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.
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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•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.
•A TMF test platform which can accurately control temperature and load was constructed.•The TMF behaviors were analyzed by macro-fine-micro multi-scale testing.•The TMF life and fracture morphologies ...of IP and OP conditions are obviously different.•Numerous fine secondary γ' phases are precipitated in the channels of the matrix phase of OP condition.
Nickel-based single-crystal superalloys are extensively used in aeroengine hot-end components owing to their unique crystal structure and outstanding high-temperature mechanical properties. In this study, a thermomechanical fatigue (TMF) test platform capable of realizing precise temperature control and coordinated control of mechanical and temperature loads was constructed. The TMF of a second-generation nickel-based single-crystal superalloy (DD6) was analyzed in a temperature range of 500–950 °C at a stress ratio of 0.1 in two phases (in-phase and out-of-phase); moreover, the TMF tests were conducted at various stress levels, and the TMF life of the single-crystal specimens was determined. The OP TMF lifetime was considerably longer than the IP TMF lifetime at identical stress levels. The failure mechanism of the material in different phases was elucidated. The fracture and microstructures of the specimens were analyzed by SEM and TEM. For the IP specimens, the fracture mode was determined to be a quasi-cleavage fracture, and the γ' phase exhibited N-type rafting prior to fracture. For the OP specimens, the fracture was relatively smooth and flat. Numerous fine secondary γ' phases are precipitated in the channels of the matrix phase.
•The high-temperature fretting fatigue performance of nickel-based single crystal tenon attachment under different load conditions have been investigated.•Microstructure deformation of shot peened ...nickel-based single crystal superalloy is revealed.•Shot peening strengthening mechanism of nickel-based single crystal tenon is revealed.
In order to reveal the shot peening (SP) strengthening mechanism on the high-temperature fretting fatigue performance of nickel-based single crystal (NBSC) tenon attachment, fretting fatigue tests under three load conditions were carried out at 600 ℃. The results showed that the fretting fatigue life of SP NBSC tenon attachment could be increased to 357.53 % of that of non-SP NBSC tenon attachment. SP changed the fretting fatigue crack initiation site from the contact surface to the subsurface. The larger the peak load was, the closer the crack initiation site was to the contact surface. SP changed the fretting fatigue crack propagation path, and the crack propagation deviated from the original direction approximately perpendicular to the contact surface. SP increased the microhardness by 28.1 %, induced −1098 MPa maximum compressive residual stress (CRS) and generated a mean geometrically necessary dislocation (GND) density of 7.55× 1014 m−2 near the surface. The micro convex bodies and greater microhardness alleviated the wear damage, and the increased dislocation density and CRS enhanced the inhibition of crack initiation and propagation, which were the strengthening mechanism of SP to improve the fretting fatigue performance of NBSC tenon. This work aimed to provide support for the long-life manufacturing of NBSC turbine blades.
