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The fatigue crack initiation process accounts for the main portion of the high-cycle fatigue (HCF) life. Although coated superalloys are well-known to be prone to surface microcracks, the influence of the coating-substrate microstructure on the damage mechanism during HCF of the coated single crystal (SX) superalloys remains unclear. In the current study, HCF failure and interrupted tests were conducted at 900 °C under a low applied maximum stress on a PtAl-coated third-generation SX superalloy using sheet specimens. The HCF crack initiation was investigated by analyzing the evolution of surface microcracks and coating-substrate microstructure. The results show that the surface microcracks nucleation and growth are controlled by the initial microstructure of the coating-substrate region and the oxidation process during the HCF crack initiation stage. The rapid nucleation and growth of the surface microcracks are promoted by grain boundaries (GBs) in the coating and the interdiffusion zone (IDZ). Comparatively, the surface microcracks are blunted by oxidation in the SX substrate due to the loss of GBs, leading to a slower microcrack growth. However, the surface microcracks can gradually grow further into the substrate by repeating the processes as follows: oxidation at the crack tip, formation of γ′-depleted region due to oxidation, recrystallization at the γ′-depleted region and cracking along the GBs in the recrystallized γ′-depleted region. Subsequently, the surface microcracks, which reach the critical size, propagate along the slip bands during the progressive propagation stage. This study will contribute to improve the HCF durability of the PtAl-coated SX superalloy.

Surface integrity is very important to the service performance and lifespan of parts, but research of the microstructure evolution mechanism of single crystal superalloy during machining is still lacking. The grinding force and temperature of single crystal superalloy were collected, and the microstructure changes and microhardness on the grinding subsurface were characterized. The characteristics of the white layer were studied as well. The results showed that in the range of test parameters, the grinding force and temperature increase remarkably with the grinding depth increasing, and the thickness of the white layer and the plastic deformation layer increases with the increase of grinding depth and feed speed. The element diffusion is found between the white layer and the plastic deformation layer, resulting in rich Al and Ta elements in the white layer. The surface microhardness increases after grinding, and the hardening degree rises with the increase of grinding depth and feed speed. [ABSTRACT FROM AUTHOR]

An interesting phenomenon is found in Nickel-based single crystal superalloy thin-wall specimens which have the same [001] primary orientation and different secondary orientation with [100], [210], and [110] at 850 °C. Generally, the testing of cylindrical samples exhibits similar creep and tensile properties regardless of their vertical orientation distribution due to the circumferential symmetry of their geometric structure. However, the 850 °C experimental results of thin-wall specimens with the same [001] primary orientation show a significant difference in tensile ductility and creep life when the secondary orientation is different. The mechanism of the secondary orientation effect is completely discussed by combining experiment, digital image correlation (DIC), and finite element modelling (FEM), which result from the strengthening effect of Lomer-Cottrel dislocations and the ability to coordinate plastic deformation in tensile, the shrinkage rate of cross-section caused by the single slip system ({111} ) and deviation from [001] with a small angle in creep.

Here we investigated the nucleation of the Re, Ru-rich δ phase affected by stacking faults in the γ phase in a 4th generation Ni-based single crystal superalloy during the service at 800°C. The stacking faults in the γ phase were rich in Re and Ru, similar to the observed δ phase. And the stacking fault regions possessed a similar structure to the δ phase, promoting the transformation from γ phase to δ phase. Furthermore, the calculation results indicated that Ru had more significant promoting effects on the δ phase precipitation than Re, providing a reference for the alloy design.

A discontinuous plastic flow is detected in a single-crystal superalloy with an [001] orientation during tensile tests over a wide temperature range. Results reveals that the synergistic impact of normal γ′ precipitates and nanoscale γ′ precipitates in γ matrix on the dislocations motion is the root of the serrated flow. [ABSTRACT FROM AUTHOR]

Hydrogen-fueled and hydrogen-hybridized aircraft engines are a new trend in the aviation industry for environmental reasons. Single crystalline Ni-based superalloys are the most commonly used engine materials and their hydrogen embrittlement properties need urgent investigation. In this study, the hydrogen embrittlement behavior and underlying fracture mechanism of a second-generation Ni-based single crystal superalloy with electrochemical hydrogen pre-charge were investigated. The superalloy showed tremendous susceptibility to hydrogen embrittlement with reduced strength and ductility. A large number of micropores and cracks on the fracture surface are found in hydrogen-charged specimens, leading to embrittlement and ultimate cracking. More dislocations, stacking faults and DSBs are observed in specimens with hydrogen uptake. Hydrogen-induced micropores first form at the γ/γ′ interface and then propagate into the γ′ phase, leading to cracking, which was analyzed using in situ environmental studies with a transmission electron microscope. Hydrogen reduces the cohesive strength between the γ- and γ′-phase and accelerates crack propagation along the voids. Hydrogen embrittlement fracture in Ni-based single crystal superalloys is due to synergistic hydrogen-enhanced local plasticity, strain-induced vacancies and decohesion in the hydrogen-induced cracking process.

This study presents a design strategy to enhance the high-temperature creep resistance of Ni-based superalloys. This strategy focuses on two principles: (1) minimizing the dimensions of γ/γ′ interfaces and γ channels by reducing the size of the γ′ phase; (2) key alloy composition control to strengthen the heterostructure γ/γ′ interfaces. This strategy proved very effective by the designed three superalloys’ prolonged creep lives. An alloy exhibits ultra-long creep life by 388 h at 1100°C/137 MPa, which runs at the highest level among those alloys without Ru addition. With Ru addition, an alloy that lasted for 748 h with a creep strain of ~6% at 1110°C/137 MPa is developed. This study provides a new route of high-temperature creep lives through heterostructure interfacial design with size effects and key alloying elements.

Here we investigated the nucleation of the Re, Ru-rich δ phase affected by stacking faults in the γ phase in a 4th generation Ni-based single crystal superalloy during the service at 800°C. The stacking faults in the γ phase were rich in Re and Ru, similar to the observed δ phase. And the stacking fault regions possessed a similar structure to the δ phase, promoting the transformation from γ phase to δ phase. Furthermore, the calculation results indicated that Ru had more significant promoting effects on the δ phase precipitation than Re, providing a reference for the alloy design. [ABSTRACT FROM AUTHOR]

Ni-based single crystal (SX) superalloy with low specific weight is vital for developing aero engines with a high strength-to-weight ratio. Based on an alloy system with 3 wt.% Re but without W, namely Ni-Co-Cr-Mo-Ta-Re-Al-Ti, a specific weight below 8.4 g/cm3 has been achieved. To reveal the relationship among the composition, mechanical properties, and thermal stability of Ni-based SX superalloys, SXs with desirable microstructures are fabricated. Tensile tests revealed that the SX alloys have comparable strength to commercial second-generation SX CMSX-4 (3 wt.% Re and 6 wt.% W) and Rene′ N5 alloys (3 wt.% Re and 5 wt.% W) above 800 °C. Moreover, the elongation to fracture (EF) below 850 °C (>20%) is better than that of those two commercial SX superalloys. During thermal exposure at 1050 °C for up to 500 h, the topological close-packed (TCP) phase does not appear, indicating excellent phase stability. Decreasing Al concentration increases the resistance of γ′ rafting and replacing 1 wt.% Ti with 3 wt.% Ta is beneficial to the stability of the shape and size of γ′ phase during thermal exposure. The current work might provide scientific insights for developing Ni-based SX superalloys with low specific weight. [ABSTRACT FROM AUTHOR]

• Vibration fatigue failure behavior of Ni-based single crystal FCH structure at high temperature were studied. • Crack initiation mechanism of FCH structure is oxidative cracking nucleation in stress concentration area. • Crack propagation mechanism is the competition mechanism of dislocation slip and dislocation climbing. • Vibration fatigue life prediction model considering stress concentration and high-temperature oxidation was proposed. • Life prediction accuracy is within 2.5-time scatter band, so life prediction model proposed has high prediction accuracy. The vibration fatigue failure behavior and fatigue life of the film cooling hole (FCH) structure of Ni-based single crystal superalloy were investigated at high temperature. The vibration fatigue test of the FCH specimens were carried out based on the paired staircase method. The cracks are mostly initiated in the stress concentration area at the edge of the FCH, and may also be initiated in non-stress concentrated areas with large defects. At high temperature, the crack initiation mechanism of the FCH specimens of Ni-based single crystal superalloy is oxidative cracking nucleation in stress concentration area, and the crack propagation mechanism is the competition mechanism of dislocation slip and dislocation climbing. Based on the Fatigue Indicator Parameter (FIP) of the critical plane method, a vibration fatigue life prediction model for FCH structures considering stress concentration and high temperature oxidation damage is proposed. The life prediction model proposed in this paper is applied to the vibration fatigue life prediction of FCH specimens. The deviation between the predicted results and the experimental results is within 2.5 times at 850℃, and the deviation is within 2 times at 980℃. Besides, the life prediction method proposed is compared with other two FIP life prediction methods, and the high accuracy and effectiveness of the proposed life prediction method are verified. [ABSTRACT FROM AUTHOR]

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. • For the first time, Ni-based single crystal superalloy was ultra-precision machined by ultrasonic vibration diamond cutting. • Machined surface morphology and tool wear generation in diamond cutting Ni-based superalloys were explored in detail. • Cutting-induced microstructure evolution was carried out to indicating the subsurface damage and work hardening tendency. [ABSTRACT FROM AUTHOR]

Abstract Ni‐based single crystal (SX) superalloys are important high‐temperature materials used for manufacturing turbine blades in aero‐engines. During service under combinational impacts of temperature and stress, the SX superalloy may reach its life due to plastic deformation, which normally accompanies time‐dependent microstructural degradation. To reveal this dynamically mechanical response, tensile tests at 1050°C are carried out to record stress‐strain curves at five stain rates as well as creep curves at four applied stresses. Deformed microstructures and defects have been analyzed to understand mechanical behaviors and the underlying mechanism by using advanced scanning electron and scanning transmission electron microscopes. Results show that the deformation mode of the alloy strongly depends on the strain rates/applied stresses under mechanical loading. The dislocation density inside the γ phase is extremely low at all tests, indicating that the γ phase is relatively weak and ready to flow at this temperature even at a very fast strain rate. The deformation behavior of the γ′ phase is much complicated. At fast strain rates or high applied stresses, the dislocation density in the γ′ phase is very high, contributing to high‐stress requirements to deform the material. At slow strain rates or low applied stresses, rafting microstructures develop and the deformation mode becomes directional coarsening/diffusion‐dominated. Our results demonstrate a comprehensive understanding of the deformation mechanism of Ni‐based SX superalloys, which may provide lifetime prediction of the mechanical failure, as well as the database for superalloy applications in mechanical systems.

The balance between the stress rupture properties and castability of a 4th-generation Ni-based single crystal superalloy can be achieved by addition of appropriate carbon content. In this work, the effects of carbon content on the microstructure and stress rupture properties of a 4th-generation Ni-based single crystal superalloy were investigated in detail. The results showed that minor carbon addition could aggravate the dendrite segregation, while the addition of carbon had no obvious effect on the rafting and distortion of γ′ phase during stress rupture deformation. At 760 °C/810 MPa and 1100 °C/210 MPa, the stress rupture lives of the three experimental alloys initially increased and subsequently decreased with the increasing carbon content. However, under condition of 1140 °C/137 MPa, the increased carbon content resulted in the reduction of the stress rupture lives of alloy. It was discovered that minor carbon elements addition (0.045 wt%) can promote formation of interfacial dislocation networks and improve creep resistance at high temperatures. At intermediate temperature, the introduction of carbon could reduce the stacking fault (SF) energy of γ matrix, thus facilitating the formation of SFs in γ matrix. The M 6 C granular carbides precipitated at high temperature, which was considered to be beneficial to stress rupture properties. [ABSTRACT FROM AUTHOR]

The effect of Pt-Al coating on low-cycle fatigue behavior of Ni-based single crystal superalloy at 760 °C was evaluated. The results show that Pt-Al coating does not change the cyclic stress response behavior of the Ni-based single crystal superalloy at 760 °C. Under the total strain amplitude from 0.6 % to 1.2 %, the cyclic stress amplitude value of Pt-Al coating samples is slightly lower than uncoated samples during cyclic deformation at the stable stage under the same applied total strain amplitude, and the fatigue life of Pt-Al coating samples is lower than uncoated ones, with an average fatigue life reduction of about 19.9 %. By the relation curve of comparing the cyclic strain amplitude with the reversals to failure, it is observed that the fatigue ductility coefficient ε′ f of the Pt-Al coating sample is higher than the uncoated sample, the fatigue ductility index c and fatigue strength coefficient σ′ f are lower than the uncoated sample, but the fatigue strength index b has little difference. It is found that the cyclic strength coefficient K′ and the cyclic strain hardening index n' of the Pt-Al coating sample are lower than the uncoated sample after fitting the curves of the relationship between cyclic stress amplitude and total strain amplitude. In addition, the cyclic stress amplitude of Pt-Al coating sample is slightly lower than uncoated sample under the same applied total strain amplitude. It was found by SEM that the cracks of both uncoated and Pt-Al coating samples initialized near the surface of the samples under low-cycle fatigue loading at 760 °C, and multiple crack sources began to spread into the substrate. It was found by TEM observation that at low strain amplitude (Δε t /2 = 0.6 %), the dislocations in both uncoated and Pt-Al coating samples mainly converged in γ, and a large number of dislocation debris gathered in the γ and dislocation entanglement was generated, while a small number of dislocations were observed to cut into γ'. The γ′ is seriously deformed and slightly rafted at Δε t /2 = 1.2 %, and a few dislocations are also observed in γ'. [ABSTRACT FROM AUTHOR]

• Fatigue crack propagation in Ni-base single crystal superalloy at room temperature. • Near-threshold fatigue cracks propagate along crystallographic slip planes. • Crystal orientations of the C(T) specimens affect the fatigue crack behavior. • Crystal plasticity FE analysis is conducted to rationalize near-threshold cracking. • Near-threshold cracking is quantified by a damage parameter based on slip. Fatigue crack propagation in the near-threshold regime was investigated experimentally and numerically in a Ni-based single crystal superalloy, namely, CMSX-4. Fatigue crack propagation tests were conducted at room temperature using four types of specimens with different primary and secondary crystallographic orientations. The results reveal that the crystallographic orientations strongly affect the fatigue crack propagation behavior, including the crack paths, fatigue crack propagation rates, and fatigue threshold. Crystal plasticity finite element analysis was conducted to quantify the slip activity of an octahedral slip system in front of the crack tip, considering the actual 3D geometry of the crack plane and elastic–plastic anisotropy. The fatigue damage parameter, considering the slip activities of the individual octahedral slip systems, provides reasonable explanation for the fatigue crack propagation rates and crack paths in the near-threshold regime, regardless of the crystallographic orientation. Furthermore, these damage parameters are equivalent at the fatigue thresholds for all specimens with different crystallographic orientation. [ABSTRACT FROM AUTHOR]

• The PtAl coating effects on the high-cycle fatigue (HCF) behavior of SX superalloy with sheet specimens were investigated. • The coated alloy had lower HCF lives than the uncoated alloy at low maximum stresses (σ max < 590 MPa). • The fatigue crack initiation site of the coated alloy could transform at higher σ max than that of the uncoated alloy. • The brittle cracking of the coating and the grain boundaries in the IDZ/SRZ leaded to surface microcrack initiation and growth, facilitating the formation of crack initiation sites. 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 (σ max). The results indicated that both coating and σ max 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 σ max decreased. However, the coating significantly increased the σ max 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 σ max , and the debit in HCF life was more pronounced with decreasing the σ max. The coating facilitated the rapid nucleation of surface cracks and promoted the transition of the fatigue crack initiation site under higher σ max. 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 σ max , 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. [ABSTRACT FROM AUTHOR]

To investigate influences of Ta content on microstructural stability during thermal exposure at 900 °C (up to 3000 h) and creep properties at 980 °C/163 MPa in Ni-based SX superalloy, alloys with 7Ta, 8Ta and 9Ta (wt.%) were studied utilizing DSC, SEM, TEM, HR-XRD and APT. The results shows that Ta addition enhanced the partitioning behavior of alloying elements. Besides, lattice misfit was controlled by both alloy composition and temperature. Ta addition increased absolute misfit both room temperature and 980 °C. The difference was that the γ/γ′ lattice misfit changed from positive to negative at room temperature. The γ′ coarsening indicated that Ta addition reduced the effective diffusion coefficient and exacerbated W segregation at γ/γ′ interface, which was beneficial to improve the microstructural stability. Merging of several adjacent γ′ precipitates in preferred direction was observed to be delayed with increasing Ta content, which occurred at 500 h for 7Ta alloy, 800 h for 8Ta alloy and 1000 h for 9Ta alloy, respectively. Ta addition affected the diffusion kinetics and greatly prolonged the steady creep duration, and then resulted in longer creep rupture life. It was discussed based on the γ matrix channels, γ′ raft thickness, the perfection degree of γ′ rafts, residual γ′ precipitates, interfacial dislocation networks and solid solution strengthening. [ABSTRACT FROM AUTHOR]

The service performance of single crystal blades depends on the crystal orientation. A grain selection method assisted by directional columnar grains is studied to control the crystal orientation of Ni-based single crystal superalloys. The samples were produced by the Bridgman technique at withdrawal rates of 100 μm/s. During directional solidification, the directional columnar grains are partially melted, and a number of stray grains are formed in the transition zone just above the melt-back interface. The grain selected by this method was one that grew epitaxially along the un-melted directional columnar grains. Finally, the mechanism of selection grain and application prospect of this grain selection method assisted by directional columnar grains is discussed. [ABSTRACT FROM AUTHOR]

The effect of thermal exposure on the microstructure and creep properties of the Ni-based single crystal superalloy in different test conditions was studied. Long-term exposure was performed at 1,000 °C and 1,100 °C for 500 h prior to the creep tests. The creep lifetime is found to be improved after the long-term exposure at 1,000 °C for 500 h as a result of the formation of secondary M23C6 in the interdendritic region. The coarsening of γ' precipitates accompanied by the formation of TCP phase lead to the degradation of alloy, which is responsible for the reduction of the creep lifetime of Ni-base single crystal superalloy after long-term exposure at 1,100 °C for 500 h. The creep lifetime of 1,000 ºC thermally exposed sample under the conditions of 1,093 ºC/137 MPa is lower than that of heat-treated state. Thermal exposure at 1,100 ºC for 500 h causes the creep lifetime to drop drastically.

In this work, four types of surfaces were prepared as follows: untreated one, dry grinding (DG), wet grinding (WG) and minimum quantity lubrication grinding (MQLG) for Ni-based single crystal superalloy. The effects of grinding conditions on the surface roughness and microstructure evolution were studied. Dry sliding tests of ground surfaces were carried out at room temperature. Through the quantitative characterization of the wear rate, the area, width and depth of the worn profiles, the friction and wear mechanism of superalloy prepared by different grinding conditions were analyzed. The results show that the MQLG surface with low surface roughness and work hardening behavior has the best wear resistance. The element transfer behavior from the GCr15 ball to the worn surface was detected by EDS analysis. The wear type is mainly abrasive wear, accompanied by slight adhesive wear and oxidation wear. It is shown that high-quality surface with nanocrystalline and high density dislocation structure produced by MQLG improves the tribological properties of superalloy, which provide theoretical guidance for the surface machining of single crystal blade to reduce fretting wear. [ABSTRACT FROM AUTHOR]

Ni-based single crystal (SX) superalloy with low specific weight is vital for developing aero engines with a high strength-to-weight ratio. Based on an alloy system with 3 wt.% Re but without W, namely Ni-Co-Cr-Mo-Ta-Re-Al-Ti, a specific weight below 8.4 g/cm3 has been achieved. To reveal the relationship among the composition, mechanical properties, and thermal stability of Ni-based SX superalloys, SXs with desirable microstructures are fabricated. Tensile tests revealed that the SX alloys have comparable strength to commercial second-generation SX CMSX-4 (3 wt.% Re and 6 wt.% W) and Rene′ N5 alloys (3 wt.% Re and 5 wt.% W) above 800 °C. Moreover, the elongation to fracture (EF) below 850 °C (>20%) is better than that of those two commercial SX superalloys. During thermal exposure at 1050 °C for up to 500 h, the topological close-packed (TCP) phase does not appear, indicating excellent phase stability. Decreasing Al concentration increases the resistance of γ′ rafting and replacing 1 wt.% Ti with 3 wt.% Ta is beneficial to the stability of the shape and size of γ′ phase during thermal exposure. The current work might provide scientific insights for developing Ni-based SX superalloys with low specific weight.

The single crystal superalloy SRR99 experimental samples were single point treated by laser shock peening (LSP) with laser pulse energy of 5–7 J. The surface morphology, micro-hardness, surface residual stress and surface microstructure of samples prior and after LSP were determined by corresponding characterization instrument. From the surface morphology, a circular pit was formed in surface, which due to the severe plastic deformation induced by LSP, and the diameter & maximum relative height of circular pit were increase with laser pulse energy. From the micro-hardness and residual stress test results of samples, these were increased after LSP treatment. In detail, the initial microhardness and residual stress were 398 HV and -498 MPa, with laser pulse energy of 5 J, the microhardness and residual stress were increased to 455 HV and -843 MPa, when the laser pulse energy was increased to 7 J, the microhardness and residual stress were increased to 512 HV and -942 MPa. In addition, the surface roughness of samples was increased too, which reflect the severe plastic deformation. By observing the microscopic morphology of the bottom of the pit, it is found that there are convex structure and crater shape defect structures on the bottom surface, and the formation of these structures may affect the surface roughness of the shock area. Although LSP caused certain degree of twist deformation of the shape of the γ' phase, the original strengthening method characteristics of the matrix phase and strengthening phase was not destroyed, which still maintained the creep strength and thermal fatigue properties of superalloy. The experiment show that LSP can cause significant work hardening effect on the surface of superalloy, and the generated high residual compressive stress can effectively delay the initiation and propagation of cracks. Therefore, LSP technology can effectively improve the fatigue life of single crystal blade components.

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. [Display omitted] • Surface/subsurface damage characteristics are formed by different crystal orientations. • The workpiece with the [110](001) crystal orientation has the optimal surface/subsurface quality. • Material stress induces the formation of dislocations and defects, and three deformation zones appear in the tool-to-workpiece contact area. • The temperature drop point is closely related to chips and side flow. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Three-dimensional atom-probe tomography was used to characterize the γ/γʹ interface structure in a third-generation Ni-based single crystal superalloy with Re addition. It was found that an element-segregation layer with Re, Co and Cr was formed in the γ phase close to the γ/γ′ interface, resulting in a more negative local interface misfit (− 0.29%) compared to the measured result (− 0.16%) from high-resolution X-ray diffraction. Furthermore, the total reduction of interfacial free energy due to the solute atom segregation based on the Gibbsian interfacial excess was calculated to indicate that Re element was the most beneficial element in producing this more negative local misfit with the largest interfacial free energy reduction (13.67 ± 0.21 mJ/m2). Simultaneously, because of the co-segregation of Re, Co and Cr, and the depletion of Ni in the γ phase close to the γ/γ′ interface, it was also deduced that some harmful topologically close-packed phases might be easier to nucleate and grow in the γ phase close to the γ/γ′ interface in service. The element-segregation layer across γ/γ′ interface resulted in a more negative local interface misfit (− 0.29%), compared to the measured result (− 0.16%) from highresolution X-ray diffraction, and the total reduction of interfacial free energy due to the solute atom segregation was calculated to indicate that Re element was the most beneficial element in producing this more negative local misfit with the largest interfacial free energy reduction (13.67 ± 0.21 mJ/m2). [ABSTRACT FROM AUTHOR]

The development of Ni-based single crystal superalloys relies heavily on the composition design with the addition of critical alloying elements, e.g., Re and Ru. Understanding the role of alloying effects require to know the configurations of the alloying element distribution between γ-Ni and γ′-Ni3Al phases and among various non-equivalent sites. This work employed first-principles density functional theory calculations to study the preference of phase and site occupancy of 11 alloying elements including Al and transition metal elements: 3d (Ti, Cr, Co, Ni), 4d (Mo, Ru), and 5d (Hf, Ta, W, Re) in Ni and Ni3Al. We calculated the substitution energies of 1298 triple-site doping configurations including 286 NiNiNi site doping of Ni, 726 AlNiNi site doping, and 286 NiNiNi site doping of Ni3Al with alloying elements Ni, Co, Ru, Cr, Re, Mo, W, Al, Ti, Ta, and Hf. In the dual-site and triple-site doping of Ni and Ni3Al, all studied alloying elements preferred to occupy Ni phase rather than Ni3Al phase. We found that the most stable defect complexes often contained the favorable substitutions of Al, Ti, Ta, and Hf for the Ni sites that stabilized the alloying elements doping at the other one or two nearest neighbor sites. The co-substitutions of various alloying elements at multiple sites are critical to understanding the strengthening mechanism of alloying elements in Ni-based single crystal superalloys. [ABSTRACT FROM AUTHOR]

The effect of thermal exposure on the microstructure and creep properties of the Ni-based single crystal superalloy in different test conditions was studied. Long-term exposure was performed at 1,000 °C and 1,100 °C for 500 h prior to the creep tests. The creep lifetime is found to be improved after the long-term exposure at 1,000 °C for 500 h as a result of the formation of secondary M23C6 in the interdendritic region. The coarsening of γ′ precipitates accompanied by the formation of TCP phase lead to the degradation of alloy, which is responsible for the reduction of the creep lifetime of Ni-base single crystal superalloy after long-term exposure at 1,100 °C for 500 h. The creep lifetime of 1,000 °C thermally exposed sample under the conditions of 1,093 °C/137 MPa is lower than that of heat-treated state. Thermal exposure at 1,100 °C for 500 h causes the creep lifetime to drop drastically. [ABSTRACT FROM AUTHOR]

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In order to research the milling subsurface layer damages of DD5 Ni-based snngle crystal superalloy, based on the directional cutting method and slot miling experiments, this paper discussed the DD5 deformation layer characteristics and recrystaliization behavoors though the massive DD5 lon gkudinal sections micro-metallographic structures and nticro^ardness tests under different miliing parameters before and after heat treatments. Then the control approaches for inhibtiing the deformation layer generation and development were proposed. The research results demonstrate that miliing parameters, cooing methods and crystal drrection condttions all influence the DD5 hardening rate largely, the hardness of the machined surface along the normal drrection show the "hardening-softernng-hardening"trend, which illustrate that the main reason for metamorplic layer formation ss work hardening under the competition mecharnsm between the thermal softennng and strann hardennng, and the metallographc structure in this area ss transformed, which improve the corrosoon ressstance of DD5 machined surfaces. The reasonable miliing trajectory along the [110] crystal drection scheme and the water based minimum quantity lubncation adoption of cooiing methods are benetidal for the inhibirion of deformation layer formation. In addttion, under the machining condttions designed herein, the mili ing processes may not provide sufficient stored energy to activate material recrystaliization under the tradirional and vacuum high temperature condmons to 1300°C. [ABSTRACT FROM AUTHOR]

In order to improve the difficult-to-machine property of DD5 Ni-based single crystal superalloys, a high-speed milling transition criteria and chip edge formation mechanism were proposed with the variable of cutting speed based on adiabatic shear theory and using the method of single factor experiments and FEM simulation. According to the measurement results of the chip sawtooth level and intervals, the chip free surface transforms from lamellar structure to serrated structure when cutting speed reaches 37.7 m/min, which shows the milling has entered into high-speed zone. The stress concentrated on the grooves intersected between the chip side and adiabatic shear band induces the longitudinal crack formation and expansion, which causes the transverse and longitudinal stress distribution variations. Then, the transverse cracks were generated. The longitudinal and transverse crack formation was the main reason of the chip edge generation. With the increasing of milling speed, the chip edge morphology transformed from flat and smooth trapezoid to narrow and defective triangle. Compared with the traditional machining, high-speed cutting is beneficial to the reduction of cutting force, chip edge height and intervals, and restrain the lateral crack expansion at the same time. [ABSTRACT FROM AUTHOR]

Creep rupture life is a key material parameter for service life and mechanical properties of Ni-based single crystal superalloy materials. Therefore, it is of much practical significance to accurately and efficiently predict creep life. Here, we develop a divide-and-conquer self-adaptive (DCSA) learning method incorporating multiple material descriptors for rational and accelerated prediction of the creep rupture life. We characterize a high-quality creep dataset of 266 alloy samples with such features as alloy composition, test temperature, test stress, and heat treatment process. In addition, five microstructural parameters related to creep process, including stacking fault energy, lattice parameter, mole fraction of the γ' phase, diffusion coefficient and shear modulus, are calculated and introduced by the CALPHAD (CALculation of PHAse Diagrams) method and basic materials structure-property relationships, that enables us to reveal the effect of microstructure on creep properties. The machine learning explorations conducted on the creep dataset demonstrate the potential of the approach to achieve higher prediction accuracy with RMSE, MAPE and R 2 of 0.3839, 0.0003 and 0.9176 than five alternative state-of-the-art machine learning models. On the newly collected 8 alloy samples, the error between the predicted creep life value and the experimental measured value is within the acceptable range (6.4486 h–40.7159 h), further confirming the validity of our DCSA model. Essentially, our method can establish accurate structure-property relationship mapping for the creep rupture life in a faster and cheaper manner than experiments and is expected to serve for inverse design of alloys. Image, graphical abstract [ABSTRACT FROM AUTHOR]

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 Cr 2 O 3. 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)Al 2 O 4 and Al 2 O 3 layer. [Display omitted] • Evolution of oxide layers depended on dominant diffusion of alloying elements. • Dominant diffusion of alloying elements changed with different oxidation stages. • Other elements diffused outwards led to passive enrichment of refractory elements. • Distribution of Re, W and Mo was mainly controlled by passive enrichment. [ABSTRACT FROM AUTHOR]

• The thermal fatigue failure mechanism of Ni-based single crystal superalloys with hole structure has been revealed. • The strain–stress constitutive equation under transient thermal shock based on the crystallographic theory was established. • The thermal stress location and evolution around the hole at different temperatures was obtained by FEM. • A thermal fatigue crack initiation life model considering transient thermal stress and cyclic oxidation has been proposed. • There is good consistency between the experimental results and the simulation results. 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. [ABSTRACT FROM AUTHOR]

• Thermal fatigue behavior of Ni-based single crystal superalloy at 25–900 °C are investigated. • The cracks only initiate at the direction of 45° from [0 0 1] direction in hot corrosion environment, which is different from the multi-point initiation of cracks in air environment. • The invisibility of crack morphology in hot corrosion environment deserves to be highlighted. • The influence mechanism of hot corrosion on the thermal fatigue behavior of Ni-based single crystal superalloy is proposed. As an important failure model for single crystal materials in service, with the increasing complexity of working environments, the thermal corrosion fatigue failure needs to be studied urgently. In this study, the thermal corrosion fatigue crack initiation and propagation behaviors of an Ni-based single crystal superalloy in both air and hot corrosion environments at 25–900 °C were studied by using a designed precise thermal fatigue test device. The microstructure characteristics and crack composition were analyzed by SEM, TEM and EDS, and the internal stress and recrystallization during thermal fatigue were characterized by EBSD. Moreover, the detailed and comprehensive crack morphology was characterized with different surface treatments. The results showed that the crack morphology was invisible in the hot corrosion environment, which is worthy of being highlighted. The initiation and propagation of thermal fatigue cracks were also accelerated in the hot corrosion environment, which contributed to the sufficient thermal stress, the accelerated material degradation of the inner/outer layers in the crack region, and the rapid forward diffusion of O 2 in the inner crack layer. Finally, the influence mechanism of hot corrosion on the thermal fatigue behavior of the Ni-based single crystal superalloy is proposed. [ABSTRACT FROM AUTHOR]

The accelerated creep failure of Ni-based single crystal superalloy by hot corrosion is confirmed by creep tests at 900 °C/550 MPa–400MPa in air and salt environments. The analysis of creep curve, microstructure and fracture type shows that there are obvious differences in hot corrosion-creep behavior under high, middle and low creep stress conditions, indicating that there is the stress dependence between the influence of hot corrosion on the creep properties of the alloy. Hot corrosion accelerates creep failure of Ni-based single crystal superalloy by decreasing the deformation resistance, reducing the effective stress area, and affecting the creep fracture mode. [ABSTRACT FROM AUTHOR]

Very high temperature creep properties of twelve different Ni-based single crystal superalloys have been investigated at 1250 °C and under different initial applied stresses. The creep strength at this temperature is mainly controlled by the remaining γ′ volume fraction. Other parameters such as the γ′ precipitate after microstructure evolution and the γ/γ′ lattice parameter mismatch seem to affect the creep strength to a lesser degree in these conditions. The Norton Law creep exponent lies in the range 6–9 for most of the alloys studied, suggesting that dislocation glide and climb are the rate limiting deformation mechanisms. Damage mechanisms in these extreme conditions comprise creep strain accumulation leading to pronounced necking and to recrystallization in the most severely deformed sections of the specimens.

Assisted by the mold preinstalled an alumina tube into seed segment, the influence of the original primary dendrite spacing of seed on the formation of stray grains at melt-back region was investigated during Ni-based single crystal casting using seeding method. The results showed that the interface reaction between the seed and mold as well as the formation of stray grains at the surface of seed were avoid using an alumina tube with surface roughness of 0.35 μm as the mold inner wall. As the original primary dendrite spacing of the seed decreased to less than 201 μm, the morphology of un-melted solid phase in semi-solid zone changed from isolated dendrite stem to the complex network, resulting in the inhibition of the formation of stray grains inside of the seed. The seed with original primary dendrite spacing of 201 μm was also successfully re-used to fabricate single crystal casting. Image 1 • A re-using seed method was proposed for Ni-based single crystal casting. • Formation of stray grains was completely suppressed at melt-back region of seed. • The influence of original microstructure of seed on stray grain was discussed. [ABSTRACT FROM AUTHOR]

Highlights • The microstructure evolution of γ′ precipitate was studied. • The coarsening rates of the γ′ precipitates is accelerated with increasing Mo:W ratio. • The coalescing of γ′ precipitates is aggravated with increasing Mo:W ratio. Abstract The phase stability of γ′ precipitates in single crystal Ni-based superalloys at the elevated temperature is inherently associated with creep performance. During high temperature in-service exposure, the coarsening of γ′ precipitates typically occurs and further results in coalescing that is regarded as an indicator of degradation of creep resistance. In this research, we design three single crystal Ni-based superalloys by means of solely tuning Mo:W ratio to investigate their coarsening behaviors upon different temperatures (950–1050 °C) and exposure times (0–500 h). The experimental results showed that the coarsening rate of γ′ precipitates increases with increasing Mo:W ratio. The coarsening kinetics are adequately characterised by the classical LSEM model. The experimental results can be used to optimize the design of alloy chemistry. [ABSTRACT FROM AUTHOR]

In this research, the hot corrosion behavior of laser repaired Ni-based single crystal superalloy in fuel gas environment is analyzed and compared with that of the substrate, and the corrosion mechanism is determined. The deposited single crystal layer is comprised of the γ(Ni) matrix and abundant spherical γ′(Ni 3 Al) nanoparticle phases. The corrosion products exhibited layered structure including the outer layer dominated by NiO, NaTaO 3 and (Ni,Co)Cr 2 O 4 , the inner layer mainly comprising Al 2 O 3 and Cr 2 O 3 , and the internal γ′ depleted layer. The hot corrosion resistance of the deposited part is superior than that of the substrate, owing to the synergistic effects of much finer γ′ phase, higher volume fraction of γ′ phase, and more compact corrosion product of the deposited component. Our work provides a practical strategy for enhancing the hot corrosion resistance of laser repaired superalloy by manipulating the size and morphology of γ′ phase. [Display omitted] • The γ′ phases in the substrate are cuboidal while those in the deposited part are spherical and much finer. • Hot corrosion products with three-layer structure are found on both the surfaces of substrate and deposited part. • Gibbs free energiesare calculated to elucidate the structure and composition of corrosion products. • Finer γ′, higher fraction of γ′, and denser product contributed to superior performance of the deposited layer. [ABSTRACT FROM AUTHOR]