Ni-based Single Crystal Superalloy Research Articles

A novel L12-strengthened single crystal high entropy alloy with excellent high-temperature mechanical properties

In this work, a novel and low-cost L12-strengthened single crystal high entropy alloy (HEA) was prepared through the seeding method of directional solidification. The microstructures, tensile properties and deformation mechanisms of the single crystal HEA were systematically investigated. The volume fraction of L12 precipitates after heat treatment was almost 80%, which was higher than that of conventional single crystal superalloys. The maximum yield strength, ultimate tensile strength and elongation, which were separately as 897 MPa, 1038 MPa and 37%, were obtained at the intermediate temperature of 800 °C. Moreover, at higher temperature (900 °C and 1000 °C), the HEA was able to maintain excellent performance, which was comparable to the commercial René N5 second-generation Ni-based single crystal superalloy. Finally, the high performance was examined by the model of strengthening contribution, which proved that the high-volume fraction of L12 precipitates in single crystal HEA was the primary reason for their significant precipitation and solid solution strengthening. Additionally, the formation of Lomer-Cottrell locks and stacking faults could enhance the high-temperature properties of the single crystal HEA.

Thermal Barrier Coatings in burner rig experiment analyzed through LAser Shock for DAmage Monitoring (LASDAM) method

This study investigates failure mechanisms in a typical thermal barrier coating (TBC) system comprising an EB-PVD columnar top coat, an aluminide bond coat, and a Ni-based single crystal superalloy substrate, simulating gas turbine operating conditions using a burner rig. TBC degradation, initiated by interfacial defects from the LASAT method, was studied during thermal gradient cycling under fast and slow cooling. In-situ optical and infrared imaging, along with ex-situ SEM cross-sectional analysis, monitored failure mechanisms. The Laser Shock for Damage Monitoring (LASDAM) technique provided insights into gradient and cooling rate impacts on columnar TBC damage. Results showed significant effects of cooling rate on delamination and localized failure at blister sites, with LASDAM revealing significant overheating at damage sites. Analysis included full-field temperature and damage assessment, emphasizing blister-driven delamination under severe thermal gradients. Discussion focused on elastic stored energy effects, noting that fast cooling induced transient conditions where reversed temperature gradients increased damage, limiting TBC lifespan.

On the control of epitaxial growth and stray grains during laser-directed energy deposited Ni-based single crystal superalloy

Laser-directed energy deposition (L-DED) is considered as an effective repair method for damaged single-crystal (SX) superalloy turbine blades. However, controlling stray grains (SGs) and epitaxial growth is still the major problem. This study investigated the influences of pre-solution treatment and intrinsic heat treatment (IHT) on the formation of SGs and epitaxial growth of L-DED René N5 SX superalloy. The γ/γ’ eutectics and coarse γ’ precipitates at the inter-dendrite region can be eliminated after pre-solution treatment procedures before the L-DED experiment. A lower temperature in pre-solution treatment resulted in the retention of residual γ/γ’ eutectics and coarse γ’ precipitates. Thus, the aggravated element segregation within the local melt pool and interface collapse during the L-DED process can result in stray grains (SGs) formation. Improved pre-solution treatment procedures can help reduce SGs caused by the residual carbides at the bottom of the molten pool. Besides, the SG formation in the deposited layers can be attributed to the recrystallization as a result of high thermal stress during the IHT in thermal cycles. The optimized pre-solution treatment procedure reduced the elemental segregation of the substrates, resulting in a larger primary dendrite arm spacing (PDAS) from the bottom of the deposited layers. Due to IHT during thermal cycles in the deposited layers, the partial aging treatment promoted the re-precipitation and growth of γ’ phase. Meanwhile, the optimized pre-solution treatment procedure decreased the size of the heat-affected zone (HAZ) caused by thermal cycles. In summary, by optimizing the substrates' pre-solution treatment procedures to eliminate the γ/γ’ eutectics, the formation of SGs resulting from substrate segregation can be controlled. This work provides the potential guidance of substrate pre-treatment for L-DED repair of SX superalloys.

Very high cycle fatigue of a notched Ni-based single crystal superalloy at high temperature

A new notched geometry has been developed for ultrasonic fatigue testing of CMSX-4 single crystal Ni-based superalloy specimens at 1000 °C. Ultrasonic fatigue tests were carried out at 1000 °C, R = -1 and 20 kHz using this new geometry. Two competing crack initiation mechanisms were identified for the studied range of fatigue lives: internal crack initiation from a casting pore associated to a rough zone (RZ) formation and surface crack initiation assisted by oxidation. Failure modes associated to the identified fatigue crack initiation mechanisms are discussed. Comparing fresh fractographic evidence with available results from the literature, it is shown that fatigue crack nucleation and first stages of crack propagation seem to govern VHCF fatigue life.

Diffusion of alloying elements during high temperature oxidation in a low-cost third generation Ni-based single crystal superalloy

Diffusion of alloying elements in a low-cost third generation Ni-based single crystal superalloy during oxidation at high temperature was investigated. The evolution of oxide layers depended on dominant diffusion of alloying elements at different stages of oxidation. Outward diffusion of Cr, Co and Ni was dominant at initial stage of oxidation, resulting in the formation of (Ni, Co)O and Cr2O3. Diffusion of Al and Ta was restricted in γ/γ' area, but dominant in γ'-free layer. Due to outward diffusion of other elements, passive enrichment of refractory elements, Re, W, Mo and Ta included, caused the formation of mixed oxides layer. Finally, selective oxidation of Al led to the formation of (Ni, Co)Al2O4 and Al2O3 layer.

Effect of Initial Position and Crystallographic Orientation on Grain Selection Procedure in Z-Form Selector for Ni-Based Single-Crystal Superalloy.

The grain selection process in a Z-form selector for Ni-based single-crystal superalloy was simulated using a macro-scale ProCAST software (2013 version) coupled CAFE module combined with an experiment to investigate the grain selection procedure and mechanism with different grain positions and crystal orientation relationships. A non-stationary solidification process was found in the Z-form selector, and the liquid-solid (L-S) interface was tilted in the same direction as the selector channel during directional solidification. Given that the grain boundary was parallel to the Z-form selector, the overgrowth rate of the bi-crystal in the selector channel was very low. The initial position of the bi-crystal in the selector channel has a greater effect on the overgrowth rate than the effects of primary and secondary orientations. The grain selection was a result of the coupling of the competitive grain growth effect and geometrical restriction effect. Finally, the selection grain mechanism within the Z-form selector was discussed, coalescing the temperature field and the grain competition growth mechanism.

Correction: Microstructures and Mechanical Behaviors of Ni-Based Single-Crystal Superalloy DD90 Joints Brazed with a Co-Based Interlayer

Correction: Microstructures and Mechanical Behaviors of Ni-Based Single-Crystal Superalloy DD90 Joints Brazed with a Co-Based Interlayer

Effect of Ru on the precipitation of the TCP phase from the γ-γʹ microstructural degradation analysis

This work investigates the effect of Ru on the precipitation of the topologically close-packed (TCP) phase from the γ-γʹ microstructural degradation analysis. The addition of Ru is found to delay the precipitation time of the TCP phase from 100 h to 1000 h after thermal exposure at 950 °C. The effect of Ru on the γ-γʹ microstructural degradation is also investigated. Adding Ru slightly decreases the coarsening rate of the γʹ size and promotes the reduction of the volume fraction of γʹ phase. The reduction of the volume fraction of γʹ phase causes the "inverse partitioning" effect and increases the solution limit of the TCP forming element in the matrix. These help inhibit the precipitation of the TCP phases in Ni-based single crystal superalloys during thermal exposure.

Effects of lattice misfit of γ/γ′ phases on hydrogen embrittlement behavior in Ni-based single crystal superalloy

The effect of γ/γ′ phase lattice misfit on hydrogen embrittlement (HE) behavior was fundamentally investigated by utilizing a Ni-based single crystal CMSX-4 superalloy with a simple microstructure that can exclude the H-trapping effects of grain boundary, c-vacancy, and misfit dislocation. Increasing isothermal aging time increased γ′ precipitate size while maintaining its volume fraction and a fully coherent interface. The magnitude of negative lattice misfit between the γ and γ′ phases increased from −0.08 to −0.22% according to γ′ precipitate coarsening, resulting in a well-developed tensile-strain field within the γ′ precipitate. The increased tensile-strain field enhanced activation energy for H-desorption in the γ′ precipitate from 31.9 to 35.1 kJ/mol. Therefore, H is preferentially distributed along the γ/γ′ interface within the γ′ precipitate even before deformation. H basically promotes the slip planarity through the H-enhanced localized plasticity (HELP) mechanism in Ni-based single crystal alloys, in which slip is the dominant deformation behavior. As the superalloy possesses a larger negative lattice misfit, H becomes trapped as diffusible-state at the tensile-strain field of the γ/γ′ interface, resulting in cuboidal brittle fracture along the {001} planes. In this process, the HELP mechanism facilitated H localization at the γ/γ′ interface. Thus, the HE behavior distinctly transitioned from HELP to an interaction between HELP and H-enhanced decohesion mechanisms (HEDE) with an increase in the magnitude of the lattice misfit. The HE behavior was investigated by correlating microstructural characterization, H-trapping behavior, and crystallographic fracture mechanism analysis.

Microstructural evolution and its influence on the wear resistance of a laser directed energy deposited Ni-based single crystal superalloy

The wear behavior of Ni-based single crystal (NBSC) superalloy SRR99 fabricated by laser-directed energy deposition (LDED) is investigated and compared with that of its cast counterpart. While γ’ precipitate size in the latter is > 400 nm, that in the former is an order of magnitude lower. Dry sliding wear tests reveal that the wear rate and coefficient of friction of the LDED alloy are 75% and 20% lower than that of its cast counterpart, respectively. Detailed transmission electron microscopy investigation of the wear-tested cast alloy indicates that there is orientation change and formation of nanoscale grains only at the top layer of the worn surface, whereas regions below undergo moderate plastic deformation via dislocation slip. In contrast, the sub-surface of the worn LDED alloy has a graded microstructure, with a composite of NiO/γ-Ni on the top, γ’ free nano-grains in the middle, and a highly deformed nanoscale layer at the bottom. The improved wear behavior of the LDED alloy is attributed to its higher dislocation density, finer γ’ precipitates, and the formation of this graded microstructure. Finally, a detailed description of mechanisms that lead to the formation of this unique graded microstructure is provided.

Experimental study on the micro-mechanical parameters of Ni-based single-crystal superalloys using nanoindentation and element distribution analysis

Ni-based single-crystal superalloys are normally prepared by directional solidification. Due to different solidification sequences, Ni-based single-crystal superalloys exhibit a typical dendritic arm structure, and the element segregation in the dendritic arm can affect the high temperature mechanical properties of the alloy. Solid solution treatment has a certain extent to reduce the inter-dendritic and dendritic arm between the concentrations of the elements, thereby changing the alloy micro-nano view parameters to improve the high temperature mechanical properties of the alloy. In this study, nanoindentation experiments were carried out on the inter-dendritic and dendritic arm of the cast and solid solution samples. The micromechanical parameters were calculated. The amounts of changes in elemental concentration were discussed with the results of the energy dispersive spectroscopy and electron probe microanalysis, so as to analyse the relationship between the micromechanical parameters and the changes in elemental concentration. The results show that solid solution treatment has a significant impact on the elastic modulus, with an increase of 114.7% of the inter-dendritic region and an increase of 142.7% of the dendritic arm. However, solid solution treatment has little effect on the hardness, with no significant change in the hardness of the inter-dendritic region and an increase of 43.86% of the dendritic arm. Solid solution treatment has a more significant effect on improving the segregation behavior of Al and Co elements, with the segregation coefficients of Al and Co elements in the inter-dendritic region decreasing and increasing by 36% and 19%, and the segregation coefficients of Al and Co elements in the dendritic arm region decreasing and increasing by 29% and 34%.

Nucleation and transition sequences of TCP phases during heat-exposure in a Re-containing Ni-based single crystal superalloy

The nucleation and transition sequences of topologically close-packed (TCP) phases in a Re-containing Ni-based single crystal superalloy were systematically investigated using in-situ transmission electron microscopy (TEM) and three-dimensional atom probe technology (3D-APT). During the initial stage of heat-exposure at 1100°C, the TCP phase forming elements (Re, Co, Cr, etc.) segregated at the γ/γ′ interface near the γ matrix side to provide the concentration undulations for the nucleation sites of TCP phases, following which the σ and P phase coherently nucleated along the (111‾)γ and (022)γ planes from the γ/γ′ interface near the γ matrix side, respectively. With prolonged heat-exposure time, transitions from σ phase to P phase, σ phase to μ phase, and P phase to μ phase occurred. Besides, the orientation relationships of TCP phase intergrowth structures indicated that the P phase grew along the (1¯01)σ plane of the σ phase by co-lattice precipitation, meanwhile, the µ phase grew with smaller lattice misfits along the (04¯0)σ plane of the σ phase and the (400)P plane of the P phase. Additionally, the result by first-principles calculation evidenced that the μ phase had the lowest system energy to make the transition of σ phase and P phase to μ phases inevitable, therefore, the TCP phase ultimately existed as the most stable μ phase. Finally, the transition sequences of TCP phase during heat-exposure could be summarized into three types: γ matrix→σ→μ, γ matrix→P→μ, and γ matrix→σ→P→μ.

Decoupling the nonthermal effect of current by electrically assisted crystal plasticity modeling of Ni-based single crystal superalloys

The electrically assisted (EA) deformation process has received considerable attention in recent years, accompanied by research on current-induced deformation mechanisms. However, there are still challenges in eliminating thermal effects, which have prevented a comprehensive understanding of the underlying current-induced mechanisms. Opting for a single crystal (SC) in research provides advantages in decoupling the nonthermal effect of electric current at smaller scales and eliminating the complex interactions that exist in polycrystalline materials. Therefore, the innovation of this work lies in decoupling the nonthermal effect of electric current and conducting a comprehensive analysis of anisotropic deformation and mechanisms within a Ni-based SC with different crystallographic axes and various current directions during electrically assisted tensile simulation. A significant tension axis direction in the SC during EA tension was induced by the combination of a higher current direction factor (|cosθ|) and a dimensionless factor for the current density (|Jα/J0α|) along the [100] axis. The stress drop within the SC due to the nonthermal effect of electric current generally increased with increasing current direction. This was attributed to the increased dislocation density differences and decreased temperature. The increased stress anisotropy of the SC at a current direction of 45° was attributed to fewer activated (111) slip systems and the pinning effect of more dislocations within these systems. This study advances our understanding of the thermal and nonthermal effects of electric current and offers valuable insights for the informed application of EA deformations in industrial and aerospace settings with SC superalloys.

Parametric evaluation of chip formation in peripheral milling of single crystal Ni-based superalloy

Parametric evaluation of chip formation in peripheral milling of single crystal Ni-based superalloy

Quantitative analysis of interfacial microstructural degradation in MCrAlY-coated superalloys during varied service conditions

MCrAlY coatings are extensively utilized to shield turbine blades from oxidative and erosive circumstances. Nonetheless, the interdiffusion between the coating and the superalloy substrate results in the genesis of topologically close-packed (TCP) particles, thereby reducing the longevity of components exposed to high temperatures during operation. In this investigation, the progression of the microstructure in MCrAlY-coated Ni-based single crystal superalloys was meticulously quantified using digital image analysis, considering a spectrum of pre-exposure temperatures, durations, and stress levels. Through digital image analysis, we observed a variation in the length of TCP particles, ranging from 3.25 to 9.8 μm, under different conditions. Interestingly, the width of TCP particles remained consistent at 0.6 μm under all tested conditions. An innovative microstructure forecasting model was formulated, encompassing standardized parameters to evaluate the impact of pre-exposure conditions. This model demonstrated a robust correlation with experimental results. This study contributes to the refinement of our understanding of interfacial microstructure deterioration caused by operational conditions and aids in the progression of coating-substrate chemical compatibility.

Thermal fatigue failure analysis and life assessment of Ni-based single crystal superalloys with film cooling holes

The transient thermal fatigue behavior of Ni-based single crystal superalloys with film cooling holes under different peak temperatures (25 °C → 980 °C and 25 °C → 1050 °C) were investigated by a combination of test and finite element simulation. The initiation position and propagation direction of thermal fatigue cracks in both single hole and multi-hole specimens exhibit the angles of ± 45° with respect to the horizontal direction at the four corners of the holes. Octahedral slip failure mainly occurred at the crack initiation location and crack tip. The cyclic oxidation around the film cooling hole promotes the initiation and propagation of cracks. Based on the crystallographic theory, the strain–stress constitutive equation of the Ni-based single crystal superalloy under transient thermal shock was established. The location and evolution of thermal stress around the hole at different temperatures were obtained by finite element calculation. The kinetic behavior of cyclic oxidation has been quantitatively characterized. A thermal fatigue crack initiation life model considering heating temperature, transient thermal stress, and cyclic oxidation has been established. The simulation results are in good agreement with the tests.

Analysis of laser-induced surface damage of single-crystal Ni-based superalloy towards improving machinability

Single-crystal (SC) nickel-based superalloys provide improved high-temperature strength, and resistance to creep, fatigue and oxidation, compared to equiaxed and columnar-grained components due to their higher solid solution strengthening, precipitation hardening and the absence of grain boundaries. However, the SC nickel-based superalloys are ‘difficult-to-cut’. To enhance the machinability, a concept of laser-induced surface damage (LISD)-assisted machining is being studied. Laser surface modification (LSM) experiments were been performed by employing the design of experiments (DoE) strategy L18, OA (orthogonal array). The LSMed geometries were characterized to identify various laser-induced surface defects including solidification cracks, microcracks, micropores, microstructural changes, phase changes, recrystallization, and deformation. Further, parametric interactions and optimization of the process parameters were performed to manufacture a larger LISD layer. The parameter-structure-property relationship was derived systematically and the mechanisms of LISD on the SC nickel-based superalloy were described in details. The results indicate that the LISD increase with an increase in laser power followed by a decrease in scan speed and beam diameter. LISD geometries possess 1 % to 4 % of crack density and lower hardness of around 27 % than the base metal. Slot milling experiments were then conducted on the LISD specimens, which showed up to a 40 % reduction in cutting forces when compared to untreated specimens. It is evident that this methodology can be adopted to further improve the machinability of various difficult-to-cut materials.

Homogenization effects by Co additions to high temperature and low stress creep performance of a 4th-generation Ni-based single crystal superalloys

The important factors limiting the fourth-generation Ni-based single-crystal superalloys (Ni-SXs) are the increases in cost and density. A cheaper and lighter potential alternative element of Ru, Co's comprehensive effects were studied. The effects of Co and inhomogeneity between dendrite core (DC) and interdendritic region (ID) on creep performance at 1100 °C and 137 MPa in a 4th generation Ni-SXs were systematically investigated. The results show that the Ni-SXs with 9 wt% Co has the longest creep lifetime. Co additions alleviate the overall element segregations between DC and ID, especially Re, Cr, and W. And reverse partitioning behavior of Co also changes the partitioning ratio of elements between γ and γ′ phases, resulting in the inhibition of TCP phases precipitation for most creep time to avoid excessive consumption of solid solution strengthening elements and local stress concentration. The decrease of the lattice constant of γ reduces lattice misfit between γ and γ′ with the addition of Co, leading to the enlargement of dislocation network spacing and less resistance for movement or shearing of dislocation. The decrease of γ′ solvus temperature with Co addition promotes the dissolution of γ′ at high working temperature, resulting in severe topological inversion of γ/γ′ microstructures at ID. In addition, the main creep cracks of all Ni-SXs appear at ID, so the inhomogeneity between DC and ID is concerned.

Microstructures and Mechanical Behaviors of Ni-Based Single-Crystal Superalloy DD90 Joints Brazed with a Co-Based Interlayer

Microstructures and Mechanical Behaviors of Ni-Based Single-Crystal Superalloy DD90 Joints Brazed with a Co-Based Interlayer

Stress redistribution and stress triaxiality effect on the fatigue life of notched Ni-based single crystal superalloy at 760℃

The multiaxial stress state caused by geometric discontinuities will result in significant differences in the fatigue performance compared to the uniaxial stress state. It is crucial importance to research the notch fatigue behaviors of nickel-based single crystal (Ni-SX) superalloys at high temperature. This study investigated the impact of stress redistribution and stress triaxiality on the fatigue life of notched Ni-based single crystal superalloy at 760℃. The high temperature fatigue tests were carried out on the specimens under different stress ratios and the stress amplitudes under the same life conditions were obtained. A novel anisotropic damage model, incorporating the influence of stress triaxiality is proposed coupled with crystal plasticity theory. Based on experimental findings, an evaluation of the test parameters was conducted.The results showed that Mises stress, maximum principal stress, normal stress of principal stress along the loading direction, maximum principal strain, and strain along the loading direction were inadequate in reflecting the significant variations of samples observed among different stress ratios. However, the stress triaxiality at the unloading valley exhibited a high sensitivity to changes in fatigue life. To evaluate the fatigue life of notched specimens under different stress ratios, an improved Basquin’s type life prediction model was proposed, utilizing the stress triaxiality at the unloading valley. The proposed model demonstrates a favorable normalization trend for the fatigue life of notched specimens subjected to various stress concentration factor (SCF) conditions.