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The harsh service environment of aeroengine hot-end components requires superalloys possessing excellent antioxidant properties. This study investigated the effect of pre-strain on the oxidation behavior of polycrystalline Ni3Al-based superalloys. The growth behaviors of oxidation products were analyzed by scanning electron microscope, transmission electron microscope, X-ray Photoelectron Spectroscopy and Raman spectroscopy. The results indicated that the 5% pre-strained alloys exhibited lower mass gain, shallower oxidation depth and more compact oxide film structures compared to the original alloy. This is mainly attributed to the formation of rapid diffusion paths for Al atoms diffusing to the surface under 5% pre-strain, which promotes the faster formation of protective Al2O3 film while continuing to increase the pre-strain to 25% results in less protective transient oxidation behavior being aggravated due to the increase in dislocation density within the alloy, which prevents the timely formation of the protective Al2O3 film, resulting in uneven oxidation behavior on the alloy. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this study is to describe the mechanism of single-crystal high-temperature creep deformation, predict the creep life more accurately and study the creep constitutive and lifetime models with microstructure evolution. Design/methodology/approach: The mechanical properties of nickel-based single-crystal superalloy are closely related to the γ' phase. Creep tests under four different temperature and stress conditions were carried out. The relationship between creep temperature, stress and life is fitted by numerical method, and the creep activation energy is obtained. The creep fracture surface, morphology and evolution of strengthening phase (γ') and matrix phase (γ) during different creep periods were observed by scanning electron microscope. With the increase of creep temperature, the rafting time is advanced. The detailed morphology and evolution of dislocations were observed by transmission electron microscope (TEM). Findings: With the increase of creep temperature, the rafting time is advanced. The detailed morphology and evolution of dislocations were observed by TEM. Dislocations are mainly concentrated in the γ channel phase, especially at high temperature and low stress. Originality/value: A creep constitutive model based on the evolution of γ' phase size and γ channel width was proposed. Compared with the experimental results, the predicted creep life is within 1.4 times error dispersion band. [ABSTRACT FROM AUTHOR]

The present study investigates the creep behavior and deformation mechanism of a third-generation single-crystal Ni-based superalloy at 1170 °C under a range of stress levels. Scanning electron microscopes (SEM) and transmission electron microscopes (TEM) were employed to observe the formation of a rafted γ′ phase, which exhibits a topologically close-packed (TCP) structure. The orientation relationship and elemental composition of the TCP phase and matrix were analyzed to discern their impact on the creep properties of the alloy. The primary deformation mechanism of the examined alloy was identified as dislocation slipping within the γ matrix, accompanied by the climbing of dislocations over the rafted γ′ phase during the initial stage of creep. In the later stages of creep, super-dislocations with Burgers vectors of a<010> and a/2<110> were observed to shear into the γ′ phase, originating from interfacial dislocation networks. Up to the fracture, the sequential activation of dislocation shearing in the primary and secondary slipping systems of the γ′ phase occurs. As a consequence of this alternating dislocation shearing, a twist deformation of the rafted γ′ phase ensued, ultimately contributing to the fracture mechanism observed in the alloy during creep. [ABSTRACT FROM AUTHOR]

With the development and wide application of nickel-based single-crystal superalloys, the effect of Ru on the microstructure stability and high-temperature properties of superalloys is becoming increasingly important. In this study, the effect of Ru on the evolution of the γ′ phase in Ni-Al-Ru ternary alloys during aging treatment was analyzed, using a scanning electron microscope and transmission electron microscope, combined with energy-dispersive spectroscopy. The relationship between chemical partition behavior and γ/γ′ lattice misfit was investigated in detail. During the aging process, Ru addition suppressed the growth rate and rafting process of γ′ precipitates, while the effect of Ru on hindering γ′ phase growth was reduced when the Ru content was over 3 at%. Ru preferentially partitioned to the γ phase, and its partitioning ratio to the γ phase increased with a variation in Ru content from 1 at% to 3 at% and decreased for the NiAl6Ru alloy. Additionally, the lattice misfit of all alloys was positive and reduced with the increase in Ru content, which hindered the Ru atoms to diffuse into the γ phase and promoted the shape of γ′ precipitates to change from cubic to spherical. [ABSTRACT FROM AUTHOR]

The variation of the tensile ductility of a directionally solidified nickel-based superalloy M4706 with temperature is investigated in the temperature range from room temperature to 980 °C. It is found that intermediate temperature brittleness occurs in M4706 at temperatures between 400 and 650 °C. Tensile tests on single crystal M4706 and in-situ nano-compression tests on the freestanding γ′ particles show that the intermediate temperature brittleness of the experimental alloy is not associated with the grain boundary and the deformation capacity of γ′ precipitates in the temperature range. Scanning electron microscope and transmission electron microscope observations disclose that the phenomenon is associated with the glide plane decohesion, which in turn corresponds to the frequent stacking fault shearing during tensile deformation. [ABSTRACT FROM AUTHOR]

The effect of pre-deformation induced by the tensile fracture at room temperature on the microstructure evolution of Ni Al model single crystal superalloy during thermal exposure at 1000 °C, was investigated by scanning electron microscope and transmission electron microscopy. Recrystallization occurs near the fracture surface where the most severe plastic deformation was suffered. The average size of recrystallization grains in front of the recrystallization area is much larger than that of at the fracture surface. Far from the fracture surface, the average size of coarsened γ′ precipitates in the slip bands is larger than that of out of the slip bands. [ABSTRACT FROM AUTHOR]

Abstract: By means of the measurement of creep curves and SEM, TEM observation, an investigation has been made into the influences of the element Re and TCP phase on the creep properties of the single crystal nickel-based superalloys. Results show that, during aging of the containing/free Re superalloys, the slice-like TCP phase identified as μ phase is precipitated along the 〈110〉 orientation on {111} planes. As the aging time prolongs, the quantities of the slice-like μ phase precipitated in the alloys increase, no spheroidized feature of μ phase is detected in 6%W alloy, but the slice-like μ phase in 4.5%Re alloy is transformed into the sphere-like morphology. During aging, the evolution process of the strip-like μ phase consists of the inhomogeneous coarsening, the appearance of the groove and the decomposition of the one in the interval distance. The higher chemical potential in the strip-like μ phase with the groove acts as the driving force for promoting the diffusion of the solute atoms, which results in the strip-like μ phase dissolved and the groove deepened up to separation to form the sphere-like morphology. During creep, the stress concentration generates easily in the regions near the strip-like μ phase, and promotes the initiation and propagation of the cracks, which is the main reason of reducing the creep lifetime of 6%W alloy to a great extent, but the stress concentration does not easily generate in the regions near the sphere-like μ phase, this is the main reason of reducing the creep lifetime of 4.5%Re alloy to a small extent. [ABSTRACT FROM AUTHOR]

In this work, the Hastelloy N superalloy was joined using a newly-designed BNiCrWBSi brazing filler alloy. Effects of brazing processing parameters and post thermal exposure on microstructure and mechanical performance of the joints were evaluated. The microstructure in the joints was revealed with the aid of scanning electron microscope, electron back-scattered diffraction and transmission electron microscope analyses. The nano-indentation technique and shear test were employed to appraise the mechanical properties (such as nano-scale elastic modulus/hardness of specific phases and joint bond strength) of the joints. The results showed that the typical joint constitution could be depicted as Hastelloy N alloy/diffusion-affected zone (DAZ) or fine-grained zone (FGZ)/isothermal solidification zone (ISZ)/diffusion-affected zone (DAZ) or fine-grained zone (FGZ)/Hastelloy N alloy. The appearance of the FGZ was dependent on the brazing temperature, irrespective of the soaking time. The optimum joint bond strength of 588.9 MPa was produced at 1240 °C for 10 min. The typical fracture procedure and mechanism were discussed based on the detailed microstructural examinations and theoretical calculations. Apart from the brazing processing parameters, the post thermal exposure could also modify the microstructure through importing tiny boride phases into the ISZ, posing a reduction in shear strength. The microstructural evolution of Hastelloy N alloy joints unveiled and relationship built between the microstructure and mechanical properties will give a significant guidance for the design and application of the superalloy brazed joint. • A higher brazing temperature imported a liquid band between the substrate and ISZ. • The optimum joint bond strength reached 588.9 MPa produced at 1240 °C for 10 min. • The fracture arose primarily on the centerline of the ISZ during the shear test. • The borides and carbides showed diverse abilities in resisting the crack propagation. • Post thermal exposure imported borides in ISZ, causing a reduction of shear strength. [ABSTRACT FROM AUTHOR]

Scanning electron microscopes (SEM) and transmission electron microscopes (TEM) were employed to investigate the topologically close-packed (TCP) phases in an experimental nickel base single crystal superalloy. Orientation relationships between P phase and matrix were found as (010) P //(1 1 ‾ 1) γ and [201] P //[110] γ , in which atomic arrangement of P phase obtains the optimal matching with that of γ/γ' matrix and the semi-coherent interface forms, achieving the lower interface energy. The elemental composition, atomic arrangement of low index planes and the atomic attachment of P phase with matrix were analyzed in detail to investigate the atomic scale structure of the fast-growing semi-coherent interface and its effects on precipitate growth. Finally, the nucleation of P phase was discussed. • μ and P phase were characterized by electron microscopes. • HRTEM micrographs and orientation relationships imply the semi-coherent P/γ interface. • Atomic attachment of P phase with matrix was discussed. • The nucleation of P phase was discussed. [ABSTRACT FROM AUTHOR]

The work investigated the effect of high- and medium-temperature thermal exposure on the microstructure and mechanical performance of the graphite/Au/Hastelloy N alloy brazed joint. The microstructure in the joint was studied with the aid of scanning electron microscope coupled with transmission electron microscope analysis. The nano-indentation was used to probe the nano-scale elastic modulus and hardness of reaction phases in the joint. The joint bond strength was evaluated by a shear test. Results indicate that: three zones (zone 1, 2 and 3) were observed in the joint with the zone 1 close to graphite while the zone 3 adjacent to Hastelloy N alloy. The precipitates in each zone were characterized and their formation mechanism was proposed. High-temperature thermal exposure was performed at 1333 K for 1–90 min. A longer soaking time facilitated the carbon diffusion and then produced a hierarchical structure with a more smooth transition of coefficient of thermal expansion, being favorable for the enhancement of the joint strength. Among the soaking time under investigation, the highest joint bond strength obtained was 34.1 MPa. Medium-temperature thermal exposure was carried out at 873–1073 K for 240 h. A high-quality joint was maintained when the joint was aged below 973 K. Over 973 K, a valid bonding was not achieved at the graphite/filler interface due to the precipitation of extensive carbon segregation in the zone 1 of the joint. The work performed will provide basic design principle for the graphite/Hastelloy N alloy brazed combinations used in molten salt reactors. • The element Mo had a separating effect on the Au-Ni system. • The elastic modulus and hardness of the Mo 2 C were 200.3 GPa and 21.2 GPa. • A hierarchical structure with a smooth CTE transition was produced in the joint. • The highest joint shear strength obtained in this work was 34.1 MPa. • Lots of carbon segregation were produced in the brazing seam when aged over 973 K. [ABSTRACT FROM AUTHOR]