Single Crystal Superalloy DD6 Research Articles

HIP diffusion bonding of FGH96-DD6 dual alloys

Diffusion bonding of powder metallurgy superalloy FGH96 to single crystal superalloy DD6 by hot iso-static pressing (HIP) was studied. The microstructure and elements diffusion at FGH96-DD6 interface were discussed. The influence of HIP process on the microstructure and mechanical properties of FGH96 and DD6 was investigated. Tensile tests of FGH96-DD6 diffusion joints performed at 750°C showed that metallurgical diffusion bonding of FGH96 to DD6 could be achieved by the appropriate HIP process. The γ′ phase of DD6 grew up with increase of HIP temperature. The tensile mechanical properties of DD6 could also be maintained up to the desired levels with the appropriate HIP process. For FGH96, the variation of HIP temperature in the experimental range did not affect obviously the tensile mechanical property at 750°C.

Creep microstructure of single crystal superalloy DD6

The interrupted creep behaviour of single crystal superalloy DD6 was studied at 980°C/220 MPa by TEM. The results show that Al and Ta diffuse to the position of the γ phase, which promotes γ′ phase growth. Meanwhile, Re diffuses from the γ phase to the γ′ phase. After creep for 150 h, γ′ phases merge to a raft structure; for the (001) plane, regular and dense dislocation networks are formed at the γ/γ′ interface and, moreover, few dislocations were observed in the γ′ phase. The thickness of the γ channel and dislocation density at the γ/γ′ interface increase with creep deformation. The correlation between creep rate and the thickness of the γ channel is linear.

Microstructure evolution of single crystal superalloy DD5 joints brazed using AWS BNi-2 filler alloy

Nickel based single crystal superalloy DD5 was brazed with the AWS classification filler metal BNi-2 at brazing temperature of 1323–1423 K for holding 10–240 min. The effect of the brazing temperature and brazing time on the microstructure of the brazed joint was investigated by scanning electron microscope (SEM) and electron probe microanalyser (EPMA). The result indicates that the joints exhibit sound bonding without any defect and crack. Three distinct regions can be identified in the joint: a diffusion affect zone composed of γ-γ′ phase and fine M3B2 precipitates, an interfacial bonding zone solidified isothermally at the brazing temperature, and braze alloy zone formed by the solidification of surplus molten filler alloy during cooling. The braze alloy zone consists of γ+γ′ eutectic phase, chromium boride, and ternary eutectic phase of γ-nickel, nickel boride and nickel silicide. With extending brazing time, the thickness of interfacial bonding zone and diffusion affect zone increased while brittle compounds reduced. The bonding mechanism was discussed with reference to the microstructure evolution of the joint.

MICROSTRUCTURE OF RECRYSTALLIZATION AND THEIR EFFECTS ON STRESS RUPTURE PROPERTY OF SINGLE CRYSTAL SUPERALLOY DD6

The specimens of single crystal superalloy DD6 were grit blasted and heat treated at 1100, 1200,and 1300 ℃ for 4 h at vacuum atmosphere respectively, then the microstructure of recrystallized DD6 alloy and their effects on the stress rupture performance were investigated. The results showed that cellular recrystallization nucleated in grit blasted samples heat treated at 1100 ℃ for 4 h, the dislocation tangles were found in the front of cellular recrystallization grain boundary in DD6 alloy, equiaxed recrystallization grains nucleated in grit blasted samples heat treated at 1300 ℃ for 4 h, and the carbides precipitate at the equiaxed recrystallization grain boundary, while the coexistence of equiaxed recrystallization grains and cellular recrystallization, defined as mixed recrystallization, occurred in the grit blasted samples heat treated at 1200 ℃ for 4 h. The cellular recrystallization reduced the stress rupture lives of DD6 alloy slightly, and the equiaxed recrystallization reduced stress rupture lives seriously, while the reduction degree of the stress rupture lives of the mixed recrystallization was between cellular recrystallization and equiaxed recrystallization. Besides this, with increase of depth of recrystallization and stress,the stresses rupture life decreased. It was also found that the fracture surface configuration was belonging to intergranular fracture with equiaxed recrystallization samples. The characteristic of the fracture surface changed to dimple fracture with cellular recrystallization samples, at all these condition the crack nucleated on the recrystallization grain boundaries of specimens during stress rupture process.

Grain Competitive Growth Prediction during Solidification in Single Crystal Superalloy DD6 Castings

Competitive growth and grain selection were simulated and analyzed during the directional solidification with the conditions for single crystal superalloy DD6 castings using Commercial software, ProCAST. A three dimensional cellular automaton (CA) model coupled with finite-element (FE) heat flow calculation was applied. Measurements at the grain scale were made using the EBSD method at the cross sections in the starter block and grain selector at an interval of 4 mm from the chill surface. The grain characteristics and the rules of competition growth were obtained. The validity of the simulation results were compared with those of the experiment. It concluded that the model-predicted tendency shows satisfactory agreement with the experiment. Increasing the distance from the chill decreases the number of grains, increases the radius of grains and drives the crystallization orientation of grains to principal stress orientation.

Effect of Directional Solidification Condition on Interfacial Reaction between DD6 Single Crystal Superalloy and Zirconia-Silica Ceramic Core

The characterization of the interfaces between DD6 single crystal superalloy and zirconia-silica (ZrO2-SiO2) ceramic cores was performed by optical microscope (OM), SEM and EDS analysis in order to study the influence of directional solidification condition on the interfacial reaction. The results showed that there were chemical reactions on interfaces between DD6 melt and ZrO2-SiO2ceramic cores and the main reaction product was Al2O3. The interfacial reaction involved a complex, interdependent system including the oxidation of Al element, the destabilization of calcia stabilization zirconia (CSZ) and the formation of liquid phase with low melting point. The intensity of interfacial reaction increased with the increase of pouring temperature and solidification time, but the number and size of reaction zones could not increase together because of the limited Al content in DD6 alloy.

Effects of Pouring Temperature on the Tensile Properties and Fracture Behavior of Single Crystal Superalloy DD6

The effects of pouring temperature on the microstructure and the tensile properties of single crystal superalloy DD6 were investigated. The results show that with the decrease of pouring temperature, the primary dendrite arm spacing increases, and the segregation ratio of main elements decreases obviously. DD6 alloy has the similar tensile behavior under the conditions of the pouring temperatures of 1520°C and 1570°C. The pouring temperature has little influence on the yield and ultimate strengths of DD6 alloy. The yield and ultimate tensile strengths of the specimens with the pouring temperature of 1520°C is little lower than the specimens with the pouring temperature of 1570°C under the testing temperatures at room temperature and 650°C, while the specimens with the pouring temperature 1520°C have higher yield and ultimate tensile strength when the testing temperature is higher than 760°C. The pouring temperature did not have an obvious influence on tensile fracture behavior. It has been observed that the tensile fracture surface belongs to quasi-cleaveage fracture mode at testing temperature of 760°C, but the mix characteristic of quasi-cleaveage fracture mode and dimple fracture mode at the testing temperature of 980°C.

Effect of Hf Content on Oxidation Resistance of Single Crystal Superalloy at 1000°C

The specimens of single crystal superalloy DD6 with 0.10% Hf and 0.47% Hf were prepared in the directionally solidified furnace. The effect of Hf content on the isothermal oxidation resistance of the second generation single crystal superalloy DD6 was studied at 1000°Cin ambient atmosphere. Morphology of oxides was examined by SEM, and their composition was analyzed by XRD and EDS. The experimental results show that the oxidation resistance of DD6 alloy with 0.47% Hf is better than that of the alloy with 0.10% Hf. The alloy with different Hf content all obeys parabolic rate law during oxidation for 100h at 1000°C. The increase of Hf content can promote the Al2O3 formation and decreases the proportion of NiO. The oxide grain size and the thickness of the oxide layer all reduce with increasing of Hf content. The oxide scale of the alloy with different Hf content is made up of an outer NiO layer with a small amount of Co3O4, inner Al2O3 and Cr2O3 layer with a small amount of TaO2.

Influence of Cast Dimension on Rotary Bending High Cycle Fatigue Properties of Single Crystal Superalloy

Samples of DD3 single crystal superalloy with different dimensions were cast in the directionally solidified furnace with high temperature gradient. The effect of cast dimension on the rotary bending high cycle fatigue (HCF) properties of the alloy was investigated at 800 °C in ambient atmosphere. SEM was used to examine the fracture surface and fracture mechanism of the alloy. The results show that the rotary bending HCF properties of the alloy decreases with increasing cast dimension. The cast dimension has little effect on the HCF fracture mechanism of the alloy. The HCF fracture mechanism of the alloy with different cast dimensions is all quasi-cleavage fracture. The fatigue cracks initiated on the surface or near the surface of the specimens. The crack would propagate along {111} octahedral slip planes. Typical fatigue arc and striation formed on fatigue crack steady propagation. The degeneration of HCF properties is due to the increase of dendrite arm spacing and size of γ′ phase particles and maximal microporosity.

Effects of Section Size on the Stress Rupture Lives of the Machined Thin-Walled Slab Specimen of DD6 Single Crystal Superalloy at 980 °C/250 MPa

The effects of section size on the stress rupture lives of DD6 single crystal superalloy at 980 °C/250 MPa were investigated adopting machined thin-walled slab specimen. Stress rupture fractograph was analyzed. The results show that stress rupture lives of machined thin-walled slab specimen decline slightly with the reduction of section size at 980 °C/250 MPa, and stress rupture lives of thin-walled specimen are slightly lower compared with that of 5 mm in diameter standard cylindrically specimen, however, the stress rupture lives of thin-walled specimen are still about 200 hours. The decrease in effective loading area caused by oxidation is the main factor for the reduction of thin-walled stress rupture lives.

Microstructure Evolution of a Single Crystal Superalloy after Tensile Fracture at Different Temperature

The tensile property tests of DD6 single crystal superalloy were performed at 25°C, 760°C and 980°C in air. Detailed microstructure evolution was carried out on the alloy to illuminate the γ phase and dislocation structure after tensile fracture by scanning electron microscope and transmission electron microscopy. The results show that the alloy has the maximum tensile strength and the minimum plasticity at 760°C. DD6 alloy has the same anomalous yield strength behavior with other single crystal superalloys. The γ phase hasve a little extension in the stress orientation after tensile fracture at 25°C. The γ phase morphology still maintains cubic after tensile fracture at 760°C. The γ phase is no longer cubic and changes into rectangular solid in the specimen tensile ruptured at 980°C. The vertical γ matrix becomes thinner and horizontal γ matrix becomes thicker slightly. The γ phase is no longer cubic and changes into rectangular solid. High density dislocations are present in the matrix channels and a lot of superlattice stacking faults are seen within γ phases in the sample tested at 25°C. A large quantities of superlattice stacking faults within γ phase and a lot of dislocations tangling in matrix channel are all present in the sample tested at 760°C. The dislocation networks have homogeneously formed at γ/γ interface in the sample tested at 980°C.

Dynamic Responses of Nickel-Based Single Crystal Superalloy DD6 Blade

The aim of this article is to address the dynamic responses properties of the second-generation nickel-based single crystal superalloy DD6 with different orientations, rotor speeds, and temperatures, and this work can lay good foundation for predicting damage behaviors of the DD6 blade and starting with the application of gas turbine in aerospace. The finite element method was applied to investigate the dynamic characteristics of the DD6 blade with [001] and [111] orientations under different rotor speeds at 760, 980, and 1070 °C. On the basis of establishing the elastic-plastic constitutive model of single crystal alloy, the dynamic responses model of a DD6 blade was educed. As a result, the corresponding static and dynamic frequencies were analyzed for different temperatures, rotor speeds, and orientations. Comparison of the computational and experimental results shows the proposed model of the DD6 blade is effective. The analyses indicate that the natural frequencies of [111] orientation is higher than that of [001], and the effect of rotor speed on natural frequencies of [001] orientation is greater than that of [111]. In other words, the values of the natural frequency are increased with the elevated rotor speed, and the values of the natural frequency are decreased obviously with the elevated temperature.

Orientation dependence of creep properties and deformation mechanism in DD6 single crystal superalloy at 760 °C and 785 MPa

The effect of orientation on the creep properties of DD6 single crystal superalloy is investigated. The results show that anisotropic creep behaviors of DD6 alloy within 20° of the [001] direction are observed at 760°C and 785MPa. The [001] direction and the [001]–[011] boundary exhibit lower primary creep strain and longer creep rupture lives than the [001]–[111] boundary. In addition, the major creep deformation mechanism of DD6 single crystal superalloy at 760°C and 785MPa is a〈112〉 dislocation ribbon passing through both γ and γ′ phases. The interaction between 〈112〉{111} systems is the principle hardening mechanism for the transition from primary creep to secondary creep, the interaction leads to the anisotropy of primary creep and creep rupture lives for the [001] direction, the [001]–[011] boundary and the [001]–[011] boundary.

Effect of Hf on Stress Rupture Properties of DD6 Single Crystal Superalloy After Long Term Aging

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 1040 °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 γ' 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 1040 °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.

Formation Mechanism of Low Angle Boundary of DD6 Single Crystal Superalloy Blades

The specimens were machined from DD6 single crystal superalloy blades with low angle boundary. The misorientation of LAB was measured with EBSD technique in scanning electron microscope. The microstructures of specimens with LAB were examined in optical microscope and scanning electron microscope. The formation mechanism of low angle boundary of DD6 single crystal superalloy blades was investigated. The results showed that he formation of LAB which is caused by the deviating orientation from ideal [001] and the angle between the crystal orientation and shell is crystal selection process acted by dendrite competitive growth rule. Part of dendrites have changed their growth orientation a little to the decreasing [001] orientation departure angle because of solidification condition fluctuating during dendrites branching process. The LAB is the obvious interface between the deforming dendrites and their surrounding dendrites.

Microstructural Evolution of the <i>[001]</i> Oriented Single Crystal Superalloy DD6 Creep at 760°C and 785 MPa

The creep and microstructures of DD6 single crystal superalloy with [001] orientation at 760°C and 785MPa were investigated. The results show that large a/2 dislocations are spread and concentrated in matrix channels during incubation period, and few of stacking faults are observed at later stage of incubation period. The stage of primary creep is characterized by the stacking faults shearing γ' phases, and the density of a/2 dislocations in the matrix continues to increase. The density of the stacking faults increases during the stages of secondary and tertiary creep.

Effect of LAB on the Stress Rupture Properties and Fracture Characteristic of DD6 Single Crystal Superalloy

The slabs of the second generation single crystal superalloy DD6 were cast using bi-seeded crystals with symmetric twist low angle grain boundaries. The influence of low angle boundary (LAB) on the stress rupture properties of the alloy was investigated at 760 °C/758 MPa, 850 °C/550 MPa and 980 °C/250 MPa. SEM was used to study the fracture surface and the failure mechanism. The results show that when the misorientation angle of LAB is relatively small it has a less effect on the stress rupture properties of the alloy. The stress rupture properties decrease with the misorientation increase on the same test conditions. The fracture characteristic of specimen shows intergranular rupture trend with the misorientation increases on the same conditions or with increasing of temperature and decreasing of stress for the same misorientation of LAB.

Suppressing the formation of SRZ in a Ni-based single crystal superalloy by RuNiAl diffusion barrier

NiAl-based bond coats applied in thermal barrier coating (TBC) systems containing Ru and Pt have been studied in recent years. Interdiffusion between the coatings and the underlying superalloy substrates usually results in degradation of the mechanical properties of the substrates. In this paper, a NiAl/Ru coating was deposited onto a single crystal superalloy DD6 by electroplating and electron beam physical vapor deposition (EB-PVD). Interdiffusion behavior of the NiAl/Ru and NiAl coated specimens at 1100°C was comparatively investigated. For the NiAl coated specimen, after 100h vacuum annealing, beneath the coating there was a 50μm thickness secondary reaction zone (SRZ) with some needle-like topologically closed packed (TCP) phases. However no SRZ was observed in the NiAl/Ru coated specimen. This may be attributed to the RuNiAl that acted as a diffusion barrier to prevent the inward diffusion of Al from the coating and the outward diffusion of refractory elements from the substrate, and hence suppressing the formation of TCP phases and SRZ in the superalloy.

Isothermal oxidation behavior of single crystal superalloy DD6

The isothermal oxidation behavior of the second generation single crystal superalloy DD6 was studied at 1050 °C and 1100 °C 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 °C and 1100 °C. The oxide scale exposed at 1050 °C 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 °C 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.

Surface Recrystallization and Twin Formation in a Single Crystal Superalloy

The samples of single crystal superalloy DD6 were grit blasted and then heat treated in the temperature range of 1100-1250°C for 4h and the DD6 alloy ‘standard heat treatment’ in vacuum furnace, respectively. The results showed that cellular recrystallization occurred in the surface layer after heating at 1100°C for 4 hours. While equiaxed recrystallization grains occurred near the surface of the samples annealed at 1200°C for 4 hours, meanwhile, cellular recrystallization located between equiaxed recrystallization grains and the original region. With the improvement of the heating temperature, the size of cellular recrystallization decreased, while the size of equiaxed recrystallization grains increased, and the shape of the coarse γ′ phase in the cellular recrystallization changed from lamellar to equiaxial. Fully equiaxed recrystallization grains nucleated after standard heat treatment. Furthermore, the twins occurred in fully equiaxed recrystallization grains, and that the γ′ phase of the twin plane appeared different from that of the equiaxed recrystallization boundary. On the contrary, the twin formation was not observed in the cellular recrystallization grains. Therefore, the differences in twin behavior between the fully equiaxed recrystallization and cellular recrystallization grains were discussed.