In response to the increased demand for very long-term service reliability of industrial gas turbine (IGT) blades, the microstructural stability of two single crystal (SX) superalloys with different ...Re addition (0Re and 2Re in wt.%, named as alloy 0Re5Ta and 2Re5Ta) were investigated during long-term thermal exposure (>5000 h) at 900 °C, with the help of atom probe tomography (APT) and high-temperature X-ray diffraction (XRD) analysis. During long-term thermal exposure, the γ′ precipitates coarsened gradually in both alloys. At the same time, the γ′ precipitates became more cuboidal for alloy 0Re5Ta and nearly spherical for alloy 2Re5Ta due to the increased positive lattice misfit for alloy 0Re5Ta and the decreased negative lattice misfit for alloy 2Re5Ta. Such lattice misfit evolution was mainly attributed to the inversion of the W partitioning from γ matrix to γ′ precipitates over time. The addition of Re can effectively decrease the coarsening rate during thermal exposure. Further investigations on the coarsening kinetics confirmed the Re effect on reducing the interfacial energy and enhancing the rate-limiting effect of Cr. Additional particle size distributions (PSDs) showed a gradual transition from the initial matrix-diffusion coarsening-controlled mechanism to the interface-diffusion coarsening-controlled mechanism at prolonged time, wherein Re triggered the interface-diffusion mechanism earlier by leading to sharper interfaces. Although Re increases the magnitude of the lattice misfit in the initial microstructure, it can significantly enhance the microstructural stability of SX superalloys for IGT application.
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The effect of 2 wt% Ru on γ/γ' microstructural evolutions and precipitation of TCP phases were investigated in two experimental alloys. The two alloys were subjected to unstressed thermal exposure at ...temperatures of 1100 °C for periods of 100, 500 and 1000 h. The microstructures of the samples before and after thermal exposures were characterized. Directional coarsening of γ' phases and the precipitation of TCP phases were observed in the both alloys after long-term exposure. The experimental results show that Ru has a very significant effect on the suppression of TCP phases by influences the partitioning of elements. Detailed analysis showed that Ru increases the lattice misfit strain, resulting in more dense and stable interfacial dislocation networks, which can promote the stability of γ/γ' microstructures by delaying topological inversion. Ledges and grooves at γ/γ' interfaces associated with the dislocations and strong segregations of Cr, Co and Re along the dislocation networks were both observed, under which a detailed explanation of the coarsening process during thermal exposure was given.
•Ru increases the microstructural stability.•Ru increases the density and stability of dislocation networks.•Ru promotes the stability of γ/γ' microstructures by delaying topological inversion.•Ru reduces the nucleation of TCP precipitates.
A new type of γ′ phase subgrain has been found in a single-crystal superalloy after exposure to 360 hours of creep at 1200 °C and 60 MPa. This signifies the capacity of dynamic recrystallization ...(DRX) of γ′ phase, overturning the widely accepted idea that it is incapable of undergoing DRX. The remarkable plasticity of the γ′-precipitate, activated at ultrahigh temperature, is likely due to the slip/rotation and cracking of the γ′-subgrains. Microstructural evidence of the subgrain boundary originating from γ′-shearing dislocations and their associated walls was obtained, which explains the subgrain formation dependence on plastic deformation accumulation. These findings help us understand the γ′-deformation mechanism and guide to improve creep resistance.
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Designing single-crystal superalloy blades to thin wall structures produces the secondary orientation which is the perpendicularity orientation of the wall surface, becoming a newly-concerned crucial ...factor of performance. Here, we have studied the influence of 110 and 112 secondary orientations on 1100 °C/137 MPa creep properties of 111-orientated single-crystal superalloys. The plastic deformation accumulation and the plastic limit during creep vary with secondary orientations. Our results give microstructural evidence of the change in the slip systems operation and micro-pore growth, and correlate the possible stress state in the thin wall to plastic deformation mechanism, and finally to properties, which can explain such influence. The results show that the activation of slip systems with different secondary orientations is different due to the different shear stress, and this difference of slip systems causes the difference of plastic deformation ability and pore growth and finally affects the creep performance.
The LCF behaviors of two Ni-based single-crystal superalloys at room temperature and 600 °C have been studied. Superalloy with 3 wt% Ru-containing (3Ru alloy) had more excellent fatigue performances ...than that of non-Ru superalloy (0Ru alloy). Microstructures observed by transmission electron microscope showed that 0Ru alloy exhibited slip bands with orthogonal slip directions after the test. (1ī1) 011 and (1ī1) 0ī1 slip systems were activated in plastic deformation process. While (111) 0ī1 and (1ī1) 0ī1 slip systems of 3Ru alloy were along the single slip direction. Therefore, adding alloying element Ru to the superalloy promoted uniform deformation and prolonged fatigue life.
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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 R2 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.
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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.
•This paper innovatively introduced low temperature into ECM of DD6.•Low temperature suppresses the formation of passive film.•Low temperature reduces the self-corrosion potential difference between ...the two phases of DD6.•TaC on the surface is removed by peeling.•The processing surface quality is better in the cryogenic environment.
Electrochemical machining (ECM) is a highly effective method for processing nickel-based single crystal (NBSC) superalloys. However, the influence of temperature on the electrochemical corrosion of NBSC remains underexplored. This study investigates the temperature dependence of the corrosion characteristics on the growth surface and its isomorphous perpendicular surface of NBSC DD6 were investigated. Results indicate that temperature significantly impacts the passivation and dissolution processes of the material: at low temperatures, the passivation effect weakens, and the differences in self-corrosion potential between phases decrease, leading to smaller variations in corrosion. Additionally, a mechanism is proposed to elucidate the temperature-dependent corrosion behaviors on different surfaces.
High and medium entropy alloys (HEAs, MEAs) have emerged as promising candidates for bond-coat materials in the thermal barrier coating systems due to their exceptional oxidation resistance. However, ...understanding the interdiffusion behavior between these alloys and single-crystal superalloys (SXs) substrates requires further investigation. In this study, diffusion couples were created between SXs and two types of MEAs, namely FeCrNiAl and FeCrNi0.4Al0.4, and the interdiffusion behavior was simulated using the commercial DICTRA code. The results reveal significant interdiffusion at 1100 °C in the couples, attributed to the substantial elemental differences between SXs and MEAs. This led to the formation of the interdiffusion zone (IDZ) and the secondary reaction zone (SRZ). Within the SRZ, phase transformation of SXs occurred, resulting in the precipitation of topologically close-packed (TCP) phases. Three types of TCP phases (σ, μ, and P) were observed. Along the {11¯02} plane of the μ phase, twins and stacking faults have formed. The glide of P phase contributed to the bending of bar-like precipitates. Notably, the amorphization of the σ phase was observed for the first time, suggesting an intermediate state during the phase transformation. Despite elemental differences between the two MEAs, the growth mechanism of IDZ and SRZ, as well as the precipitation of TCP phases, exhibited similarities. This underscores the importance of Cr and Ni diffusion alongside Al diffusion. The pronounced interdiffusion behavior raises concerns about the suitability of 3-d transition metal MEAs as bond-coat materials due to the lack of slow diffusion effect.
•The interdiffusion behaviors between two types of medium-entropy alloys were investigated.•Three types of TCP phases (σ, μ, and P) were observed within the secondary reaction zone produced by the interdiffusion.•Along the {11¯02} plane of the μ phase, twins and stacking faults have formed.•The glide of P phase contributed to the bending of bar-like precipitates.•The amorphization of the σ phase was observed for the first time.
The purity of nickel-based single crystal alloys plays an important role in the final service performance of turbine blades as it impacts the solidification structure, elemental segregation, and ...inclusions of castings. The highest purity of superalloys studied before was of altogether around 10 ppm of contents of N and O produced by the triple purification process (VIM+ESR+VAR). However, the impact mechanism still remains controversial, and meanwhile whether a lower content of N and O still keeps the impacts on castings is not yet clear due to the lack of raw materials. A superhigh purity of second generation of nickel-based single crystal superalloy DD98M with the sum of N and O contents as low as 4 ppm (hitherto the highest purity of the superalloy as published), home-produced by electron beam smelting (EBS), was put on focus in this work. The effect of three levels of purity (N-doped, commercial, and EBS, corresponding to 21, 10, and 4 ppm for the sum of N and O contents, respectively) on the microstructure evolution (including primary dendrite arm spacing (PDAS), eutectic, micropores, and γ′ phase) and the solute segregation behavior at dendritic scale and nano scale (γ/γ′ interfaces) were systematically studied and the influencing mechanisms were elucidated. The results show that the PDAS decreased from 406 μm to 322 μm with the reduction of O and N from 21 ppm to 4 ppm, which was irrelevant to precipitation of the nitride or oxide inclusions. Instead, it was due to the decrease of diffusion coefficient D or the increase of equilibrium coefficient k, or the decrease of the critical nucleation supercooling and the incubation time of dendrites. The micro-segregation, the number and volume fraction of eutectic, and the number and voltage percentage of shrinkage pores and gas pores were all greatly decreased with the increase of purity. These changes were closely related to the reduction of PDAS and the reduction of fluidity of the remained liquid in the interdendrites during solidification. The morphology and the size of the γ′ phase were independent from the purity, while the segregation of alloying elements at the γ/γ′ interface changed remarkably with the variation of the purity.
•Super-high purity of nickel-based single crystal superalloys was explored.•The PDAS, micro-segregation, eutectic and pores all greatly decreased with the increase of purity.•Clyne-Kurz model was used to predict the volume fraction of eutectics.•The elemental segregation at γ/γ′ interface changed remarkably with the variation of the purity.
