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The distribution of Rhenium (Re) in a Ni-based single-crystal superalloy is studied by sub-angstrom resolution transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). It is found that Re atoms segregate at the tensile stress regions near the interfacial dislocation cores, forming the “Cottrell atmosphere”, and the segregation process is facilitated by dislocation pipe diffusion. In situ TEM and scanning electron microscopy (SEM) straining studies reveal that the Re-decorated dislocation networks along the phase boundaries act as mechanical walls that effectively block dislocation motion and crack propagation. Furthermore, the degree of Re segregation can be regulated by thermal treatment. Theoretical analysis demonstrates that this remarkable alloying effect originated mainly from the interactions between the local composition strain of Re and the dislocation strains, leading to significantly stabilized interfacial dislocation networks. These results provide a new perspective on understanding the origin of the Re effect on mechanical properties in Ni-based superalloys and will be beneficial to both improving creep properties of Ni-based superalloys and designing high-performance Re-free superalloys. [ABSTRACT FROM AUTHOR]

The enhanced mechanical properties of Ni-based single-crystal superalloys induced by refractory elements such as Re, Wo, Mo, etc. are essential for the manufacture of advanced aero engines' turbine blades. Whereas these elements could promote the formation of the topologically close-packed (TCP) phases, and their growth would have a deleterious effect on the creep rupture life of single-crystal superalloys. Concomitantly, the formation of the brittle phase of the internal nitrides, especially the aluminium nitride (AlN), would be detrimental to superalloys as well. Nonetheless, the formation relationship between AlN and TCP phases has maintained opaque. In this study, the relationship between the TCP phase and AlN phase was analysed by microscopic methods, such as BSE, TEM and EDS. An AlN phase located near the TCP phase in the deep surface of the superalloy was found, and its formation was suggested to be related to nitrogen penetration during the thermal exposure. This accompanying growth of the AlN phase and TCP phase in the deep surface could provide a new perspective on the formation mechanism of these brittle phases in the Ni-based single-crystal superalloys. [ABSTRACT FROM AUTHOR]

In this paper, the deformation behavior of a nickel-based single-crystal superalloy with [ 1 ¯ 40] and [ 3 ¯ 40] orientations was studied at room temperature by in situ electron backscattered diffraction (EBSD) tensile equipment. The mechanical properties, slip behavior and the corresponding Schmidt factor were analyzed. The deformation bands generated during the plastic deformation stage were found and characterized by using the electron backscattered diffraction (EBSD) technique. Furthermore, the formation and propagation of deformation bands played a softening effect on stress–strain curves; however, the secondary slip bands occurring after the propagation of deformation bands played a hardening effect. Finally, the formation of deformation band and its propagation on the mechanical properties are discussed. [ABSTRACT FROM AUTHOR]

Understanding the oxidation of Ni-based single crystal (SX) superalloys is significantly important because oxidation can damage the microstructure and eventually lead to failure of the alloy. Using advanced scanning/transmission electron microscopy, oxidation effects on the microstructure of a SX superalloy during heat treatment and simulated service (stress/temperature coupling) conditions are systematically investigated in order to reveal oxide microstructures and oxidation mechanisms. Heat treatment in argon results in less weight loss, thinner γ′-free layer and fewer internal oxidation than that treated in air. But the oxide layer structures and influence depths of oxidation in both atmospheres are similar. External tensile stress not only accelerates the oxidation of alloy but also affects the oxide microstructures. Moreover, stress effects on the oxidation microstructure depend on temperature. At 750 °C, Ni–Co oxides are formed on the alloy surface, followed by the inner layer of Ni–Co–Mo–Re–W oxides at the location of original γ phase under stress free condition. As the stress increases, an oxide layer mainly containing Al, Cr, Nb, Mo and Re elements is formed between the above two oxide layers. When temperature increases to 1050 °C, as applied tensile stress increases, the thickness of oxide layer increases but the structure of oxide layer is not obviously changed. [ABSTRACT FROM AUTHOR]

To reveal if universal rules may exist in face centered cubic (FCC) concentrated solid solution (CSS) alloys for strength and plastic deformation at room and cryogenic temperatures, we select a FCC CSS superalloy Haynes 188 to demonstrate the deformation mechanisms from atomic to micrometer scales. In FCC CSS alloys, the yield strength (YS) is intrinsically determined by atomic/modulus mismatches and affected extrinsically by grain size; the tensile strength and elongation to fracture (EF) are governed by the plastic process where low stacking fault energy is beneficial. Moreover, almost all CSS alloys show that YS and EF increase simultaneously with the decrease in the temperature. Our findings may expand alloy application and can be used as a design strategy for stronger and tougher alloys. [ABSTRACT FROM AUTHOR]

Oxidation behaviors of three Ni-based model alloys and pure Ni in the temperature range of 700–1200 °C are investigated to reveal effects of Cr, Al, and Si on the oxidation resistance of Ni-based superalloys. The formation and integrity of consecutive chromia or alumina layers are important for excellent oxidation resistance. The addition of 20 at.% Cr can effectively improve the oxidation resistance of Ni-based alloys by forming a thin chromia film below 1000 °C, while adding 15 at.% Al has a beneficial effect on the oxidation resistance of Ni-based alloys at temperatures above 900 °C. The addition of 2 at.% Si to Ni-Al alloy is insufficient to form a protective SiO2 layer but can accelerate the formation of alumina, which enables Ni-Al alloy to form a consecutive inner alumina layer at a relatively low temperature of 800 °C and further improve the oxidation resistance above 800 °C. [ABSTRACT FROM AUTHOR]

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]

• β-NiAl phase precipitates in a novel high Ru-containing Ni-SC. • Ru element is found to be obviously biased in β-NiAl phase. • The partition of Ru assists nucleation of β-NiAl phase during solidification. • Studies based on the solidification path of experimental alloy were carried out. The precipitation of β-NiAl phase in a high Ru-containing Ni-based single crystal superalloys was investigated. The β-NiAl phase is mainly distributed in the interdendritic region of the as-cast structure. And it was found parts of Ru elements were polarized in β-NiAl phase as the existence of strong bonds between Ru atoms and Al atoms. In addition, the solidification path of the experimental alloy passes through the region of the L → L 1 + γ → L 2 + γ + β-NiAl → L 3 + γ + β-NiAl + precipitated γ′ → γ + β-NiAl + precipitated γ′ + γ/γ′ eutectics. It provides experimental data and theoretical guidance for the composition design of the novel Ru-containing high-generation Ni-SCs. [ABSTRACT FROM AUTHOR]

This study aimed to compare the dynamic microstructure evolution and the creep mechanical properties of second-generation nickel-based single-crystal superalloys at 780 °C/720 MPa and 850 °C/720 MPa. A method based on in situ scanning electron microscopy creep testing was used to identify the occurrence of slip bands. At the same time, the deformation mechanisms at 780 °C and 850 °C were discussed. The creep life at 780 °C was longer, almost five times that at 850 °C. Moreover, significant necking occurred in the primary creep at 780 °C, which controlled the entire creep process and accounted for almost 50% of the total life. In comparison, the primary creep at 850 °C was obviously shortened, mainly showing the characteristics of the second and third stages; the necking was mainly concentrated in the later stage of creep. This difference must be caused by the dislocation motion. The slip deformation mainly occurred at 780 °C, and the < 112 > dislocations shearing γ′ strengthening phase left obvious slip bands on the surface. The dislocations moved in the matrix channel in the early stage of creep at 850 °C, and the dislocations sheared γ′ strengthening phases in the later stage of creep while leaving obvious slip bands on the surface. [ABSTRACT FROM AUTHOR]

The important factors limiting the fourth-generation Ni-based single-crystal superalloys (Ni-SXs) are the increases in cost and density. A cheaper and lighter potential alternative element of Ru, Co's comprehensive effects were studied. The effects of Co and inhomogeneity between dendrite core (DC) and interdendritic region (ID) on creep performance at 1100 °C and 137 MPa in a 4th generation Ni-SXs were systematically investigated. The results show that the Ni-SXs with 9 wt% Co has the longest creep lifetime. Co additions alleviate the overall element segregations between DC and ID, especially Re, Cr, and W. And reverse partitioning behavior of Co also changes the partitioning ratio of elements between γ and γ′ phases, resulting in the inhibition of TCP phases precipitation for most creep time to avoid excessive consumption of solid solution strengthening elements and local stress concentration. The decrease of the lattice constant of γ reduces lattice misfit between γ and γ′ with the addition of Co, leading to the enlargement of dislocation network spacing and less resistance for movement or shearing of dislocation. The decrease of γ′ solvus temperature with Co addition promotes the dissolution of γ′ at high working temperature, resulting in severe topological inversion of γ/γ′ microstructures at ID. In addition, the main creep cracks of all Ni-SXs appear at ID, so the inhomogeneity between DC and ID is concerned. • The precipitation of TCP phases are inhibited with Co addition, which is a basis of low-cost and low-density alloy design. • The comprehensive effects of Co on high temperature creep properties are investigated and an optimal Co content exists. • The increase of Co affected the γ′ phase content, lattice mismatch, element distribution and TCP phase. • The effect of Co on microstructures evolution varies at different creep stages. • The inhomogeneity between dendrite core and interdendritic region causes creep cracks originate from interdendritic region. [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]

The effect of deformation temperature on tensile behavior of Inconel 718 alloy has been studied by a self-developed in-situ high-temperature tensile stage inside a scanning electron microscopy at a temperature range from room temperature (RT) to 750 °C. The dynamic microstructure evolution and mechanical properties at different temperatures were performed and compared by the uniaxial tensile tests. The in-situ test results showed that the mechanical properties and fracture mechanisms of Inconel 718 alloy were sensitive to deformation temperatures. From RT to 650 °C, the yield stress and ultimate tensile strength decrease slightly and the tensile ductility is comparable. While up to 750 °C, the yield stress and ultimate tensile strength decrease significantly, the elongation and reduction of cross section also showed a significant decrease from RT to 750 °C. It was found that at RT and 650 °C, tensile cracks tended to initiate around the carbide particles and the triple junctions of grain boundaries, also propagated transgranularly; at 750 °C, the cracks initiated at grain boundaries and propagated intergranularly. In fact, with the increase in deformation temperature, the fracture mechanism transformed from the ductile transgranular fracture to the brittle intergranular fracture. [ABSTRACT FROM AUTHOR]

[Display omitted] • The thermodynamic and dynamic factors influencing the nucleation and growth of topologically close-packed (TCP) phases are summarized in detail. • The possible mechanism of the interface structure affecting TCP phase growth is discussed. • The corresponding research directions of TCP phase precipitation and alloy design are proposed. The nucleation and growth of topologically close-packed (TCP) phases in Ni-based single crystal superalloys (SCs) are controlled by composition, temperature, time, interface structure and stress. The formation of TCP phases causes detrimental effects on the mechanical properties of SCs. Therefore, it is crucial to study the formation mechanism and control factors of TCP phases to develop new-generation Ni-based SCs. Different alloying elements influence the TCP phase nucleation by adjusting the chemical driving force. The interface structure determined by interface energy and strain energy mainly affects the growth mechanism and morphology of the TCP phase. Various temperature determines the driving force for TCP nucleation and diffusion rates of elements during TCP phase growth. Holding time is also crucial to the growth and evolution of TCP phases. Applied stress also has an impact on the precipitation of TCP phases. Evolution between different types of TCP phases depends on the temperature, composition and crystal structure of TCP phases. TCP phases damage the mechanical properties mainly by causing stress concentration and weak interface strength. The review aims to establish a link among TCP phase formation, alloy design and microstructure from the macroscopic view to the microscopic view, providing some perspectives for future research. [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]

This work investigates the evolution of the crystal structure of the topologically closed packed orthorhombic P phase precipitate in a Re-containing Ni-based single crystal superalloy during thermal exposure. The P phase is formed with a thin needle morphology. The precipitate is formed from the matrix with an initial complex atomic arrangement which continues to evolve during the process of thermal exposure. Based on the experimental evidence of this study and the theoretically predicted structure reported in the literature, a mechanism of the structural transformation is proposed. The initial structure is composed of a parallelogram (P) atomic arrangement configuration, which gradually evolve into a rectangle (R) atomic arrangement configuration in the [100] P projection. In the [010] projection, the initial structure is composed of alternating rows of a larger parallelogram (P') and a larger rectangle (R') configurations, which gradually evolve into an intricate structure of P'-R' along the length [001] P direction and P'P'-R'R' along the transverse [100] P direction. The initial structure is formed for its structural similarities to the γ phase to minimize lattice mismatch. The final structure is evolved over time to conform to its thermodynamically more stable state. The structural evolution is achieved by collective atomic shuffling. These intricate atomic arrangements gives rise to the very faint and highly dense parallel striation lines along the transverse [010] P direction in [100] P view and along the length [010] P direction in [010] P view under transmission electron microscopy observation. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The cyclic oxidation behavior of fourth-generation nickel-based single crystal superalloys with 2–4 wt% Mo content were studied at 900 ℃ and 1100 ℃. At 900 ℃, with the increase of Mo content, the oxide layer accumulates more growth stress, resulting in the interface between the outer oxide layer and the inner oxide layer more prone to crack initiation. At 1100 ℃, the oxidation layer accumulates more growth stress and thermal stress, so the oxidation layer occurs large scale spalling, which is more severe in 4Mo alloy. The results show that the increase of Mo content of alloy promotes the oxidation of containing W-Re alloy, which promotes the volatilization of oxides and the spalling of oxide, decreases the protection of oxide layer, and deteriorates the oxidation resistance of the alloy. • The increase of Mo content of alloy promotes the oxidation of containing W-Re alloy. • With the increase of Mo content, the volatilization of oxides and the spalling of oxide of alloy was promoted. • Mo has more severe harm to the oxidation resistance of the alloy at higher temperature. • The alloy with 2 wt% Mo addition has the highest oxidation resistance. [ABSTRACT FROM AUTHOR]

Advanced single-crystal superalloys, developed to improve the high temperature capability of gas turbines, often underperform in low-temperature conditions. This limitation compromises the overall reliability of turbine blades under complex service environments. This study introduces a novel strategy to enhance the overall performance of a fourth-generation single-crystal superalloy. Rapid liquid-nitrogen quenching following the aging treatment promotes extensive nucleation of ultrafine γ′ particles (γ′ γ) with an average size of ∼10 nm, precipitated under extremely low supersaturation, as evidenced by high-resolution transmission electron microscopy (HRTEM) and atom probe tomography investigations. The combination of differential scanning calorimetry analysis, density functional theory calculations, and kinetic models verifies the high stability of γ′ γ particles at low-temperature conditions around 900 °C. HRTEM observations reveal Orowan bypassing and dislocation cutting of γ′ γ particles, leading to additional precipitation hardening by limiting dislocation motion in the γ phase and shear of primary precipitates. Consequently, a bimodal precipitation microstructure comprising γ′ γ particles and primary γ′ phase significantly reduces the strain rate of low-temperature creep and nearly doubles the creep rupture life at 800 °C/735 MPa. Simultaneously, high-temperature properties at 900 °C/392 MPa and 1100 °C/137 MPa remain comparable to those of the original microstructure as the primary γ/γ′ microstructure is minimally affected by rapid cooling. This advancement improves overall performance and redefines the importance of small precipitates, broadening microstructure design concepts for future superalloy development. [Display omitted] [ABSTRACT FROM AUTHOR]

This work investigates the effect of Ru on the solution heat treatment window of the Ni-based single crystal superalloy. The addition of the Ru influences the solidification microstructure and widens the window. The solidification process is established to occur in the sequence of γ (dendrite) → γ + γ' grid eutectic (inter-dendrite) → γ + γ' lamella eutectic (inter-dendrite) → γ' (inter-dendrite). This is followed by the γ → γ + γ' decomposition of the γ dendrite into the common channel-cuboid structure upon cooling. The solvus of γ' and the solidus of the alloy define the window. In alloys containing inter-dendrite eutectic, the window is in principle zero. In this case, only the dynamic window created by homogenization diffusion may allow heat treatment. Adding Ru decreases the elements' partition ratios between the dendrite core and dendrite edge regions. It also promotes the enrichment of Re at the γ/γ' phase interfaces, which helps to refine the γ + γ' channel-cuboid structure and reduce the differences in both the size and volume fraction of the γ' phase between dendrite core and dendrite edge. Ru is found to inhibit the formation of (Ta, X)C carbide. These help widen the solution treatment temperature window of the Ni-based single crystal alloy. [Display omitted] • The solidification of the Ni-based single crystal superalloy is established. • The addition of the Ru can widen the solution heat treatment window. • The metallurgical principle behind the window of solution heat treatment is explained. • The effect of Ru on the window and the microstructure in cross-scale is clarified. [ABSTRACT FROM AUTHOR]

Abstract Topologically close-packed (TCP) phases in Ni-based single crystal superalloys are known to exhibit different morphologies and orientation relationships within the matrix. This study investigated the influence of chemical composition on lattice parameters, which in turn determine the interface lattice misfit between TCP phase and matrix and thus shape and orientation of the TCP phase. In this study, TCP phase particles of different morphologies were formed by ageing at different temperatures. The level of TCP-phase element enrichment was found to increase with increasing growth time and ageing temperature, and thus leads to a same evolution behavior of the atomic volume. Graphical abstract Unlabelled Image [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]

The precipitation and growth morphology of topologically close-packed (TCP) phases in a Re-containing Ni-based single crystal superalloy during thermal exposure were studied. The alloy was found to develop acicular-shaped coherent μ phase particles along <110> γ directions of the matrix after exposure at 950 °C. P phase precipitates were found to form with increased time and at higher temperatures, and in different morphologies, including acicular shaped particles along <112> γ and <110> γ directions, and plate-like particles in {110} γ planes of the matrix. The different P phase precipitates were found to have different lattice constants and different lattice correspondences with the matrix. The μ phase precipitate was also found to have different lattice correspondences with those reported in the literature. The multiplicity of lattice correspondences for the same TCP phase is attributed to the different lattice parameters, which render the precipitates to have different coherency relationships with the matrix. [ABSTRACT FROM AUTHOR]

Amorphous Al 2 O 3 coating was prepared on Ni-based single crystal superalloy via atomic layer deposition (ALD) for high temperature oxidation protection. The oxygen inward transport and outward-cation diffusion were hindered by amorphous Al 2 O 3 coating, and the coated alloy showed high oxidation resistance. The low oxygen partial pressure at the alloy/coating interface altered the alloy oxidation behavior, allowing the priority oxidation of protective Al and Cr elements. The oxidation process was controlled by the growth of Cr 2 O 3 and Al 2 O 3. The coating exhibited high spallation resistance. Degradation of the coating, the formation of NiAl 2 O 4 phase in amorphous Al 2 O 3 layer, was observed. • Amorphous Al 2 O 3 coating was proposed and prepared on Ni-based single crystal superalloy. • The amorphous Al 2 O 3 coating improved oxidation resistance and exhibited high adhesion. • The amorphous Al 2 O 3 coating altered the oxidation behavior of the alloy. • The effect mechanism of amorphous Al 2 O 3 coating on the alloy oxidation was discussed. [ABSTRACT FROM AUTHOR]

The oxidation behavior of a fourth-generation nickel-based single crystal superalloy with different heat treatments were studied at 1100 °C in air. The specimens were divided into three groups, one as-cast group, and two heat-treated groups: solution treated and fully heat treated. The elemental distribution is mainly affected by the solution temperature, the higher the solution temperature, the more uniform the distribution of elements. Heat treatment can effectively reduce the difference in oxidation behavior between dendritic region and interdendritic region. However, due to the inability to form a continuous Al 2 O 3 layer, the oxidation resistance of heat-treated specimens degrades. • The elements distribution will affect the oxidation behavior of superalloys. • The Al content of the region will affect the morphology of the internal Al 2 O 3 layer. • Heat treatment can reduce the oxidation difference between dendritic region and interdendritic region. • After heat treatment, the alloy is more prone to large-scale oxide spalling. [ABSTRACT FROM AUTHOR]

For further composition optimization, the microstructure evolution of the fourth-generation nickel-based single crystal superalloys, which contains different Co (Cobalt) additions, is studied at three thermal exposure temperatures. Based on Lifshitz, Slyozov and Wagnerz (LSW) theory and trans-interface diffusion-controlled (TIDC) theory, the effects of Co content and aging temperatures on coarsening rate of γ′ phase are investigated in detail. The results show that the diffusion coefficient of solute is accelerated, but the γ/γ′ interfacial energy decreases with the addition of Co, thus changing the coarsening rate. Furthermore, the thermodynamic equilibrium solubility difference of elements between γ′ and γ, and the segregation of elements at the γ/γ′ interface are alleviated with increase of Co, resulting in the suppression of TCP phase after long-time thermal exposure, which guides the economical design of nickel-based single crystal superalloys. • Co additions have a dual effects on microstructure evolution of the superalloys. • The effect of Co on coarsening rate of γ′ depends on temperature. • The accuracy of two coarsening prediction models is tested. [ABSTRACT FROM AUTHOR]

[Display omitted] • The promoting effect of pre-precipitated δ phase on deformation twinning and recrystallization behavior of Inconel 718 superalloy during hot compression was studied systematically. • The promoting effect on the formation of deformation twins was attributed to the lower stacking fault energy (SFE) and the local stress concentration. • A novel heteroepitaxial recrystallization (HERX) mechanism was found based on inverse precipitation. The delta (δ) processing strategy is an efficient heat treatment to refine and homogenize the grain microstructure of Inconel 718 superalloy (IN718). However, the microstructure becomes complicated by the effect of pre-precipitated δ phases, due to the coupling of precipitations, high temperature, and loading stress. The present study aims to clarify the effect of δ phases on the microstructure evolution of IN718, especially the promoting effect on deformation twins (DTs) and dynamic recrystallization (DRX) behavior. The results show that the promoting effect of δ phases on the formation of DTs was attributed to the lower stacking fault energy (SFE) caused by the precipitation of δ phases and the local stress concentration caused by the distribution of δ phases. The area fraction of DRX grains in the compressed δ-containing specimen was 21% higher than that in the compressed δ-free specimen, which was associated with the discontinuous dynamic recrystallization (DDRX) nucleation stimulated by the single δ phase and the continuous dynamic recrystallization (CDRX) nucleation promoted by the distribution of δ phases. Furthermore, a novel heteroepitaxial recrystallization (HERX) mechanism was found based on inverse precipitation from the undissolved δ phase to γ matrix phase grains with a crystallographic orientation relationship of (0_10)[1 0 2] δ //(__111)[0 1 1] HERX. [ABSTRACT FROM AUTHOR]

[Display omitted] • Structure of μ phase in Ni-Cr-Mo ternary model superalloy was investigated by atomic scale HAADF-STEM. • The priority order of Mo element in μ phase structure is 6c3 > 6c2 > 6c1. • The priority order of Ni element in μ phase structure is 18 h > 3a. • When the Mo element is insufficient, the smaller Cr atoms will occupy the same sites as Mo atoms. Topologically close-packed (TCP) phases seriously endanger the properties of Superalloy. Investigating the microstructure of the TCP phases will provide a crystallographic reference for the design of superalloys with better properties. The μ phase is a common TCP phase in superalloys. It belongs to the space group R-3m, and has a hexagonal structure with five Wyckoff sites of 3a, 18h, 6c1, 6c2 and 6c3. In this paper, occupation of the three kinds of atoms at five Wyckoff sites of the μ phase in Ni-Cr-Mo model superalloy was determined using aberration-corrected High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM), QSTEM software simulation and first-principles calculations. It was found that Mo atoms with larger size and smaller electronegativity tend to occupy 6c3, 6c1 and 6c2 sites in order of priority. Ni atoms mainly occupy the sites with coordination number (CN) of 12 at 3a and 18 h, and 3a sites contain more Cr atoms than 18 h sites. [ABSTRACT FROM AUTHOR]

In this study, we investigate the movement of dislocations in Ni-based superalloys under the influence of lattice misfit stresses between the ordered γ'-cuboids and the disordered γ-matrix during thermal exposure in the absence of an applied load. This study focuses on a different condition than the conventional creep testing, and thus offers a unique opportunity to study the intrinsic behavior of the alloy. The dislocation density increases substantially with time during thermal exposure, leading to the formation of various configurations of dislocation networks on the {100} γ/γ′ interfaces, including <110> diamond-shaped, <110>/<100> mixed polygon-shaped and <100> square-shaped networks. During thermal exposure, the b = 1 2 < 110 > native dislocations first move and evolve into 60° mixed dislocations along the <110> directions on the {100} γ/γ′ interfaces, forming the <110> diamond-shaped dislocation networks. In the case of longer thermal exposures, the dislocations further evolve into pure edge dislocations along the <100> on the {100} γ/γ′ interface, leading to the evolution of the <110> diamond-shaped dislocation networks into <100> square-shaped networks, with the mixed <110>/<100> dislocation networks as an intermediate stage during the transition. These movements occur by sweep glide in the {111} planes and diagonal climb on the {100} planes for the edge and mixed dislocations and by cross-slip for the screw dislocations. The driving force for all of these movements is the interaction between the normal misfit stresses and the edge components of the Burgers vectors of the dislocations to relax the misfit stresses. An analysis based on the elastic strain energy considerations is presented to explain the driving forces for the dislocation movements. [ABSTRACT FROM AUTHOR]

Two distinct evolutionary processes of secondary γ′ precipitates in a Ni-based single crystal superalloy were revealed by means of Cs-corrected high-resolution transmission electron microscopy (HRTEM) and corresponding X-ray energy dispersive spectroscopy analyses. Upon creep testing at elevated temperatures, it was determined that large quantities of secondary γ′ precipitates were formed. The precipitates were found to select two distinctive forms and routes: one randomly dispersed in the γ matrix (2nd γ′ γ ), and the other formed at nodes of dislocation networks (2nd γ′ d ) in regular arrays. These two types of secondary γ′ precipitates exhibited different evolutionary morphologies and mechanisms. The 2nd γ′ γ precipitates progressively evolved in shape from spherical to cuboidal and then to butterfly-like shapes, and the mean size also increased gradually from 11 nm to 34 nm. The 2nd γ′ d precipitates exhibited stable cuboidal morphology and a mean size of ∼30 nm. These differences are attributable to the different element transport conditions for the two types of precipitates, i.e., an uphill diffusion for the 2nd γ′ γ precipitates and a dislocation network pipe transportation for the 2nd γ′ d precipitates. The 2nd γ′ γ precipitates show severe segregation of γ′-rich elements of Ni and Al and deficiency of γ′-poor elements of Cr compared with the 2nd γ′ d precipitates. [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]

[Display omitted] • Co alleviates large-scale heterogeneity of multiple microstructures of the alloys. • Co significantly reduces the partitioning coefficient of γ forming elements, which alleviates small-scale heterogeneity. • Co additions suppress TCP precipitation and increase microstructure stability. The effects of Co on multi-scale elemental distributions were studied in an advanced Ru-containing Ni-based single crystal superalloys. Two types scales of microstructural heterogeneities, the large-scale dendritic segregations and small-scale elemental partitions between γ/γ' phase were compared in as-cast, heat-treated and thermal-exposed alloys. The elemental homogeneities on both large and small scale were shown to be enhanced with Co additions, giving rise to the restricted precipitation of TCP phase. It is thus expected the potential role of Co to replace part of Ru in advanced superalloys. [ABSTRACT FROM AUTHOR]

This work studied the relationship between the fatigue behavior and microstructural evolution of a second-generation nickel-based single-crystal superalloy with [001], [102], and [314] orientations at room temperature. Strain-controlled low-cycle fatigue experiments were executed using an in-situ Scanning electron microscope (SEM) fatigue test system. Digital image correlation (DIC) mapping combined with SEM images were used to identify strain concentration and predict areas of crack initiation. Results showed that the deformation of specimens with different orientations was dominated by planar slip and followed Schmid law. Cross-slips were observed in the specimen with the [001] oriented orientation, while a single slip was observed for the other two orientations. Fatigue crack propagation was dominated by one slip system in the [001] and [314] oriented specimens. While in the [102] oriented specimen, it was alternately dominated by two slip systems. The fatigue life of Nickel-based single-crystal superalloys (NBSCs) had a high orientation dependence, and was related to the elastic modulus and crack propagation path. The cyclic softening that occurred during fatigue was caused by dislocation motion. • The fatigue behavior of three oriented nickel-base single-crystal superalloys was investigated by in-situ SEM technology. • The strain distribution, slip system, and the location of crack initiation were determined combining with DIC technique. • Effects of orientation and dislocation movements on fatigue behavior were discussed. [ABSTRACT FROM AUTHOR]

The plastic deformation behavior of three various orientation specimens of a nickel-based single-crystal (SX) superalloy were investigated by in-situ electron back-scattered diffraction (EBSD) at room temperature. Tensile properties, slip morphology, the evolution of Schmid factor, lattice rotation, and local strain were thoroughly discussed. In [1 1 13] specimen, the interaction of multi slip behavior induced local strain concentration, which restricted the plasticity but enhanced the strain hardening. Due to the single slip behavior, lower tensile strength and higher elongation were discovered in [9 1 7] and [2 1 3] specimens. Furthermore, during the plastic deformation of [1 1 13] specimen, lattice rotation is confined around [0 0 1] pole and shows a dispersive distribution, which is triggered by the multi slip behavior. A modified Sachs model considering multi slip behavior has been developed to predict the lattice rotation, this model is also applicable to severely deformed grains in polycrystalline materials. [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]

Creep properties of a 4th generation nickel-based single crystal superalloy (Ni-SX) are investigated at 1150 °C and 1100 °C. Fracture mechanism is revealed as competitive microcrack initiation induced by topologically close-packed (TCP) phase formation and creep inconsistency on dendritic structures showing the strong temperature-and-time dependent relations. At 1150 °C, alloy displays the high nucleation rate of TCP phases. Dense TCP particles dissolve the surrounding γ/γ' microstructures forming large depleted zone where is easily stress-concentrated. Dislocations are found to emit from the interface between TCP and depleted zone that promotes dislocation shearing and microcrack initiation in the large depleted zone, as well as the fracture of alloy. On contrary, alloy crept at 1100 °C suffers to relatively restricted TCP nucleation. But the longer creep time gives to larger size of TCP precipitates. Compared to alloy crept at 1150 °C, the depleted zone is much smaller that is not prone for stress concentration. For this creep inconsistency induced by dendritic segregation plays the main role in determining the creep properties. The larger local strain rate forms microcracks primarily in interdendritic region causing the final fracture. • Creep behaviors of a new 4th generation Ni-SX are compared at 1150 °C and 1100 °C. • Temperature-and-time dependent TCP precipitation and creep inconsistency affect the creep. • Depending on temperature, the two factors display competitive behaviors. • Higher temperature of 1150 °C enlarges the influence of TCP precipitation. • Creep inconsistency on dendritic structures dominants the creep at 1100 °C. [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]

In their comment (Scripta Mater. (2016)), Cormier et al. raised questions on the findings reported in our earlier publication (Acta Mater. (2016) of the formation of 2nd γ′ d and 2nd γ′ γ precipitates in a Ni-based single crystal superalloy during creep testing at elevated temperatures, and suggested that the 2nd γ′ d particles were mere observations of surface protuberances of the primary γ′ rafts and that the 2nd γ′ γ precipitates were possibly formed during cooling after the creep test. To clarify these, we conducted new experiments, and present the new evidence and our responses to the points raised in this short communication. [ABSTRACT FROM AUTHOR]

The chemical and structural evolution within dislocation networks are investigated in a nickel-based single crystal superalloy crept under 1373K and 137MPa. The local region within dislocation networks can be divided to two separated parts: gridlines which compose the arrangements of dislocation networks and mesh regions as intervals between gridlines. With evolution of dislocation networks, precipitation of nano-sized γ′ phase (γ′ n) are observed in mesh region, while gridlines always display the FCC feature of γ phase. The formative mechanism of γ′ n is revealed as increased segregations of Cr, Co and Re along gridlines which enhance the contents of γ′-rich elements such as Al in mesh region to promote the local phase transition from FCC to highly ordered L1 2. However, growth of γ′ n requires massive diffusion of γ′-rich elements that is confined by the chemical barrier along gridlines with strong segregations of Cr, Co and Re. The mesh region of γ′ n and gridlines of γ reinforce each other forming potential γ/γ′ substructures to stabilize the dislocation networks during creep. Denser dislocation networks formed in dendrite core under larger local lattice misfit are accompanied with facilitated formation of γ/γ′ substructures which derive higher creep resistance than interdendritic region. Image, graphical abstract [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]

It is known that Ni-based superalloys experience microstructural rafting during high temperature straining. This paper presents a comparative study of the different rafting models by means of finite element modeling and experimentation investigation. It was found that uniaxial elastic loading does not alter the misfit or misfit isotropy of the structure, but causes repartitioning of the misfit into elastic strains in the γ and γ' phases. This causes an elastic strain energy anisotropy and hydrostatic stress anisotropy among the vertical and horizontal γ phase channels. Both anisotropies predict a raft structure that is contrary to the experimental observation. Through a carefully designed pre-treatment at 750 °C and under a uniaxial stress of 750 MPa, an anisotropic dislocation structure was created on the {001} γ'/γ interfaces without altering the cuboidal morphology of the γ' phase. This allows the evaluation of the influence of dislocations on the rafting behavior of the alloy. It was found that rafting occurred in the pre-treated samples containing anisotropic dislocation structures during thermal exposure without applied stress, instead of isotropic corsening, and that the raft structures conform to the expectations base don the anistropic dislocation strucutres. This demonstrates that an anisotropic dislocation structure on the {001} γ'/γ interfaces is a direct cause of rafting. This is attributted to the fact that dislocations on γ'/γ interfaces help to relax the local misfit strains, causing some to expand at the expense of others. At the same time, the formation of the anisotropic dislocation structure is a direct result of the anisotropy of elastic strains induced by the applied bia stess in during the pre-treatment. These explain the complex interplay of the elastic and plastic models reported the literature. Unlabelled Image • The elastic stretching alters the partitioning of elastic strains in two phases, whereas the misfit remains unchanged. • The elastic strain energy and hydrostatic stress anisotropies are ruled out to be responsible for rafting. • The anisotropic dislocation on γ'/γ interfaces wtith γ' phase cuboid morphology unchanged were created by pre-straining. • The dislocation anisotropy is effective in causing rafting in Ni-based superalloys by causing misfit anisotropy. [ABSTRACT FROM AUTHOR]

L1 2 structured γ′ phase is the main strengthening phase of Ni-base superalloys, which contain multiple alloying elements. Locations of these elements in the lattice are crucial for understanding the mechanical/physical properties of γ′ phases. Here, we reveal the site occupancy of various elements in γ′ phases of two multi-component Ni-base single crystal superalloys by using advanced scanning transmission electron microscopy and energy disperse spectroscopy. Cr, Mo, Re, Ta, W and Ti atoms preferentially locate at Al sublattices, and Co and Ru atoms prefer Ni sublattices. Moreover, the addition of Ru does not affect the site occupancy of other alloying elements. Image 1 • EDS mapping directly shows atomic distribution of all elements in the γ′ phases. • Cr, Mo, Re, Ta, W and Ti atoms preferentially occupy Al sublattices. • Co and Ru atoms prefer Ni sublattices. • Ru has limited effect on site occupancy of other alloying elements in γ′ phase. [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]

Abstract This study investigated a novel hypothesis on the influences of residual or pre-dislocation structures on creep life of Ni-based single crystal superalloys. Different types of anisotropic dislocation structures were created by pre-straining the samples at a relatively low temperature under different constant bias load conditions. After such pre-straining treatment, the samples were subjected to creep testing at higher temperatures under uniaxial tension. It was found that the pre-straining treatment was effective in creating different anisotropic dislocation structures without affecting the γ′ phase cuboid morphology and that the samples containing different anisotropic dislocation structures have all significantly reduced creep lives. The difference in the anisotropy of the dislocation structures has little influence on the reduction of the creep life or the rafting morphology of the γ′ phase cuboids. By using a Re-free alloy, the influence of the TCP phase precipitation on creep life is also clarified. These observations suggest that interruption to high temperature straining is the main contributor to the reduced creep lives. This finding raises a serious concern on the validity, or reliability, of creep life assessment based on continuous laboratory creep tests for real life applications, where the services are almost always interrupted into intermittent sessions. Graphical abstract Unlabelled Image Highlights • Different anisotropic dislocation network structures were created by pre-straining at an elevated temperature. • The pre-existing anisotropic dislocation structures have no influence on the rafting behavior of the γ′ phase cuboids. • The pre-existing dislocations promote the formation of TCP phases during creep testing. • All pre-strained samples showed significantly reduced creep life. [ABSTRACT FROM AUTHOR]