Ni-based Single Crystal Superalloy Research Articles

Effect of thermal barrier coatings on the fatigue behavior of a single crystal nickel-based superalloy: Mechanism and lifetime modeling

Coatings are typically used to protect engineering materials from oxidation, corrosion, and erosion. However, it also changes the surface condition of substrate materials and causes a mechanical mismatch between the substrate and coatings. This study investigated the effect of thermal barrier coatings (TBCs) on the fatigue behavior of a second-generation single crystal (SX) Ni-based superalloy. The experimental results showed that the impact of the TBCs on the fatigue lifetime was loading-dependent. The coating was beneficial under low stresses but had no effect under high stresses. The scanning electron microscope (SEM) observation showed that the TBCs did not change the fatigue crack initiation location. For most coated and uncoated samples, the cracks are more likely to generate from the internal defects in the substrate. In addition, an equivalent strain energy density-based lifetime prediction model was proposed for coated samples. The prediction capability of the proposed model agreed well with experimental results. This study could provide insight into the failure mechanism for TBCs-coated single-crystal superalloys.

Microstructure characteristics of a René N5 Ni-based single-crystal superalloy prepared by laser-directed energy deposition

This work focused on the microstructure and mechanical properties of nickel-based single crystal (SX) superalloy René N5 prepared by laser-directed energy deposition (L-DED) using a flat-top laser beam. A comparison was conducted with the conventional directional solidification technique. Results show that the crack-free single-crystal superalloys with a fraction of low-angle grain boundary (LAGB) higher than 98.5% were obtained by epitaxial growth under different laser powers. The flat fusion line using a flat-top laser beam can suppress the formation of stray grain and high-angle grain boundaries (LAGB) during the L-DED process. Microstructure observation shows the extremely refined dendrites in L-DED SX superalloys with primary dendrite arm spacing (PDAS) values around 20–30 µm, which is significantly lower than as-cast ones (∼350 µm). PDAS increases with the increasing laser power. Segregation of refractory elements of Re, W, and Ta increases with increasing laser power, and the SX specimens with laser power of 1100 W and 1300 W have lower elemental segregation compared with as-cast specimens. The SX specimens exhibit a uniform square γ′ phase at the dendrite core, with an average size of 79–101 nm and a volume fraction of 57–68%. Besides, the carbides with discontinuous and refined shapes show a uniform distribution in the L-DED specimens in comparison with as-cast specimens. The microhardness of as-deposited specimens increases with the increasing laser power, which is higher than that of as-cast ones. This work can provide potential guidance for the microstructure control in the laser additive manufacturing of Ni-based SX superalloys and a further improvement in high-temperature mechanical performance.

Microstructure evolution and dynamic recrystallization mechanism induced by grinding of Ni-based single crystal superalloy

Ni-based single crystal superalloy has experienced high temperature and extreme strain rate during grinding process, resulting in white layer and plastic deformation layer on the subsurface. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and transmission Kikuchi diffraction (TKD) techniques were used to characterize the microstructure changes such as dislocation distribution, crystal structure and orientation information at the nanoscale of the ground subsurface. The mechanism of dynamic recrystallization and grain refinement of the single crystal superalloy were obtained. The results show that the high-density dislocation structure is easy to form in the stress concentration areas, which are the nucleation sites of dynamic recrystallization. With the accumulation of misorientation, the subgrain boundaries (SGBs) are transformed into low-angle grain boundaries (LAGBs) and high-angle grain boundaries (HAGBs). Moreover, equiaxed nanograins are formed near the top surface due to the cooperation mechanism of dislocation movement and twins. Under the condition of high strain rate, the deformation mechanism of Ni-based single crystal superalloy is continuous dynamic recrystallization (CDRX) dominated by subgrain rotation. This work illustrated the grinding-induced microstructure evolution and mechanism of dynamic recrystallization, which provides theoretical guidance for machining of single crystal blade.

Effects of solidification rate and heat treatment on microstructure characterization and hot tensile strength of single crystal CMSX-4 superalloy

Single crystal CMSX-4 Ni-based superalloy, consisting of rhenium and hafnium elements, were directionally solidified at three different withdrawal rates of 2, 3, and 4 mm/min through a Bridgman furnace. After that a full solution heat treatment was followed by two step aging at 1140 °C/6 h and 870 °C/20 h respectively. Microstructural characteristics, e.g. the primary and secondary dendrite arm spacing, the size and volume fraction of γ ́ phase, and percentage of porosity were analyzed using the optical microscopy (OM) and scanning electron microscopy (SEM) equipped with an energy dispersive spectroscopy (EDS) detector. It was found that an increase in the withdrawal rate of the as-cast specimens led to a reduction in the values of primary and secondary dendrite arm spacing, the percentage of porosity and also size of γ ́. In addition, at higher withdrawal rates, the γ/γ ́ eutectic phase is distributed from a continuous and blocky shaped to fine scattered islands. Nevertheless, the minimum volume fraction of eutectic was obtained at the withdrawal rate of 3 mm/min. Heat-treatment of the specimens associated with formation of a noticeable cuboidal γ’-precipitates, which was about 70% at withdrawal rate of 3 mm/min. Furthermore, the aged specimens indicated the lower γ/γ ́ eutectic phase compared to as-cast specimens, while the percentage of porosity increased for aged specimens. Eventually, the highest ultimate tensile strength of the single crystal CMSX-4 at 870 °C was measured to be 1170 MPa which was obtained at a withdrawal rate of 3 mm/min, due to the high volume fraction of the γ ́ phase and the lower values of γ/γ ́ eutectic phase.

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.