Single Crystal Superalloy DD6 Research Articles

Microstructure evolution and elemental diffusion behavior near the interface of Cr2AlC and single crystal superalloy DD5 at elevated temperatures

As one of the promising MAX phase materials for high-temperature applications, Cr2AlC is considered as a potential substitution bond coat material in thermal barrier coating systems. In this paper, the microstructure evolution and elemental diffusion behavior near the interface of the diffusion couple composed of Cr2AlC and single crystal superalloy DD5 were investigated at 1100 °C, 1150 °C, and 1200 °C. Elemental interdiffusion between Cr2AlC and DD5 occurs significantly, resulting in the formation of a thick layer of Kirkendall holes after 20 h heat treatment at 1100 °C and higher temperatures. The outward diffusion of Ni into Cr2AlC and the inward diffusion of Al into DD5 alloy causes the formation of β-NiAl matrix embedded with dispersed Cr7C3 phase. Simultaneously, the precipitation of σ-TCP phase and degradation of the γ/γ′ matrix occurs in the alloy. Additionally, TaC, M2C (where M = Ta, W, Cr), and M23C6 (M = Cr, Re, W) compounds are formed near the interface along with the dissolution of σ-TCP phases. It is further found that Al in Cr2AlC exhibits the highest average effective diffusion coefficient among the four dominant diffusing elements. It also displays the lowest diffusion activation energy which is due to its relatively weak CrAl and AlAl bonds.

Effect of Etching on Fatigue Properties of DD6 Single-Crystal Superalloy

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.

Influence of Drawing Velocity on the Inner Quality of Single-Crystal Superalloy

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.

Creep behavior and a deformation mechanism based creep rate model under high temperature and low stress condition for single crystal superalloy DD5

This work reports the investigation on creep behavior under high temperature and low stress condition for single crystal superalloy DD5 and the subsequent establishment of a deformation mechanism based creep rate model. The γ/γ′ microstructure evolution and the precipitate-dislocation interaction mode were investigated systematically during creep test at 1100 °C and 120 MPa, for a better understanding of underlying creep deformation characteristics. The γ/γ′ microstructure becomes rafted and topological inverted eventually as creep deformation continues. The formation of dense interfacial dislocation network that impedes dislocations from cutting into γ′ precipitates is assumed to be one of the main strengthening mechanisms during creep. The climbing over rafted γ′ precipitates is seen as a recovery/softening process which annihilates dislocations, receding the strengthening effect of interfacial dislocation network. The cutting through rafted γ′ precipitates is proposed to account for the acceleration of creep, a strong dependence of creep properties on rafting (the effect of widening horizontal γ channel on the degraded back stress of cutting through rafted γ′ precipitates) then arises. The established model involving these characteristics was found to predict the creep behavior (below 1% creep strain) under different temperatures and stresses for single crystal superalloy DD5 accurately. Several microstructure characteristics and physical properties emerge from the model, which is promising for alloy design and compositional optimization.

Transient thermal fatigue crack propagation behavior of a nickel-based single-crystal superalloy

Thermal fatigue crack propagation behaviors of a V-notched Ni-based single-crystal superalloy DD6 were tested at 25 °C ↔ 760 °C/900 °C/1000 °C. The propagation directions of the two main cracks were approximately 45° with the dendrite orientation, and the {1 1 1} 〈1 1 0〉 slip family was mainly activated. Based on 3D transient thermo-mechanical coupling theory and rate-dependent crystal plasticity theory, the stress intensity factor and J-integral were calculated to predict the thermal fatigue crack propagation behavior. Three surface heat transfer stages were considered. The predicted crack propagation rates at three heating temperatures all showed good agreement with those obtained in the experiments.

Investigation on geometric precision and surface quality of microholes machined by ultrafast laser

The geometric precision and surface quality of film cooling holes has significant influence on the cooling efficiency and fatigue life of turbine blades. In this paper, the investigation about fabrication process of film cooling holes on the single crystal superalloy DD6 plate specimen by ultrafast laser is carried out. By comparing three different processing paths, minimum roughness of hole wall could be obtained by concentric circular scanning, which was used in the followed-up experiment. The influence of process parameters on the geometric precision and surface quality of microholes was analyzed, and the physical mechanism was examined. The results show that among several process parameters, focus position has the most significant influence on the microholes geometric precision and surface quality, and that the divergent beam (where the focal plane is above the machined surface) can lead to smaller taper, roundness, and surface roughness. In addition, besides a layer of solidified debris adheres to the inner wall at the hole entrance, the microholes machined by ultrafast lasers have no other defects such as a recast layer, microcrack or heat affected zone.

Quantitative study of the microstructure evolution along the thickness direction in the nickel-based single crystal superalloy DD6 at 1323 K thermal exposure

The wall thickness of turbine blade is becoming thinner because of the need for cooling, which leads to the thickness debit effect. The microstructure evolution along the thickness is deemed one of the major factors. The present work quantitatively investigates the microstructure evolution along the thickness of a second generation nickel-based single crystal superalloys DD6 under pure thermal condition. Experiments are performed on specimens with thickness of 0.3 mm, 0.6 mm and 1.5 mm. Microstructure is analyzed using scanning electron microscopy (SEM) in combination with quantitative image analysis. The interplay of oxidation and substrate causes apparently different microstructures along the thickness which are classified into three layers except the base material layer. The growth of each layer is barely influenced by the specimen thickness and their evolutions with time are fitted by both logarithmic and power equations. The microstructure along the thickness is not obvious different in the base material layer by naked eyes. Quantitative analysis of the average size and probability distribution function of the γ′ particle area and circularity reveals the variation of microstructure along the thickness. The average size of the γ′ particle area increases with the increase of the distance from the thickness symmetry axis. The average value of circularity at the position with the same distance from the thickness symmetry axis is larger in the thin specimen than that in the thick specimen, which represents more severe rafting in the thin specimen. A diffusional mechanism is used to interpret the observed microstructure changes at the base material layer. The new findings are important for mechanical performance analysis of thin-walled structure in turbine blade applications.

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 °C 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 °C, 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%.

Experimental study on grinding surface properties of nickel-based single crystal superalloy DD5

Nickel-based single crystal superalloy DD5, a typical difficult-to-machine material, is an important material for blades of aero-engine and gas turbine. The major characteristic of single crystal material from polycrystalline material is that the single crystal material shows significant anisotropy within the material. In order to investigate the grinding surface properties of DD5 material, some basic experiments have been conducted. The shear modulus G(001) and elastic modulus E(001) in the (001) crystal plane of nickel-based single crystal superalloy along different crystal orientations were deduced firstly in this paper. The shear modulus G(001) and elastic modulus E(001) along different crystal orientations in the (001) crystal plane of nickel-based single crystal superalloy change periodically with the cycle of π/2. The DD5 grinding anisotropy experiment and grinding surface quality orthogonal experiment were conducted then. When the angle θ is 45°, the grinding surface roughness Ra is the minimum, obtaining the best surface quality. In the grinding process of nickel-based single crystal superalloy DD5, the influence order of each process parameter on surface roughness Ra is as the following: grinding wheel linear speed > feed rate > grinding depth. The optimal process parameters of the nickel-based single crystal superalloy DD5 plane grinding selected by the range and variance method are the grinding wheel linear speed is 30 m/s, the grinding depth is 20 μm, and the feed rate is 0.4 m/min. The grinding subsurface microstructure and debris surface morphology were observed finally. Under the process parameters above, a plastic deformation layer of about 2 μm appears on the grinding subsurface, and the γ phase and γ' phase in the plastic deformation layer twist seriously. A work hardening layer of about 0.5 μm appears between the grinding surface and the plastic deformation layer. The free surface of the debris is discontinuous, showing a serrated feature of a section.

Precipitation characteristics of a nickel-based single-crystal superalloy after long-term thermal exposure

Abstract Microstructural stability is of great importance for the application of rhenium-containing nickel-based single-crystal superalloys. The microstructures of single-crystal superalloy DD6, especially the precipitation characteristics before and after thermal exposure, were characterized using scanning electron microscopy, transmission electron microscopy, and related techniques. Acicular M 6C carbides, rod-like M 6C carbides, and granular μ phases have been observed in the aged alloys, and the orientation relationships of these precipitates have been identified. During the early stage of M 6C precipitation, the carbide maintains a cube-on-cube orientation relationship with the matrix, whereas further growth of the M 6C carbides leads to a twin-related orientation relationship. For the granular μ phases containing a high concentration of Re, no special relationship has been observed. The results of the study will be beneficial for the understanding and improvement of nickel-based single-crystal superalloys.

Thermal fatigue behavior of a nickel-base single crystal superalloy DD5 with secondary orientation

The effect of cycle temperatures on the thermal fatigue behavior of a second generation Nickel-based single crystal superalloy DD5 with different secondary orientations was investigated. The experimental results show that the crack propagation rate increases with the rise of the upper limit temperature of the thermal fatigue test. It was found that the same initiation sites and propagation orientations of the thermal fatigue cracks were observed in samples with different secondary orientations. Thus, in samples with the second orientation of [110] and [100], thermal crack initiated at the edge of the holes with a directionally solidified direction of 45° and propagated along a direction of 45° with directional solidification.

Mechanical behavior of a DD5 single crystal superalloy with different misorientations under quasi-static and dynamic compression

Abstract

Validity analysis of materials' dynamic tensile SHTB experimental technique at ultrahigh temperature

In this work we investigated several key issues in view of the dynamic tensile experimental technique used in the split Hopkinson tension bar at ultra high temperature by performing numerical simulation, experimental verification and tests of several typical materials' dynamic tensile property at high temperature. The results show that the stress distribution was uniform for the flat tensile specimen with a hook joint after its gauge section size was optimized. The flow stress curve of the hook joint flat tensile specimen coincided well with that of the thread specimen, and no evident shake was observed in the strain rising stage. Through accurate pneumatic control, effective rapid synchronous assembly and loading of the specimen could be achieved at the same time when the loading wave arrived. When the temperature of the specimen reached 1 200 ℃, the average temperature of the specimen only dropped about 1.3% and the temperature rise of the loading bars kept below 180 ℃ during the whole cold contact between the high temperature specimen with the cold loading bars as well as in the process of the stress wave loading the specimen. To validate this experimental technique, tests were conducted at the temperature as high as about 1 200 ℃ for the dynamic tensile mechanical properties of a few materials such as 3D printed TC4 and single crystal nickel-base superalloy DD6.

Discussion of a Characteristic Temperature in Single Crystal Superalloy DD6

It is concluded that there is a characteristic temperature present between 800 ºC and 850 ºC in the single crystal superalloy DD6, symbolized as Tc after the longterm crept microstructures are analysed, with investigation into the fracture mechanism of the specimens with low angle grain boundary (LAB) as well as the relationship between modulus and temperature. When the temperature rises beyond Tc, the rafting microstructure comes to arise in the long-term crept specimens. The cohesive strength of the LAB will be weaker than that of alloy at the condition of the temperature beyond Tc so that the inter granular fracture would arise in the specimens with LAB.

Characterisation of surface recrystallisation in a grit-blasted single-crystal superalloy

ABSTRACTIn this work, detailed experimental insights regarding the microstructural features on surface recrystallisation of a grit-blasted single crystal superalloy DD6 have been obtained by transmission electron microcopy (TEM), scanning transmission electron microcopy (STEM), scanning electron microscopy and electron backscattered diffraction (EBSD) techniques. The broadened diffraction ring arcs typical for micro-textures revealed the fragmented finer grains after surface grit-blasting. Energy dispersive spectroscopy analysis and STEM mapping confirmed the characteristic elements for differentiation of γ′ and γ phase, and the precipitated secondary or tertiary γ′ in the recrystallised region was characterised by dark field TEM. Moreover, the recrystallisation depth was feasibly revealed by ion-etching and further used for EBSD imaging without polishing. The resulted EBSD pole figures and misorientation profiles confirmed that the recrystallised region exhibited a rather random orientation, destroying the original single crystal type.

EBSD characterization of boundary types after discontinuous precipitation in a grit-blasted nickel-based superalloy

In the present work, direct and detailed insights with respect to the statistical information on boundary type of discontinuous precipitates and their advancing reaction fronts in a Re-containing single crystal superalloy DD6 have been obtained by EBSD. With the annealing time prolonging, the reaction fronts proceeded into the original matrix for gradual release of grit-blast induced stress. The average recrystallized grain diameter and the average depth of recrystallization were respective 0.83μm and 3.98μm for the specimen annealed at 1050°C for 10min, which, after 16h, had grown to the average values of 3.22μm and 31.17μm. The reaction fronts of discontinuous precipitation have been confirmed to be high-angle grain boundaries, but do not always correspond to CSL type. Besides the role of strain energy dissipation for recrystallization, the formation of twin structure has no direct relationship with the proceeding of reaction fronts. The information of boundary type of discontinuous precipitation and its advancing reaction fronts would provide new insight into the cellular recrystallization behavior.

The Precipitation Behavior of γ′ Phase in Single Crystal Ni-Based DD6 Superalloy for Turbine Blade

The precipitation behavior of γ′ precipitates in typical section dimensions of DD6 single crystal superalloy turbine blade was investigated experimentally during directional solidification process. The phase transformation temperatures in the single crystal Ni-based DD6 superalloy from DSC analysis and JmatPro simulation were basically in consistent with the isothermal solidification experiments. The solidification route of DD6 single crystal superalloy could be described as follows: L1 → γ + L2; L2 → (γ + γ′)eutectic + MC; γ → γ′/γ. With increasing continuous cooling rates, the primary γ′ precipitates tended to be refined, and the size distributions of the primary γ′ precipitates at every temperature measuring position followed the normal distribution. In comparison to the interdendritic regions, nearly a 60% reduction in the average sizes of the primary γ′ precipitates was measured in the dendritic core regions. The result of the primary γ′ size difference was strongly affected by the multi-component segregations between the interdendritic and dendritic regions, where the γ′ forming elements of Al and Ta segregated towards the interdendritic regions. Furthermore, the secondary γ′ precipitation was found to occur within a relatively wide corridor of γ matrix for low cooling rates (12.6, 23.3 and 29.7 °C/min) during the directional solidification process. The occurrence of the secondary γ′ precipitation resulted from the complex interaction of multiple thermodynamic and kinetic factors in the γ′ nucleation and the diffusion rate of γ′ forming elements.

Influence of Heat Treatment Temperature on the Microstructures and High Cycle Fatigue Properties of DD6 Single Crystal Superalloy

The effect of heat treatment temperature on the microstructure and the high cycle fatigue property of DD6 single crystal superalloy was investigated. After standard heat treatment, the alloy was hold at 1200°C and 1300°C for 1h respectively, and then air-cooled. The results showed that after heat treatment at 1200°C for 1h, the size of γ′ particles became slightly larger and quite unevenly, at the same time, the serrated γ/γ′ interface appeared. After heat treatment at 1300°C for 1h, the small irregular γ′ particles re-precipitated. In addition, a small number of fine secondary γ′ particles were also observed in the matrix channel of the alloy after primary aging of 1200°C/1h and 1300°C/1h. The high cycle fatigue properties of the alloy by standard heat treatment were higher than that of the alloy by 1200°C/1h and 1300°C/1h, but the decreased degree of 1200°C/1h was much bigger than that of 1300°C/1h. Analysis on fracture surfaces of the alloy at 800°C demonstrated a quasi-cleavage mode.

Microstructure Evolution and Stress Rupture Properties of DD6 Single Crystal Superalloy after Overheating

The second generation single crystal superalloy DD6 after standard heat treatment was respectively overheated at 1100°C, 1150°C, 1200°C, 1250°C, 1300°C, 1320°C for 1h and air cooled. The effect of overheating on the microstructure and stress rupture properties at 980°C/250MPa of the alloy was investigated. The results showed that the size of γ′ phase was slightly increased overheating at 1100°C, 1150°C and 1200°C. The size of γ′ phase had a big increase and its size distribution was very uneven after overheating at 1250°C. The small part of γ′ phase has serrated γ′/γ phase surface as a result of un-completely solution and the irregular small γ′ phase was in the majority when overheated at 1300°C. While all the irregular small γ′ phase precipitated again after completely solution when overheated at 1320°C. There was no fine second γ′ phase in the γ matrix channel of the alloy after standard heat treatment and overheating at 1320°C. But the fine second γ′ phase precipitated in the γ matrix channel after overheating at every temperature of 1100 °C~1300°C. No obvious change of the stress rupture life was found after overheating at 1100°C, 1150°C, 1200°C and 1250°C. The stress rupture life considerably reduced after overheating at 1300°C, whereas slightly reduced after overheating at 1320°C. The appearance of the raft had almost no change after overheating at 1100°C. With increasing of overheating temperature from 1100°C to 1250°C, the length of raft became shorter and the width thickening. The γ phase formed the wavy raft after overheating at 1300°C and 1320°C and the thickness of latter was larger than that of the former. Finally, the relationship between the microstructural evolution and stress rupture properties of the alloy after overheating was discussed.

Effects of Thermal Exposure on Structures of DD6 Single Crystal Superalloy with Thermal Barrier Coatings

Effects of Thermal Exposure on Structures of DD6 Single Crystal Superalloy with Thermal Barrier Coatings