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Being the final procedure in the blade shape processing, the material removal depth of abrasive belt grinding ultimately affects the blades' final shape precision and their servicing performance. The advanced blade material, single crystal superalloy, exhibits anisotropic mechanical characteristics, which adds complexity in blade material removal mechanism. In this work, based on crystallographic elastoplastic theory and abrasive belt's surface geometry, a microscopic penetration depth-contact force function is derived considering the anisotropic nature of the material through the multi-scale analysis of the contact between flexible wheel and the workpiece, Furthermore, combining with the dynamic contact zone profile, a flexible removal depth model of anisotropic materials is established. The proposed model was evaluated by the robotic belt grinding experiments. Experimental results demonstrate the model's excellent predictive capabilities. The results illustrate the significant effect of crystallographic anisotropy on MRD, and verified the tangential force and MRD predicting accuracies of the proposed model. [ABSTRACT FROM AUTHOR]

Single crystal superalloy turbine rotor blade is one of the core hot-end components of the aero-engine， which has a decisive role in the thrust and performance of the aero-engine. Additive manufacturing for repair technology is one of the most challenging tasks in the special machining of aviation equipment. In this paper， the repair processes and their application for single crystal superalloy turbine rotor blades were systematically reviewed. Aiming at the problems of hot cracking defect， the cracking formation mechanism， key influencing factors， and control methods were summarized. In addition， the research progress in microstructure and mechanical properties of single crystal superalloys repaired by additive manufacturing technology are summed up. Furthermore， the prospective developing direction of single crystal superalloy turbine rotor blade repair is indicated. Specific filler material composition design， new process development， and multi-objective collaborative optimization based on deep learning are considered to be important future research directions.

Currently， the surface working temperature of second-generation or more advanced single crystal superalloy blade is above 1050 ℃， resulting in serious interdiffusion between coating and substrate. The interdiffusion not only consumes the beneficial elements of the coating and then reduces its service life， but also leads to the formation of secondary reaction zone in the substrate， which will seriously damage the high-temperature mechanical performance of the single crystal superalloy. Preparing diffusion barriers and designing the low inter-diffusivity coatings are effective ways to control the interdiffusion between thermal barrier coatings and single crystal superalloy substrates. Firstly，based on the latest international and domestic research progresses， the design principles， characteristics and classification of diffusion barriers were introduced briefly， and then the main problems and related research studies of metal diffusion barriers， ceramic diffusion barriers and active diffusion barriers were analyzed. Secondly， the application and latest progress of low inter-diffusivity coatings were reviewed， and the high-temperature protection properties and anti-diffusion mechanisms of nanocrystalline coatings， equilibrium coatings and γ′-based coatings were discussed in detail. Finally， the future development directions of different diffusion barriers and low inter-diffusivity coatings were pointed out.

The solution-treated second-generation single crystal (SX) superalloys underwent hot isostatic pressing (HIP) at 1316 °C and 105 MPa for varying durations. This study investigates the effect of HIP on micropores and stress-rupture behavior under conditions of 980 °C and 250 MPa. The results illustrate that the application of HIP treatment significantly reduces the porosity and volume of micropores, thereby increasing the rupture life of the alloys. With the disappearance of micropores, the crack initiation site shifts from the micropores to the γ/γʹ interface, resulting in reduced crack dimensions and decelerated propagation. Consequently, the smaller size of cracks in HIPed specimens slow down fracture. Nonetheless, the crack propagation pattern remains consistent: cracks expand perpendicular to the stress axis, connecting with cracks at other heights via secondary cracks along the direction.

Turbine blades are mainly subjected to thermal stress and gradient centrifugal stress in operational conditions. The effects of centrifugal stress on the microstructural degradation of single crystal superalloys under a multiaxial stress state remain unclear. Here, the NiAlReRu component was exposed to high-speed rotation at 20,000 rpm and a high temperature of 1000 °C for 100 h. The analysis indicates that the centrifugal stress causes the distortion of dendrite stems and the aggregation of micropores. The centrifugal stress promotes the formation of interfacial dislocation networks, accompanied by more severe rafting of γ′ phases. These findings provide a comprehensive experimental understanding of the microstructural evolution under simulated turbine blade working conditions, which provides a reference for the subsequent tests under near-service conditions.

Copyright of Journal of Materials Engineering / Cailiao Gongcheng is the property of Journal of Materials Engineering Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The preparation of a single crystal superalloy surface overlay welding coating to improve its high-temperature mechanical properties is of great significance for prolonging the service life of blades. This work selected two types of welding wire alloys, CoCrMo and CoCrW, to prepare coatings on the surface of a single crystal superalloy. A comparative study was conducted on their mechanical properties, such as tension, compression, fatigue, durability, and wear at a high temperature of 900 ℃, aiming to reveal the high-temperature mechanical properties of the two types of welding coatings. Results showed that the average high-temperature tensile strength of the CoCrMo welded specimen was smaller than that of the CoCrW welded specimen; the average high-temperature duration of CoCrMo welded specimens at 150 MPa was lower than the average duration of CoCrW welded specimens; the high-temperature fatigue life of CoCrMo welded specimens at 220 MPa was 7.186 × 105; and the average high-temperature wear rate of CoCrMo sample was 3.64 × 10−6 mm3·N−1·m−1. The CoCrW alloy was more wear resistant than CoCrMo. The hardness of CoCrMo welded joints gradually increased from the substrate to the heat-affected zone and then to the fusion zone, and was much higher in the fusion zone than in the CoCrW alloy. [ABSTRACT FROM AUTHOR]

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.

Size effect of the nickel-based single crystal superalloy DD6 is investigated in this study by performing nanoindentation experiments and finite element (FE) simulations by emphasizing the strain rate effect. An analytical method is proposed to identify whether a material has the indentation size effect (ISE) based on incremental formulations of the indentation strain rate. By taking ISE as a critical factor, the contact area is further described by considering the curvature radius of non-ideal indenters due to wear issues in practice. In order to emphasize the strain rate effect during indentations, the material parameters dominating mechanical properties are adopted from the strain rate dependent Johnson–Cook model, particular for the nickel-based single crystal superalloy. Consequently, the dimensional ISE analysis is performed by proposing a theoretical framework to establish the relationship among the indentation strain rate, the indenter tip curvature radius and the applied load–penetration depth (P–h) curve for characterizing nanoindentation responses. By identifying the indenter curvature radii and loading time as the important external factors of hardness measurement, FE simulations are validated to achieve good agreement with experimental data. The validated FE predictions are reliably employed to calibrate the dimensionless model of hardness with different indenter curvature radii and loading rates. In the proposed form of the dimensional hardness model, it is found that the indenter curvature radius can be perfectly independent of the material constitutive properties and loading rates. It reveals the potential and advantageous determination of characteristic length of materials by using nanoindentation.

The development of clean and green manufacturing of superalloys has attracted widespread attention. The returned scrap of superalloys, however, contains a significant amount of oxygen (O), nitrogen (N), and inclusions, which cannot be used effectively, and thus resulting in substantial waste generation. In this study, the electron beam drip melting (EBDM) is used to deeply remove the O, N and inclusions in a single crystal superalloy. The concentration of O and N can be reduced to 0.57 and 0.55 ppmw after drip melting, with removal rates of 97.9 % and 83.8 % respectively. The number density of inclusions experienced a significant decrease, dropping from 12.90 to 0.18 mm−2, representing a reduction of 98.6 %. The intense Marangoni convection will gather the surface inclusions in the central area of the electron beam spot. The inclusions get close to each other to form cavities, and thus the aggregation reaction occurs to form inclusion clusters. Due to the dissolution and decomposition reactions caused by the local overheating and bombardment of electron beam, the inclusions can be removed effectively. The generated O and N are eliminated through mass transfer and evaporation reactions, while the remaining inclusions on the surface can be artificially removed. The electron beam drip melting can be used for the preparation of superalloys with extremely low levels of O, N and inclusions.

The primary dendrite spacing (PDS) and segregation of directionally solidified single crystal (SC) superalloy under the longitudinal magnetic field (LMF) were investigated based on the analysis of the whole cross-sectional microstructure at different solidification distances. The results show that the PDS under the LMF remains basically unchanged at different solidification distances, and it is greater than that under no LMF. With the increase of magnetic field intensity, the PDS increases and the macrosegregation decreases. The increasing PDS and reducing segregation under the LMF can be attributed to the increase of solute boundary layer, which expands the non-equilibrium freezing temperature range and brings the effective partition coefficient closer to 1. The increase of the solute enrichment layer thickness could be caused by the downward secondary circulation generated by the thermoelectric magnetic convection (TEMC) near the interface, which drives the migration of solutes towards the interdendritic region. This work not only clarifies the mechanism of LMF controlling PDS and reducing segregation by TEMC, but also provides theoretical guidance for producing high-quality SC superalloys using magnetic fields. • The longitudinal magnetic field makes the primary dendrite spacing more uniform at different solidification distances. • The longitudinal magnetic field reduces the macrosegregation. • The increasing primary dendrite spacing and reducing segregation are attributed to the increase of solute boundary layer. • The increased solute boundary layer could be caused by secondary circulation generated by TEMC. [ABSTRACT FROM AUTHOR]

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Single crystal plate castings of superalloy CMSX-4 were prepared, providing simplified geometries of turbine blades featured with five stage platforms. The recrystallization behavior in the plate castings was investigated after solution heat treatments at different temperatures. The results show that after solution heat treatment, recrystallization occurs at the abrupt cross-section of every platform except the lowest one. The recrystallization area increases and then decreases as the platform's height increases. The outside platform of the casting is also more likely to result in recrystallization than the inside one. The nucleation and growth of the recrystallization grains are significantly influenced by the solution heat treatment temperature (TH). When TH is raised from 1303 ℃ to 1315 ℃, the recrystallization area increases by 30 times, from 9.7 mm2 to 293 mm2. Because of the abrupt section transition, the platform part of the casting will produce more plastic deformation and accumulate a large amount of deformation energy during solidification and cooling, and the abrupt change in cross-section will easily become the source of recrystallization during the subsequent heat treatment. Meanwhile, the difference in heat exchange conditions, cooling rate and deformation rate during solidification leads to uneven stress distribution. As a result, different recrystallization behaviors appear after solution heat treatment.Furthermore, increasing TH can effectively promote the dissolution of γ/γ' eutectics and coarse γ' phase, reduce the pinning effect of residual γ/γ' eutectics and coarse γ' phase in the recrystallization grain boundary migration process, and significantly increase the recrystallization defect area.

The low cycle fatigue（LCF） properties of DD6 single crystal superalloy were investigated at 700 ℃ and R of 0.05. SEM was used to study the fracture surface and fracture microstructure. The results show that the LCF life of the alloy decreases with the increase of strain amplitude. LCF properties of the alloy are excellent under asymmetrical cyclic loading. The alloy has no transition fatigue life during LCF tests at all total strain amplitudes. LCF fatigue damage can be dominantly contributed to elastic damage and the plastic deformation is very minimal. The plastic damage increases with the increase of total strain amplitude. The crack initiation site, the fatigue crack propagation area and the final fracture zone can be observed in the fracture surface and all specimens is similar to quasi-cleavage fracture. The fatigue cracks are initiated from the micro-pores on the surface, sub-surface or far from the surface. Far from the surface crack fractures have fish-eye feature. The fatigue crack propagates perpendicularly to main stress at first and then along {111} plane. Typical fatigue striation, cleavage steps and river pattern characteristic are formed on fatigue crack propagation zone. The cleavage plane, slip band and tearing ridge are seen in the final fracture zone. Fracture microstructure analysis shows that the γ′ phase morphology far from the fracture surface still maintains cubic shape， and the slip bands are visible seen near the fracture surface， and secondary cracks are formed along slip bands.

The effect of secondary γ' phase evolution on the creep properties of single crystal superalloy DD6 was investigated at 760℃/785 MPa and 980℃/250 MPa by FESEM and TEM. The results indicate that the secondary γ' phases in DD6 alloy with standard heat treatment are precipitated in the matrix channel after exposure at 1120℃/4 h/AC. Since the secondary γ' phases in the matrix prevent the a/2〈011〉 dislocation gliding in the matrix channel and promote the {111}〈112〉 slip operated in the primary γ' phases at the beginning of creep at 760℃/785 MPa, the secondary γ' phases in the matrix channel decrease the incubation period and significantly improve the primary strain and creep rate. The secondary γ' phases in the matrix will dissolve rapidly at the beginning of creep at 980℃/250 MPa, therefore, the secondary γ' phases have no influence on the creep behaviour of 980℃/250 MPa.

High-performance joining especially brazing has been highly expected in the manufacture or maintenance of single crystal blades. In this paper, a Ni-40Cr-10Ta (at.%) alloy was developed as filler material for single crystal superalloys. High-performance brazing of DD5 superalloy was achieved. The joints brazed at 1260 °C contained an isothermal solidification zone (ISZ) with “γ + γ′” structure and an athermal solidification zone (ASZ) composed of γ, γ', γ/γ′-eutectics, TaCr2 and TaC. Unlike the conventional filler materials with melting point depressants of B and Si, the Ni-Cr-Ta filler material was free of B and Si, and thus the resulting joints contained no B-rich and Si-rich compounds and eutectics with low melting points, which enhanced the brazed joint performance. The tensile strength at 980 ℃ of the heat-treated joints reached 654 MPa, which is 88.5 % that of the base metal.

The mechanism of oxidation-assisted initiation of surface cracks of a fourth-generation Ni-based single crystal superalloy was systematically investigated during low cycle fatigue at 900 °C and 980 °C. The results show that cracks initiate near the surface defects at 900 °C, while they initiate in the surface oxide layer at 980 °C. At 900 °C, the oxidation microcrack initiation in the thicker inner oxidation layer is difficult to connect with the surface oxidation crack, which is an essential reason for the crack growth rate being slower and not becoming the main crack. At 980 °C, microcracks form in the outer oxide layer and quickly connect with microcracks at the surface and inner/outer oxide layer interface, growing into long cracks that become channels for rapid oxygen transport. This accelerates the crack growth rate, and eventually the oxide crack becomes the main crack. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Materials Engineering / Cailiao Gongcheng is the property of Journal of Materials Engineering Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Journal of Materials Engineering / Cailiao Gongcheng is the property of Journal of Materials Engineering Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Creep characterization of different diameter silica bar core during directional solidification of the single crystal superalloy was investigated by suspension method. The scanning electron microscope (SEM) was employed to observe the microstructure of the surface and transversal section of crept silica bar. The energy dispersive spectroscopy (EDS) and X-ray diffraction technology were used to analyse and determine the composition of the reaction product. The experimental results show that the crept deformation amount increases with prolonging of creep time and decreasing of the diameter of the silica bar. When the creep time is 60 min, the diameter 0.5 mm silica bar has the largest creep deformation, and the average deformation is 30 mm, while the 2 mm silica bar is 24 mm. The interfacial reaction of SiO2 with C and Al, which are deposited on the silica bar surface due to the elevated temperature and low vacuum, induces the formation of porous layer. The volume fraction of the porous reaction products leads to the different crept deformation amount of different diameter silica bars. The creep deformation amount of silica bars has a linear relationship with the volume fraction of surface reaction products.

Thermal-mechanical fatigue (TMF) tests and isothermal fatigue (IF) tests were conducted using thin-walled tubular specimens under strain-controlled conditions. The results of TMF tests showed a strong correlation between mechanical behavior and temperature cycling. Under different phases of temperature and mechanical loading, the hysteresis loop and mean stress of the single crystal superalloy showed noticeable variations between the stress-controlled and strain-controlled conditions. In the strain-controlled TMF test, temperature cycling led to stress asymmetry and additional damage, resulting in a significantly lower TMF life compared to IF life at the maximum temperature. Moreover, the OP TMF life is generally lower than that of the IP TMF at the same strain amplitude. The Walker viscoplastic constitutive model based on slip systems was used to analyze the TMF mechanical behavior of the single crystal superalloy, and the change trends of the maximum Schmid stress, the maximum slip shear strain rate, and the slip shear strain range were analyzed, and their relationship with the TMF life was investigated. Finally, a TMF life prediction model independent of the loading mode and phase was constructed based on meso-mechanical damage parameters. The predicted TMF lives for different load control modes and phases fell within the twofold dispersion band. [ABSTRACT FROM AUTHOR]

In order to investigate the solidification microstructures in typical section dimensions of a third generation single crystal superalloy DD9 turbine blade with 1500℃ and 1540℃ mould temperatures during directional solidification process, Optical microscopy (OM) and scanning electron microscopy(SEM) were used to analyse the solidification structure of typical sections of the blade. The results show that with increasing mould temperatures, the dendrite patterns have a tendency of becoming more refined, and the secondary dendritic arms tend to be highly developed. With the same mould temperature, the dendrite patterns in the blade aerofoil section are more refined than those in the tenon section. Also with increasing mould temperatures, the γ' precipitates of the interdendritic regions and the dendritic cores tend to be refined, the γ' precipitation size dispersion decreases, and the size distributions of the γ' precipitates follow the normal distribution law. Compared with the interdendritic regions, almost 61% reduction of the γ' precipitation average sizes is measured in the dendritic core. With the same mould temperature, the γ' precipitation sizes in the aerofoil section are more refined than those in the tenon section. Compared with the decreasing sectional areas, the increasing mould temperatures bring down the γ' precipitation sizes obviously. The sizes and contents of the γ-γ' eutectics decrease with increasing mould temperatures. The morphologies of γ-γ' eutectics show both sunflower-like shape and plate-like shape.

The impact of a Pt-Al bond coat on the high-cycle fatigue (HCF) behaviour of fourth-generation single crystal superalloys at 760℃ and 900℃ was investigated. The Pt-Al bond coat was found to be almost negligible under low stresses at 760℃; however, the bond coat effect was detrimental at 900℃ or under high stresses at 760℃. At 760 °C with increasing high-amplitude stress, the tip cracks preferentially nucleated from defects within coat and propagated inwards, accelerating the fracture. The fatigue life of the coated alloys was considerably decreased at 900 °C, which was attributed to the damage accumulated in the bond coating via oxidation, crack-induced oxide cracking and interfacial microstructure degradation. An Elevated temperature led to the crack-induced oxides shifting from a small fan-shape to a large umbrella-shape, increasing the density of slip bands and quantity of secondary cracks in the substrate. Furthermore, more Ru diffused outwards at 900℃ than that at 760℃, which caused the disappearance of L-C dislocation locks and tertiary γ′ phases and aggravated TCP phases precipitation in the substrate at 900℃. Ultimately, to assess the degradation of the HCF life, an empirical life prediction method was developed, and the calculations results well matched the test results.

To expand the selection range of materials for blades in gas turbines, new single crystal superalloys with different Re contents were developed and the microstructures, high temperature tensile properties and fracture behaviors were investigated. The dendrites and γ+γ′ eutectic are observed in the as-cast superalloys. After heat treatment, the γ+γ′ eutectic in the interdendritic regions is completely eliminated and cubic γ′ phase with regular morphology maintains a coherent relationship with γ matrix. The size of γ′ phase decreases with the increasing Re due to its inhibition effect on the growth of γ′ phase. The three alloys have little differences in strength below 850 °C and possess the maximum tensile strength at 760 °C. However, the strength of 3Re and 0Re alloys is obviously higher than that of 1.5Re alloy at 980 °C due to the higher content of refractory elements. In addition, all alloys have a very obvious work hardening peak below 760 °C before fracture. With the increasing temperature, the characteristics of plastic fracture become more obvious due to the changes of dislocation motion. At 650 °C, the alloys show quasi-cleavage fracture. At 980 °C, numerous dimples can be clearly seen while no slip deformation trace is found, showing obvious dimple aggregation fracture. The γ′ phase elongates along the direction of stress loading, and the γ channel perpendicular to the direction of the stress axis widens. The rafting phenomenon of γ′ phase in the all three single crystal superalloys does not appear, indicating the alloys have good microstructure stability.

Based on three-dimensional atom probe technique and Ardell method, the influences of temperature (850℃∼1150℃) on microstructure, compositions and solid solution strengths of γ and γ′ phases, γ/γ' partitioning behaviors of alloying elements and γ/γ' interfacial energy of a Mo-rich Ni based single crystal superalloy were investigated. After quenching from 1100℃, microstructure was basically similar as heat-treated state. However, after quenching from 1150℃, γ′ phase obviously re-dissolved, and γ channel obviously widened. Meanwhile, with increasing temperature, Co, Cr, Mo and Re contents in γ phase decreased, while Al and Ta contents in γ phase increased. With increasing temperature, Co, Cr and Re contents in γ′ phase slightly decreased, Mo content in γ′ phase significantly decreased, while Al and Ta contents in γ′ phase increased. Meanwhile, with increasing temperature, solid solution strength of γ phase decreased, while solid solution strength of γ′ phase increased. In addition, with increasing temperature, partitioning ratios of Co, Cr and Re elements decreased, partitioning ratios of Mo and Al elements increased, while partitioning ratio of Ta firstly decreased and then increased. Moreover, both γ/γ′ interfacial width and γ/γ′ interfacial energy decreased with increasing temperature. • Compositions of γ and γ' phases in heat-treated and quenched Alloy Mo were measured by high-precision 3DAP technique. • With increasing temperature, Co, Cr, Mo and Re contents in γ phase decreased, Al and Ta contents in γ phase increased. • With increasing temperature, Co, Cr, Mo and Re contents in γ′ phase decreased, Al and Ta contents in γ′ phase increased. • With increasing temperature, solid solution strength of γ phase decreased, solid solution strength of γ′ phase increased. • Both γ/γ′ interfacial width and γ/γ′ interfacial energy of Alloy Mo decreased with increasing temperature. [ABSTRACT FROM AUTHOR]

This paper comprehensively explores the causes of intermediate-temperature brittleness of a fourth-generation Nickel-based single-crystal superalloy through a systematic study of tensile properties and deformation mechanisms at intermediate-temperatures. The research was performed at temperature of 700 °C, 800 °C, 850 °C and 900 °C. The experimental alloy demonstrated the highest yield strength of 914 MPa and worst plasticity at 800 °C. It was found that pure-shearing fracture occurred at 700 °C and 800 °C, while the fracture characteristics of shear fracture and ductile fracture were found at 850 °C and 900 °C. Then, the slip bands extended in the same direction at 700 °C and 800 °C. The phenomenon was different at 850 °C and 900 °C. These deformation traces extended in the different directions. At 700 °C and 800 °C, the partial dislocation with Burgers Vector of a/3 <112> shearing into the γ′ phases was the predominate deformation mechanism, while both the partial dislocation with Burgers Vector of a/3 <112> and the super-dislocation with Burgers Vector of a <110> and a <010> shearing into the γ′ phases were present at 850 °C and 900 °C. Nevertheless, the mechanism of the super-dislocation with Burgers Vector of a <110> and a <010> shearing γ′ phases pervaded in the alloy at 900 °C. In general, it was concluded that the alloy underwent intermediate-temperature brittleness at 800 °C in terms of the changes of fracture features, slip bands and dislocation configurations. The results of this study provided an experimental reference and guidance for improving the safe serviceability of the fourth-generation single crystal superalloy. • Tensile test was conducted on a fourth-generation SX superalloys at intermediate-temperatures. • Tensile fracture mechanism was discussed from macroscopic and microscopic perspectives. • The increase in the brittleness of the alloy at intermediate-temperatures were analyzed. • This work guided future optimization in Ni-based SX superalloys. [ABSTRACT FROM AUTHOR]

The effects of Ta on the high temperature creep behavior and deformation mechanism in a Ni-based single crystal (SX) superalloy were investigated at 1120 °C/137 MPa. Ta was regarded as an essential strengthening element of the γʹ phase in the alloy, and the results indicated that the increase of Ta prolonged the creep rupture life. In this work, the addition of Ta increased the volume fraction of the γʹ phase and narrowed the γ channels, making the dislocation bowing out at the primary stage of creep more difficult. As creep continued, with a lower effective diffusion coefficient of alloying elements and larger γʹ raft thickness, the alloy with 6.5 wt% Ta addition was considered to possess the lower dislocation climbing rate, which reduced the minimum creep rate of the alloys. Subsequently, it was noted that the density of superdislocations in γʹ phase was clearly reduced with the increase of Ta. This was supposed to be the elevated Ta content in the alloy, which increased the lattice misfit and led to the formation of a denser dislocation network, thereby enhancing the interfacial strengthening effect and effectively preventing the cutting of superdislocations. The Ta effect on high-temperature creep was systematically summarized in this study, which was an important guideline for further composition design and optimization of Ni-based SX superalloys. [ABSTRACT FROM AUTHOR]

In this paper, the creep properties and deformation mechanisms of a low-cost 3rd generation nickel-based single crystal superalloy deviating 1.5°, 7.8°, and 14.7° from [001] direction were studied at 800 °C/760 MPa and 1100 °C/137 MPa. The microstructure after full heat treatment and creep rupture of specimens were observed by scanning electron microscopy (SEM). The dislocation configurations of specimens after creep rupture were observed by transmission electron microscopy (TEM). The results indicate that specimens with different orientation deviations exhibit strong anisotropy at 800 °C/760 MPa, and the creep lives of specimens decrease with the increase of θ angle. In specimens A (1.5°) and B (7.8°), the creep deformation is controlled by multiple <112>{111} slip systems, which would produce working hardening. Multi-directional stacking faults are apt to form stacking fault (SF) locks, which inhibit the slipping and cross-slip of dislocations and then increase creep life of the specimen. In specimen C (14.7°), the creep deformation is controlled by single <112>{111} slip system, unidirectional stacking faults cut into γ′ particles, causing rapid failure of the alloy. The creep anisotropy of specimens with different orientation deviations is insignificant at 1100 °C/137 MPa. The same N-type rafted structures and the dislocation networks with similar spacing form in three specimens, which have the same ability to hinder dislocations cutting into γ′ particles. [ABSTRACT FROM AUTHOR]

• Investigating elastic modulus of single crystal superalloys with MCrAlY coatings for different preexposure durations. • Introducing a novel life prediction model using strain energy density, factoring in inter-diffusion and microstructural changes. • The proposed model is applicable beyond MCrAlY coatings, proving effective for single crystal superalloys with varied compositions. MCrAlY coatings, extensively utilized for safeguarding turbine blades against oxidation and erosion, encounter impediments due to inter-diffusion between the coating and substrate, thereby exacerbating fatigue life degradation at elevated temperatures. In this study, we introduce a novel approach involving the modification of critical depth in interfacial strain energy density to elucidate the impact of interfacial microstructure evolution on mechanical properties. Building upon this concept, we propose a fatigue life prediction model, which incorporates the dynamic evolution of interfacial structure and mechanical characteristics. Validation against empirical data underscores the commendable precision of the model. Our inquiry not only advances the comprehension of mechanical-chemical coupled behaviors but also yields significant insights for the optimization and maintenance of turbine blades. [ABSTRACT FROM AUTHOR]

During the creep deformation, damage mainly occurs in the tertiary creep stage. Thus, creep life is essentially controlled by the tertiary stage. To find out the impact of the dislocation microstructure on the creep rate in this stage, and in particular its thickness-dependence, creep tests of Ni-based single crystal superalloy are carried out at 1100 °C/130 MPa with sheet samples of varying thickness. The creep rate in tertiary creep clearly increases and rises faster in thinner sample sheets. The observed microstructures demonstrate that the γ/γ' phase distribution shows no topological inversion. It is found that the variety of dislocations in γ' rafts decrease with decreasing sample thickness. The γ' rafts in thin samples are sheared by only two types of superdislocation involving individual screw superdislocations and double Lomer-Cottrell structures, while the γ' rafts in thick samples are sheared by three types of superdislocations, again individual screw superdislocations and double Lomer-Cottrell structures, but also Kear-Wilsdorf structures. Also the density of disclocations varies significantly. A higher density of individual screw superdislocations and of mobile dislocation pairs, but lower density of locked non-planar structures promotes the creep of thinner samples. [Display omitted] • γ' rafts are sheared by individual screw superdislocations in both thin and thick samples during tertiary creep. • Only one type of non-planar structure, namely the double Lomer-Cottrell structure is detected in γ' rafts of thin samples. • Two types of non-planar superdislocation structures are found in γ' rafts of thick samples. • Higher density individual screw superdislocations and less variants non-planar ones promotes the creep of thinner samples. [ABSTRACT FROM AUTHOR]

Nickel-based single crystal (SX) turbine blades typically experience varying load conditions during operation. It is essential to consider the effects of changing stress and temperature when evaluating creep damage. The commonly used time-fraction method, based on the linear damage rule (LDR), has been demonstrated to be inconsistent with experimental results under certain loading conditions. There is a lack of research on damage accumulation under variable loading conditions specifically for SX superalloy. This paper conducted two-stage creep experiments for SX superalloy DD6 at 980 °C with stress levels of 230 MPa and 320 MPa, and at 1050 °C with stress levels of 135 MPa and 230 MPa. The load for the experiment was set based on the typical operating conditions of the SX turbine blades. The evolution of the microstructure under variable loading was observed using scanning electron microscope (SEM) and transmission electron microscope (TEM). The result of creep experiments revealed that the damage accumulation behavior of the SX superalloy does not conform to the LDR. Furthermore, in two-stage loading tests, the creep damage accumulation behavior differs from the classical sequence effect of fatigue loading. SEM and TEM analyses suggested that this phenomenon might be associated with the varying degradation rates of microstructures in SX superalloy. • Investigated damage accumulation in SX superalloy under variable loading. • The evolution of microstructure under variable loading was observed. • A novel creep damage model was proposed and validated for SX superalloy. [ABSTRACT FROM AUTHOR]

• A unique clamping system was designed for LCF tests of thin-walled coupons. • Coatings were detrimental to LCF lifetime, especially at low-medium temperatures. • The lifetime degradation was related to the coating-induced premature cracking. • A CDM-based life prediction model was proposed for different surface conditions. Within this work, the effect of different oxidation-resistant coatings on the low-cycle fatigue (LCF) behavior and damage mechanisms of third-generation single crystal (SC) Ni-based superalloy was investigated. Stress-controlled LCF tests of bare alloy, MCrAlY-coated and PtAl-coated thin-walled specimens were carried out at 760 °C, 850 °C and 980 °C. A unique clamping system was meticulously designed to overcome the clamping challenge in high-temperature furnaces. The cracking behaviors and failure modes were determined by advanced microstructure characterization. The results show that the coatings are detrimental to the LCF lifetime of thin-walled specimens, especially at low-medium temperatures of 760 °C and 850 °C. For bare alloy, the tip cracks nucleate from the surface oxide layer, whereas cracks in the coating-substrate system nucleate prematurely from the coating surface and propagate to the substrate. The oxygen diffusion and the frontward propagation of cracks are synergistically enhanced in the SC substrate, resulting in oxidation-assisted crack propagation and ultimate failure. Finally, A fatigue life prediction model is proposed by introducing coating-induced crack initiation and oxidation effects, where the predictive results fit well with experimental results. This study provides a comprehensive insight into the damage mechanisms and coating-induced LCF life degradation of the coating-substrate system. [ABSTRACT FROM AUTHOR]

• The VHCF behavior of [1 1 1]-oriented single crystal superalloy was studied at 760 °C and 1000 °C. • Lower octahedral shear stress and additional cross-slip systems result in increased deformation inhomogeneity in [1 1 1]. • The high climb force leads to similar fatigue life for [0 0 1] and [1 1 1] orientations at low stress at 1000 °C. Aimed at research the deformation orientation in VHCF regime under different temperatures, fatigue behavior of [1 1 1]-oriented DD6 single crystal superalloy at 1000 °C and 760 °C was studied. Using the stress range criterion, the [1 1 1] orientation shows higher fatigue resistance. This difference decreases with lower stress ranges and almost vanishes near 109 cycles. In contrast, the [1 1 1] orientation has lower fatigue resistance than the [0 0 1] orientation by using the total strain range criterion. Dislocations are restricted from shearing the γ' phase due to the lower octahedral resolved shear stress in the [1 1 1]-oriented, decreasing the deformation in γ' phase. Combined with distinctive cross-slip behavior, this further increased the deformation along the γ-channel, significantly enhancing deformation inhomogeneity between the γ' phase and γ-channels. At elevated temperatures, thermally activated dislocation climb and higher climb stress factor lead to a greater reduction in fatigue performance for [1 1 1] orientation at lower stress level. [ABSTRACT FROM AUTHOR]

The mechanism of oxidation-assisted initiation of surface cracks of a fourth-generation Ni-based single crystal superalloy was systematically investigated during low cycle fatigue at 900 °C and 980 °C. The results show that cracks initiate near the surface defects at 900 °C, while they initiate in the surface oxide layer at 980 °C. At 900 °C, the oxidation microcrack initiation in the thicker inner oxidation layer is difficult to connect with the surface oxidation crack, which is an essential reason for the crack growth rate being slower and not becoming the main crack. At 980 °C, microcracks form in the outer oxide layer and quickly connect with microcracks at the surface and inner/outer oxide layer interface, growing into long cracks that become channels for rapid oxygen transport. This accelerates the crack growth rate, and eventually the oxide crack becomes the main crack.

In order to control the forming surface quality of signal crystal turbine blade tenon teeth in the creep feed grinding, the influence of the creep feed grinding parameters on the grinding surface integrity of DD5 nickel-based single crystal superalloy was investigated via orthogonal experiment. The results showed that the surface roughness along vertical grinding direction was ranged at 0.56-0.74 μm at the grinding wheel speed range of 15-30 m/s, feeding velocity range of 120-210 mm/min and grinding depth range of 0.1-0.7 mm, and the surface roughness in the grinding direction is about 1/5 of that in the vertical grinding direction. The surface topography and texture results showed that there were the obvious grooves and ridges on the grinding surface caused by the grain ploughing and scratching, the length and height of grooves and ridges on the grinding surface changed obviously under different processing parameters, and the three-dimensional topography of the grinding surface fluctuated obviously. The length of grooves and ridges along the grinding direction were sensitive to the speed of grinding wheel, waviness of grooves and ridges along the vertical grinding direction were sensitive to the grinding depth and workpiece feed rate. The different degrees of work hardening effect were presented at the grinding surface, the biggest work hardening effect achieved at 11.6%, and the maximum depth of work hardening effect was 110 μm. The distinct plastic deformation appeared at the grinding surface. The γ phase presented slip deformation along the grinding direction with various degrees, and the γ' phase presented skewing, twisting, broken and fracture, the maximum depth of plastic deformation was 2.92 μm. The work hardening effect of DD5 creep feed grinding mainly due to the plastic deformation degree at the grinding surface. The experimental conclusions provided theoretical guidance for DD5 signal crystal turbine blade tenon teeth.

In this study, in the respect of rafting behavior of Ni-based single crystal (SC) superalloy under creep and thermal mechanical fatigue (TMF), an image processing program is developed to investigate the probability characteristic of γ channel width, and a channel width evolution model considering non-quasi-static modification is proposed. Firstly, the fractured and interrupted tests are conducted on SC superalloy. The channel width evolution behavior under different load conditions is observed via scanning electron microscope. Then, an image processing program based on image binarization is developed to conduct statistics on channel width. The statistical results show that the stress and temperature have significant effects on rafting rate, while the phase difference has a small effect. Meanwhile, the channel width obeys lognormal distribution, and the mean values follow linear relationship with standard deviations. In the modeling part, a static model is established to predict the mean value evolution of channel width under creep condition. To be generalized to TMF condition, the model is modified by considering the non-quasi-static effect. After that, the channel width evolution model is further derived to describe the probability density distribution of channel width. The predicted channel width evolutions are in good agreement with the experimental results. The statistical results and models can provide the basis for multiscale modeling of SC superalloy. [ABSTRACT FROM AUTHOR]

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In this study, the third-generation rhenium containing single crystal superalloy was brazed by mixed powder filler, in which the mixed powder filler was consisted of nickel-based powder filler and the superalloy powder with the same composition of the base metal. SEM and EPMA were used to analyze the influence of the proportion of the third-generation single crystal superalloy powder in the mixed filler on the microstructure of the joint, and the high temperature stress rupture properties of the brazed joint with four kinds of solder were tested. The result indicates that the microstructures and phase compositions of the Ni-based powder filler and mixed powder filler are both consisted of γ-Ni, γ′, CrB, Ni3B and M3B2 type boride, but the residue of the mixed powder filler is molten ball-type superalloy. With the thickness of brazing gap constant and increasing the ratio of the third-generation single crystal superalloy powder in the mixed filler, the precipitation of M3B2 type boride and low-melting point phases in the joint can be inhibited, and the distribution of borides becomes more uniform and the size becomes smaller, thus improving the uniformity of the composition and microstructure of the joint. When the proportion of alloy powder increased from 0% to 50%, the endurance life of the joint increased from 15 min to 34 hours. However, when the proportion of alloy powder increased to 60%, there are a lot of void defects in the joint, resulting in the endurance life of the joint decreased to 4 hours.

Copyright of Journal of Materials Engineering / Cailiao Gongcheng is the property of Journal of Materials Engineering Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In this work, we investigate the effect of hot isostatic pressing on the intermediate temperature tensile property of nickel-base single crystal superalloy. The experimental results indicate that the hot isostatic pressing improves the intermediate temperature plasticity of single crystal superalloy from 6.4% to 9.3%, but the yield strength remains unchanged. In addition, the fracture surface is changed by hot isostatic pressing, the microporosities disappear in the fracture surface under hot isostatic pressing, and an initial crack is formed from the region of plenty of stacked slip planar instead of microcracks by microporosities. The mechanism of the improvement of plasticity and the change of fracture behavior is explored in detail. The application of hot isostatic pressing reduces the size and volume faction of microporosities, which makes it difficult for the crack initiation and propagation, and further leads to the improvement of plasticity.

To meet the development needs of advanced aeroengines,the structure of aeroengine turbine blades is becoming increasingly complex, and the content of refractory elements is increasing in single crystal superalloys, which are the preferred materials for turbine blades. As a result, the tendency to form the grain defects increases during the preparation of single crystal turbine blades, which directly affects the quality of single crystal turbine blades. In this paper, a kind of grain defect that appears in the directional solidification process of single crystal superalloys—freckle was discussed. The research works on the formation mechanism, the criterion model and the control method of freckles formation during the directional solidification of single crystal superalloys in recent years was reviewed. The influence of the alloy composition, blade structure, directional solidification process and crystal orientation of single crystal castings on the formation of freckles was analyzed. Considering the influence of the alloying elements in different alloy systems and the parameters of the directional solidification process on the freckle formation, further studying the freckle formation mechanism of the single crystal turbine blade with complex structures, establishing an effective method for prediction and control of freckles are the future research directions.

Generally, interactions of stacking faults (SF) have been considered to have a strengthening effect for single crystal (SX) superalloys during creep at intermediate temperatures. However, results in this work show that V-like SF interactions are more effective than T- or X-like interaction in preventing further expansion of SFs in the experimental Re -low and Re -free SX superalloys. Under the creep condition of 760 °C/780 MPa, dislocations entanglement near concave γ/γ′ interfaces, enrichment of Re atoms along SFs and more V-like SF interactions within γ′ phase are conductive to lowering the creep rate of the Re -low alloy. Finally, based on interface and two-phase evolution, configurations of matrix dislocations as well as the expansion and interactions of SFs, we propose new understandings to improve intermediate-temperature creep resistance for SX superalloys. [Display omitted] [ABSTRACT FROM AUTHOR]