Roles of Zr and Y in cast microstructure of M951 nickel-based superalloy

The influence of Zr and Y on the cast microstructure of a nickel-based superalloy was investigated by optical microscopy （OM）,scanning electron microscopy（SEM）,electron probe micro-analysis（EPMA）and X-ray diffraction（XRD）.Theγ＋γ′eutectic volume in the superalloy rises notably with the increase of Zr or Y content.Meanwhile,the morphologies of primary MC carbides change from needle and platelet-like to blocky shape with increasing Zr and Y doped.The XRD results show that the primary MC carbide lattice constant increases with Zr and Y additions,and EPMA investigation shows that the platelet-like MC carbides contain primarily Nb and C,while those carbides in blocky shape have 39.2%Zr and 39.6%Nb in average,.These influences on the cast microstructure can be attributed to the atomic size effects of Zr and Y.

Kinetics and microstructural evolution during recrystallization of a Ni_3Al-based single crystal superalloy

The recrystallization kinetics and microstructural evolution of a Ni3Al-based single crystal superalloy were presented, especially the different recrystallization behaviors between the dendrite arm and the interdendritic region. The single crystal alloy was deformed by grit blasting. A succeeding annealing under inert atmosphere at 1280 ℃ for different time led to the formation of recrystallized grains close to the grit blasting surface. It was found that the recrystallization depth and velocity in the dendrite arm were respectively deeper and faster than those in the interdendritic region where the Y-NiMo phase existed. The recrystallization process in the interdendritic region was significantly inhibited by the Y-NiMo precipitates. However, the pinning effect gradually weakened with the annealing time due to the dissolution of the Y-NiMo phase, and the recrystallization depth in the dendrite arm was deeper than that in the interdendritic region.

Effect of Co on oxidation and hot corrosion behavior of two nickel-based superalloys under Na_(2)SO_(4)-NaCl at 900℃

Nickel-based superalloys with and without Co by partial replacement of W were prepared using double vacuum melting.A comparison of the oxidation in air and hot corrosion behaviors under molten 75 wt.%Na2 SO4+25 wt.%NaCl at 900℃were systematically investigated.The results showed that partial replacement of W with Co promoted the formation of chromia scale and consequently decreased the oxidation rate.Besides,the addition of Co also retarded the internal oxidation/nitridation of Al and consequently promoted the growth of Al_(2) O3 scale,which further decreased the scaling rate and improved the adhesion of scale.Moreover,the addition of Co also further improved the hot corrosion resistance under molten Na2 SO4-NaCl salts.

EFFECTS OF TRACE ELEMENTS ON THE OXIDATION OF LOW SEGREGATION Ni－Cr－Al BASED SUPERALLOYS

The oxidation behaviors of two kinds of low segregation Ht-Cr-Al based superalloys have been studied between 1000-1100℃, and compared with that of general Mt-Cr-Al based superalloys. The results indicated that the simultaneous additions of 0.1 wt% S and 0. 1 wt% Zr to low segregation alloys increase the oxidation rate of Al2O3-forming alloy and improve the scale adherence. The addition of 0.1 wt% Zr can ,minimize the negative effects of S on the adherence of Al2O3 scale. Low amounts of S(≤50 ppm wt) have no obviously negative effects on the adherence of Cr2O3 scale formed on one of the low segregation superalloys.

Surface recrystallization of a Ni_3Al based single crystal superalloy at different annealing temperatures and blasting pressure

The effects of annealing temperature and grit blasting pressure on the recrystallization behavior of a Ni3Al based single crystal superalloy were studied in this work. The results show that the precipitation of the Y-NiMo phase occurs at 900 and 1000 °C, which precedes recrystallization. The initial recrystallization temperature was between 1000 and 1100 °C. Cellular recrystallization was formed at 1100 and 1200 °C, which consisted of large columnar γ′ and fine γ + γ′. The dendrite arm closed to the interdendritic region may act as nucleation sites during initial recrystallization by a particle simulated nucleation mechanism at 1280 °C. The size of the grains first turned large and then became small upon the pressure while the recrystallization depth increased all the time.

Determination of Aluminum in Nickel-Based Superalloys by Using Laser-Induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy （LIBS） was developed to detect aluminum in nickel-based superalloys （K417, GH4033, DZ125L, З ∏742y） using a non-intensified, non-gated, low-cost detection system. The precision of LIBS depends strongly on the experimental conditions. The calibration curves of Al（I）394.4 nm and Al（I）396.2 nm under the optimum experimental parameters are presented. Finally the limit of detection （LOD） for aluminum is calculated from the experimental data, which is in the range of 0.09% to 0.1% by weight.

Morphological evolution of γ'precipitates in a nickel-based superalloy during various solution treatments

The morphological evolution of the γ' phase in nickel-based superalloy жc6y during various solution heat treatments was investigated. The significant changes of the γ' precipitates were observed in the solution-treated samples. The coarsening and dissolution of γ' phase simulta-neously occurred at intermediate temperatures. In some areas, the primary precipitates became blunt and the adjacent ones were intercon-nected with each other via a diffuse neck, indicating a coarsening process of the primary γ' population. The coarsening was dominated by the precipitate agglomeration mechanism (PAM) rather than by the well-known Ostwald ripening mechanism. In other areas, the partial dissolu-tion of the γ' precipitates began to occur, spreading gradually from dendrite cores to interdendritic regions. In addition, a flower-like γ' struc-ture was developed during the subsolvus solution treatments. The observable long filaments composed of erraticly shaped precipitates were caused by the heterogeneous nucleation of the cooling precipitates during water quenching.

TEMPERATURE EFFECT ON LOW-CYCLE FATIGUE BEHAVIOR OF NICKEL-BASED SINGLE CRYSTALLINE SUPERALLOY

The low-cycle fatigue （LCF） behavior of a nickel-based single crystal superalloy with [001] orientation was studied at an intermediate temperature of T0℃ and a higher temperature of To ＋ 250℃ under a constant low strain rate of 10^-3 s^-1 in ambient atmosphere. The superalloy exhibited cyclic tension-compression asymmetry which is dependent on the temperature and applied strain amplitude. Analysis on the fracture surfaces showed that the surface and subsurface casting micropores were the major crack initiation sites. Interior Ta-rich carbides were frequently observed in all specimens. Two distinct types of fracture were suggested by fractogaphy. One type was characterized by Mode-I cracking with a microscopically rough surface at To ＋ 250℃. Whereas the other type at lower temperature T0℃ favored either one or several of the octahedral {111} planes, in contrast to the normal Mode-I growth mode typically observed at low loading frequencies （several Hz）. The failure mechanisms for two cracking modes are shearing of γ＇ precipitates together with the matrix at T0℃ and cracking confined in the matrix and the γ/γ＇interface at To - 250℃.

Effect of a High Magnetic Field on the Shape of the γ' Precipitates in Cast Nickel-based Superalloy K52

The shape change of the γ＇ precipitates of cast Ni-based superalloy K52 after aging treatment under a high magnetic field was investigated. The results show that duplex γ＇ precipitates are present in the γ matrix after aging treatment with or without the magnetic field. One is the coarse particles with average size of 500 nm; the other is fine spherical γ＇ precipitates with average of 100 nm in diameter. The application of a 10T magnetic field only results in the shape of the coarse γ＇ particles changing from spherical to cuboidal when the alloys subjected to the same heat treatments. This shape change was mainly discussed based on the strain energy increase caused by the difference in magnetostriction between the γ matrix and γ＇ precipitates. The fine γ＇ particles still keep spherical. Further TEM observations shows that a number of γ particles in nano-scale precipitate in the coarse γ＇ particles in the specimens treated without the magnetic field. In addition, it was found that the magnetic field caused the decrease of the hardness in the alloy, and the hardness was associated with the field direction.

Effect of solidification parameters on the microstructures of a single crystal Ni-based superalloy AM3

A single crystal Ni-based superalloy AM3 was processed at withdraw rates of 3.5, 10, 50, 100, 200, and 500 μm·s-1, respectively.The as-cast microstructures and solidification segregation ratio were characterized with various withdraw rates.The shape and size of carbide microstructures were determined.As expected, the primary and secondary dendrite arm spacings (PDAS and SDAS) decrease with the increase of withdraw rate.The highest volume fraction of eutectic γ/γ' is observed at the 100 μm·s-1 withdraw rate.The volume fraction of eutectic γ/γ' does not appear to be a strong function of the withdraw rate.With increasing withdraw rate, interface morphologies change in the sequence of planar, cellular, and dendrite.There is a general refinement of the microstructure as the withdraw rate increases.EPMA analysis showed that withdraw rate does not have obvious influence on the segregation of elements.

Surface characteristics of rapidly solidified nickel-based superalloy pow-ders prepared by PREP

The surface microstructure and the surface segregation of FGH 95 nickel-basedsuperalloy powders prepared through plasma rotating electrode processing (PREP) have beeninvestigated by using SEM and AES. The results indicate that the surface microstructure of powderschanges from dendrite into cellular stricture as the particle size of powders decrease, and thepredominant precipitates solidified on the particle surfaces were identified as MC' type carbidesenriched with Nb and Ti. It was also indicated that along with the depth of particle surfaces, thesegregation layer of S, C and O elements are thick, and that of Ti, Cr elements are thin for largesire powders while they are in reverse for median size particles.

First principles study of the diffusional phenomena across the clean and Re-doped γ-Ni/γ'-Ni_3Al interface of Ni-based single crystal superalloy

Density functional theory calculations in conjunction with the climbing images nudged elastic band method are conducted to study the diffusion phenomena of the Ni-based single crystal superalloys.We focus our attention on the diffusion processes of the Ni and Al atoms in the γ and γ ’ phases along the direction perpendicular to the interface.The diffusion mechanisms and the expressions of the diffusion coefficients are presented.The vacancy formation energies,the migration energies,and the activation energies for the diffusing Ni and Al atoms are estimated,and these quantities display the expected and clear transition zones in the vicinity of the interface of about 3–7（002） layers.The local density-of-states profiles of atoms in each（002） layer in the γ and γ ’ phases and the partial density-of-states curves of Re and some of its nearest-neighbor atoms are also presented to explore the electronic effect of the diffusion behavior.

Influence of thermal exposure on microstructure and stress rupture properties of a new Re-containing single crystal Ni-based superalloy

In this study, the long-term thermal microstructural stability and related stress rupture lives of a new Re-containing Ni-based single-crystal superalloy, DD11, were investigated after high-temperature exposure for different lengths of time. The results show that the γ' precipitates retained a cuboidal morphology and the γ' size increased after short thermal exposure for 50 h at 1,070℃. As the thermal exposure time was prolonged to 500 h, the cuboidal γ' gradually changed into irregular raft-like morphology due to particles coalescence, and the morphology of the microstructure was almost unchanged after further thermal exposure up to 3,000 h. The stress rupture experiments at 1,070℃ and a tensile stress of 140 MPa showed that the rupture lives increased significantly after thermal exposure for 50 h and dropped dramatically with increasing exposure time up to 500 h but decreased slowly after exposure for more than 500 h. These results imply that stress rupture properties did not decrease when the γ' remained cuboidal but degraded to different extents during the γ' coarsening process. The coarsening of the γ' precipitates and change in morphology were regarded as the main factors leading to the degradation of the stress rupture lives. This study provides fundamental information on the high-temperature longterm microstructural stability and mechanical performance, which will be of great help for DD11 alloy optimization and engineering aeroengine applications.

An anisotropic micromechanics model for predicting the rafting direction in Ni-based single crystal superalloys

An anisotropic micromechanics model based on the equivalent inclusion method is developed to investigate the rafting direction of Ni-based single crystal superalloys. The micromechanical model considers actual cubic structure and orthogonal anisotropy properties. The von Mises stress, elastic strain energy density, and hydrostatic pressure in dif- ferent inclusions of micromechanical model are calculated when applying a tensile or compressive loading along the [001] direction. The calculated results can successfully pre- dict the rafting direction for alloys exhibiting a positive or a negative mismatch, which are in agreement with pervious experimental and theoretical studies. Moreover, the elastic constant differences and mismatch degree of the matrix and precipitate phases and their influences on the rafting direction are carefully discussed.

Microstructural evolution and castability prediction in newly designed modern third-generation nickel-based superalloys

The present research aims to establish a quantitative relation between microstructure and chemical composition （i.e., Ti, Al, and Nb） of newly designed nickel-based superalloys. This research attempts to identify an optimum microstructure at which the minimum quanti- fies of γ/γ＇ and γ/γ＂ compounds are achieved and the best castability is predicted. The results demonstrate that the highest quantity of inter- metallic eutectics （i.e., 41.5wt%） is formed at 9.8wt% （Ti ＋ A1）. A significant quantity of intermetallics formed in superalloy 1 （with a com- position of7 - 9.8wt% （Ti ＋ A1））, which can deteriorate its castability. The type and morphology of the eutectics changed and the amount considerably decreased with decreasing Ti ＋ A1 content in superalloy 2 （with a composition ofy - 7.6wt% （Ti ＋ A1）, 1.Swt% Nb）. Thus, it is predicted that the castability would improve for superalloy 2. The same trend was observed for superalloy 4 （with a composition of 7 - 3.7wt% （Ti ＋ A1）, 4.4wt% Nb）. This means that the amount of Laves increases with increasing Nb （to 4.4wt%） and decreasing Ti ＋ A1 （to 3.7wt%） in su- peralloy 4. The best castability was predicted for superalloy 3 （with a composition ofy - 5.7wt% （Ti ＋ A1）, 2.8wt% Nb）.

Microstructure and Mechanical Properties of Spray Deposited Ni-based Superalloys

Three kinds of superalloys were prepared by spray deposited process. The analysis results of microstructures and mechanical properties indicate that the spray deposited preforms with higher integral densification and the oxygen content in each kind of superalloy was very low. The microstructures are consisted of fine grain without dendritic equi-axed. The spray deposited superalloys possessed good ductility. The forging experiment displays that even though the once deformation of spray deposited GH742 alloy more than 60%, the crack can not be found. Meanwhile, the mechanical properties of spray deposited superalloys are significantly increased.

Effect of H impurity on misfit dislocation in Ni-based single-crystal superalloy:molecular dynamic simulations

The effect of H impurity on the misfit dislocation in Ni-based single-crystal superalloy is investigated using the molecular dynamic simulation. It includes the site preferences of H impurity in single crystals Ni and Ni3Al, the interaction between H impurity and the misfit dislocation and the effect of H impurity on the moving misfit dislocation. The calculated energies and simulation results show that the misfit dislocation attracts H impurity which is located at the γ/γ′ interface and Ni3Al and H impurity on the glide plane can obstruct the glide of misfit dislocation, which is beneficial to improving the mechanical properties of Ni based superalloys.

Low-cycle Fatigue Behavior of Ni-based Superalloy GH586 with Laser Shock Processing

Low-cycle fatigue behavior of Ni-based superalloy GH586 with laser shock processing（LSP） was investigated. The residual stress of the specimens treated with LSP was assessed by X-ray diffraction method. The microstructure and fracture morphology were characterized by using an optical microscope（OM）, a scanning electron microscope（SEM）, and a transmission electron microscope（TEM）. The results indicated that the maximum residual compressive stress was at about 1 mm from the shocking spot center, where the residual compressive stress was slightly lower. High density tangling dislocations, dislocation walls, and dislocation cells in the microstructure of the specimens treated with LSP effectively prevented fatigue cracks propagation. The fatigue life was roughly twice as long as that of the specimens without LSP. The fatigue crack initiation（FCI） in specimens treated with LSP was observed in the lateral section and the subsurface simultaneously. The fatigue striation in the fracture treated with LSP was narrower than that in the untreated specimens. Moreover, dimples with tear ridges were found in the fatigued zones of the LSP treated specimens, which would be caused by severe plastic deformation.

Isothermal oxidation behavior and mechanism of a nickel-based superalloy at 1000°C

The oxidation behavior of a nickel-based superalloy at 1000°C in air was investigated through X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy analysis. A series of oxides, including external oxide scales（Cr_2O_3,（TiO_2 ＋ Mn Cr_2O_4）） and internal oxides（Al_2O_3,Ti N）, were formed on the surface or sub-surface of the substrate at 1000°C in experimental still air. The oxidation resistance of the alloy was dependent on the stability of the surface oxide layer. The continuity and density of the protective Cr_2O_3 scale were affected by minor alloying elements such as Ti and Mn. The outermost oxide scale was composed of TiO_2 rutile and Mn Cr_2O_4 spinel, and the growth of TiO_2 particles was controlled by the outer diffusion of Ti ions through the pre-existing oxide layer. Severe internal oxidation occurred beneath the external oxide scale, consuming Al and Ti of the strength phase γ′（Ni_3（Al,Ti）） and thereby severely deteriorating the surface mechanical properties. The depth of the internal oxidation region was approximately 35 μm after exposure to experimental air at 1000°C for 80 h.

Molecular dynamics study of mosaic structure in the Ni-based single-crystal superalloy

The mosaic structure in a Ni-based single-crystal superalloy is simulated by molecular dynamics using a potential employed in a modified analytic embedded atom method. From the calculated results we find that a closed threedimensional misfit dislocation network, with index of （011）{100} and the side length of the mesh 89.6A, is formed around a cuboidal γ′ precipitate. Comparing the simulation results of the different mosaic models, we find that the side length of the mesh only depends on the lattice parameters of the γ and γ′ phases as well as the γ/γ′ interface direction, but is independent of the size and number of the cuboidal γ′ precipitate. The density of dislocations is inversely proportional to the size of the cuboidal γ′ precipitate, i.e. the amount of the dislocation is proportional to the total area of the γ/γ′ interface, which may be used to explain the relation between the amount of the fine γ′ particles and the creep rupture life of the superalloy. In addition, the closed three-dimensional networks assembled with the misfit dislocations can play a significant role in improving the mechanical properties of superalloys.