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%.

Superb creep lives of Ni-based single crystal superalloy through size effects and strengthening heterostructure γ/γ' interfaces

This study presents a design strategy to enhance the high-temperature creep resistance of Ni-based superalloys.This strategy focuses on two principles:(1)minimizing the dimensions ofγ/γ′interfaces andγchannels by reducing the size of theγ′phase;(2)key alloy composition control to strengthen the heterostructureγ/γ′interfaces.This strategy proved very effective by the designed three superalloys'prolonged creep lives.An alloy exhibits ultra-long creep life by 388 h at 1100°C/137 MPa,which runs at the highest level among those alloys without Ru addition.With Ru addition,an alloy that lasted for 748 h with a creep strain of~6%at 1110°C/137 MPa is developed.This study provides a new route of high-temperature creep lives through heterostructure interfacial design with size effects and key alloying elements.

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.

New insights into the microstructural stability based on the element segregation behavior atγ/γ′interface in Ni-based single crystal superalloys with Ru addition

A new insight into the microstructural stability was proposed in Ni-based single crystal superalloys with Ru addition,and the element segregation behavior atγ/γ′interface was investigated by three-dimensional atom probe technology(3D-APT).After standard heat treatment,it was found that Ru addition barely altered the element partitioning coefficient betweenγmatrix andγ′phase,and no element-segregation layer was observed atγ/γ′interface.During the heat exposure at 1100°C,Ru addition obviously promoted the rafting of theγ′precipitates and inhibited the precipitation of topological close-packed(TCP)phases.It was more important that an element-segregation layer containing Re,Co,and Cr was formed in theγmatrix close to theγ/γ′interface due to an“uphill diffusion”effect,and its concentration was obviously reduced after Ru addition.Finally,the microstructural stability based on the element segregation behavior atγ/γ′interface was discussed.This element-segregation layer increased theγ/γ′interfacial energy by increasing the absolute value of the lattice misfit ofγ/γ′interface to promote the rafting of theγ′precipitates after Ru addition.On the other hand,the decrease of the segregation concentration of Re,Co,and Cr elements as TCP phase-forming elements near theγ/γ′interface due to a“reverse partitioning”effect inhibits the precipitation of TCP phases in Ni-based single crystal superalloys after Ru addition.

Effect of carbon addition on carbide morphology of single crystal Ni-based superalloy

Single crystal superalloys of AM3 with different carbon levels were prepared at withdraw rate of 50μm/s. The effect of carbon addition on the carbide morphology was investigated. It was found that there were four types of MC-type carbides, acicular, nodular, blocky, and Chinese script-type in the crystals. With an increase in carbon level, the volume fraction of carbide increased significantly while the volume fraction of eutectic decreased significantly. Furthermore, the size of carbide in high level carbon alloy became much larger.

Effect of thermal exposure on the microstructure and creep properties of a fourth-generation Ni-based single crystal superalloy

The effect of thermal exposure on microstructure and creep properties of a fourth-generation nickelbased single crystal superalloy was investigated.The thermal exposure of samples after the full heat treatment was carried out at 1000℃,1100℃ and 1140℃ for 100 h and 200 h.The γ’ coarsening,γ’ rafting and γ channel widening were observed in samples after thermal exposure.When the thermal exposure time was constant,the morphology of γ’ phase in the alloy evolved significantly with increasing aging temperature.The interracial dislocation networks in aged samples after creep ruptured gradually became irregular and sparse with the increase of exposure temperature.When the higher exposure temperature was used,enla rgement of the defect pores was observed in samples,the microcracks were more likely to initiate and propagate at the corner of these pores.After aging at 1000℃ for 100 h,the creep life at 1140℃/137 MPa was slightly longer than that of heat-treated sample,which could be attributed to the slightly coarsened γ’ phase,homogenization of refractor elements.In contrast,the creep life of sample exposed at 1140℃ for 100 h was greatly decreased.The decrease of creep life was dominated by the rafting of γ’phase,the irregular interfacial dislocation networks as well as the enlargement of homogenization pores.

Effect of Skew Angle of Holes on the Thermal Fatigue Behavior of a Ni-based Single Crystal Superalloy

In the present work,holes of various skew angles were electrochemically machined in the middle of the plate specimens in a Ni-based single crystal superalloy and crack initiation and propagation around holes during thermal fatigue cycles（20–1100 ℃） were investigated.It was demonstrated that the skew angles had a significant effect on the initiation and propagation of thermal fatigue cracks.During thermal fatigue process,stress concentration occurred at the edge of the holes.As for skew angles,the maximum stress concentration appeared at the acute side of holes.The maximum stress concentration resulted in plastic deformation at the acute side of the 30° hole,driving the thermal fatigue cracks to initiate after 220 cycles and propagate along [011] direction.However,the stresses concentrated at the edge of 90° or 60° holes were not large enough to initiate cracks even after 580 thermal cycles.This work will help to understand the local deformation behavior in the vicinity of cooling holes with various skew angles and have serious design implications for turbine blades.

Microstructure evolution and mechanical behavior of Ni-based single crystal superalloy joint brazed with mixed powder at elevated temperature

Brazing of a Ni-based single crystal superalloy has been investigated with the additive Ni-based superalloy and filler Ni–Cr–W–B alloy at 1260℃, and attentions were paid to the microstructure evolution during brazing and the stress-rupture behavior at 980℃ of such brazed joints after homogenization. Microstructure in the brazed joint generally includes brazing alloy zone（BAZ）, isothermally solidified zone（ISZ） and diffusion affected zone（DAZ）. Microstructure evolution during this brazing process is discussed at the heating stage, the holding stage and the cooling stage respectively, according to the diffusion path of B atoms. Initially well-distributed γ’/γ’ microstructure in the homogenized bonded zone after heat treatment and substantial γ’ rafts enhance the post-brazed joint to obtain a stress-rupture lifetime of more than 120 h at 980℃/250 MPa. On the other hand, the decreased stress-rupture behavior of post-brazed joint, compared with parenting material, is ascribed to the presence of inside brazing porosity and stray grain boundary, which not only reduces the effective loading-carrying area but also offers preferential sites for creep vacancy aggregation to further soften stray grain boundary. And finally an early fracture of these post-brazed joints through the intergranular microholes aggregation and growth mode under this testing condition was observed.

First-principles study of multiple-site substitutions of alloying elements in Ni-based single crystal superalloys

The development of Ni-based single crystal superalloys relies heavily on the composition design with the addition of critical alloying elements,e.g.,Re and Ru.Understanding the role of alloying effects require to know the configurations of the alloying element distribution betweenγ-Ni andγ′-Ni3Al phases and among various non-equivalent sites.This work employed firstprinciples density functional theory calculations to study the preference of phase and site occupancy of 11 alloying elements including Al and transition metal elements:3d (Ti,Cr,Co,Ni),4d (Mo,Ru),and 5d (Hf,Ta,W,Re) in Ni and Ni3Al.We calculated the substitution energies of 1298 triple-site doping configurations including 286 Ni Ni Ni site doping of Ni,726 Al Ni Ni site doping,and 286 Ni Ni Ni site doping of Ni3Al with alloying elements Ni,Co,Ru,Cr,Re,Mo,W,Al,Ti,Ta,and Hf.In the dual-site and triple-site doping of Ni and Ni3Al,all studied alloying elements preferred to occupy Ni phase rather than Ni3Al phase.We found that the most stable defect complexes often contained the favorable substitutions of Al,Ti,Ta,and Hf for the Ni sites that stabilized the alloying elements doping at the other one or two nearest neighbor sites.The co-substitutions of various alloying elements at multiple sites are critical to understanding the strengthening mechanism of alloying elements in Ni-based single crystal superalloys.

Quantitative study on interactions between interfacial misfit dislocation networks and matrix dislocations in Ni-based single crystal superalloys

The interactions between the moving dislocation within matrix channel and the interfacial misfit dislocation networks on the two-phase interfaces in Ni-based single crystal superalloys are studied carefully via atomic modeling, with special focus on the factors influ- encing the critical bowing stress of moving dislocations in the matrix channel. The results show that the moving matrix dislocation type and its position with respect to the interfacial misfit dislocation segments have considerable influences on the interactions. If the moving matrix dislocation is pure screw, it reacts with the interracial misfit dislocation segments toward dislocation linear energy reduction, which decreases the critical bowing stress of screw dislocation due to dislocation linear energy release during the dislocation reactions. If the moving matrix dislocation is of 60^-mixed type, it is obstructed by the interaction between the mixed matrix dislocations and the misfit interfacial dislocation segments. As a result, the critical bowing stress increases significantly because extra interactive energy needs to be overcome. These two different effects on the critical bowing stress become in- creasingly significant when the moving matrix dislocation is very close to the interracial misfit dislocation segments. In addition, the matrix channel width also has a significant influence on the critical bowing stress, i.e. the narrower the matrix channel is, the higher the critical bowing stress is. The classical Orowan formula is modified to predict these effects on the critical bowing stress of moving matrix dislocation, which is in good agreement with the computational results.

High Temperature Stress Rupture Anisotropy of a Ni-Based Single Crystal Superalloy

High temperature stress rupture anisotropies of a second generation Ni-base single crystal（SC） superalloy specimens with [001], [011] and [111] orientations under 900 ℃/445 MPa and 1100 ℃/100 MPa have been investigated in the present study, with attentions to the evolution of γ/γ′ microstructure observed by scanning electron microscopy and the dislocation configuration characterized by transmission electron microscopy in each oriented specimen. At 1100 ℃/100 MPa as well as 900 ℃/445 MPa, the single crystal superalloy exhibits obvious stress rupture anisotropic behavior. The [001] oriented specimen has the longest rupture lifetime at 900 ℃/445 MPa, and the [111] oriented sample shows the best rupture strength at 1100 ℃/100 MPa. While the [011] oriented specimen presents the worst rupture lifetime at each testing condition, its stress rupture property at 1100 ℃/100 MPa is clearly improved, compared with900 ℃/445 MPa. The evident stress rupture anisotropy at 900 ℃/445 MPa is mainly attributed to the distinctive movement way of dislocations in each oriented sample. Whereas, at 1100 ℃/100 MPa, together with the individual dislocation configuration, the evolution of γ/γ′ microstructure in each orientation also plays a key role in the apparent stress rupture anisotropy.

An anisotropic micromechamcal model for calculation of effective elastic moduli of Ni-based single crystal superalloys

An anisotropic micromechanical model based on Mori-Tanaka method is developed to calculate the effective elastic moduli of

Coarsening kinetics of γ' precipitates in a Re-containing Ni-based single crystal superalloy during long-term aging

The morphological evolution and coarsening kinetics ofγ'precipitates in a Re-containing Ni-based single crystal superalloy were investigated during isothermal aging at 900,950 and 1000℃.After heat treatment,well-defined cuboidalγ'precipitates with low misfit was obtained within the experimental alloy.Then coarsening rate constants and particle size distribution(PSD)ofγ'phases were calculated and specified based on the measured precipitate sizes for va rying periods of aging times from 100 to 2000 h.After aging for 2000 h,γ'precipitates maintained cubical shape at 900℃,while exhibited sphere at 950 and 1000℃.Coarsening models based on diffusion-controlled process with a functional relationship of r^(3) vs.t(classic Lifshitz-Slyozov-Wagner coarsening model)and interface-controlled model with a function of r^(2) vs.t(trans-interface diffusion-controlled coarsening model)were investigated to fit between the experimental results and theoretical analysis.It was found that Re as the slowest diffusing solute in the alloy constituted the rate-limited step for coarsening based on LSW model,while the process limiting coarsening as governed by an interface diffusion process could possibly be related to the Al diffusion through theγ/γ'interface.The PSDs and coarsening exponent were discussed by comparing the experimental data with predictions of LSW and TIDC models.Finally,coarsening mechanism could be divided into four regimes:(i)coarsening by diffusion-controlled;(ii)coarsening by diffusion and interface cocontrolled;(iii)coarsening by interface-controlled;(iv)coarsening by interface-controlled accompanied withγ'coalescence.

A review of composition evolution in Ni-based single crystal superalloys

Due to the outstanding creep performance, nickel-based single crystal superalloys(Ni-SXs) are extensively applied in modern aero-engine and industrial gas turbine. Apart from the special single crystal structure which is disadvantageous to extension of creep cracks, Ni-SXs derive the creep strength from intrinsic two-phase microstructure(γ phase and γ’ phase). Main microstructural parameters including volume fraction of γ’ phase and the lattice misfit, and the formation and distribution of precipitated phase are determined by the compositions of alloys. Besides, the creep properties are greatly influenced by these microstructural parameters and precipitated phase. This review has summarized the relationships between different alloying elements and microstructures and indicated their influence on creep properties of Ni-SXs. In addition, with the improvements of experimental methods and characterization technique, some recent discoveries have provided additional evidence to support or challenge the pervious creep theories of superalloys. In view of these new discoveries, this review has provided some perspectives which can be referenced in future compositional design of Ni-SXs.

Stress dependence of the creep behaviors and mechanisms of a third-generation Ni-based single crystal superalloy

Elevated temperature creep behaviors at 1100℃ over a wide stress regime of 120-174 MPa of a thirdgeneration Ni-based single crystal superalloy were studied. With a reduced stress from 174 to 120 MPa, the creep life increased by a factor of 10.5, from 87 h to 907 h, presenting a strong stress dependence. A splitting phenomenon of the close-(about 100 nm) and sparse-(above 120 nm) spaced dislocation networks became more obvious with increasing stress. Simultaneously, ao<010> superdislocations with low mobil让ies were frequently observed under a lower stress to pass through γ precipitates by a combined slip and climb of two ao<110> superpartials or pure climb. However, ao<110> superdislocations with higher mobility were widely found under a higher stress, which directly sheared into y precipitates. Based on the calculated critical resolved shear stresses for various creep mechanisms, the favorable creep mechanism was systematically analyzed. Furthermore, combined with the microstructural evolutions during different creep stages, the dominant creep mechanism changed from the dislocation climbing to Orowan looping and precipitates shearing under a stress regime of 137-174MPa, while the dislocation dim bing mechanism was operative throughout the whole creep stage un der a stress of 120 MPa, resulting a superior creep performanee.

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.

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.

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.

Formation of the Surface Eutectic of a Ni-based Single Crystal Superalloy

The microstructure, size, elemental composition and hardness of the surface eutectic layer formed during directional solidification of a Ni-based single crystal superalloy were studied. The formation mechanisms of the surface eutectic on the outer surface of the casting were also discussed. The metal/mould interactions did not play any role in the formation of the surface eutectic. The formation cause of surface eutectic layer was attributed to the interdendritic residual liquid for excretion caused by solidi？cation shrinkage.