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The elastoplastic behaviors of a nickel-based single crystal superalloy at 760 °C, 927 °C and 1100 °C were studied, respectively, after 815 °C/250 MPa lasting for 2000 h, 927 °C/205 MPa lasting for 2000 h and 1100 °C/30 MPa lasting for 1000 h. After 815 °C/250 MPa lasting for 2000 h, the $\gamma '$ phases still maintain the cubic shape, but their size increases compared with the original microstructure. The values of $\sigma _{\mathrm{b}}$ , $\sigma _{0.2}$ , $\delta _{5}$ and $\psi $ increase. The tensile fracture surface is 45° with the [001] direction and the fracture mechanism is octahedral slip. After 927 °C/205 MPa lasting for 2000 h and 1100 °C/30 MPa for 1000 h, N-type plate-like rafting structure formed, and the latter owns a higher degree. The values of $\sigma _{\mathrm{b}}$ and $\sigma _{0.2}$ decrease obviously, while $\delta _{5}$ and $\psi $ basically are unchanged. The mechanism is the mixed fracture of octahedral slip and I-type. The fatigue verification test and simulation further prove the strengthening effect.

Stress-rupture properties of a Ni-base Re-containing single-crystal superalloy with three orientations have been tested under 900 °C/445 MPa. An obvious anisotropy of stress-rupture property is attributed to orientation reliant deformation microstructure. The good strength in [001] orientation is attributed to the rapid multiplication of dislocations active in horizontal channels and later γ’ cutting via dislocations pair coupled with anti-phase boundary. The microtwin formation largely limits the strength and plasticity as a result of the continuous shearing across γ/γ’ microstructure by {111}〈112〉 slip activated in [011] orientation. The property in [111] orientation results mainly from the lateral cross-slip movements of the screw dislocations within connected matrix channels as well as the precipitate shearing by coplanar dislocations. Microcracks all initially originate from the interdendritic micropores in three orientations. The critical temperature of stress-rupture anisotropy could be increased by a high level of refractory solutes especially Re.

The effects of etching on the surface integrities and the high cycle fatigue properties of DD6 single-crystal superalloy were investigated. The standard heat treated DD6 specimens were etched twice and four times using HCl/H2O2 solution, respectively, and the three-dimensional surface topography of specimens etched for different times was observed by Leica DCM8 confocal microscope and SEM. After that, the specimens with different surface states were subjected to high cycle fatigue testing at 760 °C and 980 °C in ambient atmosphere, respectively. The results showed that the surface of un-etched specimens had many longitudinal and parallel fine scratches which were caused by polishing, and the surface roughness was very low. Etching caused the occurrence of etching pits in the interdendritic areas, and with the increase in etching time, both the depth of the etching pits and the surface roughness increased. Moreover, the fatigue properties of the alloy were reduced after etching. The fatigue properties decreased with the increase in etching time. Etching was found to play a significant role on the high stress amplitude zone, while a smaller influence on the low stress amplitude zone. Compared with the results at 980 °C, the fatigue properties of the alloy at 760 °C were more affected by etching. The fatigue limits at 760 °C of the alloy after etching can be well predicted by the $$ \sqrt {\text{area}} $$ parameter model.

This study investigates the inner quality including microdefects, orientation deviation and dendrite spacing of DD6 single-crystal superalloy with different drawing velocities. The XRD, SEM, electron probe microanalyzer methods and mechanical properties were used to evaluate the defects and microstructures, related to their evolution mechanism in DD6 superalloy. It was found that the drawing velocity of 80 μm/s can obtain a 4° deviation angle from the 〈001〉 orientation, less microdefects and well tensile properties. With drawing velocity increasing from 30 to 150 μm/s, PDAS decreases from 471 to 359 μm, respectively. Moreover, the shrinkage porosity decreases firstly and then increases due to different feeding conditions. In addition, the segregation of the solute elements increases and promotes the content of γ/γ′ eutectic, which deteriorate the mechanical properties. The research results can provide a basis for the development of high-quality single-crystal blades.

Three-dimensional atom-probe tomography was used to characterize the γ/γʹ interface structure in a third-generation Ni-based single crystal superalloy with Re addition. It was found that an element-segregation layer with Re, Co and Cr was formed in the γ phase close to the γ/γ′ interface, resulting in a more negative local interface misfit (− 0.29%) compared to the measured result (− 0.16%) from high-resolution X-ray diffraction. Furthermore, the total reduction of interfacial free energy due to the solute atom segregation based on the Gibbsian interfacial excess was calculated to indicate that Re element was the most beneficial element in producing this more negative local misfit with the largest interfacial free energy reduction (13.67 ± 0.21 mJ/m2). Simultaneously, because of the co-segregation of Re, Co and Cr, and the depletion of Ni in the γ phase close to the γ/γ′ interface, it was also deduced that some harmful topologically close-packed phases might be easier to nucleate and grow in the γ phase close to the γ/γ′ interface in service. The element-segregation layer across γ/γ′ interface resulted in a more negative local interface misfit (− 0.29%), compared to the measured result (− 0.16%) from highresolution X-ray diffraction, and the total reduction of interfacial free energy due to the solute atom segregation was calculated to indicate that Re element was the most beneficial element in producing this more negative local misfit with the largest interfacial free energy reduction (13.67 ± 0.21 mJ/m2).

Superdislocation configurations in a [011]-oriented single crystal superalloy after creep at 1100 °C/150 MPa have been investigated. Besides a〈110〉 superdislocations, a[100] superdislocations are observed. TEM analysis shows that these a[100] superdislocations are pure edge dislocations and mainly lie on (100) plane with 〈011〉 directions. They form by the reactions of two matrix dislocations with different Burgers vectors, and they may move in a similar way to a〈100〉 dislocations in [001] specimens owing to the zero Schmid factors.

Ni-based single crystal superalloy IC21 with different Ce contents was prepared by screw selection method, and Al–Si coatings were prepared by pack cementation. Cyclic oxidation and transient oxidation were carried out at 1150 °C to investigate the effect of Ce on the high-temperature oxidation behavior of Al–Si coatings. After cyclic oxidation at 1150 °C, the addition of trace Ce decreased the mass gain of the Al–Si-coated samples and effectively improved oxidation performance and spallation resistance. In specimens without Ce doping, microstructure examination revealed that bulky γ′-Ni3Al precipitated in oxidized coating because of high Al consumption, and this finding suggests that Ce inhibits the consumption of Al in the coating during oxidation. Ce slows down the transformation of θ-Al2O3 to α-Al2O3 and improves the adhesion of the oxide scale.

We investigate the formation mechanism of stray grain of nickel-based single-crystal superalloy under a high magnetic field. It was shown that the high magnetic field induced the formation of stray grains on the side that the dendrite diverges the wall. In addition, results showed a sloping solid/liquid interface and a further increased sloping magnitude under a high magnetic field. Further, the formation process of stray grain in the high magnetic field is analyzed in detail.

The formation of low-angle grain boundaries (LABs) in the rejoined platforms of a Ni-based single crystal superalloy under different directional solidification rates was investigated by the experimental investigation and the ProCAST simulation. The results showed that the growth morphology and orientation evolution of dendrites in the platforms were different under the withdrawal rates in the range of 60–100 μm/s and then resulted in different types of LABs. At lower withdrawal rates, the longitudinal LABs were common in the rejoined platforms. Both the sliver defects and the orientation deviation of original primary dendrites from two independent growth paths could cause the longitudinal LABs in the platforms. At higher withdrawal rates, the dendrite growth patterns were more complex and the secondary branches with lateral growth tended to deviate from their original orientation, eventually leading to the formation of some transverse LABs. Finally, some suggestions to prevent the formation of different LABs are provided.

A third-generation single-crystal (SX) superalloy was heat treated at 1350 °C for 24 hours while being exposed to nickel vapor. It is observed that a layer of 60-µm thickness deposited onto the alloy. This prevented the formation of TCP phases and the occurrence of melting, which were reported in previous studies. The interdiffusion between the alloy and deposited layer shows strong indications of cross-diffusion effects, as shown from the composition profile measurements and simulations.

An approach to determination of squareness of initial γ′ precipitates (S2D) is proposed to evaluate its effect on creep-rupture life (tr) of nickel-base single crystal (SC) superalloys. It is found that the 760/982 °C rupture life varied with the change in regional S2D caused by redistribution of W when 1st-step aging temperature changed in full heat treatment on superalloy DD83 investigated. The longest creep-rupture life occurred at the highest value/the lowest difference in S2D in the interdendritic regions/between the typical dendritic regions in DD83. It is also found that S2D is a weighted function of the area fraction (F2D), spacing (h), and size (d) of γ′ precipitates and is closely related to tr in a series of SC superalloys. In addition, the variation of S2D with F2D (here, thermodynamic mole fraction is approximately expressed by F2D) through lattice misfit (δ) in the SC superalloys with F2D ranging from ~ 60 to 75 pct is well correlated. Therefore, to reveal and to better understand these relationships and correlations may help to optimize the phase variables in order to achieve a long rupture life of SC superalloys. In addition, functions to reveal the interrelationships of F2D, volume fraction (F3D), S2D, and cuboidness (S3D) of initial γ′ precipitates are derived considering their shape changes. All of these are hoped to be helpful in practical applications and in understanding the true meaning of the related variables.

Rare-earth elements in Ni-based single-crystal superalloys are easily lost during the casting process. The effects of incorporating Ca on the loss of Y from an Ni-based superalloy and the cyclic oxidation of this alloy have been investigated. The addition of Ca increases the Y content from 10.87 to 72.78 ppm, which has been rationalized by thermodynamic calculation of the Gibbs free energy. Ca also reduces the falling off of the protective oxide film of Y-containing Ni-based alloy and enhances the cyclic oxidation performance.

γ′ + β Binary-phase Ni–Al coatings were prepared on second-generation single-crystal superalloy Rene N5 (N5) substrates by electron beam physical vapor deposition. Inter-diffusion behavior between coatings and substrates at 1100 °C and its effects on microstructure stability of the substrates were investigated. A 3-μm-thick substrate diffusion zone (SDZ) layer forms beneath the coating/substrate interface after 5-h heat treatment. The SDZ layer is composed of γ′-Ni3Al phases and needle-like precipitates growing along (100) $$\left\langle {110} \right\rangle$$ or (100) $$\left\langle { 1 {\bar{\text{1}}\text{0}}} \right\rangle$$ direction. After 100-h annealing, the thickness of the SDZ layer increases to ~17 μm, much lower than that of the single β-NiAl-coated substrate, while keeping single-crystal microstructure.

The effect of a transverse magnetic field on stray grain formation during directional solidification of superalloy was investigated. Experimental results indicated that the transverse magnetic field effectively suppressed the stray grain formation on the side the primary dendrite diverges from the mold wall. Moreover, the quenched experimental results indicated that the solid/liquid interface shape was obviously changed in a transverse magnetic field. The effect of a transverse magnetic field on stray grain formation was discussed.

Single crystal superalloys are widely used in high temperature sections of turbines, where they are able to withstand extended exposure to high temperatures. As these alloys are commonly cast and directionally solidified via the Bridgman process, the knowledge of their solidification path is of great importance. This paper gives an overview of aspects of the solidification path in superalloys, as studied experimentally and from a modeling and simulation point of view. The implications of microsegregation, and sequence of phase formation are discussed. Attention is given to the processes leading to the formation of interdendritic γ′, also often referred to as the γ/γ′ eutectic.

The effects of a high magnetic field on microstructures during seed melt-back of superalloy were investigated. Experimental results indicated that the high magnetic field significantly modified the melt-back interface shape and the melt-back zone length. In addition, stray grain on the edge of sample was effectively suppressed in the high magnetic field. Based on experimental results and quantitative analysis, the above results should be attributed to the increasing temperature gradient in a high magnetic field.

An advanced technology for joining powder metallurgy (P/M) superalloy and single crystal (SC) superalloy is highly needed for fabricating a high-strength turbine bladed disk and the joints must meet the requirements of harsh working conditions. In the present paper, P/M superalloy FGH96 and SC superalloy DD6 were joined together using a Ni–Cr–B brazing alloy. The microstructure of the joint brazed at 1120 °C for 10 min showed that massive Ni3B phase was formed in the central part of the brazing seam. In the FGH96 matrix near the brazing seam, a small bright phase which contained some element B, Cr, Mo, and W distributed along with grain boundary. The joints were subjected to a diffusion heat treatment at 1000 °C for 1, 4, and 16 h, respectively. The results showed that the diffusion depth of B is increased from 80 μm at 1000 °C/1 h to 140 μm at 1000 °C/16 h. In addition, the bright phase was refined obviously, implying the homogenization of the joint microstructure. Compared with the joints treated for 1 and 4 h, the strength of the joints treated for 16 h was increased remarkably from 604∼630 to 913 MPa.

The fracture mode of an experimental Ni-based single crystal superalloy containing topologically close-packed phases at ambient temperature was investigated by a simple and practical testing method. Microstructural investigation and compositional analyses were performed on the matching fracture surfaces of a small scale specimen after the impact experiment. It is suggested that the weak TCP/γ′ interface coherency resulted in the interfacial decohesion, which was the main fracture mode for the investigated superalloy under the impact load at ambient temperature.

Stray grains were found during the preparation of single crystal superalloy AM3 by seeding technique and the researching on the competitive growth of bi-crystal. Stray grains were mainly observed at the diverging 〈001〉 corner of the mold wall. Therefore, increasing orientation deviation angle would intensify the possibility of the formation of stray grains. This was because the solute was inclined to enrich at the diverging 〈001〉 corner of mold wall, leading to the relatively large undercooling, and accordingly resulted in the formation of stray grains.

A model of typical turbine blade shape with different platforms was designed to study the nucleation and growth of stray grains in the platforms by both experimental investigation and a ProCAST simulation based on a cellular automaton finite-element model. The results show that at the withdrawal rate of 5 mm/min, no stray grains nucleate in the small dimensional platform. However, the primary grain grows into the inner and outer sections of this platform in different manners due to different thermal conditions in these sections. Furthermore, with the increase of platform dimension, stray grains with random orientations gradually nucleate in the corners of the platforms. It is found that these stray grains tend to nucleate either in the inner corners or at a faster withdrawal rate, which is associated with the corresponding thermal condition. Based on these results, the rule of the critical platform dimension and withdrawal rate without stray grain formation has been proposed. Besides, the simulation results are in accordance with experimental findings.

The specimens of the second generation single crystal superalloy DD6 with different Hf contents were prepared in the directionally solidified furnace with a high temperature gradient. The long term aging of the specimens after full heat treatment was performed at 1 040 °C for 800 h. The effect of Hf on the microstructure and stress rupture properties under 980 °C/250 MPa of the alloy after long term aging was investigated. The results show that the y coarsening and rafting and no topologically close packed phase (TCP) are observed in the microstructures of DD6 alloy with different Hf contents after aged at 1 040 °C for 800 h. It indicates that DD6 alloy with different Hf contents all possesses good microstructure stability. With increasing Hf content the rupture life after long term aging turns shorter and the elongation represents the increasing first and decreasing afterwards. The fracture mechanism of the alloy with different Hf contents at 980 °C/250 MPa all shows dimple model. The influence of the microstructures on the stress rupture properties of the alloy is also discussed.

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.

The influence of both topologically close-packed (TCP) phase precipitation and pores on the creep life of a single-crystal superalloy has been studied at 1323 K (1050 °C)/160 MPa. Despite very reproducible primary and secondary creep stages, the creep life is scattered for this specific condition where a very steep tertiary creep stage is observed, corresponding to a highly localized failure process. Image processing was performed after failure to determine the stereological parameters characterizing pores and TCP-phase particles. It was determined that pores are major determinants of creep life under these temperature and stress conditions. It was also observed that the average surface area or the density of pores is not sufficient to explain creep life variability. A homogenization method including modified γ/γ′ microstructure area surrounding pores and TCP-phase particles was developed and correlated to creep life. It is shown that the greater the extent of the modified microstructure, the lower the creep life. Moreover, a better understanding of the TCP-phase role in controlling the creep life was obtained: TCP-phase particles modified the local stress field and disturbed the local γ/γ′ microstructure. They enhance the generation of vacancies and subsequent nucleation and growth of pores.

The effect of different cooling rate on microstructure and creep properties of DD3 single crystal superalloy after solution treatment was investigated. 10 °C/min cooling rate is adverse to the creep properties performed at all the conditions of 760 °C/786 MPa, 982 °C/210 MPa and 1038 °C/161 MPa. The best creep lives can be obtained with 149 °C/min cooling rate specimens at 760 °C/786 MPa. At 982 °C/210 MPa and 1038 °C/161 MPa, a cooling rate of 38 °C/min shows the best creep properties in the four kinds of bars. The experimental results show that the size of γ′ precipitates decreases with the increasing of cooling rate from 10 °C/min to 149 °C/min. During the creep test, the directional coarsening of γ′ precipitates occurred and γ′ rafting is formed.

The effect of surface recrystallization by heating after shot-peening on the creep rupture 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 μm 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 recrystallized layer, inhomogeneous deformation between the recrystallized layer and the matrix, and stress concentration caused by notch effect resulted in the decrease in creep rupture life of the single-crystal superalloy.

Single crystal superalloy turbine blades are currently widely used as key components in gas turbine engines. The single crystal turbine blade casting’s properties are quite sensitive to the grain orientation determined directly by the grain selector geometry of the casting, A mathematical model was proposed for the grain selection during directional solidification of turbine blade casting. Based on heat transfer modeling of the directional withdrawing process, the competitive grain growth within the starter block and the spiral of the grain selector were simulated by using the cellular automaton method (CA). Validation experiments were carried out, and the measured results were compared quantitatively with the predicted results. The model could be used to predict the grain morphology and the competitive grain evolution during solidification, together with the distribution of grain orientation of primary dendrite growth direction, with respect to the longitudinal axis of the turbine blade casting.

The microstructure and phase composition of single crystals of refractory nickel alloys bearing 9% Re are studied after many-stage heat treatment, long-term high-temperature annealing, and testing for long-term strength.

During the thermal-mechanical loading of high temperature single crystal turbine components, all three creep—stages: primary, secondary and tertiary, manifest themselves and, hence, none of them can be neglected. The development of a creep law that includes all three stages is especially important in the case of non-homogeneous thermal loading of the component where significant stress redistribution and relaxation will result. Thus, local creep analysis is crucial for proper design of damage tolerant airfoils.

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°C. 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.

A new configuration of a superdislocation in the γ′ phase of a fourth-generation single-crystal TMS-138 superalloy was found after creep rupture in a [001] tensile test at 1150 °C and 137 MPa. The segments of the superdislocation lie in four directions, i.e., [110], $$[1\overline{1}0]$$ , [100], and [010], strictly on a (001) plane with a Burgers vector b = [001]. This superdislocation is pure edge in character and does not dissociate into superpartials. Microstructural evidence shows that this kind of superdislocation is formed by combination of two interfacial dislocations with different Burgers vectors, i.e., 1/2[011] + 1/2 $$[0\overline{1}1]$$ → [001].

Effects of the hot-isostatic pressing (HIP) process on microstructures and mechanical properties of Ni-base single crystal superalloy CMSX-4 were investigated. In the overall heat treatment process, the HIP treatment remarkably induced the healing behavior of micropores and decreased the pore size and porosity of superalloy CMSX-4 compared with normally treated specimens. The microstructure of γ′ phase after the HIP process showed rather a coarsening tendency and could be developed by the partially solution and aging treatment. Consequently, the elimination of cast micropores using the HIP step resulted in the inhibition of crack initiation in microstructure and improved the stress-rupture lives of Ni-base single crystal superalloy by 185%.

When deforming bulk material, micromechanisms involving moving defects result in mechanical characteristics observed at a macroscopic scale.In situ straining of microsamples in a Transmission Electron Microscope, provides the unique advantage of observing the dislocation dynamics involved in such micro-deformation processes under the combined effects of stress and temperature. Here the efficiency of this technique is illustrated by describing the different obstacles controlling the movement of dislocations in a two-phase industrial single crystal superalloy. At 25‡ and 850‡C, different core structures of the moving dislocations as well as several ways of crossing obstacles are described, which concern the movement of dislocations inγ channels, at γ/γ′ interfaces and while shearing γ′ precipitates. From these observations, a quantitative analysis is developed leading to the evaluation of the critical propagation stresses involved in the channels of the matrix and when crossing the interfaces. This allows to discuss the various sites of resistance opposed to the dislocation movements and controlling the macroscopic deformation.

Rafting of the γ/γ′ morphology of nickel-base superalloys is a well-known phenomenon during high-temperature deformation. The initial stages of this type of directional coarsening were modeled two-dimensionally by the method of finite elements (FEs) using an energy-perturbation approach. In addition to the elastic energy density, the effect of the local difference of the hydrostatic stresses in γ and γ′ in combination with the different lattice parameters of the two phases was considered in the calculations as a further driving force. From the results of modeling, the deformation-induced internal stresses and strains were determined and used to evaluate the direction-dependent lattice parameters and lattice misfits of the two phases. The results agree well with experimentally determined values.

The single crystal superalloy CMSX2 was studied after a creep exposition at high temperature by neutron diffraction topography and neutron diffractometry. Topographs clearly displayed the dendritic structure of the sample. The topographs and the rocking curves indicate an influence of the creep both on the dendritic structure of the sample and on the individual dendrites.

acad sinica,inst met res,shenyang 110015,peoples r china.;wang, hm (reprint author), beijing univ aeronaut & astronaut,beijing 100083,peoples r china