Solute atom migration in GH4169 superalloy under electrostatic fields

Electric field treatment (EFT) was applied on GH4169 alloy during aging at 500–800℃ to investigate the microstructure and property variation of the alloy under the action of EFT. The results demonstrate that the short-distance diffusion of Al, Ti, and Nb atoms can be accelerated by EFT, which results in the coarsening of γ′ and γ″ phases. Meanwhile, lattice distortion can be caused by the segregation of Fe and Cr atoms, owing to the vacancy flows migrating toward the charged surfaces of the alloy. Therefore, the alloy is hardened by the application of EFT, even if the strength of the alloy is partly reduced, which is caused by precipitation coarsening.     

Effect of cold rolling and first precipitates on the coarsening behavior of γ″-phases in Inconel 718 alloy

The coarsening behaviors of γ″-phase particles in Inconel 718 alloy aged at 750, 800, and 850°C were investigated by scanning electron microscopy (SEM). Detailed observations and quantitative measurements were conducted to characterize the coarsening behavior of the γ″-phase under various aging conditions. The experimental results indicate that the existence of the δ-phase retards the formation and coarsening of the γ″-phase, without influencing its final particle size or amount. Moreover, when cold rolled with a reduction of 50%, the dimensions of the γ″ particles in Inconel 718 alloy decrease with increasing aging time. Furthermore, the coarsening behavior of the γ″-phase in the Inconel 718 alloy after a normal aging treatment (sample A) and that of the primary δ-phase (sample B) follow the Lifshitz–Slyozov–Wagner (LSW) diffusion-controlled growth theory; the thus-obtained activation energies for the γ″-phase are 292 kJ·mol-1 and 302 kJ·mol-1, respectively.     

Phase-field simulation on microstructure evolution of D0_(19) phase in γ/γ′structure of Co–Al–W superalloys

The morphological evolution and precipitation kinetics of γ′ and D0_(19)(Co_3 W) phase in Co–Al–W alloys at 900 °C have been studied by applying Phase-field method and experiment in order to understand the transformation process of γ′ phase and D0_(19) phase. The growth processes of D0_(19) phase and precipitation of γ′ phase under elastic fields were simulated through coupling with thermodynamics and dynamics databases. The simulation results indicate that the misfit δ≥ 0.53% has a greater impact on γ′ particle morphology in γ/γ′ structure.Co–Al–W alloy with low Al and high W is one of the factors to promote the precipitation of D0_(19) phase. Three stages during aging, namely the γ′ phase incubation stage, the γ′ phase fast nucleation and growth stage, and the transformation from γ′ phase to D0_(19) phase stage can be observed with the non-constant coarsening rate that varying with the decrease of γ′ phase. The particle size distribution(PSD) during the precipitation of D0_(19) phase is more in line with MLSW theory than LSW theory. This simulation results are in good agreement with the experiment results to help analyze microstructure evolution of Co–Al–W alloy.     

Effects of Ru on the microstructure and phase stability of a single crystal superalloy

Two experimental single crystal superalloys, the Ru-free alloy and the Ru-containing alloy with[001] orientation, were cast in a directionally solidified furnace, while other alloying element contents were kept unchanged. The effects of Ru on the microstructure and phase stability of the single crystal superalloy were investigated. γ' directional coarsening and rafting were observed in the Ru-free alloy and Ru-containing alloy after long-term aging at 1070℃ for 800 h. Needle-shaped σ topologically close packed (TCP) phases precipitated and grew along the fixed direction in both the alloys. The precipitating rate and volume fraction of TCP phases decreased significantly by adding Ru. The compositions of γ and γ' phases measured using an energy-dispersive X-ray spectroscope (EDS) in transmission electron microscopy (TEM) analysis showed that the addition of Ru lessened the partition ratio of TCP forming elements, Re, W and Mo, and decreased the saturation degrees of these elements in γ phase, which can enable the Ru-containing alloy to be more resistant to the formation of TCP phases. It is indicated that the addition of Ru to the Ni-based single crystal superalloy with high content of the refractory alloying element can enhance phase stability.     

不同相构型改性718型合金的高温稳定性评价

Inconel 718 is a Ni–Fe-based superalloy widely used in aerospace engines because of its excellent mechanical properties. However, the inferior stability of the γ″ phase limits the application of Inconel 718, which coarsens rapidly at temperatures greater than 650°C. Further improving the temperature tolerance of Inconel 718 requires optimization of the phase configuration via modification of the alloy’s chemical composition. Given the aforementioned objective, this work was conducted to study the precipitation behavior and thermal stability of the strengthening phases with various structures in modified Inconel 718 alloys by tailoring the Al/Ti ratio. With increasing Al/Ti ratio, three particle configurations were formed: γ′/γ″ composite, isolated γ′, and γ′/γ″/γ′ composite particles. The results of aging tests demonstrate that the isolated γ′ and the γ′/γ″/γ′ composite structure exhibited better thermal stability at temperature as high as 800°C. The isolated γ′ exhibited a reduced coarsening rate compared with the γ′/γ″/γ′ composite particles because the isolated γ′ phase was rich in Al, Ti, and Nb. However, the γ′/γ″ composite particles coarsened and decomposed rapidly during aging at temperatures greater than 700°C because of the lower stability resulting from the larger number of γ″ particles. The obtained results provide necessary data for the compositional optimization of novel 718-type alloys.     

Evaluation on elevated-temperature stability of modified 718-type alloys with varied phase configurations

Inconel 718 is a Ni–Fe-based superalloy widely used in aerospace engines because of its excellent mechanical properties. However,the inferior stability of the γ″ phase limits the application of Inconel 718, which coarsens rapidly at temperatures greater than 650°C. Further improving the temperature tolerance of Inconel 718 requires optimization of the phase configuration via modification of the alloy's chemical composition. Given the aforementioned objective, this work was conducted to study the precipitation behavior and thermal stability of the strengthening phases with various structures in modified Inconel 718 alloys by tailoring the Al/Ti ratio. With increasing Al/Ti ratio, three particle configurations were formed: γ′/γ″ composite, isolated γ′, and γ′/γ″/γ′ composite particles. The results of aging tests demonstrate that the isolated γ′ and the γ′/γ″/γ′ composite structure exhibited better thermal stability at temperature as high as 800°C. The isolated γ′ exhibited a reduced coarsening rate compared with the γ′/γ″/γ′ composite particles because the isolated γ′ phase was rich in Al, Ti, and Nb. However, the γ′/γ″composite particles coarsened and decomposed rapidly during aging at temperatures greater than 700°C because of the lower stability resulting from the larger number of γ″ particles. The obtained results provide necessary data for the compositional optimization of novel 718-type alloys.     

Atomic-scale computer simulation for early precipitation process of Ni75Al10V15 alloy

The kinetic model for a ternary system is introduced based on the microscopic diffusion form of the phase-field equations for a binary alloy. The equation is solved in the reciprocal space. This model is used to investigate the early precipitation process of Ni75-Al10V15 by simulating the atomic pictures of the two ordered phases and calculating the order parameters of γ′(Ni3Al) phase. Simulation results show that the γ′ ordered phases precipitate from the disordered matrix by a non-classical nucleation mechanism, and the nonstoichiometric γ′ ordered phase appears first and then transforms into the stoichiometric one. Clusters of V atoms appear at the γ′ phase boundaries followed by the formation of the nonstoichiometric θ ordered phase. The farther the location from γ′ phase boundary is, the lower the order degree of θ phase is. There exist two kinds of DO22 ordered domains: a horizontal one and a vertical one, related to their adjacent γ′ phase boundaries. The model could describe the atomic ordering and composition clustering simultaneously, and any a priori assumption about the new phase structure and precipitation mechanism etc. is unnecessary.     

High temperature creep mechanisms of a single crystal superalloy: A phasefield simulation and microstructure characterization

The high temperature creep behavior of a single crystal Ni-based superalloy was studied by combined experimental and numerical methods. The creep test results showed that the creep curves exhibited a three-stage feature. The qualitative explanations for each stage of the creep curves were carried out based on the microstructure characterizations of γ/γ′ phases and dislocations. An elastoplasticity incorporated phase-field model was developed to provide quantitative understanding on directional coarsening(rafting) of γ′ phase. The simulation results showed that the directionality of γ′ coarsening was induced by both dislocation activity in γchannels and elastic inhomogeneity between γ and γ' phases, therein the dislocation activity played a major role.This findings provide new insights into the design of novel single crystal superalloys with improved creep properties.     

Morphology and kinetics evolution of γ′ and γ phases in Ni-xAl-(20-x)Cr at % alloys

The morphology and kinetics evolution of γ' and γ phases in Ni-xAl-(20-x)Cr at%(x=7.5-13.7) alloys aged at700℃ were studied by utilizing the phase-field simulation.The separated γ' phases from γ matrix in low Al content(x 13) alloys transferred to the interconnected morphology with separated y phases in high Al content(x 13) alloys,and long-time aging produced near cubic γ' phase.The coarsening rate of γ' phase declined firstly when the Al content x=7.5-9.0,then increased sharply as x ranges from 9 to 12,while the coarsening rate of γphase declined directly.Additionally,the average edge-to-edge distance of neighboring γ' phases showed a threestage variation when Al content x 9,it decreased to minimum value and increased slowly then decreased again with aging time.In contrast,when 9 ≤ x 13,the average particle distance of γ' phase decreased to a nearly constant value.With the coarsening of y phase,their average particle distance increased.The kinetics evolution was clarified with the combined variations of the particle number density,average particle radius and volume fractions in the continuous composition Ni-xAl-(20-x)Cr at% alloys.     

Low Carbon Low Alloy Submicro-Steel with Nano-Precipitates

A submicro-steel sheet was successfully fabricated by severe warm-wiling at 773 K through a single pass. The microstructure was characterized first and the thermal stability of the submicro-steel was investigated by annealing the steel at different temperatures.Results indicate that grains are nearly equiaxial with an average diameter of 300 nm for grains near to the surface and 600 nm for grains at the center of the sheet and the submicro-steel can be subjected to annealing at 773 K without obvious grain growth. The formation of the submicro-structure can be related to a severe plastic deformation-induced grain refinement mechanism. The unusually high thermal stability can be attributed to the pinning effect of numerous uniformly-distributed nano-precipitates in the steel. The average diameter of the larger precipitates is about 30 nm and the smaller less than 10 inn.     

Effect of heating rate on the densification of NdFeB alloys sintered by an electric field

This study introduces a novel method of electric field sintering for preparing NdFeB magnets. NdFeB alloy compacts were all sintered by electric fields for 8 min at 1000℃ with different preset heating rates. The characteristics of electric field sintering and the effects of heating rate on the sintering densification of NdFeB alloys were also studied. It is found that electric field sintering is a new non-pressure rapid sintering method for preparing NdFeB magnets with fine grains at a relatively lower sintering temperature and in a shorter sintering time. Using this method, the sintering temperature and process of the compacts can be controlled accurately. When the preset heating rate increasing from 5 to 2000℃/s the densification of NdFeB sintered compacts gradually improves. As the preset heating rate is 2000℃/s, Nd-rich phases are small, dispersed and uniformly distributed in the sintered compact, and the magnet has a better microstructure than that made by conventional vacuum sintering. Also, the maximum energy product of the sintered magnet reaches 95% of conventionally vacuum sintered magnets.     

Cathodic micro-arc electrodeposition of yttrium stabilized zirconia （YSZ） coatings on FeCrAI alloy

The formation of ceramic coatings on metal substrate by cathodic electrolytic deposition (CELD) has received more attention in recent years. But only thin filmscan be prepared via CELD. Yttrium stabilized zirconia (YSZ) ceramic coatings were deposited on FeCrAI alloy by a novel technique--cathodic micro-arc electrodeposition (CMED).The result shows that, when a high pulse electric field is applied to the cathode which was pre-deposited with a thin YSZ film, dielectric breakdown occurs and micro-arc discharges appear. Coatings with reasonably thickness of-300μm and crystalline structure can be deposited on the cathode by utilizing the energy of the micro-arc. The thickness of the as-deposited coating is dominated by the voltage and the frequency. Y2O3 is co-deposited with ZrO2 when Y(NO3)3 was added to the electrolyte, which stabilize t-phase, t′-phase and c-phase of ZrO2 at room temperature. The amount of the m-ZrO2 in the coating is diminished by increasing the concentration of Y(NO3)3 in the electrolyte.This report describes the processing of CMED and studies the microstructure of the deposited YSZ coatings.     

High temperature strength and ductility of the (C+N) strengthening Fe-Cr-Mn(W,V) steels

Fe-Cr-Mn(W, V) austenite steels used as low radioactive structural materials in fusion reactor have been investigated. The resultsshow that the high temperature strength and the creep fracture life of Fe-Cr-Mn(W, V) steels can be effectively improved through (C+N) complex-strengthening, so can be the high temperature ductility. The strength and ductility of the steels are superior to that of SUS316 steels and JPCAS below 673K. The relationship between strength, ductility andthe formation temperature is related to the evolution of deformation microstructure. The fracture and microstructure observation above 673Kindicates that the main way to further improve ductility at high temperature is the control of carbide coarsening at the grain boundaries.     

Nonrelativistic phase in γ-ray burst afterglows

The discovery of multiband afterglows definitely shows that most γ-ray bursts are of cosmological origin. γ-ray bursts are found to be one of the most violent explosive phenomena in the Universe, in which astonishing ultra-relativistic motions are involved. In this article, the multiband observational characteristics of γ-ray bursts and their afterglows are briefly reviewed. The standard model of γ-ray bursts, i.e. the fireball model, is described. Emphasis is then put on the importance of the nonrelativistic phase of afterglows. The concept of deep Newtonian phase is elaborated. A generic dynamical model applicable in both the relativistic and nonrelativistic phases is introduced. Based on these elaborations, the overall afterglow behaviors, from the very early stages to the very late stages, can be conveniently calculated.     

Nonrelativistic phase in γ-ray burst afterglows

The discovery of multiband afterglows definitely shows that most γ-ray bursts are of cosmological origin. γ-ray bursts are found to be one of the most violent explosive phenomena in the Universe, in which astonishing ultra-relativistic motions are involved. In this article, the multiband observational characteristics of γ-ray bursts and their afterglows are briefly reviewed. The standard model of γ-ray bursts, i.e. the fireball model, is described. Emphasis is then put on the importance of the nonrelativistic phase of afterglows. The concept of deep Newtonian phase is elaborated. A generic dynamical model applicable in both the relativistic and nonrelativistic phases is introduced. Based on these elaborations, the overall afterglow behaviors, from the very early stages to the very late stages, can be conveniently calculated.     

Glass forming ability of Zr– Al –Ni(Co,Cu) understood via cluster sharing model

Clusters are shared atoms in different ways with their neighboring clusters in the crystalline phases.Cluster formula [effective cluster]1(glue atom)xcan be used to describe crystalline phases,and the effective cluster means the true cluster composition due to cluster overlapping in the phase structure.Degree of cluster sharing of Zr6Al2Ni(InMg2),Zr2Co(Al2Cu) and Zr2Cu(MoSi2) phases is investigated in this paper.Ni3Zr9,Co3Zr8 and Cu5Zr10clusters are highlighted because they have the least degree of sharing and can best represent the local atomic short-range order features of the formed phases.It is pointed out that the least sharing clusters are correlated with metallic glass formation and are verified by experiments.     

The effects of the guide field on the structures of electron density depletions in collisionless magnetic reconnection

Two-dimensional particle-in-cell simulations are performed to investigate the formation of electron density depletions in collisionless magnetic reconnection.In anti-parallel reconnection,the quadrupole structures of the out-of-plane magnetic field are formed,and four symmetric electron density depletion layers can be found along the separatrices due to the effects of magetic mirror.With the increase of the initial guide field,the symmetry of both the out-of-plane magnetic field and electron density depletion layers is distorted.When the initial guide field is sufficiently large,the electron density depletion layers along the lower left and upper right separatrices disappear.The parallel electric field in guide field reconnection is found to play an important role in forming such structures of the electron density depletion layers.The structures of the out-of-plane magnetic field By and electron depletion layers in anti-parallel and guide field reconnection are found to be related to electron flow or in-plane currents in the separatrix regions.In anti-parallel reconnection,electrons flow towards the X line along the separatrices,and are directed away from the X line along the magnetic field lines just inside the separatrices.In guide field reconnection,electrons can only flow towards the X line along the upper left and lower right separatrices due to the existence of the parallel electric field in these regions.     

Microstructure and stress rupture properties of polycrystal and directionally solidified castings of nickel-based superalloys

A new directionally solidified Ni-based superalloy DZ24, which is a modification of K24 alloy without rare and expensive elemental additions, such as Ta and Hf, was studied in this paper. The microstructure and stress rupture properties of conventionally cast and directionally solidified superalloys were comparatively analyzed. It is indicated that the microstructure of K24 alloy is composed of γ, γ′, γ/γ′ eutectics and MC carbides. Compared with the microstructure of K24 polycrystalline alloy, γ/γ′ eutectic completely dissolves into the γ matrix, the fine and regular γ′ phase reprecipitates, and MC carbides decompose to M₆C/M₂₃C₆ carbides after heat treatment in DZ24 alloy. The rupture life of DZ24 alloy is two times longer than that of K24 alloy. The more homogeneous the size of γ′ precipitate, the longer the rupture life. The coarsening and rafting behaviors of γ′ precipitates are observed in DZ24 alloy after the stress-rupture test.     

Observation and simulation of flux rope structures at the dayside magnetopause

The signatures of flux ropes with obvious core magnetic field are detected by Cluster Ⅱ at the dayside magnetopause during 11:00--11:15 UT on Mar. 2, 2001. The similar characteristics can be found from the magnetic fiel dvariations recorded by the four spacecrafts (Cluster Ⅱ C1--C4). All the three (-/ ) bipolar signatures in the BN component are accompanied with enhancements of BM and magnetic field strength B in the boundary normal coordinates (LMN coordinates). A MHD simulation with two dimensions and three components is performed to explore the reconnection process driven by the incoming flow of solar wind at the dayside magnetopause. The numerical results can illustrate the recurrent formation of magnetic structures with a core magnetic field. The time history of the magnetic field B and three components Bx, By and Bz at a given point of the current sheet can reproduce the observational features of the events mentioned above.     

Characterization of Bi2/3Cu3Ti4O12 ceramics synthesized by semi-wet route

Bi_(2/3)Cu_3Ti_4O_(12)(BCTO) ceramic was synthesized by the semi-wet route using metal nitrate solutions and solid TiO_2 powder in a stoichiometric ratio. Fourier transform infrared(FTIR) study of BCTO precursor powder and calcined ceramic showed the presence of alcoholic functional groups and the stretching band of Ti-O and Cu-O respectively. X-ray diffraction(XRD), scanning electron microscope(SEM) and energy dispersive x-ray spectroscopy(EDX) were employed to characterize the structure, surface morphology and purity of the sintered BCTO ceramic respectively. X-ray diffraction study confirmed the single phase formation of BCTO ceramic at1073 K. The average dimension of grains calculated by SEM and AFM was found to be in the range of 0.73 ±0.2 μm with clear grain boundaries. Magnetic property was investigated over a wide temperature range 2–300 K at a magnetic field of 7 tesla. The Curie temperature was calculated by zero field cooled(M~(ZFC)) and field cooled(M~(FC)) magnetization at 100 Oe applied field which was found to be 125 K. The sintered BCTO ceramic shows high dielectric constant(ε'=2.9×10~4) at 323 K and 100 Hz.