High-performance joining especially brazing has been highly expected in the manufacture or maintenance of single crystal blades. In this paper, a Ni-40Cr-10Ta (at.%) alloy was developed as filler ...material for single crystal superalloys. High-performance brazing of DD5 superalloy was achieved. The joints brazed at 1260 °C contained an isothermal solidification zone (ISZ) with “γ + γ′” structure and an athermal solidification zone (ASZ) composed of γ, γ', γ/γ′-eutectics, TaCr2 and TaC. Unlike the conventional filler materials with melting point depressants of B and Si, the Ni-Cr-Ta filler material was free of B and Si, and thus the resulting joints contained no B-rich and Si-rich compounds and eutectics with low melting points, which enhanced the brazed joint performance. The tensile strength at 980 ℃ of the heat-treated joints reached 654 MPa, which is 88.5 % that of the base metal.
The influence of orientation on the creep rupture properties of a single crystal superalloy DD6 under multiaxial stress was carried out at 980 °C and 400 MPa. A circumferential V-type notched ...specimen had been designed to investigate the effect of multiaxial stress state on the creep behavior. It was found that the creep lifetimes of the 111 oriented notched specimen were slightly longer than that of the 011 orientation and 1.36 times longer than that of the 001 orientation. The computational results showed that the strain and damage distribution revealed fourfold symmetry, double symmetry and threefold symmetry for 001, 011 and 111 orientation respectively, which was confirmed by experimental observation. At high temperature the creep anisotropy in three different orientations exhibited mainly in primary and secondary creep stages. Through the study of notched specimens by SEM, the morphology evolution of γ′-phase proved that the directional coarsening was strongly dependent on the local effective stress and the direction of the local max principal stress with respect to loading axis. Fracture morphology displayed uneven cleavage configuration with multi-level feature, and the cleavage planes parallels (001), (011) and (111) crystal plane for 001, 011 and 111 orientation, respectively. The cleavage plane, which is attributed to the cracks propagated along the interfaces of γ/γ′phases, displayed square-like, rhombus-like and hexagon-like feature for 001, 011 and 111 orientation, respectively. Due to the higher density in dislocations of {111} planes, it is more easily as the secondary crack when the crack reach the {111} planes, which is thought to be the main reason of forming different feature on primary cleavage plane.
•The order of creep lifetime for different orientation is 111>011>001.•Fourfold symmetry, double symmetry and threefold symmetry was found in FE result.•The symmetry was confirmed by fracture morphology.•The uneven cleavage displayed square-like, rhombus-like and hexagon-like feature.•Explaining how the cleavage planes formed different features.
Due to the excellent thermo-mechanical properties, nickel-based single-crystal superalloys (Ni-SXs) are widely used as a turbine blade material in aero-engine and gas turbine. Ni-SXs consist of ...regularly arranged γ′ precipitates, which exhibit an ordered L12 structure. As the main source of precipitation strengthening, the morphology of γ′ precipitates, including the size, shape, volume fraction, and spatial arrangement, plays a critical role in mechanical properties. Aging treatment is one of the most effective heat treatment processes on regulating the microstructure of Ni-SXs to bring the strengthening potential into full play, which usually consists of two or more steps. This article presents a review on the effects of multiple-stage aging processes on the morphology of γ′ precipitates including the temperature and time of the primary aging and secondary aging, also dissects the essential principle of the morphological change during aging from the perspective of thermodynamics including the combined contribution of interfacial energy and elastic energy. The morphological evolution behavior of γ′ precipitates during the aging process is summarized subsequently. In addition, coarsening theoretical models of γ′ precipitates and its application are briefly described. This review aims to establish the link between the aging treatment process and the micro-morphological evolution. Thus, some perspectives are put forward in this article which can provide the guiding references for the design of the aging treatment process of Ni-SXs.
•Degradation mechanisms during VHCF of coated and grit-blasted CMSX-4 elucidated.•VHCF life reduced by a thick RX layer without contribution of environment.•Roughness degrades the oxidation ...resistance and the VHCF life if combined with RX.•Oxidation damage in the interdiffusion zone governs VHCF life of coated specimens.
The very high cycle fatigue properties of the Ni-based single-crystal superalloy CMSX-4 coated with an MCrAlY type coating and grit-blasted were studied at 1000 °C, 20 kHz and R = -1. A recrystallized layer thicker than the size of the larger casting pores (>50 μm) drastically reduces the fatigue life without environmental influence. The recrystallized layer and roughness introduced by grit-blasting degrades the environmental resistance of the alloy and the resultant fatigue life. The fatigue life of the coated specimens is also greatly reduced from damage accumulation in the interdiffusion zone by grain boundary cracking, oxidation and internal nitridation.
The multiaxial stress state caused by geometric discontinuities will result in significant differences in the fatigue performance compared to the uniaxial stress state. It is crucial importance to ...research the notch fatigue behaviors of nickel-based single crystal (Ni-SX) superalloys at high temperature. This study investigated the impact of stress redistribution and stress triaxiality on the fatigue life of notched Ni-based single crystal superalloy at 760℃. The high temperature fatigue tests were carried out on the specimens under different stress ratios and the stress amplitudes under the same life conditions were obtained. A novel anisotropic damage model, incorporating the influence of stress triaxiality is proposed coupled with crystal plasticity theory. Based on experimental findings, an evaluation of the test parameters was conducted.The results showed that Mises stress, maximum principal stress, normal stress of principal stress along the loading direction, maximum principal strain, and strain along the loading direction were inadequate in reflecting the significant variations of samples observed among different stress ratios. However, the stress triaxiality at the unloading valley exhibited a high sensitivity to changes in fatigue life. To evaluate the fatigue life of notched specimens under different stress ratios, an improved Basquin’s type life prediction model was proposed, utilizing the stress triaxiality at the unloading valley. The proposed model demonstrates a favorable normalization trend for the fatigue life of notched specimens subjected to various stress concentration factor (SCF) conditions.
•The LCF performances with thickness debit effect of nickel-based single crystal superalloy DD6 with 001 orientation are investigated.•Based on SEM observation, the formation mechanism of thickness ...debit effect and the damage mechanism under LCF loads are revealed.•A thickness-sensitive LCF damage model of nickel-based single crystal superalloy DD6 is proposed.•The stress-controlled LCF deformation behaviours including the whole-lifetime ratcheting behaviours are well predicted.
Through stress-controlled low cycle fatigue (LCF) experiments, LCF performances for thickness debit effect of nickel-based single crystal superalloy DD6 with 001 orientation are investigated. Based on scanning electron microscope (SEM) observation, the formation mechanism of thickness debit effect and the damage mechanism under LCF loads are revealed. Then, considering the thickness debit effect of nickel-based single crystal superalloy DD6, an improved slip-based damage model is developed. Predicted LCF lifetimes are mainly within a scatter band of factor 2, and the predicted laws between LCF lifetimes and wall-thicknesses are in good agreement with the experimental data, which verifies the rationality and accuracy of the thickness-sensitive LCF damage model established in this paper. Furthermore, a damage-coupled crystallographic constitutive model reflecting thickness debit effect is developed. For DD6 specimens with different wall-thicknesses, the predicted results of the stress-controlled LCF deformation behaviours including the whole-lifetime ratcheting behaviours show good agreement with the experimental data.
•The tensile behavior of single-crystal nickel superalloy with differently structured cooling holes was investigated.•The cooling hole increases the tensile strength but decreases the ductility of ...the plate specimen.•Highly localized stress and plastic strain concentration occurred around a diffusive hole structure.•Crystal plasticity simulation revealed that larger accumulated plastic slips near diffusive holes accelerated final fracture.•Asymmetric plastic deformation around a diffusive hole caused complex pathed crack propagation.
Film cooling hole structures significantly influence the mechanical behavior of a single crystal turbine blade. This study investigates the tensile behavior of nickel-based single crystal plate specimens with three different structure holes, including straight, inclined, and diffusive holes, by in-situ tensile tests and crystal plasticity simulation. The results show the cooling holes play a strengthening effect on the single crystal superalloy, and the tensile strengths of the single crystal specimen with holes were increased. At the same time, fracture strains were decreased compared to those without hole. In addition, the cooling hole structure causes the difference in stress concentration and subsequent local plastic deformation around a hole, which further affects the tensile strength, fracture strain, crack initiation and propagation of the single crystal specimens. The asymmetric diffusive cooling hole induces larger resolved shear stresses and localized plastic deformation near the hole, which results in lower tensile strength and decreased fracture strain. Surface microcracks are observed along the acute angle zone for the inclined and diffusive holes, while at an angle of 65° to the loading direction for the straight hole. These orientations are consistent with the maximum accumulated plastic slip in the crystal plasticity simulations. And all the surface microcracks propagated along the direction of the dominant slip system. In addition, the non-uniform distribution of accumulated plastic slips along the hole depth causes the macrocrack initiation on the specimen surface, which propagates along direction, and leads to final failure on {111} plane.
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This study analyzes the effect of shot peening on the surface integrity of nickel-based single crystal superalloys (SX) and the evolution of stress concentration factors caused by surface finishing ...and casting micropores near the surface before and after shot peening by numerical simulation. On one hand, SX forms a surface gradient plastic deformation structure after shot peening. The surface yield strength is 82% higher than that of the matrix by nanoindentation fitting. The deformation of the surrounding materials of the near-surface casting micropores is significantly greater than that of the materials at the same depth by EBSD. On the other hand, the series parameters of surface roughness after shot peening are higher than those of grinding. However, the stress concentration Kt1.32 at the bottom of the smooth crater is significantly lower than that of grinding "furrow" Kt1.94. The surface gradient deformation caused by shot peening flattens the surface casting micropores so that the stress concentration at the upper end of the micropores is significantly smaller than that in the grinding state. Based on the above two effects, the rotary bending fatigue limit of the single crystal alloy after shot peening at 650 °C is increased by 13.0% compared with the ground state. The peened sample no longer originates from the casting micropores.
•Stress concentration reduction by casting pores deformation and tool-marks elimination.•Yield strength of surface metal increased by 82%.•Fatigue limit increased by 13%.
Based on the physical phenomenon that the fatigue cracks initiate along specific slip plane, a slip plane damage‐based low‐cycle fatigue (LCF) lifetime model for the nickel‐based single‐crystal ...superalloy is established. The predicted results indicate that the lifetime model can reflect the orientation effect. In addition, in order to characterize the dwell‐time dependence of the LCF lifetime, creep damage and compression‐creep damage are introduced to the lifetime model. Finally, the lifetime predictions under LCF loading with tensile dwell time, compressive dwell time and tensile‐compressive dwell time are conducted by employing the lifetime model, respectively. The predicted lifetimes show a good agreement with the experimental data, which verifies the accuracy of the developed lifetime model in this paper.
The effect of grit-blasting on the recrystallization configuration of a single-crystal nickel-based DD6 superalloy has been investigated. Both coated (with NiCoCrAlYHf bond coat) and uncoated alloys ...exhibit a double-layered recrystallized microstructure consisted of the consecutive equiaxed recrystallization (ERX) layer and the inconsecutive cellular recrystallization (CRX) structure. A chemical gradient of γ and γ’ phase promoting elements was observed in the interdiffusion zone, and the Cr profile was used to figure the average diffusion coefficient. With the increase of grit-blasting pressure, the overall recrystallization depth for both coated and uncoated alloys gradually increases at first, and then flattens out; the residual stress varies with the grit-blasting pressure in the same way.
•Effect of grit-blasting pressure on recrystallization of DD6 alloy was investigated.•Recrystallization zone is composed of equiaxed and cellular structures.•Recrystallization depth increases with the increasing blasting pressure.•Cellular structure extends from the equiaxed recrystallized region.
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