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The alloying effects on the microstructure, especially precipitation of harmful phases and solvus temperature of L1 2 -structured γʹ phase, etc., are essential for alloy design and development of precipitation strengthened Co-based superalloys. Based on the newly reported Co30Ni9Al5Mo10Cr2Ta0.03B alloy, alloying elements Mo, Cr, Al, Ta and Ni are systematically changed and alloys are fabricated to investigate their microstructures in two heat treatment statuses, namely homogenization and aging, respectively. Results show that Mo and Cr are γ forming elements and have limited effect on γʹ solvus temperature, but high concentration of Mo and Cr facilitates the formation of harmful μ phase. Al, Ta, and Ni are all γʹ stabilizers and can effectively increase γʹ solvus temperature, but roles of Al, Ta and Ni are different. Increasing Al and Ta would result in other harmful phases, such as the μ phase in the 6Ta and 4Ta alloys, and μ + B2 phases in the 13Al and 11.5Al alloys. However, Ni addition significantly increases solvus temperature and area fraction of γʹ phase without occurrence of harmful phases. The current findings might provide guidelines for design and development of precipitation strengthened Co-based alloys. [Display omitted] • The alloying effects of Mo, Cr, Ta, Al & Ni are systematically investigated. • Mo & Cr are γ formers, which facilitate the formation of μ phase. • Ta is a strong γˊ former, but excessive Ta facilitates the formation of TCP phase. • Al is necessary for γˊ phase, but excessive Al results in B2 & TCP phases. • Ni stabilizes γˊ phase without forming the harmful phases in Co-based alloys. [ABSTRACT FROM AUTHOR]

To model the centrifugal loads and complex geometries experienced by turbine blades during service, the nickel-based single crystal superalloy DD6 samples with a groove structure are designed and tested under the coupling conditions of high-speed rotating of 20,000 r/min and high temperature of 980 ℃. Scanning electron microscopy and transmission electron microscopy are used to analyze the microstructural evolution of γ-γ' phases and dislocation morphologies of 400 h and 1000 h samples. The results indicate that microstructural degradation of the γ-γ' phase attributes to the larger centrifugal tensile stress S 11 in the applied physical stress compositions until deformation occurs. Our observations suggest that a/2 < 101 > -type dislocations initiate plastic deformation by slipping within the γ phase. However, their movement is constrained at the interface between the γ and γ' phases, resulting in their tendency to aggregate at this boundary. After the formation of stable interfacial dislocation networks, these dislocations then react to generate either a< 010 > -type dislocations or a/2 < 112 > -type dislocations, which cut into the γ' phase. However, under high stress conditions, the a/2 < 101 > -type dislocations are primarily cut directly in the form of dislocation pairs. • The γ-γ' phase degradation and dislocation evolution of DD6 superalloy was investigated. • Creep tests are conducted under high-speed rotating and high temperature condition. • Microstructural degradation of the γ-γ' phase attributes to the larger centrifugal tensile stress S 11. • Interfacial dislocation networks and superdislocations accelerate the separation and disappearance of the γ phase. [ABSTRACT FROM AUTHOR]

In order to make up for the deficiency that the laboratory testing of standard samples cannot approximately simulate the turbine blade geometry and service environment, the creep experiment of the nickel-based single crystal superalloy NiAlReRu components with film cooling holes (FCHs) is carried out under the coupling high-speed rotating and high temperature condition. The component with the FCHs exposed to the coupling high-speed rotating of 20000 rpm and high temperature of 1000 ℃ continuously lasts 255.92 h until it breaks. The microstructural evolution and fracture process are analyzed. The results indicate that the existence of the FCHs evidently changes the stress distribution of the region with the FCHs, which makes the microstructural evolution of γ′ phase significantly different. The component is fractured along the second row of the FCHs and the normal direction of the fracture surface is basically parallel to the centrifugal force direction. The fracture process can be divided into three typical stages. The coupling effect of temperature and stress on the failure mechanism is discussed. • The failure mechanism of NiAlReRu superalloy under high-speed rotating and high temperature was investigated. • The microscopic stress concentration induced by film cooling holes contributes to the change of rafting behavior. • Film cooling holes become the initial source of cracks and are responsible for dissociation fracture. [ABSTRACT FROM AUTHOR]

The microstructure, thermal and mechanical stability of Ni-based single crystalline superalloys depend strongly on the alloying elements and their concentrations. Alloying has been the main design strategy for stabilizing the compositions, microstructures and thermal-mechanical properties. This article presents a review on the effects of some common alloying elements on the microstructural and mechanical property stability control of Ni-based superalloys. The various alloying elements are divided into four categories according to their main effects on these properties, comprising base elements, mechanical strengthening elements, long term stability elements and the oxidation resistance elements. The mechanical strengthening elements can further be divided into precipitation, solid solution and grain boundary segregation elements. The precipitation elements strengthen the alloys by forming the L1 2 structured γʹ phase. The solid solution elements strengthen primarily the γ phase, by increasing the solidus temperatures and decreasing the stacking fault energy, which in turn influences the thermal stability of the phases and the resistance of dislocation movement. The grain boundary elements strengthen the alloys by the formation of carbides and borides along the grain boundaries during solidification, which help to prevent the formation of casting pores and hot tearing and to strengthen low angle boundaries. The long-term stability elements inhibit the precipitation of topologically closed-packed phases causing deterioration of the mechanical properties. The oxidation resistance element, mainly Al, promotes the formation of protective Al 2 O 3 surface layer. [ABSTRACT FROM AUTHOR]

To determine the influence of Re and Ru on the phase morphologies and microstructure in Ni-based superalloys, a series of model alloys derived from the 4th generation TMS-138 superalloy have been investigated with systemically changing Re and Ru additions. All alloys are subjected to solid solution and aging treatments to achieve the typical γ/γ′ two phases microstructure. Phase transition temperatures are measured by using differential scanning calorimetry. The Re is found to increase the γ′-solvus temperature with a slope of roughly 5.4 ℃ per wt% Re in Ru-free alloys and 2.3 ℃ per wt% Re in 2Ru alloys, while the Ru exhibits much weaker effect. Microstructure and composition of the two phases are examined to understand the Re and Ru effects on γ′ size, shape, volume fraction, the elemental partition behavior, and the γ/γ′ lattice misfit. The results show that the addition of Re affects size and shape of γ′ precipitates, and also reduces γ/γ′ lattice misfit significantly. Meanwhile, Re increases partition coefficients of Cr and Ru. In comparison, the addition of Ru seems to have little effect on morphology of γ′ precipitates and the γ/γ′ lattice misfit, but Ru appears to alter partitioning behaviors of other elements. Because Re and Ru are two key elements for high generation Ni-based single crystal (SX) superalloys, this work might provide guidelines for designing next generation of SX superalloys. [Display omitted] • Re effectively increases the γ′ solvus temperature of superalloys, while Ru has slight effect on the γ′ solvus temperature. • Re addition decreases the size of γ′ phase and γ/γ′ lattice misfit, facilitating cubic precipitation of γ′ phase. • Ru addition only has little effect on the γ′ morphology and γ/γ′ lattice misfit of superalloys. • Partitioning behaviors of other alloying elements depend on the Re and Ru additions, especially Ru. [ABSTRACT FROM AUTHOR]

Oxidation induced phase transformation from γ′ to γ phase can severely degrade creep properties of superalloys and has been studied by in-situ environmental transmission electron microscopy, energy dispersive spectrometer and electron energy loss spectroscopy in detail. The in-situ observation showed that, during oxidization, the ordered γ′ phase (L1 2) gradually transformed to disordered γ phase (face-centered cubic (fcc)) with the appearance of oxidation product consisted of γ-Al 2 O 3 crystallites with a strong crystallographic texture. This directly proves that γ′-γ phase transformation are induced by aluminum oxidation with aluminum depleted in γ′ phase. Furthermore, the oxygen was demonstrated to diffuse preferentially through γ′-γ phase interface which is different from previously thought (γ matrix). [ABSTRACT FROM AUTHOR]

Here, we investigate a new type of kink structure that is found at γ/γ′ interfaces in nickel-based single crystal superalloys. We studied these structures at the atomic and elemental level using aberration corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The core of the dislocation absorbs high-Z elements (i.e., Co and Re) that adopt hexagonal arrangements, and it extrudes elements (i.e., Ni and Al) that adopt face centered cubic (fcc) structures. High-Z elements (i.e., Ta and W) and Cr, which is a low-Z element, are stabilized in body centered cubic (bcc) arrangements; Cr tends to behave like Re. High-Z elements, which migrate and adopt a hexagonal structure, induce kink formation at γ/γ′ interfaces. This process must be analyzed to fully understand the kinetics and dynamics of creep in nickel-based single crystal superalloys. [ABSTRACT FROM AUTHOR]

It is important and necessary to understand the micro-mechanism of rhenium (Re) effect in the deformation process. The in-situ tensile experiments of Ni–Al and Ni–Al-Re model single crystal superalloys have been carried out in scanning electron microscope at 750 °C. It is found that Re can promote the complex stacking fault (CSF) formation, which is similar to the previous results after tensile experiments at room temperature. The micro-mechanism of Re promoting the CSF formation has been investigated by means of advanced probe-corrected scanning transmission electron microscopy and energy dispersive X-ray spectroscopy. The local high concentration of Re is observed at CSF regions after tensile experiments both at room temperature and 750 °C. The first principles calculation results further indicate that low concentration of Re increases CSF energy, while high concentration of Re decreases CSF energy, which is consistent with the experimental results. • In-situ tensile experiments of model single crystal superalloys are carried out. • Re can promote the complex stacking fault (CSF) formation. • High concentration of Re is observed in local region along CSF. • The calculation results show high concentration of Re decreases CSF energy. [ABSTRACT FROM AUTHOR]

Nickel-based single crystal superalloys (Ni-SXs) are of great interests in aircraft industry due to their excellent high temperature creep strength deriving from the rigid gamma/gamma prime (γ/γ′) microstructure and the single crystal nature. The creep properties of Ni-SXs depend strongly on the degradation rate of the original rigid γ/γ′ microstructure with the dislocation movements in the microstructure. Since the applied temperature and stress strongly affect the dislocation movements, the different creep mechanisms are displayed to Ni-SXs during low temperature (∼750 °C), mid-temperature (∼950 °C) and high temperature (∼1100 °C) creep. The creep deformation during low temperature creep is mainly caused by the shearing of γ′ phase by a limited number of dislocations, while high temperature creep mainly occurs to the accumulation of creep strains with the increased dislocation activities in γ phase. To enhance the creep properties of alloy, a variety of strategies of the microstructural optimization need to be considered. One is to increase the volume fraction of γ′ phase to enhance the precipitation strengthening effects. But excessive amounts of γ′ phase are disadvantageous to low temperature strength of alloy. The coarsening of γ′ phase and the rafting process strongly correlates to dislocation movements. It is of importance to discuss the specific mechanisms of coarsening and rafting as well as their influence to the creep properties of alloy. The lattice misfit between the γ and γ′ phase plays an important role in influencing the coarsening of γ′ phase and the creep properties of alloy. A proper lattice misfit helps to form the cubic γ′ phase which is advantageous to alloy. But a too large lattice misfit can promote the coarsening of γ′ phase that damages the structural stability of alloy at high temperatures. The increased additions of refractory elements in alloy promote the formation of topologically close-packed (TCP) phase. TCP phase facilitates the nucleation and propagation of micro-cracks around it that damages the creep properties of alloy. This review aims to summarize some aspects of microstructural evolution during creep of alloy with considering their effects to creep properties. To guide the design of Ni-SXs in future, some perspectives about the microstructural optimization are properly provided. • The creep properties of Nickel-based superalloys are greatly decided by the microstructural evolution during creep. • The creep mechanisms are determined by the intrinsic dislocation activities occurred in the microstructure. • The coarsening of γ′ phase is emphasized, especially the rafting process. • Lattice misfit strongly affects the formation of dislocation networks and coarsening of γ′ phase. • The formation of TCP phase which is detrimental to creep properties need to be strictly restricted in superalloy design. [ABSTRACT FROM AUTHOR]