anisotropy of the modulus of face-centered cubic cells, the nickel-based single crystal alloys exhibits substantial anisotropy at high temperatures. Their creep properties, especially the creep rupture lives, are related to the actuation of the slip system and the magnitude of the shear stress on the slip plane. The creep orientation sensitivity has been analyzed through creep experiments, as well as the microstructure and morphology of the fracture. On the macroscale, the necking sections of [001], [011] and [111] orientations are circular, elliptical and circular, respectively. Meanwhile, on the microscopic scale, the damage evolution caused by the expansion of the internal micropores in the single crystal alloy under the three orientations shows fourfold symmetry, double symmetry and triple symmetry, respectively. Based on the crystal plasticity theory, an anisotropic creep constitutive model and damage model are established to reflect the difference in creep deformation and damage evolution caused by orientation. The numerical simulations of the whole creep process of a second generation nickel-based single crystal superalloy show that the creep constitutive model and damage model can simulate the deformation and damage of the specimen under different orientations, and the creep life can be obtained.

Damage tolerance of low angle grain boundary in a fourth generation Nickel-based single crystal superalloy at 1100 °C service conditions

The effect of different low angle grain boundaries (LAGBs) on the microstructure and mechanical properties in a fourth generation Nickel-based single crystal superalloy was studied. Double seed crystals techniques were used to obtain the specimens with LAGBs. The results showed that there were coarse γ′ phases at LAGBs with the grain boundary angle increasing, the shape of which changed from block to long strip at as-cast and the size of which increased gradually after heat treatment. After thermal exposure at 1100 °C for 100 h, the microstructure stability deteriorated when the angle exceeded 12.6°, where the discontinuous precipitation (DP) colonies region occurred along the grain boundary because of high interfacial energy and mobility at LAGBs. With the grain boundary angle increasing, the ultimate tensile strength of the sample rose slightly and then fell, while the elongation and creep rupture life decreased significantly exceeded 4.9°. Under 1100 °C service condition, the carbides and topological close-packed (TCP) phases were precipitated at LAGBs, which seriously impaired the grain boundary bonding strength. On the other hand, the misorientation at LAGBs led to the stress concentration, which increased the difficulty of grain boundary coordinated deformation. Finally, the damage tolerance of the LAGBs in this fourth generation Nickel-based single crystal superalloy was approximately estimated as about 7° at 1100 °C/150 MPa creep rupture condition based on a 70% creep rupture life standard.

Evolution of phase interface microstructure and its effects on stress rupture property of a 4th generation nickel-based single crystal superalloy after thermal exposure

Lattice constants, phase interface microstructure and stress rupture properties of nickel-based single crystal superalloy after thermally exposed at 1100 °C for different time were investigated using synchrotron radiation diffraction and transmission electron microscopy techniques. The results show that the lattice constants of γ and γ' phase at room temperature decrease and the misfit at room temperature becomes more negative after 200 h exposure, and these parameters remain unchanged after exposure for 500 h. The release of misfit stress during thermal exposure is completed by the transformation from coherent γ/γ' interface to semi-coherent interface as well as the formation of misfit dislocation. The misfit dislocation network generated during thermal exposure is mainly rectangular shape, and its formation process is analyzed in detail. Additionally, it is found that the stress rupture lives decrease obviously after thermal exposure, while the morphology and size of interfacial dislocation network of thermally exposed alloys after rupture are very similar with that of unexposed alloy. The relation of γ' phase morphology and interfacial dislocation network to the stress rupture property deterioration has been discussed.

Aging process design based on the morphological evolution of γ′ precipitates in a 4th generation nickel-based single crystal superalloy

In order to meet the design requirements of the aging treatment process of a 4th generation nickel-based single crystal superalloy (Ni-SX) developed independently, the effects of aging temperatures and aging times on the precipitation and morphological evolution of γ′ precipitates are studied. The morphological evolution behavior of γ′ precipitates during the aging process is summarized subsequently and the coarsening behavior of γ′ precipitates is discussed by comparing with the Lifshitz-Slyozov-Wagner model (LSW) and the trans-interface diffusion-controlled model (TIDC). It is demonstrated that primary aging temperature and secondary aging time dominate the size and squareness of γ′ precipitates respectively, a narrow primary aging temperature range and a suitable secondary aging time are allowed to obtain the optimized morphology of γ′ precipitates. The optimal aging process of the Ni-SX investigated in the present work is obtained for 1100-1120 °C/4 h and 870 °C/16 h, confirmed by the corresponding creep tests. The coarsening growth of γ′ precipitates in short-term aging also conforms to the LSW model well. Besides, the aging process design rules of various Ni-SXs of different generations are also summarized.

Effect of Withdrawal Rate on Microstructure and Mechanical Properties of a Third Generation Nickel Based Single Crystal Superalloy

The influence of withdrawal rate (3, 4.5,6,9 and 12 mm/ min) on microstructure and high temperature rupture properties of A3 alloy was investigated by testing the microstructure and properties of the third generation nickel-based single crystal superalloy. The results show that with the increase of withdrawal rate, the dendrite spacing decreased exponentially, the segregation degree of main elements and the content of (γ + γ) eutectic structure decreased, and the size of γ phase decreased. After heat treatment, the eutectic structure was dissolved basically, and the segregation of the forming elements of γ phase is eliminated at different withdrawal rates. The size of γ phase in dendrite is smallest of 372.3μm at 4.5 mm/ min, the pore content is significantly larger than that of as-cast alloy, and decreased firstly and then increased with the increasing of withdrawal rate, which is 0.112% at 4.5 mm/ min and 0.182% at 3 mm/ min. The rupture life of the alloy at 1100 °C/140MPa is the longest at 4.5 mm/ min, which is mainly determined by the precipitation of TCP phase, pore content and γ size.

Effect of heating processing on the formation of micro-pores during solution treatment of high generation nickel-based single crystal superalloy

With the development of nickel-based single crystal superalloys, the quantity of micro-pores generating in solution treatment has increased, and there is an urgent need to develop a solution treatment design method that takes account of both residual eutectics and micro-pores. Therefore, the effect of heating processing on the formation of micro-pores during solution treatment of a 4th generation nickel-based single crystal superalloy has been investigated using high-resolution transmission X-ray tomography and electron probe microscopic analyzer. In the temperature range below interdendritic γ' phases solvus, decreasing the heating rate can limit the formation of micro-pores significantly. However, heating method (continuous or stepwise), has a little effect on micro-pores formation. The influence of heating processing on micro-pores formation is associated with composition homogenization degree of the alloy before eutectics dissolve. Moreover, it is found that the dissolution of eutectics is most likely the reason for substantial formation of micro-pores. Finally, a designing thinking for heating processing of solution treatment for high generation nickel-based single crystal superalloys is proposed.

Creep anisotropy of a 3rd generation nickel-base single crystal superalloy in the vicinity of [001] orientation

The effect of orientation on the creep behavior of a 3rd generation Ni-base single crystal superalloy with orientation deviated from [001] direction 10°∼20° was studied at 1100 °C/150 MPa and 850 °C/650 MPa. It is shown that the creep anisotropy almost disappears at 1100 °C/150 MPa for samples within 20° of [001]. Rafts with waving structures are formed in all samples which eliminate the various γ/γʹ microstructures caused by orientation deviation and hinder the matrix dislocation movement in similar ways. While the creep properties are highly anisotropic at 850 °C/650 MPa. The creep lives of samples close to [001]-[011] boundary are much longer than those of samples close to [001]−[1‾11] boundary, which is mainly caused by the activated slip systems at primary creep stage. Multiple <112>{111} slips are more likely to be activated in samples close to [001]-[011] boundary, and single <112>{111} slip system dominates the deformation of samples close to [001]−[1‾11] boundary. The phenomenon is rationalized in view of the nucleation and propagation of <112>{111} slip systems by means of the nominal resolved shear stress (RSS) analysis.

Competitive deformation induced by TCP precipitation and creep inconsistency on dendritic structures in a nickel-based single crystal superalloy crept at high temperatures

Creep properties of a 4th generation nickel-based single crystal superalloy (Ni-SX) are investigated at 1150 °C and 1100 °C. Fracture mechanism is revealed as competitive microcrack initiation induced by topologically close-packed (TCP) phase formation and creep inconsistency on dendritic structures showing the strong temperature-and-time dependent relations. At 1150 °C, alloy displays the high nucleation rate of TCP phases. Dense TCP particles dissolve the surrounding γ/γ' microstructures forming large depleted zone where is easily stress-concentrated. Dislocations are found to emit from the interface between TCP and depleted zone that promotes dislocation shearing and microcrack initiation in the large depleted zone, as well as the fracture of alloy. On contrary, alloy crept at 1100 °C suffers to relatively restricted TCP nucleation. But the longer creep time gives to larger size of TCP precipitates. Compared to alloy crept at 1150 °C, the depleted zone is much smaller that is not prone for stress concentration. For this creep inconsistency induced by dendritic segregation plays the main role in determining the creep properties. The larger local strain rate forms microcracks primarily in interdendritic region causing the final fracture.

Enhanced creep resistance induced by minor Ti additions to a second generation nickel-based single crystal superalloy

Over the past decades, the evolution of high-performance nickel-based single crystal (SX) superalloys has advanced along the pathway of continuous increment in the Re content, which leads to microstructure instability, as well as increased density and cost. This paper proposes an alternative approach to enhance the creep resistance by slight composition modification instead of increasing the Re content. We systematically investigated the effects of minor Ti additions on the microstructure, lattice misfit, and elemental partitioning behavior of a novel second generation nickel-based SX superalloy, whose creep performance exceeded that of René N5. The addition of 0.5 wt.% Ti resulted in more and finer γ′ precipitates, narrower γ channels, and a higher magnitude of the γ/γ′ lattice misfit. In addition, all partitioning coefficients of Re, Mo, W, and Cr elements in the γ matrix were increased. Thicker γ′ and thinner γ lamellae were formed in the raft structure. The creep stain rate decreased significantly, and the creep life extended twice that of the base alloy at 1030 °C and 230 MPa. The temperature capacity of the Ti-containing alloy was superior to that of some commercial second generation SX superalloys and similar to that of René N6. Denser dislocation networks accumulated at the γ/γ′ interface, confirming the dislocation strengthening mechanism during the creep process. Stronger solid solution strengthening, impeded cross-slip, and more sluggish diffusion dynamics were considered to contribute to the enhanced creep resistance of the Ti-containing superalloy.

Temperature dependence of lattice mismatch and γ′ volume fraction of a fourth-generation monocrystalline nickel-based superalloy

AbstractThe strain distribution in a new generation nickel-based single crystal superalloy has been determined in the temperature range 293 K– 1598 K. Measurements have been performed using diffraction of high energy synchrotron radiation (150 keV,λ= 0.008 nm) to determine the variations with temperature of the γ/γ′ lattice mismatch and of the γ′ volume fraction. The chemical segregation at the dendrite scale and the internal stress induce a large range of values for the lattice mismatch. The measurements of the γ′ volume fraction determined by X-ray diffraction are in agreement with those obtained by atom probe tomography, image analysis or computation.

High temperature shakedown of a 2nd generation nickel-base single crystal superalloy under tension-torsion loadings

Components that face aggressive thermomechanical loadings that are cyclic and multiaxial often employ nickel-base single crystal superalloys to achieve acceptable lifetimes. These superalloys are commonly used with first-yield design rules, however their use under inelastic design, leveraging safe elastoplastic shakedown behaviors at elevated temperatures, has remained largely unexplored. This paper is the first to combine cyclic test results with some microstructural observations to establish conditions for the macroscopic shakedown response of a 2nd generation nickel-base single crystal superalloy under tension-torsion loadings at 600 °C. Microstructural studies are performed to identify and compare deformation mechanisms in specimens that exhibit shakedown and ratchetting. Based on the results, directions for future materials development, to directly exploit shakedown in design, are suggested.

Nanoindentation study and mechanical property analysis of nickel-based single crystal superalloys

Based on uniaxial tensile and compressive creeps, the mechanical properties of the third generation nickel-based single crystal superalloy were studied via the combination of nanoindentation, crystal plasticity theory, and finite element analysis. The Young’s moduli and hardness of γ, γ′ and the topologically close-packed (TCP) phases are related to chemical composition and applied stress. The maximum values of the Von Mises equivalent stress and damage are at the tip of the TCP phase. These values are inversely proportional to the distance to the TCP phase. Furthermore, the rafting of the γ′ phase can be predicted according to the distribution of the TCP phase and stress.

The formation and evolution of NiAl phase in a fourth generation nickel-based single crystal superalloy

The formation mechanism of the Ru-rich β-NiAl phase in a nickel-based single crystal superalloy and its evolution course during the long-term aging were investigated. The β-NiAl phase confirmed via the XRD and TEM analysis was observed in the solidification process for casting alloy. The intensive study on the formation mechanism of the β-NiAl phase was conducted based on the analyses of the phase diagram, the incipient melting experiment, and the segregation behavior. In addition, a new phase identified as the δ phase was also observed in the β-NiAl phase, which was enriched in Re, Co, W, and Ni. The solidification path was determined as L → β, L → β + δ, L + β → γ′, L → γ′, L → (γ + γ′), which took into consideration the influence of the β-NiAl phase and the δ phase. Besides, the high melting point of the β-NiAl phase induced impediment in homogenizing heat treatment, long-term aging at 1100 °C from 100 h to 1000 h was carried out to eliminate the β-NiAl phase. Hence, the γ’/β-NiAl and γ/β-NiAl diffusion couples were put forward to interpret the evolution of the β-NiAl phase during the long-term aging. The TCP phase which was determined as μ phase via the selected diffraction spot was examined in the full heat-treated and long-term aging specimens. The morphology of the TCP phase could be divided into three types: needle-like, rod-like, and bulk-like. The influence of the β-NiAl phase on the formation of the TCP phase was researched in detail.

Cyclic inelastic behavior and shakedown response of a 2nd generation nickel-base single crystal superalloy under tension-torsion loadings: Experiments and simulations

Nickel-base single crystal superalloy components are designed to experience extreme and often cyclic multiaxial loads. While much of the literature has provided the foundation to use these materials near critical yield conditions, their use under inelastic design, exploiting safe elastoplastic shakedown behaviors is largely unexplored. In particular, this paper focuses on establishing the conditions for the macroscopic shakedown response of a 2nd generation nickel-base superalloy under tension-torsion loadings from numerical and experimental perspectives. A numerical model is developed to more broadly evaluate shakedown limit loads and it is found that by allowing shakedown to occur, an 18% increase in feasible design load space is expected compared to traditional first-yield design criteria. Experiments on hollow cylindrical specimens are also conducted and used to assess the prediction capabilities of the numerical finite element model.

Creep anisotropy of a 3rd generation nickel-base single crystal superalloy at 850 °C

The influence of orientation on the creep behaviors of a 3rd generation single crystal superalloy was studied at 850 °C. It is found that the creep properties are highly anisotropic for [001], [011] and [111] specimens. The [111] samples show the longest creep life. The [011] specimens display the shortest creep life of less than 1 h and the longest elongation. And [001] specimens exhibit the intermediate creep life. Electron backscattered diffraction (EBSD) analyses and transmission electron microscopy (TEM) observations show that the creep anisotropy is mainly related to the slip systems that activated, especially the <112>{111} slip systems. Single slip of <112>{111} system results in a high creep strain and poor property, which controls the primary creep of [001] and entire creep stage of [011] specimens. While interaction of primary and secondary <112>{111} systems has an effect on work hardening, which occurs during steady and tertiary creep of [111] orientation and hence results in the best creep property of [111] specimens. By comparing with SRR99 and CMSX-4 alloys, higher Re content in DD33 alloy may facilitate the formation of continuous stacking faults in [011] specimens and activation of multiple <112>{111} slip systems in [111] specimens, which will affect the creep property and anisotropy.

Formation of Secondary Phases in the Boundary between Surface Defect Grains and Matrix in Third Generation Nickel-Based Single Crystal Superalloy Turbine Blades

Ni-based single crystal superalloy turbine blades have excellent mechanical strength and resistance to corrosion and oxidation due to a uniformly distributed gamma prime phase in a gamma matrix. However, defect grains have been often found on the surface of turbine blades after manufacturing, which can be potential sites of crack initiation. In this study, several different types of surface defect grains formed in third generation Ni-based single crystal turbine blades, such as stray grains, freckle chain grains, equiax grains, and a new grain formed in surface scale, had been investigated. The grain boundary regions were observed by high resolution electron microscopy. Although the formation mechanism of each grain defect is different, secondary phases, such as rhenium-rich particles, have been always found in each grain boundary. In addition, depending on the existence of the secondary phases as well as the size of defect grains, different microstructures were observed even in the same defect grain boundary. Finally, the observed results suggest that if there is any boundary region in a turbine blade, secondary phases, such as Re-rich particles, can be found.

Thermal fatigue behavior of a nickel-base single crystal superalloy DD5 with secondary orientation

The effect of cycle temperatures on the thermal fatigue behavior of a second generation Nickel-based single crystal superalloy DD5 with different secondary orientations was investigated. The experimental results show that the crack propagation rate increases with the rise of the upper limit temperature of the thermal fatigue test. It was found that the same initiation sites and propagation orientations of the thermal fatigue cracks were observed in samples with different secondary orientations. Thus, in samples with the second orientation of [110] and [100], thermal crack initiated at the edge of the holes with a directionally solidified direction of 45° and propagated along a direction of 45° with directional solidification.

Investigation on a ramp solution heat treatment for a third generation nickel-based single crystal superalloy

The solution effects of a series of traditional stepwise solution heat treatments (SSHTs) and a ramp SHT were investigated. The results show that as the dwelling time increases, the residual segregation reduces after SSHT and the uniformity of size distribution of γ′ particles increases after subsequent aging treatment. Less residual segregation was obtained after the ramp SHT than SSHT. The advantages of ramp SHT are proposed as follows: higher permissible solution temperature without incipient melting, lower residual segregation due to more effective dissolution, more homogenous size distribution of γ′ after aging treatment, and 1.3-times longer rupture life at 1100 °C and 150 MPa. The dynamic ramp solution heat treatment is more appropriate due to the continuously increasing incipient melting temperature controlled by kinetic diffusion.