Ni-based Single Crystal Superalloy Research Articles

Homogenizing effects of Co on multi-scale elemental distribution in Ru-containing Ni-based single crystal superalloys

• Co alleviates large-scale heterogeneity of multiple microstructures of the alloys. • Co significantly reduces the partitioning coefficient of γ forming elements, which alleviates small-scale heterogeneity. • Co additions suppress TCP precipitation and increase microstructure stability. The effects of Co on multi-scale elemental distributions were studied in an advanced Ru-containing Ni-based single crystal superalloys. Two types scales of microstructural heterogeneities, the large-scale dendritic segregations and small-scale elemental partitions between γ/γ' phase were compared in as-cast, heat-treated and thermal-exposed alloys. The elemental homogeneities on both large and small scale were shown to be enhanced with Co additions, giving rise to the restricted precipitation of TCP phase. It is thus expected the potential role of Co to replace part of Ru in advanced superalloys.

High-reliability repair of single-crystal Ni-base superalloy by selective electron beam melting

Here, we report the utilisation of a powder-bed-fusion additive manufacturing to achieve the CMSX-4 single-crystal repair with René N5 powders. The single-crystal repair is characterised by a fine columnar dendritic substructure, limited in-grain misorientation over tens of millimetres in height, and strong deposit-to-substrate bonding. The brighter contrast region of 300 to 400 μm thick, formed by the cellular solidification front, sits above the melted back region with a horizontal low-angle boundary (4° misorientation) in-between. Its higher hardness of 450 HV is attributed to the fine cuboidal primary γ′ of 135 ± 7 nm. Their presence is a combined effect of high cooling rate and limited local compositional difference. The Scheil-Gulliver predicted solute partitioning of Ta and Hf to the inter-dendritic region agrees well with the MC carbides appearing at the columnar dendritic and cellular boundaries. Computational-fluid-dynamics (CFD) simulations, combined with the minimum undercooling criterion, helps to determine the vertical distance between the melt-pool bottom and the oriented-to-misoriented transition borderline, providing a convenient guideline to assess the single-crystal likelihood, in a sense that this distance needs to be larger than the hatch depth. The CFD-calculated solidification velocities and thermal gradients agree with the columnar-to-equiaxed transition curve and the experimental observation.

Super-Solidus Hot Isostatic Pressing Heat Treatments for Advanced Single Crystal Ni-Base Superalloys

Super-solidus hot isostatic pressing (SSHIP) heat treatment has first been developed and applied to the third-generation Ni-base single crystal superalloy CMSX-10 K. This new type of heat treatment aims at significantly reducing the total time required for the solution heat treatment and at enhancing mechanical properties as compared to conventional heat treatment routes. The SSHIP is an innovative, economical and sustainable approach that can be applied to all types of Ni-base SX superalloys. It is especially interesting for alloys with a high content of refractory elements and a large volume fraction of eutectic microstructure in the as-cast state.

Enhanced machinability of Ni-based single crystal superalloy by vibration-assisted diamond cutting

The machined surface integrity of Ni-based single crystal superalloy is one major factor that affects the performance of its component. It is extremely difficult to achieve a nanometric surface quality by only using ordinary cutting (OC) technology due to the rapid tool wear. Hence, in this work ultrasonic vibration cutting (UVC) with single crystal diamond tool is applied to investigate the machinability of Ni-based single crystal superalloy, including a series of comprehensive investigations on the surface integrity machined in OC and UVC, such as surface roughness, surface morphology, chip formation, tool wear, sub-surface damage, and so on. The experimental results indicated that as compared to OC where finished surface deteriorates seriously due to rapid tool wear, the surface roughness by applying UVC is decreased from Sa 60 nm in OC to Sa 4.815 nm in UVC due to the efficient suppression of tool wear. And the maximum profile error of 1.314 μm achieved in OC is significantly reduced to 0.038 μm with eliminated anisotropic machining effect in UVC. It is also clarified that there are obvious grain refinements and severe plastic deformation near the sub-surface in OC, which is mainly caused by the huge thermo-mechanical loading driven by the tool wear. Moreover, due to the grains are significantly refined in OC, the workpiece material is seriously work hardened. In contrast, the thickness of sub-surface damage is decreased from 2.27 μm in OC to 0.1902 μm in UVC due to the small loading forces, accompanied with more uniform evolution of microstructures.

Insights into thin film blistering of gold coating on metal substrate

Au thin films, up to h ≈ 90 nm thick, have been deposited by direct current sputtering at room temperature to address the effect of coating thickness on film blistering that occurred on Ni-based single crystal (NBSC) superalloy rather than on Si. Transmission electron microscopy provides evidence that the blisters nucleated at the Au/NBSC interface. Statistically, the diameters of the blisters increases while their density decreases with the increase in h; however, the height-to-diameter ratios of the blisters measured from the film surface are rather constant and independent of h. When h is increased from ≤63 nm to ∼90 nm, the size distribution of the blisters turns from a single mode to a bimodal along with an emergence of larger circular blisters and edge-like ones. Mechanical modelling and density-function calculations provide evidence that the formation of blisters is driven by pockets of energy concentration while the nucleation site is influenced by surface absorbents of the substrate.

Substrate stimulating technique for Ni-based single crystal superalloy preparation during direction solidification

The grain selector method is a prevailing technique in the manufacturing process of Ni-based single crystal superalloy, however, it cannot control the orientation of single crystal within an appropriate scope. In this study, a novel technique named substrate stimulating method was proposed and DD32 Ni-based single crystal superalloy was prepared. The microstructure evolution was characterized by EBSD and OM, and the temperature field and temperature gradient distribution were predicted by the finite volume method. With a substrate height of 18 mm, the proportion of grains with deviation angles less than 12° and 8° account for 100 % and 99 % at the entrance of spiral selector, compared with 80 % and 51 % by grain selector method, which makes significance sense for optimizing the orientation of single crystal casting and improving the yield of single crystal casting. The morphology and the orientation of the substrate stimulated grains, which are mainly determined by temperature gradient and melt undercooling, could be optimized by adjusting the substrate height and melt pouring temperature.

Anisotropic Tensile Properties in Ni-Based Single-Crystal Superalloy DD6 near [001] Orientation at 760°C

This work is concerned the tensile properties of the secondary Ni-based single-crystal superalloy DD6 near [001] orientation at 760°C. In this study, anisotropic tensile properties of DD6 alloy within 10° of the [001] orientation were exhibited at 760°C. The yield strength and ultimate tensile strength of DD6 alloy oriented close to [001] direction was the highest. As the deviation off the [001] orientation increased, both 0.2% yield strength and ultimate tensile strength was decreased. The specimens oriented close to [001]-[111] boundary exhibit higher yield strength and ultimate tensile strength than the specimens oriented close to [001]-[011] boundary. Numerous of dislocations can be found in the γ matrix channels during the tensile deformation. A number of dislocation pairs and few of stacking faults are found in the γ' precipitates after the tensile at 760°C. The morphology of γ' phases in DD6 alloy maintained cubical during the tensile deformation at 760°C. With Schmid's Law, the mechanism of anisotropic tensile properties in DD6 alloy near [001] orientation is analyzed.

Stress state mechanism of thickness debit effect in creep performances of a Ni-based single crystal superalloy

Weight reduction and advanced cooling schemes to improve aircraft engine efficiency drive the design of turbine blades toward thin-walled structures. The decrease of wall thickness leads to a decrease of creep rupture life and/or the increase of creep rate, which is termed as the thickness debit effect. To capture the nature of this effect and improve the reliability of turbine blades, creep tests at 1100 °C/130 MPa are carried out on sheet samples of a Ni-based single crystal superalloy with thicknesses from 0.60 to 2.17 mm. The creep rupture life is found to decrease with the decrease of sample thickness linearly, exhibiting an obvious thickness debit effect. The analysis of the microstructure evolution demonstrates that the rafting mechanism of the γ' phase of all samples remains the same (i.e. stress-induced, diffusion-controlled process) and is independent of the sample thickness. The fracture morphology reveals that the fracture mechanism of thin sample is a mixed fracture including micro voids and cleavage-like planes, while that of thick sample is micro voids-dominated. Environmental degradation suggests that surface damage due to oxidation and nitridation is not the primary causes for thickness debit effect, yet promotes the effect integrally. Finite element analyses and the observed dislocation configurations reveal that the thin sample is in a plane stress state, that activates six slip systems of the type {111}<110> and one slip system of the type {111}<112> during creep, while the thick sample is close to the ideal uniaxial stress state which activates all possible eight slip systems of the type {111}<110> and two slip systems of the type {111}<112>. Less slip systems activated in thin sample induce less work hardening, which results in heterogeneous deformation and shorter rupture life.

Hydrogenated vacancy induced changes in thermodynamic stability and mechanical properties of Re-containing γ-Ni/γ'-Ni3Al phase interface

Re is an important alloying element to improve the high temperature performance of Ni-based single crystal superalloys, and has a segregation tendency to the γ/γ' phase interface. Hydrogen embrittlement introduced by hydrogenated vacancies (H-Vacs) severely reduces the service life of alloys. However, the formation mechanism of hydrogenated vacancy (H-Vac) and its effects on the Re-segregated γ/γ' phase interface are still unclear. In this work, the first-principles simulations are applied to clarify the formation of H-Vac and its effects on the thermodynamic stability and mechanical properties of Re-containing γ/γ' phase interface. We find that at the Re-containing γ/γ' phase interfaces, the hydrogen atom is more likely to dissolve in the first-nearest-neighbor (1st-NN) octahedral interstices of vacancy than in the 2nd-NN octahedral interstices. The Vac-containing γ/γ' phase interface can be stabilized when hydrogen atom dissolves in 1st-NN octahedral interstices to form H-Vac complex. For the Re-containing γ/γ' phase interface, the formation of the H-Vac complex does not change its preferential cleavage plane. Moreover, the fracture strength of the Re-containing γ/γ' phase interface is always higher than that of the Re-free γ/γ' phase interface. Thus, Re can enhance the interfacial fracture strength, which is also verified by the first-principles computational tensile tests.

Effect of Mo on microstructural stability of a 4th generation Ni-based single crystal superalloy

The microstructural stability during thermal exposure at 900 °C of Ni-based single crystal superalloys with various Mo content was investigated. The coarsening of γ′ precipitates and formation of TCP (topologically close-packed) phases were discussed in detail. The LSW theory and TIDC theory were applied to study the coarsening behavior of γ′ particles, and both models fitted well with the experimental results. Increasing Mo content enhanced the partitioning behavior of alloying elements, thus increasing the lattice misfit and interfacial stress of γ′/γ phases. Additionally, Mo addition also decreased γ′ coarsening rate by retarding the diffusion process of elements. However, Mo addition significantly promoted the precipitation of TCP phases, consuming massive solution strengthener elements, such as Re, W and Mo. TCP precipitation may significantly limit the creep life at 900 °C.

Composition design and microstructure of Ni-based single crystal superalloy with low specific weight—numerical modeling and experimental validation

Composition design and microstructure of Ni-based single crystal superalloy with low specific weight—numerical modeling and experimental validation

Time-dependent crack growth mechanism in Ni-based single crystal superalloys at high-temperature

The microscopic mechanism of time-dependent crack growth (TDCG) in single crystal (SC) Ni-based superalloys under a relatively high-temperature condition (900 °C) is critically investigated. Two alloy types are compared: alloy A containing 3% rhenium (Re) and alloy B containing no Re. With an initial stress intensity (K) value of 40 MPam1/2, both alloys show similar two step TDCG, i.e. a steady stage followed by an acceleration stage. The total life to failure (tf) of alloy B is, however, an order of magnitude shorter than alloy A. In addition to the conventional fractography, cross-sectional analyses (EBSD and EDS) are also conducted for an interrupted crack tip. Two distinct fracture modes are then identified in both alloys: {111}<112> twin-induced shear fracture and non-crystallographic fracture along dendrite boundaries. The twin systems observed are the secondary ones having a relatively smaller Schmid factor compared to the primary ones that remain inactive under the <100> tension. In the case of alloy A, the twin induces local phase transformation involving TCP precipitation, decomposition of γ (Ni)/γ’ (Ni3Al) structure and recrystallization, which eventually enhances local ductility and dominates the crack growth rate. In the case of alloy B, the macroscopic appearances of these fracture modes are much the same, but they are significantly facilitated by micro-voids formed exclusively in γ channel whose stability is much lower than that in alloy A. The apparently brittle TDCG nature of alloy B compared to alloy A can be interpreted as an enhanced localization of plasticity at the crack tip.

Epitaxial growth and oxidation behavior of the NiCoCrAlYTa/Y2O3 coating on a nickel-based single-crystal superalloy blade tips, produced by electro spark deposition

The characteristics of the working surface are often related to the safety and longevity of the machine, therefore, various functional coatings are used to meet the special needs of particular components. Ni-based single crystal superalloy blade tips frequently suffer from oxidation, corrosion, and wear under harsh conditions. To improve these problems, the NiCoCrAlYTa/Y2O3 metal matrix composite coating was prepared by electro spark deposition. In this work, the microstructure and epitaxial growth behavior of the NiCoCrAlYTa/Y2O3 coating were investigated, and the oxidation resistance of the substrate and the coating were evaluated and compared. The coating showed ultra-fine columnar crystal structure. The diffusion between the substrate and the coating elements caused by the instantaneous high temperature generated by the electro spark deposition endowed the coating with extremely high metallurgical bonding strength. The ultra-high cooling rate and approximately one-dimensional unidirectional rapid solidification made the Y2O3 nanoparticles evenly dispersed in the coating and realized directional epitaxial growth. The NiCoCrAlYTa/Y2O3 coating exhibited excellent oxidation resistance compared to the substrate. Y2O3 nanoparticles changed the diffusion path of the elements and accelerated the formation of the Al2O3 film. The synergistic effect of ultra-fine microstructure and nanoparticles on the adhesion between the oxide scale and the coating was improved.

Effects of Mo addition on microstructure of a 4th generation Ni-based single crystal superalloy

The effects of Mo addition on the microstructure of a 4th generation Ni-based single crystal superalloy were investigated. Mo addition significantly promoted elements Mo, W and Re partition into γ phase and enhanced absolute lattice misfit at 1100 ​°C. The increase of Mo concentration from 2 ​wt% to 4 ​wt% also decreased the content of eutectic islands and the segregation ratios of other alloying elements in the as-cast state, especially Re and W. After heat treatments, the size of γ′ phase and width of γ channels decreased slightly with higher Mo content. More Mo additions slightly enhanced the segregation behavior of Re while reducing the segregation behavior of Mo. However, it revealed that the primary and secondary dendrite arm spacings were barely affected by Mo addition.

Effect of Coating Pre-Treatment on Surface Recrystallization of DD6 Single Crystal.

Thermal barrier coatings (TBCs) are widely used to protect high-temperature components against harsh environments, such as extremely high temperatures. In this work, a second generation Ni-based single crystal superalloy (DD6) was treated in two ways: (1) via simple surface sandblasting under different pressures with no additional coating, and (2) through simple surface sandblasting under different pressures and then by applying NiCoCrAlYHf (HY5) coatings. The effects of pre-treatment (sandblasting) and the HY5 coating on the surface recrystallization of the alloy were thoroughly investigated. According to the results, both sandblasting pressure and the presence or absence of a coating significantly influence surface recrystallization. In particular, the critical sandblasting pressure for recrystallization increased the maximum recrystallization depth in both the coated and uncoated samples. Meanwhile, the recrystallization depth of the alloy with a coating was reduced compared to that without a coating. In addition, the number of recrystallized cells in the coated alloy was decreased, which indicated that the HY5 coating effectively reduced the degree of recrystallization.

Experimental Study at the Phase Interface of a Single-Crystal Ni-Based Superalloy Using TEM.

The single-crystal Ni-based superalloys, which have excellent mechanical properties at high temperatures, are commonly used for turbine blades in a variety of aero engines and industrial gas turbines. Focusing on the phase interface of a second-generation single-crystal Ni-based superalloy, in-situ TEM observation was conducted at room temperature and high temperatures. Intensity ratio analysis was conducted for the measurement of two-phase interface width. The improved geometric phase analysis method, where the adaptive mask selection method is introduced, was used for the measurement of the strain field near the phase interface. The strained irregular transition region is consistent with the calculated interface width using intensity ratio analysis. An intensity ratio analysis and strain measurement near the interface can corroborate and complement each other, contributing to the interface structure evaluation. Using TEM in-situ heating and Fourier transform, the change of dislocation density in the γ phase near the two-phase interface of the single-crystal Ni-based superalloy was analyzed. The dislocation density decreases first with the increase in temperature, consistent with the characteristics of metal quenching, and increases sharply at 450 °C. The correlation between the variation of dislocation density at high temperatures and the intermediate temperature brittleness was also investigated.

Oxidation behaviors and compressive strength evolution of DD6 Ni-based single-crystal superalloy at 1100 ℃

In this work, the oxidation behaviors of DD6 Ni-based single-crystal superalloy and its compressive strength evolution after oxidation were investigated. The results showed that the thermal exposure time has a significant effect on the oxidation products, oxidation kinetics, Al elements diffusion and compressive strength evolution of DD6 superalloy. Firstly, the relationship between grain size change of (Ni0.9,Co0.1)O, the thickness of γ′-free zone and the exposure time was approximate power exponential law, while the relationship between weight change of DD6 superalloy and exposure time was approximate parabolic law since the densification of inner continuous Al2O3 layers hindered the diffusion of oxygen ions. Moreover, the black phases of AlN in the γ′-free zone formed during the oxidation process, and its quantity and size increased first and then decreased with the oxidation time extending, since the inner Al2O3 not only blocked the ion but also reacted with AlN. In addition, with the increase of oxidation time, the compressive yield strength of DD6 in [001] direction first decreased rapidly and then gradually stabilized at about 740 MPa.

Crystal plasticity constitutive modeling of tensile, creep and cyclic deformation in single crystal Ni-based superalloys

A microstructure-sensitive crystal plasticity constitutive framework is proposed for simulating the tensile, cyclic, and creep response of single crystal Ni-based superalloys. In this framework, a non-Schmid model is used to account for the orientation- and temperature-dependent yield anisotropy. A model for the evolution of the slip system-level backstress is developed to simulate the cyclic response. The model accounts for the effect of microstructural features like precipitate volume fraction and size, and matrix channel width, on the associated hardening mechanisms. A physical model for creep deformation has also been developed that accounts for dislocation climb normal to the slip plane and the microstructure evolution due to rafting and isotropic coarsening of the precipitates. Application of the model is demonstrated by predicting the orientation- and temperature-dependent thermo-mechanical deformation of two single crystal superalloys, CMSX-4 and PWA-1484. The model is calibrated to the experimental tensile, cyclic and creep response for these alloys. Finally, the model is qualitatively validated by predicting the creep-fatigue interactions for different loading orientations and a range of thermo-mechanical conditions for CMSX-4 and PWA-1484.

Fatigue life evaluation for notched single-crystal Ni-based superalloys considering inhomogeneous rafting microstructure

Hot-section components made by single crystal (SC) Ni-based superalloys with geometrical discontinuities such as film holes suffer from spatially inhomogeneous rafting of the γ'/γ microstructure due to harsh circumstances. Therefore, low cycle fatigue (LCF) life prediction is critical for ensuring the safety of engineering structures. Within this regard, an energy-based LCF life model is proposed based on the concept of the strain energy gradient by introducing the effect of rafting into the plastic and elastic parts of the strain energy density. The inhomogeneous microstructure degradation of a centre-hole specimen is depicted by stress-assisted rafting and stress-free isotropic coarsening, whereas the inelastic deformation behaviour of materials near the central hole is simulated by a microstructure-sensitive constitutive model. Combined with the concepts of modified strain energy density, effective damage zone and weight function, the proposed life model achieves a good prediction accuracy in ± 2 scatter bands against the experimental data of the centre-hole specimens with different rafting treatments. Finally, the uncertainty of the proposed life prediction model induced by the statistical dispersion of the microstructure is estimated. The results show that the uncertainty of the predicted lifetimes decreases with the rafting extent and fatigue life, even though the distribution of the γ channel width of the alloy with heavily rafting alloys is highly dispersed.

A novel Re-free Ni-based single-crystal superalloy with enhanced creep resistance and microstructure stability

Rhenium (Re) is a paramount strengthening element in nickel-based single crystal superalloys because of its low diffusion coefficient, high melting point and high partition coefficient between γ and γ' phases. However, its detrimental influence on phase stability has remained a long-standing problem to be solved. In this paper, we report a new design strategy to improve creep resistance and phase stability of nickel-based single crystal superalloys by replacing Re by osmium (Os), which contradicts the common belief. The experimental observations clearly show that the creep rate of newly designed CSU-Os-2 alloy is lower than that of commercial CMSX-4 alloy at 980 °C/200 MPa/100 h. Moreover, the CSU-Os-2 alloy also owns the better phase stability after exposure at 1100 °C for 1000 h, compared with CMSX-4 alloy. The striking properties can be well explained by higher partition coefficient between γ and γ' phases and very likely lower diffusion coefficient of Os in γ matrix than Re.