Single Crystal Superalloy Research Articles

Effects of rafting on the meso deformation and fracture behaviour of a Ni-based single crystal superalloy at room temperature: In-situ observation and simulation

Effects of rafting on the meso deformation and fracture behaviour of a Ni-based single crystal superalloy at room temperature: In-situ observation and simulation

Out-of-phase thermomechanical fatigue of a single crystal Ni-base superalloy

The present work studies the out-of-phase thermomechanical fatigue (OP-TMF) behavior of the precisely oriented [001] single crystal Ni-base superalloy ERBO/1 (CMSX-4 type). The OP-TMF tests are performed between minimum and maximum temperatures of 1023 and 1223 K and a cyclic mechanical strain amplitude of ±0.5 %. In accordance with previous findings, the present study confirmed that isothermal low cycle fatigue (LCF) tests at 1023 and 1223 K show significantly longer fatigue lives than the OP-TMF tests, specimens with rougher surfaces fail earlier than polished specimens and deformation bands and cracks form in the specimen surface. Two new results were obtained: First, when comparing OP-TMF cycles with fast cooling (tensile part of cycle) and fast vs. slow heating (compressive part of cycle) one finds that the slower heating/compressive cycle is more damaging. Second, analytical scanning electron microscopy of the flanks of an OP-TMF crack allows to study the diffusion-controlled growth kinetics of the oxide and the underlying zone, which is depleted by the oxide forming elements. These results are discussed in the light of the microstructural, mechanical and chemical results of the present study considering previous work on OP-TMF of superalloy single crystals (SXs).

Research on the effect of remelting on the epitaxial growth process of single crystal superalloy

In this paper, the remelting process is applied to Laser Directed Energy Deposition (L-DED) to repair single crystal superalloys. The main formation modes of stray grains in single crystal epitaxial growth were analyzed, and the inhibitory effect of remelting process on stray grains in single crystal epitaxial growth using L-DED was studied. Comparative experiments between remelting and non-remelting processes have been conducted. Three process parameters in the remelting process: power, z-axis lift and cladding-remelting interval time were studied. The impact of different remelting process parameters on single crystal epitaxial growth and the formation of stray grains has been studied. Remelting has been shown to effectively prevent the formation of stray grains during the solidification of the melt pool, which are otherwise caused by the presence of unmelted powder during the powder feeding phase of the L-DED process. A cladding layer without stray grains on both sides is obtained. After remelting, the epitaxial growth structure can achieve larger cellular and columnar dendritic zones and smaller primary dendrite arms. A favorable epitaxial growth structure was obtained in this paper, under the optimized remelting process window.

Microstructure based model for creep of single crystal superalloys in the high temperature and low stress creep regime

A new constitutive creep model for Ni-base superalloys has been developed that links the instantaneous creep response to incremental microstructure changes occurring during creep. The structure starts with cuboidal γ′- particles embedded in the γ-phase, and transitions to a rafted structure. In the present Kocks-Mecking type of model, the dislocation density ρ is considered as a state variable that evolves during creep. The increase of ρ depends on the γ-channel width in the γ/γ′-microstructure. Recovery, i.e., the reduction of ρ, is modelled from thermally activated, mechanically assisted climb. The strain rate is also modelled from the very same recovery process. The model successfully describes creep curves in a range of temperatures and load stresses with a maximum of five physical parameters. Once the model has been calibrated, it can be used to predict creep curves for other temperatures, stresses, and microstructures if the channel width evolution is known. The model is even capable of making reasonable predictions when fitted only to a single experiment. Extrapolations to different alloys with moderately varying compositions are possible.

A Study of the Internal Deformation Fields and the Related Microstructure Evolution during Thermal Fatigue Tests of a Single-Crystal Ni-Base Superalloy.

Ni-base superalloys operate in harsh service conditions where cyclic heating and cooling introduce deformation fields that need to be investigated in detail. We used the high-angular-resolution electron backscatter diffraction method to study the evolution of internal stress fields and dislocation density distributions in carbides, dendrites, and notch tips. The results indicate that the stress concentrations decay exponentially away from the notch, and this pattern of distribution was modified by the growth of cracks and the emission of dislocations from the crack tip. Crack initiation follows crystallographic traces and is weakly correlated with carbides and dendrites. Thermal cycles introduce local plasticity around carbides, the dendrite boundary, and cracks. The dislocations lead to higher local stored energy than the critical value that is often cited to induce recrystallization. No large-scale onset of recrystallization was detected, possibly due to the mild temperature (800 °C); however, numerous recrystallized grains were detected in carbides after 50 and 80 cycles. The results call for a detailed investigation of the microstructure-related, thermally assisted recrystallization phenomenon and may assist in the microstructure control and cooling channel design of turbine blades.

Monotonic tensile and cyclic deformation of a Ni-based single crystal superalloy with anisotropic microstructural rafting patterns at high temperature: Experiment and constitutive modelling

Monotonic tensile and cyclic deformation behaviours are investigated under different microstructural rafting states of a SC Ni-based superalloy, with emphasis on the influences of the rafting extent, type and loading orientation. The deformed microstructures and the dislocation configurations are characterized to give a micro-based understanding on the varying of deformation behaviours due to rafting. It is found that the decreases in the initial yield point and cyclic stress amplitude are only related to the rafting extent. Nevertheless, the rafting type (namely, the plate-like and needle-like morphology) has an undeniable contribution to the shape of hysteresis loops, where the plate-like rafting morphology results in more significant Bauschinger effect than needle-like rafting morphology. The variation of monotonic and cyclic deformation induced by rafting shares affinity with the alteration of internal stress and the movement of dislocations. Afterwards, a microstructure-sensitive constitutive model with two-phase flow rules has been developed. The effect of rafting on the monotonic and cyclic stress-strain responses is captured by introduce a series of microscopic mechanisms and a micromechanics-based back stress model that considers the morphology and size of the γ'/γ two-phase structures. The developed model is used to simulate the macroscopic stress-strain responses of the SC Ni-based superalloy under different rafting states. Model predictions are in good agreement with tests, capturing the reduction of cyclic stress amplitudes and the change in hysteresis loops. Finally, the impacts of the two-phase flow rules and the micromechanics-based back stress on the simulation capability have been discussed.

Effect of crystal orientation on surface/subsurface damage characteristics of nano-cutting Ni-based single crystal superalloy

In ultra-precision machining, material anisotropy has an essential impact on surface generation and subsurface damage. In this work, the molecular dynamics method is applied to create the diamond nano-cutting models of Ni-based single crystal (NBSX) superalloy workpieces with different crystal orientations, and the surface/subsurface damage characteristics, crystal structure changes, cutting force, and atomic temperature and stress are discussed. The results show significant differences in surface/subsurface quality, crystal structure transformation, cutting force, atomic temperature and stress distributions and dislocation density obtained from nano-cutting workpieces along different crystal orientations. In four groups of workpieces, the surface/subsurface quality is used as an evaluation index, which shows that the [110](001) crystal orientation has the best cutting effect, and the [111](1‾ 10) crystal orientation is the worst cutting direction. This work reveals the damage generation mechanism of the machined surface/subsurface of NBSX superalloy on an atomic scale, which provides technical support for improving machining quality and optimizing machining parameters.

Investigation on the interdiffusion mechanism between single-crystal superalloy and FeCrNiAl-based medium entropy alloys

High and medium entropy alloys (HEAs, MEAs) have emerged as promising candidates for bond-coat materials in the thermal barrier coating systems due to their exceptional oxidation resistance. However, understanding the interdiffusion behavior between these alloys and single-crystal superalloys (SXs) substrates requires further investigation. In this study, diffusion couples were created between SXs and two types of MEAs, namely FeCrNiAl and FeCrNi0.4Al0.4, and the interdiffusion behavior was simulated using the commercial DICTRA code. The results reveal significant interdiffusion at 1100 °C in the couples, attributed to the substantial elemental differences between SXs and MEAs. This led to the formation of the interdiffusion zone (IDZ) and the secondary reaction zone (SRZ). Within the SRZ, phase transformation of SXs occurred, resulting in the precipitation of topologically close-packed (TCP) phases. Three types of TCP phases (σ, μ, and P) were observed. Along the {11¯02} plane of the μ phase, twins and stacking faults have formed. The glide of P phase contributed to the bending of bar-like precipitates. Notably, the amorphization of the σ phase was observed for the first time, suggesting an intermediate state during the phase transformation. Despite elemental differences between the two MEAs, the growth mechanism of IDZ and SRZ, as well as the precipitation of TCP phases, exhibited similarities. This underscores the importance of Cr and Ni diffusion alongside Al diffusion. The pronounced interdiffusion behavior raises concerns about the suitability of 3-d transition metal MEAs as bond-coat materials due to the lack of slow diffusion effect.

The origin of the straight propagation of the σ phase in a Ni-based single crystal superalloy at elevated temperature

Topologically close-packed (TCP) phases, although inherently hard, are commonly perceived as detrimental in various alloys. In Ni-based single crystal superalloys (Ni-SXs), the primary adverse impact of TCP phases on material mechanical properties stems from their direct propagation, disrupting the original γ/γ′ dual-phase structure and leading to cleavage along the TCP phases under applied stress. Understanding the specific reasons for the straight propagation of TCP phases through the complex phase structure is essential for mitigating or potentially reversing their detrimental effects on material performance, a challenge that remains elusive. This study addressed this challenge by employing ultra-high spatial and energy resolution transmission electron microscopy (TEM) characterization, coupled with in situ scanning transmission electron microscopy (STEM) heating experiments in a fourth-generation Ni-based single crystal superalloy. Our research unveiled a significant phase-separation process during the incubation stage preceding σ phase formation at elevated temperatures, driven by the diffusion of atoms. Notably, domains of the γ′ phase, characterized by a scale of tens of nanometers, emerged within preexisting γ phases, and vice versa. This phenomenon reconstructed the original γ/γ′ dual-phase structure, leading to a finely refined dual-phase structure. The refined structure effectively reduced the diffusion distance of alloy elements necessary for forming σ phases. Consequently, the σ phases nucleated randomly and propagated directly through the newly refined γ/γ′ dual-phase structure. Throughout the thickening process, the notable difference in the diffusivity of alloying elements eventually resulted in the growth of a highly defective σ phase structure.

Effect of purity on solidification structure and micro-segregation in a nickel-based single crystal superalloy

The purity of nickel-based single crystal alloys plays an important role in the final service performance of turbine blades as it impacts the solidification structure, elemental segregation, and inclusions of castings. The highest purity of superalloys studied before was of altogether around 10 ppm of contents of N and O produced by the triple purification process (VIM+ESR+VAR). However, the impact mechanism still remains controversial, and meanwhile whether a lower content of N and O still keeps the impacts on castings is not yet clear due to the lack of raw materials. A superhigh purity of second generation of nickel-based single crystal superalloy DD98M with the sum of N and O contents as low as 4 ppm (hitherto the highest purity of the superalloy as published), home-produced by electron beam smelting (EBS), was put on focus in this work. The effect of three levels of purity (N-doped, commercial, and EBS, corresponding to 21, 10, and 4 ppm for the sum of N and O contents, respectively) on the microstructure evolution (including primary dendrite arm spacing (PDAS), eutectic, micropores, and γ′ phase) and the solute segregation behavior at dendritic scale and nano scale (γ/γ′ interfaces) were systematically studied and the influencing mechanisms were elucidated. The results show that the PDAS decreased from 406 μm to 322 μm with the reduction of O and N from 21 ppm to 4 ppm, which was irrelevant to precipitation of the nitride or oxide inclusions. Instead, it was due to the decrease of diffusion coefficient D or the increase of equilibrium coefficient k, or the decrease of the critical nucleation supercooling and the incubation time of dendrites. The micro-segregation, the number and volume fraction of eutectic, and the number and voltage percentage of shrinkage pores and gas pores were all greatly decreased with the increase of purity. These changes were closely related to the reduction of PDAS and the reduction of fluidity of the remained liquid in the interdendrites during solidification. The morphology and the size of the γ′ phase were independent from the purity, while the segregation of alloying elements at the γ/γ′ interface changed remarkably with the variation of the purity.

Unveiling the initial oxidation mechanism of a single crystal Ni-based superalloy with pre-deformation

The role of pre-deformation on the initial oxidation mechanism of a single crystal Ni- was revealed. With pre-deformation, the recrystallized grain boundaries and dislocations provided sites for the nucleation of Al2O3 and enhanced the diffusion flux of Al irrespective of γ/γ′ interface, γ′, and γ phase. Ultimately, a thin compact α-Al2O3 layer formed, improving the oxidation resistance. Without pre-deformation, Ni and Al atoms diffused along the γ'/γ interface, however, the Al-flux is insufficient to maintain the growth of Al2O3. As a result, a loose multilayer film formed on the surface, which decreased the oxidation resistance.

Effect of PtAl coating on the HCF property of a coated third-generation single crystal superalloy with sheet specimens at 900 ℃

High-cycle fatigue (HCF) is one of the primary failure modes for turbine blades in aero-engines. Therefore, comprehending the effect of coating on the HCF property of single crystal (SX) superalloys is vital for the safe service of turbine blades. In order to mimic the degradation of the fatigue property for the turbine blades during service, this study investigated the effect of PtAl coating on the HCF property of a third-generation Ni-based SX superalloy using sheet specimens at 900 ℃ and different maximum stresses (σmax). The results indicated that both coating and σmax affected the fatigue properties and crack initiation process of SX superalloy. A transition in fatigue crack initiation site from the internal micropores to the surface cracks for both uncoated and coated alloys has been observed as the σmax decreased. However, the coating significantly increased the σmax required for the transition of the crack initiation site. Meanwhile, the HCF property of the SX superalloy was reduced with the deposition of PtAl coating at lower σmax, and the debit in HCF life was more pronounced with decreasing the σmax. The coating facilitated the rapid nucleation of surface cracks and promoted the transition of the fatigue crack initiation site under higher σmax. On the other hand, since the HCF life is controlled by the crack growth rate, grain boundaries in the interdiffusion zone (IDZ) and secondary reaction zone (SRZ) accelerated the growth of the surface crack under lower σmax, resulting in a shorter crack initiation stage and lower fatigue life compared to the uncoated alloy. This study will be helpful in accurately predicting the HCF life of the coated Ni-based SX superalloys.

The Microstructure characteristic and Its influence on the stray grains of Nickel-based single crystal superalloys prepared by Laser directed energy deposition

The microstructure adjustment and the stray grains inhibition are important for the preparation of Nickel-based single crystal (SX) superalloys by laser directed energy deposition (L-DED). In this work, the single channel monolayer and single channel five-layers DD6 SX superalloys have been prepared by L-DED. The macroscopic simulations in the deposition processing and multiscale characterizations of deposition structure were used to analyze the formation and evolution mechanisms of unique precipitations and stray grains. Results shows that the larger laser power and higher powder feed rate have a positive impact on the epitaxial growth of columnar dendrite and the elimination of stray grains. The residual stress caused by the complex thermal cycles is not the direct reason that induce the formation of SGs, but the rapid movement of dislocations result in the formation of sub-grain boundary. According to the columnar transition to equiaxed theory (CET), temperature field simulation and analysis of element segregation behaviors, the element segregation results in the unique microstructure, include petal-shaped γ/γ′ eutectic and demarcation band with a thickness of a single γ/γ′ eutectic. Besides, the geometric size of γ/γ′ eutectic decrease with respect to the deposition height. With the change of height at deposition region, dendritic orientation deviations affect the uniformity of element segregation, resulting in variations in γ′ phase and γ/γ′ eutectic dimensions between dendritic core and the inter-dendrite zone at the microscopic level and manifesting as grain boundaries at the mesoscale. At the same time, the formation mechanisms of carbide and adjacent γ/γ′ eutectic leads to the misorientations between the dendrite core and the inter-dendrite zone, which also appears as stray grains at the mesoscopic scale.

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 also help reduce SGs caused by the carbides in the substrate. 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 equivalent heat treatment process 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,coarse precipitaes and decrease the carbides in the subsrtates, the formation of SGs resulting from substrate segregation and heterogeneous microstructure 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.

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.

Development of thermal barrier coating on single crystal superalloy CMSX-4 by two-source evaporation EB-PVD and hot corrosion performance of the coating in a simulated aero-engine environment

Superalloys are crucial in high-temperature applications, particularly in the gas turbine industries. With the development of high-temperature superalloy materials, efforts are still being made to improve the turbine entry temperature (TET), which will ultimately raise the efficiency of gas turbines. In this research, a second-generation single-crystal superalloy CMSX-4 based on nickel was prepared. The surface was coated by thermal barrier coating (TBC) with a bond coat of NiCoCrAlY with vacuum plasma spray (VPS) and a top coat with Yttria Stabilized Zirconia (YSZ) by Electron Beam Physical Vapor Deposition (EB-PVD) with a minimal current of 2.3 A with two sources compared to the conventional high-current that has been currently employed. These CMSX-4 specimens, both the as-cast and TBC coated, were used to test the effects of cyclic oxidation and hot corrosion at 1000 °C, simulating an aircraft engine. Also, a hot corrosion test using salt compositions (Wt%) of 60Na2SO4 - 40NaCl was performed simulating an aviation engine environmental condition. A FE-SEM/EDS analysis was used to examine the surface and cross-section microstructures of oxidized, hot-corroded as-cast, and hot-corroded TBC-coated CMSX-4 and its corrosion products. Using the XRD technique, the phases that were found in the oxidized and hot-corroded samples were identified. The outcome demonstrated that the EB-PVD TBC-coating on CMSX-4 with beam switching technique and a lower electron beam current has adequate protection without any damage to the coatings at 1000 °C.

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.

Effect of temperature on the tensile deformation mechanisms of a fourth nickel-based single crystal superalloys

This work conducted tensile experiments from room temperature to 1100 °C on a new fourth-generation single crystal superalloys along [001]. The effect of temperature on the tensile properties and deformation mechanism of the alloy were investigated. Results showed that the yield strength of the alloy reached its peak value (1017 MPa) at 850 °C. From room temperature to 760 °C, the occurrence of stacking faults (SFs) was observed in the γ matrix channel. From the room temperature to 850 °C, the occurrence of stacking faults was observed in the γʹ phase. The interface dislocation networks appeared at 980 °C and above. At room temperature, stacking faults sheared γʹ phase, in which three types of stacking faults occurred, which controlled deformation process. The main deformation mechanism from 650 °C to 850 °C was the shearing of γʹ phase by stacking faults and super dislocations, and at 850 °C, stacking faults in different directions of propagation corresponded to the formation of Lomer-Cottrell locks (L-C locks), which greatly increased the yield strength. At 980 °C and 1100 °C, dislocation climbing and bypassing and anti-phase boundary (APB) coupled dislocation pair shear γʹ phase became the main deformation mechanism.

Phase-field approach assisted solution heat treatment efficient design for novel multicomponent Co-based superalloys

Fast homogenization without incipient melting is the key to designing solution heat treatment (SHT) process for single crystal superalloys. In this paper, a multicomponent phase-field model coupled with solidus temperature is developed for dynamically tracking incipient melting temperature, TIM, of alloy during SHT. The simulation process runs completely automatically only by inputting the composition information of as-cast specimen. Based on the obtained TIM-time profile, the holding temperature and the time of SHT are determined. Applying this approach to Co–30Ni–11Al–4W–1Ta–4Ti–5Cr single crystal specimen, a gradual increased TIM from 1247 °C to 1293 °C within 72,000 s is obtained, and thus a stepwise SHT process with a short time of less than 20 h is designed. The experimental verification shows that after SHT, the segregation coefficient of the slowest diffusing element W decreased from more than 2 to 1.065, the content of harmful secondary phase has decreased to below 0.1%, and no incipient melting occurs.