Oxidation behavior of 4774DD1 Ni-based single-crystal superalloy at 980℃ in air

The oxidation behavior of a novel Ni-based single-crystal 4774DD1 superalloy for industrial gas turbine applications was investigated by the isothermal oxidation at 980℃ and discontinuous oxidation weight gain methods.The phase constitution and morphology of surface oxides and the characteristics of the crosssection oxide film were analyzed by XRD,SEM and EDS.Results show that the oxidation kinetics of the 4774DD1 superalloy follows the cubic law,indicating its weak oxidation resistance at this temperature.As the oxidation time increases,the composition of the oxide film evolves as following:One layer consisting of a bottom Al_(2)O_(3)sublayer and an upper(Al_(2)O_(3)+NiO)mixture sublayer after oxidized for 25 h.Then,two layers composed of an outermost small NiO discontinuous grain layer and an internal layer for 75 h.This internal layer is consisted of the bottom Al_(2)O_(3)sublayer,an intermediate narrow CrTaO_(4)sublayer,and an upper(Al_(2)O_(3)+NiO)mixture sublayer.Also two layers comprising an outermost relative continuous NiO layer with large grain size and an internal layer as the oxidation time increases to 125 h.This internal layer is composed of the upper(Al_(2)O_(3)+NiO)mixture sublayer,an intermediate continuous(CrTaO_(4)+NiWO_(4))mixture sublayer,and a bottom Al_(2)O_(3)sublayer.Finally,three layers consisting of an outermost(NiAl2O_(4)+NiCr2O_(4))mixture layer,an intermediate(CrTaO_(4)+NiWO_(4))mixture layer,and a bottom Al_(2)O_(3)layer for 200 h.

High cycle fatigue behavior of the second generation single crystal superalloy DD6

The second generation single crystal superalloy DD6 with 0.10%Hf and 0.34%Hf （in mass fraction） was subjected to high-cycle fatigue （HCF） loading at temperatures of 700 ℃ in ambient atmosphere. SEM was used to determine the initiation site and the failure mechanism. Evolution of the microstructure was investigated by TEM observation. The results show that fatigue limit of DD6 alloy with 0.34%Hf is a little smaller than that of the alloy with 0.10%Hf. The fatigue cracks initiated on the surface or near the surface of the specimens. The crack would propagate along { 111 } octahedral slip planes, rather than perpendicular to the loading axis of specimen. Typical fatigue striation formed in steady propagation of fatigue crack. The fracture mechanisms of the high cycle fatigue of DD6 alloys with 0.10%Hf and 0.34%Hf are quasi-cleavage fracture. Different types of dislocation structures were developed during high cycle fatigue deformation.

Orientation dependence of transverse tensile properties of nickel-based third generation single crystal superalloy DD9 from 760 to 1100 ℃

At temperatures ranging from 760 to 1100 °C, the tensile properties of a nickel-based third generation single crystal superalloy DD9 with [100],[120] and [110] orientations were studied. The microstructures and fracture surfaces were observed by OM, SEM and TEM. Results show that the tensile strength of [100] specimen is higher than that of [120] and [110] specimens at 760 and 850 °C;while at the temperatures higher than 980 °C, the tensile strength of all specimens has little difference. The fracture mechanisms of [100],[120] and [110] specimens are the same at 760 and 980 °C. At 1100 °C, the fracture surfaces of [100] and [120] specimens are characterized by dimple features;while [110] specimen shows mixed quasi-cleavage and dimple featured fracture surfaces. At 760 °C, obvious superlattice stacking faults (SSFs) are observed only in [100] specimen;while at 1100 °C, the dislocation configurations of all specimens are similar. The difference in the number of potential active slip systems in [100],[120] and [110] specimens during the tensile deformation process is the main reason for the transverse tensile anisotropy.

Effect of ruthenium on γ' precipitation behavior and evolution in single crystal superalloys

The effect of Ru on γ＇ precipitation behavior and evolution in single crystal superalloys with different Ru contents were investigated by scanning electron microscopy with energy dispersive spectroscopy,3D atomic probing,differential scanning calorimetry.The results show that the solvus of the γ＇ phase decreases gradually with increasing Ru content in the alloys by casting or by the same solution and aging treatments,the alloy with a larger Ru content yields a smaller γ＇ phase.The addition of Ru increases the growth rate and coarsening rate of the γ＇ phase.Ru mainly distributes in the γ phase,which causes more Re and Mo partition into the γ＇ phase,increasing the absolute value of mismatch and the rafting rate of the γ＇ phase.

Isothermal oxidation behavior of single crystal superalloy DD6

The isothermal oxidation behavior of the second generation single crystal superalloy DD6 was studied at 1050 ℃ and 1100 ℃ in ambient atmosphere.Morphology of oxides was examined by SEM and their composition was analyzed by XRD and EDS.The experimental results show that DD6 alloy obeys subparabolic rate law during oxidation of 100 h at 1050 ℃ and 1100 ℃.The oxide scale exposed at 1050 ℃ is made up of an outer NiO layer with a small amount of Al2O3 and an inner Al2O3 layer.The oxide scale exposed at 1100 ℃ is made up of an outer Al2O3 layer with a small amount of NiO,an intermediate layer,mainly composed of Cr2O3 and TaO2,and an inner Al2O3 layer.The γ＇-free layer was formed under the oxide scale at two temperatures.

Effect of surface recrystallization on the creep rupture property of a single-crystal superalloy

The effect of surface recrystallization by heating after shot-peening on the creep rapture property and fracture behavior of a single-crystal superalloy was investigated. The results show that the creep rupture property of the single-crystal superalloy was greatly influenced by surface recrystallization. A recrystallized surface layer with a depth of 101 ~m resulted in a decrease in creep rupture life by nearly 50%, and an almost linear reduction of creep rupture life was observed with the increase of recrystallization depth. A lower strength of the recrystal- lized layer, inhomogeneous deformation between the recrystallized layer and the matrix, and stress concentration caused by notch effect resuited in the decrease in creep rupture life of the single-crystal superalloy.

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, y＇ directional coarsening and rafting were observed in the Ru-free alloy and Ru-containing alloy after long-term aging at 1070~C for 800 h. Needle-shaped o 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 ofy and y＇ 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 satu- ration degrees of these elements in y 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.

Microstructual evolution and stability of second generation single crystal nickel-based superalloy DD5

The microstructual evolution and stability of a second generation single crystal (SC) nickel-based superalloy DD5 with minor grain boundary (GB) strengthening elements (C, B and Hf) were studied as a function of as-cast, heat treatment and thermal exposure. The microstructure and composition of the alloy were investigated by optical microscopy, scanning electron microanalysis (SEM), electron probe microanalysis (EPMA), energy dispersive spectrometry (EDS) and extraction analysis. In the as-cast condition,the microstructure observations and composition analysis showed that γ phase was the primary solidification phase and there were three microsegregations in the metal matrix. The morphology of these microsegregations depended on element segregations. After heat treatment, the dendrite cores contained fine and cuboidal-shaped γ′ particles with an average edge length of about 0.5 μm, whileinterdendritic regions contained irregularly-shaped γ′ particles and MC/M23C6 carbides. The mass fraction of γ′ phases was 61.685%.After exposure at 980 °C for 1000 h, no TCP phase was observed in both dendritic and interdendritic regions, indicating a good microstructual stability of the DD5 alloy at 980 °C.

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.

Microstructure - properties relationship of transient liquid phase diffusion bonded a third generation single crystal super alloy joint

Microstructure of transient liquid phase( TLP) diffusion bonded a third generation single crystal superalloy joint was investigated using scanning electron microscopy( SEM),and mechanical properties test of joint was carried out,for obtaining relationship between microstructure and mechanical properties of joint. The results showed that the joint contained bonding zone and base metal. The diffusion zone was obviously observed. When it was not finished for isothermal solidification process,the bonding zone would contain isothermal solidification zone and rapid solidification zone. Metallographic examination revealed that isothermal solidification zone was consisted of γ and γ' phase. Rapid solidification zone was consisted of two different structures,which were ternary eutectic of borides,γ and γ' phase developing at the edge of joint,binary eutectic of γ and γ' phase appearing in the portion of joint. When it was not enough for homogenization process under the condition of finishing isothermal solidification process,the bonding zone would contain isothermal solidification zone and borides at the interface. Under the conditions of relatively high welding temperature and long welding time,average tensile strength of joint was equivalent to that of parent material.

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.

Evolution of interfacial dislocation networks during long term thermal aging in Ni-based single crystal superalloy DD5

Interfacial dislocations found in single crystal superalloys after long term thermal aging have an important effect on mechanical properties. Long term thermal aging tests for DD5 single crystal superalloy were carried out at 1,100 ℃ for 20, 100, 200, 500 and 1000 h, and then cooled by air. The effect of long term thermal aging on the dislocation networks at the γ/γ' interfaces was investigated by FE-SEM. Results showed that during the long term thermal aging at 1,100 ℃, misfit dislocations formed firstly and then reorientation in the(001) interfacial planes occurred. Different types of square or rectangular dislocation network form by dislocation reaction. Square dislocation networks consisting of four groups of dislocations can transform into octagonal dislocation networks, and then form another square dislocation network by dislocation reaction. Rectangular dislocation networks can also transform into hexagonal dislocation networks. The interfacial dislocation networks promote the γ' phase rafting process. The dislocation networks spacings become smaller and smaller, leading to the effective lattice misfit increasing from-0.10% to-0.32%.

Crack growth behavior of SRR99 single crystal superalloy under thermal fatigue

The thermal fatigue behavior of a single crystal superalloy SRR99 was investigated. Specimens with V-type notch were tested at the peak temperatures of 900, 1000, and 1100℃. The crack growth curves as a function of the number of cycles were plotted. With the increase of peak temperature, the crack initiation life was shortened dramatically. Through optical microscopy （OM） and scanning electron microscopy （SEM） observation, it was found that multiple small cracks nucleated at the notch tip region but only one or two of them continued to develop in the following thermal cycles. The primary cracks generally propagated along a preferential direction. Microstructure changes after thermal fatigue were also discussed on the basis of SEM observation.

Simulation of stray grain formation at the platform during Ni-base single crystal superalloy DD403 casting

The mechanism of stray grain formation at the platform of turbine blade simulator and the effect of withdrawal rate （V） on the stray grain phenomenon have been investigated using a macro-scale ProCAST coupled with a 3D Cel ular Automaton Finite Element （CAFE） model. The results indicate that the stray grains nucleate at the edges of platform at V=150μm·s-1 and 200μm·s-1. Using ProCAST computer simulation software, it was proven that the stray grain formation is signiifcantly dependent on the undercooling and the temperature ifeld distribution in the platform. The macroscopic curvature of the liquidus isotherm becomes markedly concave with an increase in the withdrawal rate. The probability of stray grain formation at the edges of platform can be increased by increasing the withdrawal rate in the range of 70μm·s-1 to 200μm·s-1.

Effect of carbon additions on the microstructure of a single crystal Ni-based superalloy AM3

The microstructure of experimental nickel-base single crystal superalloys with different levels of carbon has been studied. The results indicated that with increasing carbon addition, the liquidus temperature decreased obviously and the as-cast microstructures exhibited a decrease in the amount of γ/γ′ eutectic structure and an increase in the volume fraction of carbides. The carbides formed in these alloys were most script-type MC carbides which appeared continuous dendritic networks in the interdendritic region. The segregation behavior of element W was influenced by the carbon addition.

Creep feed grinding induced gradient microstructures in the superficial layer of turbine blade root of single crystal nickel-based superalloy

The service performance of the turbine blade root of an aero-engine depends on the microstructures in its superficial layer.This work investigated the surface deformation structures of turbine blade root of single crystal nickel-based superalloy produced under different creep feed grinding conditions.Gradient microstructures in the superficial layer were clarified and composed of a severely deformed layer(DFL)with nano-sized grains(48–67 nm)at the topmost surface,a DFL with submicron-sized grains(66–158 nm)and micron-sized laminated structures at the subsurface,and a dislocation accumulated layer extending to the bulk material.The formation of such gradient microstructures was found to be related to the graded variations in the plastic strain and strain rate induced in the creep feed grinding process,which were as high as 6.67 and 8.17×10^(7)s^(−1),respectively.In the current study,the evolution of surface gradient microstructures was essentially a transition process from a coarse single crystal to nano-sized grains and,simultaneously,from one orientation of a single crystal to random orientations of polycrystals,during which the dislocation slips dominated the creep feed grinding induced microstructure deformation of single crystal nickel-based superalloy.

Effect of long-term thermal exposure on microstructure and creep properties of DD5 single crystal superalloy

The effect of thermal exposure on the microstructure and creep properties of the Ni-based single crystal superalloy in different test conditions was studied.Long-term exposure was performed at 1,000 ℃ and 1,100 ℃ for 500 h prior to the creep tests.The creep lifetime is found to be improved after the long-term exposure at 1,000 ℃ for 500 h as a result of the formation of secondary M_(23)C_(6) in the interdendritic region.The coarsening of γ’ precipitates accompanied by the formation of TCP phase lead to the degradation of alloy,which is responsible for the reduction of the creep lifetime of Ni-base single crystal superalloy after long-term exposure at 1,100 ℃ for 500 h.The creep lifetime of 1,000 oC thermally exposed sample under the conditions of 1,093 ℃/137 MPa is lower than that of heat-treated state.Thermal exposure at 1,100 ℃ for 500 h causes the creep lifetime to drop drastically.

MICROMECHANICS OF THE DAMAGE-INDUCED CELLULAR MICROSTRUCTURE IN SINGLE CRYSTAL Ni-BASED SUPERALLOYS

An analytical method to investigate the morphological evolution of the cellular mi-crostructure is explored and proposed. The method is essentially based on the Es-helby 's micromechanics theory, and it is extended so as to be applied for a material system containing inclusions with high volume fraction, by employing the average stress field approximation by Mori and Tanaka. The proposed method enables us to discuss a stable shape of precipitate in the material system, which must be influenced by many factors: e.g., volume fraction of precipitate; Young's modulus ratio and lattice misfit between matrix and precipitate; external stress field in multiaxial state; and heterogeneity of plastic strain between matrix and precipitate. A series of numerical calculations were summarized on stable shape maps. The application of the method to predict the γ' rafting in superalloys during creep showed that the heterogeneity of plastic strain between matrix and precipitates may play a significant role in the shape stability of the precipitate. Furthermore, it was shown that the method was successfully applied to estimate the morphology of the cellular microstructure formed in CMSX-4 single crystal Ni-based superalloy.

A CRYSTALLOGRAPHIC MODEL FOR THE ORIENTATION DEPENDENCE OF LOW CYCLIC FATIGUE PROPERTY OF A NICKEL-BASE SINGLE CRYSTAL SUPERALLOY

Fully reversed low cyclic fatigue (LCF) tests were conducted on [0 0 1], [0 1 2], [(1) over bar 1 2], [0 1 1] and [(1) over bar 1 4] oriented single crystals of nickel-bared superalloy DD3 with different cyclic strain rates at 950 degrees C. The cyclic strain rates were chosen as 1.0 x 10(-2), 1.33 x 10(-3) and 0.33 x 10(-3) s(-1). The octahedral slip systems were confirmed to be activated on all the specimens. The experimental result shows that the fatigue behavior depends an the crystallographic orientation and cyclic strain rate. Except [0 0 1] orientation specimens, it is found from the scanning electron microscopy(SEM) examination that there are typical fatigue striations on the fracture surfaces. These fatigue striations are made up of cracks. The width of the fatigue striations depends on the crystallographic orientation and varies with the total strain range. A simple linear relationship exists between the width and total shear strain range modified by an orientation and strain rate parameter. The nonconformity to the Schmid law of tensile/compressive flaw stress and plastic behavior existed at 95 degrees C, and an orientation and strain rate modified Lall-Chin-Pope ( LCP) model was derived for the nonconformity. The influence of crysrallographic orientation and cyclic strain rate on the LCF behavior can be predicted satisfactorily by the model. In terms of an orientation and strain rate modified total strain range, a model for fatigue life was proposed and used successfully to correlate the fatigue lives studied.

Influence of Co, W and Ti on the Stress-rupture Lives of a Single Crystal Nickel-base Superalloy

The influence of Co, W and Ti on stress-rupture lives of a Ni-Cr-AI-Mo-Ta-Co-W-Ti single crystal nickel-base superalloy has been investigated using a L9 （34） orthogonal array design （OAD） by statistical analysis. At a selected composition range, Ti content was the most important factor to the effect of the stress-rupture lives and then followed by Co content. W content had the minimum effect on stress-rupture lives. The optimal alloy should contain 10 wt pct Co, 8 wt pct W and zero Ti. The optimized alloy also had good microstructural stability during thermal exposure at 870℃ for 500 h.