NUMERICAL STUDY OF THE NOTCH EFFECT ON THE CREEP BEHAVIOR AND LIFE OF NICKEL-BASE SINGLE CRYSTAL SUPERALLOYS

Numerical calculations of creep damage development and life behavior of circular notched specimens of nickel-base single crystal had been performed. The creep stress distributions depend on the specimen geometry. For a small notch radius, von Mises stress has an especial distribution. The damage distribution is greatly influenced by the notch depth, notch radius as well as notch type. The creep crack initiation place is different for each notched specimen. The characteristics of notch strengthening and notch weakening depend on the notch radius and notch type. For the same notch type, the creep rupture lives decrease with the decreasing of notch radius. A creep life model has been presented for the multiaxial stress states based on the crystallographic slip system theory.

Influence of Pre-Compressive Creep on Internal Friction Stress and Creep Parameters of Nickel-Base Single Crystal Superalloys

By means of pre-compressive creep treatment, the cubic γ′ phase in a nickel base single crystal superalloy is transformed into the P-type rafted structure. And the influence of the pre-compressive creep on the internal friction stress and creep lifetimes of the superalloy are investigated by means of the measurement of the creep curves and microstructure observation. Results show that, compared to the P-type structure alloy, the full heat treated state alloy displays a bigger internal friction stress value of dislocation motion during steady state creep and a longer creep lifetimes. The creep activation energies of the full heat treated and P-type structures alloys are measured to be 462 kJ/mol and 412 kJ/mol, respectively. Thereinto, the P-type rafted γ′ phase in the alloy is transformed into the N-type structure during tensile creep. And the N-type γ′ phase transformed from the P-type structure displays a shorter size in length, this is a main reason of the P-type structure alloy possessing a shorter creep lifetimes due to creep dislocation moving easily over the rafted γ′ phase.

High temperature creep-rupture features of nickel-base single crystal superalloy

The creep and rupture behavior of a nickel-base single crystal superalloy with [001] orientation was investigated at temperature of 10001040℃ and stress in the range of 150320MPa. The creep features and microstructure were studied by means of the measurement of creep curves and TEM observation. The results show that all creep curves exhibit a short primary and a dominant accelerated creep stage. From the creep parameters and TEM observations, it is suggested that the primary deformation mechanism has a change from precipitatation shearing by pairs of dislocation in the high applied stress region to dislocations climb around the γ′ particles in the low applied stress region. Furthermore, the detailed failure process and fracture surfaces were analyzed by SEM observation.

High-cycle fatigue behavior of nickel-base single crystal superalloy

High-cycle rotating bending fatigue behavior of SRR99 nickel-base single crystal alloy at 700 and 900℃ was investigated. The fatigue strengths for 107 cycles are 350 and 335MPa at 700 and 900℃, respectively. The total fatigue life becomes shorter when the temperature increases regardless of the loading stress and frequency. With the number of cycles decreasing, the difference in fatigue strength at the two temperatures becomes smaller. Typical fatigue rupture process including crack initiation site, crack propagation region and final rupture region exhibits at 700℃. The fracture surface is basically characterized by cleavage rupture at 900℃.

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.

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.

Surface recrystallization of a single crystal nickel-base superalloy

The recrystallization behavior of a single crystal nickel-base superalloy was investigated by shot peening and subsequent annealing. Two kinds of recrystallization microstructures, which are intensively dependent on the annealing temperature, are shown in the nickel-base superalloy after shot peening and subsequent annealing. Surface recrystallized grains are obtained when the superalloy is annealed at solution treatment temperature. The nucleation of recrystallization originates from the dendritic core, where rapid dissolution of γ＇ particles occurs. Cellular recrystallization is observed after annealing at lower temperatures. Cellular structures induced by high diffusivity of the moving boundary and more γ＇ particles dissolution led by residual stress are developed from the surface region. Recrystallized kinetics of the shot-peened alloy annealed at 1050°C accords with the Johnson-Mehl-Avrami-Kolmogorov equation. The low Avrami exponent is caused by the inhomogeneous distribution of stored energy, the decreasing of stored energy during recovery, and the strong resistance of boundary migration by γ＇ particles.

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℃.

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.

SOLIDIFICATION OF NICKEL-BASED SINGLE CRYSTAL SUPERALLOY BY ELECTRIC FIELD

The crystal growth of a nickel-based single crystal superalloy DD3 was researched via controlled directional solidification under the action of a DC electric field. The cellular or dendrite spacing of the single crystal superalloy is refined and microsegregation of alloying elements Al, Ti, Mo and W, is reduced by the electric field. The electric field decreases the interface stability and reduces the critical growth rate of the ceUular-dendritic translation because of Thomson effect and Joule heating. The precipitation of the γ＇ phase is more uniform and the size of the γ＇ phase is smaller with the electric field than that without the electric field.

Evolution mechanism of recrystallization in a nickel-based single crystal superalloy under various cooling rates during heat treatment

The recrystallization behaviors of a nickel-based single crystal superalloy during heat treatment at 1,200℃ for 4 h with various cooling rates were studied.Results show that the thickness of recrystallization layer decreases with the increase of cooling rate.In addition,the microstructures ofγ′phase in the recrystallization region are different in various cooling rates.In the high cooling rates(70,100℃·min^(-1)),small size and high volume fraction ofγ′phases are formed in the recrystallization region.It is also found that irregular fine secondaryγ′phases are precipitated between matrix channels with an average size of 150 nm in the original matric(100℃·min^(-1)).The sizes of the secondaryγ′phase decrease with the increase of cooling rate.In contrast,large size and small volume fraction ofγ′phases are formed in the recrystallization region,and a grain boundary layer is formed under a low cooling rate(10℃·min^(-1)).The evolution mechanism of recrystallization at various cooling rates during heat treatment is analyzed.

SOLIDIFICATION AND MICROSEGREGATION BEHAVIORS OF NICKEL-BASE SINGLE CRYSTAL SUPERALLOY SOLIDIFIED AT MEDIUM COOLING RATE

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

压力破裂上的公司， W 和 Ti 的影响 aNi-Cr-AI-Mo-Ta-Co-W-Ti 单身者水晶镍底生活超级合金用 L_9 (~) 被调查了由统计分析的直角的数组设计(OAD ) 。在一个选择作文范围， Ti 内容是到压力破裂生活的效果的最重要的因素然后由 Cocontent 列在后面。W 内容在压力破裂生命上有最小的效果。最佳的合金 contain10 wt 应该爱抚公司， 8 wt 宠物 W 和零 Ti。优化合金也为 500 h 在 870 deg C 在热暴露期间有好微结构稳定性。

Antiphase Boundary Strengthening in a Single Crystal Nickel-base Superalloy

An investigation has been made into strengthening mechanism in a single crystal nickel-base superalloy DD8 by transmission electron microscopy. The results show that the stress rupture strength of the alloy increases with decreasing misfit, and the antiphase boundaries (APBs) formed in the ordered γ' phase, rather than the misfits, play a dominate role in strengthening of the single crystal Ni-base superalloy DD8.There are three kinds of mechanisms for forming the APBs which were observed in the present materials. One is mis-arrangement of the local ordered atoms in the γ' precipitates due to the local strain; the second arises from the 1/2<110> dislocations cutting into the γ', and the third is the formation of the APBs induced by the 1/2<110> matrix dislocation network. The contribution of the antiphase boundary energy to the strength of the alloy can be expressed by:where τ is the resistance to deformation provided by the APB energy; S is the long-range order degree in γ'; Tc is the transition temperature from order to disorder; f is the volume fraction of γ'; rs is the radius of γ'; b is the Burgers vector; a is the lattice constant; G is the shear modulus, and k is the proportional constant.

Effect of competitive crystal growth on microstructural characteristics of directionally solidified nickel-based single crystal superalloy

Directionally solidified single crystal superalloy test bars were prepared by the spiral grain selection method.The microstructural evolution and orientation characteristics of the starter block and spiral part were studied,and the influence of the competitive growth of crystals on the microstructural characteristics was analyzed.The results show that the divergent grain groups,with small size and randomly oriented grains,appear at the bottom of the start block due to the chilling effect,which is an important area for competitive growth.As the height of the starter block increases,the primary dendrite spacing increases,and the grain density decreases;furthermore,the proportion of grains with an orientation deflection angle less than 10°gradually increases.The<001>texture gradually becomes stronger as the height of the starter block increases,which indicates that the competitive growth of crystals gradually weakens.At the initial stage of the crystal selection in the spiral part,the obstacle of adjacent grains and spiral passage is the main working mechanism.The grains located at the inner side of the front edge of the spiral passage have the growth advantage.The single crystal screening process is achieved at about two-thirds of the spiral height,and the single crystal with the orientation deviation angle of 6.7°from the casting axis is prepared.

Microstructure Evolution of a Single Crystal Nickel-Base Superalloy During Heat Treatment and Creep

Microstructure evolution of a single crystal nickel-base superalloy during heat treatment and tensile creep at 1010°C and 248 MPa for 30h was observed and analyzed. Internal stresses because of lattice mismatch between and / phase provided the driving force for / shape evolution during heat treatment. More than 65 vol. % distorted cubic phase keeping coherency with the y matrix precipitated after solution at 1295°C for 32h. The shape of phase was perfectly cubic with increasing precipitate size during the two-step aging treatment. Due to the applied stress and internal stress field the continuous y1/ lamellar structure perpendicular to the applied stress was formed after 30h tensile creep.

RAFTING PREDICTION CRITERION FOR NICKEL-BASE SINGLE CRYSTALS UNDER MULTIAXIAL STRESSES AND CRYSTALLOGRAPHIC ORIENTATION DEPENDENCE OF CREEP BEHAVIOR

Based on the relief of interfacial energy density by dislocation generation at the gamma - gamma ' interfaces, a rafting prediction criterion has been developed for nickel-base single crystals under multiaxial stresses. The diagrams of rafting have been presented, and confirmed by experimental results. The rafting process have been analyzed quantitatively by the relief of interfacial energy. The criterion has been applied to study the creep behavior. The example of creep life analysis shows that the criterion can be correlated greatly to the crystallographic orientation dependence of creep behavior. (Author abstract) 11 Refs.

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.