Using the first-principles calculations based on the density functional theory, we studied the site occupancy of Re in the Co7W6 μ phase and its effects on the electronic properties. The results of binding energy and defect formation energy show that the stability of the system was enhanced after Re doping, which indicates that Re is prone to the Co7W6 μ phase formation. The results of electronic properties calculations (the density of states and charge density difference) reveal that there exists a strong interaction between Re and its NN(nearest neighborhood) atoms, which was attributed to the d-d hybridization. Besides, the bonding strength becomes stronger as Re substituting the Co sites, which further elucidate that Re preferentially occupies the Co sites.

Dislocation substructures were investigated in a nickel-base single crystal superalloy subjected to 85 h creep at 1100 °C and 130 MPa using conventional and high resolution transmission electron microscopy. Both hexagonal and square-like interfacial dislocation networks were observed. The square-like networks, which were caused by two perpendicular sets of a/2<110> dislocations forming a[100] superdislocations, were tested. It was concluded that the a[100] interfacial dislocation occurred in the form of a dislocation dipole. The creation and destruction of dislocation dipoles proceeded reversibly via a slip-and-climb mechanism.

•Dislocation networks at γ/γ′ interface can be observed after 85 h creep.•The involved dislocation reaction is a/2[110]+a/2[1¯10]→a[010].•The a[010] dislocation is of mixed type with an a/2[010] edge component.•The square-like (hexagonal) dislocation network owns a 3D structure.Detailed characterizations of interfacial dislocation networks in a nickel-base single crystal superalloy after creep loading for 85 h at 1100 °Cand 130 MPa were carried out at larger magnifications. The “square-like” dislocation network is formed by dislocation reaction including a[010], a/2[110], a/2[11¯0] dislocations and is different from square (and hexagonal) networks in formation mechanism and geometric structure. The atomic structure of core region of dislocations indicates their individual features and highlights the three-dimensional geometry of “square-like” dislocation network for the first time.Figure optionsDownload full-size imageDownload as PowerPoint slide

•The best relative distance between Cr and Re atoms is a (the lattice constant).•Re can decrease the systemic binding energy, while Cr will increase the binding energy.•There exist the sequence of diffusion between Cr and Re atoms.•the PBE + U approach is introduced and relativistic effect is discussed.The diffusion behavior of Cr and Re in nickel-based superalloys was explored at an atomic level using the Vienna Ab-inito Simulation Package. To improve the accuracy of the calculation, the PBE + U approach is introduced and relativistic effect is discussed. The relative position and sequence of diffusion between Cr and Re atoms are studied using first-principles calculations. The best relative distance between Cr and Re atoms is a (the lattice constant). Furthermore, there is a sequence of diffusion between Cr and Re atoms. It is easy for a Re atom to diffuse towards a Cr atom, resulting in a small, stable group. However, it is difficult for a Cr atom to diffuse into a region containing a Re atom. Our results were minimally affected by the Coulomb U and relativistic effects. This theoretical study may aid in experimental design.

•Dislocation morphology and stress distribution around dislocation in Ni3Al are observed on the atomic scale by molecular dynamics simulation.•The dislocation morphology demonstrates that influenced area of dislocation is pipe-like.•3D colormap surface is used to present the stress distribution around dislocation.Dislocation is very important for the properties of materials. In this work, morphology and stress distribution around dislocation in Ni3Al are observed in atomic scale by the molecular dynamics. We obtained the dislocation morphology by the simulation and found that the atom distribution around the dislocation core is irregular. Layers of atoms on both the left and right sides of the dislocation core leans to the core, and about six layers of atoms are fractured due to the missing of the half plane of atoms. Along the [1 1 1] direction, there are about eight layers of atoms deviating from its equilibrium position. These results demonstrated that influenced area of dislocation is pipe-like. The stress distribution agrees with the results from the formula. Both normal stress and shear stress components exist in the edge dislocation stress field, normal stress mainly concentrating on two sides of the dislocation line, while the shear stress exist several angstrom away from the dislocation line.

The microstructural characteristics of nickel-based polycrystalline superalloy GH4037 turbine blades that have been in service for 1600 h have been studied by transmission electron microscopy. In particular, emphasis has been placed on paired dislocations and their interactions with γ′ precipitates. Paired dislocations are universal and coexist with Orowan loops occasionally. The attractive force due to anti-phase boundary energy and the repulsive force between two 1/2<110> dislocations act on paired dislocations simultaneously, causing different morphologies of edge dislocations. Since the shear stress required by paired dislocations to cut through γ/γ′ structure is much lower than Orowan stress of single matrix dislocation, the major formation mechanism is dislocation pairing. A large number of paired dislocations slipping on the same {111} plane can form slip band and can shear off γ′ particles, which may directly lead to the formation of microcracks.

The characteristics of microstructural evolution along the transverse axis of the 1/3 position of turbine blade body have been investigated with the help of transmission electron microscopy. Dislocation configurations change with the evolution of local stress from the leading edge to the trailing edge: slip bands → dislocations → only γ/γ′ structure → dislocations → slip bands. The formation mechanism of slip bands illustrates that dislocations are generated in pairs and glide on the same plane continuously. Finite element analysis is made to assess the stress distribution along the transverse axis of turbine blade body. The instantaneous and inhomogeneous stress at the leading and trailing edges of blade body becomes the driving force for the formation of slip bands.

A FeCO3/graphene nanocomposite (FCO/GNS), in which FeCO3 nanoflakes aligned by nanowires well dispersed in graphene matrix, is successfully synthesized via a simple one-pot hydrothermal route. With the electronic conductor, buffer, dispersion and synergetic lithium storage effect of graphene, the composite exhibits significantly improved electrochemical activity and cycling stability relative to bare flower-like FeCO3 microspheres, as well as excellent rate performance and a great recovery capability, delivering a charge capacity of 934.4, 798.6, 661.4, 451.9, 403.4, 268.9 and 193.1 mAh g−1 at 0.2, 0.4, 0.8, 2, 3, 4 and 5 C, respectively, and a recovery capacity of up to 1166 mAh g−1 after 255 cycles from 0.1 to 5 C. Combining the experimental data with voltage profiles and CV curves, conversion reactions of the resulted FeCO3 are inferred to be not limited to the transition between FeCO3 and Li2CO3 but involving further reduction of Li2CO3. All the results suggest that the obtained FCO/GNS nanocomposite is a very promising anode material for lithium ion batteries.

•Critical resolved shear stress for twinning in superalloy are calculated theoretically.•Finite element method analyzes the stress distribution around micropore near crack-tip.•Deformation twinning occurs due to the stress concentration.Thermo-mechanical fatigue (TMF) test was performed in a [0 0 1] oriented single crystal superalloy TMS-75. After TMF tests, detailed microstructural evolution were observed from the interior and outer surfaces of the specimens. The path of the crack initiation and propagation on the failure surface of the ruptured specimen has a concrete representation. It was found that the distribution of deformation twins occurs around the micropores near the crack-tip. Considering the twinning dislocation slip mechanism, the critical tensile stress value 1.34 GPa for twinning in single crystal superalloy was obtained by means of theoretical calculations. Associated with crack propagation in varying degrees, finite element method was performed in an effort to clarify the stress fields around the micropore near the crack-tip region. The finite element method analysis results reveal that the stress in the region between the micropore and crack-tip is larger than the critical tensile stress value to promote the nucleation and propagation of deformation twins. Both theoretical calculations and finite element method analysis results are well consistent with the experimental results. The stress concentration around the micropore near the crack-tip region results in a high density of deformation twins.

•Distinguished Al(OH)3 and Al2O3 as aluminum precursors doped LiMn2O4.•Ratio of Raman intensity for Mn3 +/Mn4 + of LiMn2O4 with Al precursors varies.•Al(OH)3 as Al precursor substitutes into LiMn2O4 more readily than Al2O3.In this paper, the effect of different aluminum precursors, i.e., Al(OH)3 and Al2O3, on the structural and electrochemical properties of spinel LiMn2 − xAlxO4 (x = 0, 0.05, 0.1, 0.15) is investigated by X-ray diffraction (XRD), micro-Raman spectroscopy (RS), field-emission scanning electron microscopy (FE-SEM) and charge–discharge cycling test. In particular, the ratio of Raman intensity at 625 cm− 1 to Raman intensity at 585 cm− 1 provides crucial insights into the Al substitution in spinel LiMn2O4 lattice. The XRD patterns and FE-SEM images show that the crystal structure and the particle morphologies of LiMn2 − xAlxO4 are not altered by use of either Al(OH)3 or Al2O3 as Al precursors. However, the Raman spectra show that Al doping behavior is significantly affected by Al precursors, with Al(OH)3 serving as a more effective Al precursor than Al2O3. Such conclusion is further supported by the electrochemical cycling results.

•A reasonable σ phase model was adopted in our calculation.•The site preference of refractory elements in σ and γ phases was investigated.•The bonding characteristic was analyzed on the basis of electronic microstructures.The impurity formation energies of the σ and γ phases of Ni-based single crystal superalloys doped with W, Cr and Co in different sublattices have been investigated using first-principles based on the density functional theory. The bonding characteristics of the doped σ phase were analyzed with the valence charge densities and the density of the states. The results of the calculations indicated that the typical refractory element W, which has a large atomic size, preferentially partitions into the σ phase due to the nature of the bonding and the unique crystal structure with close-packed planes and large interstitial spaces. In addition, the site preference of refractory elements in γ phase was in the order of W, Cr and Co.

The thermomechanical fatigue performances of two superalloys, TMS-82+ and TMS-196, were comparatively studied. The stress relaxation during the hold period and the plastic deformation in TMS-196 were both less than those in TMS-82+. The microstructural evolution was observed using scanning electron microscope and transmission electron microscope. It was found that numerous thin twins were formed during the tests in TMS-196, while in TMS-82+, few thick twins were developed. Moreover, the fracture surface of TMS-82+ is relatively flat, whereas the fracture surface of TMS-196 is rather rough. This difference was attributed to their different twin morphologies. Given their different compositions and properties, this research may provide useful guidance for the further development of superalloy.

This paper provides further insight into the formation of deformation twins at different stages during the whole thermomechanical fatigue cycling in a nickel-base single-crystal TMS-82 superalloy. In general, it is found that twinning behaviors can always be associated with the applied stress orientation. The preferred twinning direction at the primary stage is 〈001〉-compression since the tangled dislocations which appear after the first plastic deformation provide an opportunity for twinning nucleation in compression. At the intermediate stage, the applied stress required for formation of twins in tension is much larger than that in compression; hence, twinning behaviors show distinct tension/compression asymmetry. A thick twin plate and a great many dislocations can be found after fatigue failure, and one can rationalize the reason for this twinning being associated with the TMF procedure. Twins at the tip of the crack in tension occur owing to the existence of compressive strain field.

•Studies of pressure effects on hexagonal systems remain scarce.•The underlying structural evolution that occurs under applied pressure is determined.•The elastic properties under different hydrostatic pressures are studied in detail.Using hybrid density functional theory, we investigated the structural, electronic and elastic properties of the C14 NbCr2 Laves phase under different hydrostatic pressures. Our results for the structural properties are consistent with calculated and experimental results. The analysis of the density of states and the charge difference density are used to determine the underlying structural evolution that occurs under applied pressure. Hexagonal NbCr2 is mechanically stable according to the elastic stability criteria and exhibits low ductility according to the B/G ratio. Moreover, the elastic properties, including the elastic constants and elastic moduli are studied in detail.

Thermomechanical fatigue behavior of a nickel base single crystal superalloy TMS-82 has been investigated. It was found that the micropores in the alloy tend to initiate large amounts of deformation twins and the crack causes many twins to occur during its propagation. It is postulated that these twins around the micropores or the crack are induced by the stress field of the micropores or the crack tip, respectively. A thermodynamic model is adopted linking the twin formation energy with the stress field around the crack tip, which explains the observed results with good consistency. The influencing factors for a crack to stimulate twins around it are shown by the model, which might provide possible ways to enhance the alloy's thermomechanical fatigue performance.

This article has been retracted: please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy).This article has been retracted at the request of the authors and the Coordinating Editor.The calculations at the core of the article were based on an assumption that the crystal structure of the σ phase was incorrectly approximated. Further research and intensive discussion with colleagues has helped the authors to realize the assumptions made with their calculations cannot stand. This precludes from determining the site occupancy of each element and the segregation in the σ phase cannot be correctly approximated as a multicomponent phase by W8Cr10NiCo11 in the calculations.The authors have expressed deep regret and have asked for retraction of their article.

By means of high resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field image technique (HAADF), morphological of plate-shaped σ phase and interfacial characteristics between plate-shaped σ phase and γ phase in Ni-based single crystal superalloys have been studied. On the basis of HRTEM observations, an atomic structural interface between σ phase and γ phase with a step has been proposed. σ Phase has the relationship of [0 0 1]γ//[1 1 2¯]σ, (2¯ 2 0)γ//(1¯ 1 0)σ, (2¯2¯ 0)γ//(1 1 1)σ; [0 1 1]γ//[1 1 0]σ, (1 1¯ 1)γ//(0 0 1¯)σ with the γ phase. The compositional characteristics of the σ phase which are rich in more heavy elements, such as W, Re, Cr and Ru, has also been clearly revealed by the HAADF imaging technique and Energy Dispersive X-Ray Spectroscopy (EDS) mapping analysis.Graphical abstract(a) TEM micrograph of σ phase; (b) HRTEM image of σ/γ interface corresponding to the area of the white frame in (a); (c) an enlarged image of area from the white frame in (b). The combination of σ/γ interface appears very well, and a two-atomic-layer step is shown on the σ/γ interface. In addition, σ phase has the orientation relationship of [0 0 1]γ//[1 1 2¯]σ, (2¯ 2 0)γ//(1¯ 1 0)σ, (2¯2¯ 0)γ//(1 1 1)σ; [0 1 1]γ//[1 1 0]σ, (1 1¯ 1)γ//(0 0 1¯)σ with the γ phase.Highlights► Elemental characteristic of σ phase is studied by HAADF techniques and EDS analysis. ► Interfacial characteristics of σ/γ interface are revealed by HRTEM. ► An atomic structural σ/γ interface with a two-atomic-layer step has been proposed.

Dislocation configurations at different creep stages (1100 °C and 137 MPa) in a superalloy TMS-75(+Ru) were studied in transmission electron microscopy (TEM) and the movement path of these creep-produced dislocations could be fully illustrated. Due to the small value of γ/γ′ lattice misfit, these dislocations cannot glide in the horizontal γ matrix channels by cross slip, but they mainly move by climbing around the γ′ cuboids. In the primary stage, the dislocations first move by slip in the γ-matrix channels. When they reach the γ′ cuboids, they move by climbing along the γ′ cuboid surfaces. In the secondary creep stage, dislocation reorientation in the (001) interfacial planes happens slowly, away from the deposition orientation of 〈110〉 to the misfit orientation of 〈100〉. The velocity of the reorientation is lower and a perfect γ/γ′ interfacial dislocation network cannot be formed quickly. This factor results in a large creep rate of the alloy during the secondary creep stage. The path for dislocation motion during the early creep stages consists of the following sequences: (i) climbing along the γ′ cuboid surface, (ii) deposition onto the (001) γ/γ′ interfacial plane, and (iii) reorientation from the 〈110〉 direction to the 〈100〉 direction.

•Dislocation morphology and stress distribution around dislocation in Ni3Al are observed on the atomic scale by molecular dynamics simulation.•The dislocation morphology demonstrates that influenced area of dislocation is pipe-like.•3D colormap surface is used to present the stress distribution around dislocation.Dislocation is very important for the properties of materials. In this work, morphology and stress distribution around dislocation in Ni3Al are observed in atomic scale by the molecular dynamics. We obtained the dislocation morphology by the simulation and found that the atom distribution around the dislocation core is irregular. Layers of atoms on both the left and right sides of the dislocation core leans to the core, and about six layers of atoms are fractured due to the missing of the half plane of atoms. Along the [1 1 1] direction, there are about eight layers of atoms deviating from its equilibrium position. These results demonstrated that influenced area of dislocation is pipe-like. The stress distribution agrees with the results from the formula. Both normal stress and shear stress components exist in the edge dislocation stress field, normal stress mainly concentrating on two sides of the dislocation line, while the shear stress exist several angstrom away from the dislocation line.