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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]

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]

Herein, a kind of the ultrastrong low alloy wear‐resistant steel suitable for excavator bucket teeth is prepared. The ultrastrong steel with good toughness and wear resistance is successfully prepared by the forging, quenching, and tempering processes. The yield strength, tensile strength, elongation, and hardness of the ultrastrong steel reached up to 1700, 1960 MPa, 7%, and 563HV, respectively. Optical microscope (OM), scanning electron microscope (SEM), and transmission electron microscope (TEM) are used to observe the microstructures of the ultrastrong steel. After the forging, quenching, and tempering, two types of martensite laths are formed in the matrix of the ultrastrong steel, i.e., the coarse laths with low‐density dislocations and the thin laths with high‐density dislocations and some ε‐carbides. Low‐temperature tempering allows the carbides to precipitate in large quantities, and maintains the matrix with high density dislocations in the thin laths, which is the micromechanism of the steel with high strength and high toughness, and good wear resistance. [ABSTRACT FROM AUTHOR]

In the study we investigated the primary structure and physical properties of the new generation of superalloys based on Co10Al5Mo2Nb type cobalt. The microstructural investigations were carried out with a scanning electron microscope on conventionally prepared electrolytically etched metallographic microsections. The studies of microstructure using a transmission electron microscope were performed using thin films. The testing of mechanical properties in the as-received condition included the performance of static tensile test at room temperature using a testing machine with a maximum load of 200 kN. These materials may replace nickel-based superalloys in the future due to their excellent properties at elevated temperatures relative to nickel-based superalloys. Currently, nickel-strengthened 0 phase steels are still unrivalled in aerospace applications. However, cobalt-based superalloys are a response to their existing limitations, which do not allow maintaining the current rate of development of aircraft engines. [ABSTRACT FROM AUTHOR]

Microstructural evolutions of EK61 superalloy during long-term aging until 1000 h at 700°C and 750°C, respectively, are studied by combination of Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM). Impact fracture morphologies after aging for different time are observed by the SEM. The microstructure is found to be relatively stable during aging at 700°C, and the fracture morphologies are characterized by transgranular fracture. At 750°C, the coarsening of γ ′ phase leads the reduction of the quantity of dimples, the chainization of carbides on grain boundaries leads to intergranular fracture, and the netting of η phases within grains leads to the formation of lamellar cleavage steps. It is obvious that the destabilization of precipitated phases affects fracture morphology significantly. The relationship between fracture morphology and the microstructure promotes the evaluation of service reliability of EK61 superalloy. [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 The creep behavior, creep damage and creep life of 718Plus alloy were investigated at temperatures ranging from 650 to 750 °C in air. The fracture surface, microstructure evolution and deformation mechanisms were studied by optical, scanning and transmission electron microscope (SEM and TEM). The results show that creep curves of 718Plus alloy consist of primary stage and tertiary stage without steady-state region. The Monkman-Grant relationship of 718Plus alloy can be described by t r = 9.32E-2/( ε ̇ min)0.75 and apparent creep activation energy of 718Plus alloy is about 572.7 kJ/mol. Further investigation shows creep damage of alloy at 650 °C can be mainly ascribed to the effect of grain boundary sliding (GBS) and microtwinning. The alloy shows transgranular fracture at high stress, while it presents ductile fracture at low stress due to the nucleation, growth and linkage of creep voids. At 750 °C, the alloy failed in the form of ductile fracture mainly due to the microstructure instability and dislocation glide. At higher stress of 700 °C, the effect of GBS is mainly responsible for the creep damage of 718Plus alloy, while GBS, dislocation glide and microstructure instability work together and lead to the creep damage of alloys at lower stress of 700 °C. The fracture mechanism map and fracture models for 718Plus alloy were constructed. Remaining life assessment of 718Plus alloys was also evaluated by Larson-Miller parameter method. [ABSTRACT FROM AUTHOR]

The effect of long-term isothermal aging of Haynes 230 super-alloy in an advanced ultra-supercritical (A-USC) power plant at 700 °C for up to 10,000 h on microstructural evolution and tensile behavior. And utilizing a combination of the scanning electron microscope (SEM), X-ray diffraction analysis (XRD), and transmission electron microscope (TEM). Although the grain size and grain boundary M23C6 carbides did not change significantly, the dissolution of intercrystallite primary M 6 C carbides and the precipitation of intracrystalline secondary lamellar M 6 C carbides were observed at 2000 h to 10,000 h. According to the SEM and TEM results, it is shown that the dissolution of primary M 6 C carbides makes the W, Mo, and Cr in the γ matrix increase, while the precipitation process of secondary M 6 C carbides is M+ 6 C→M 6 C. To reduce the interfacial energy of the γ matrix, secondary M 6 C carbides are precipitated in the crystal under long-term aging at 700 °C, with a gradual increase in time content. the best tensile and yield strengths at 750 °C occur after 4000 h of aging, with microscopic voids forming mainly around the granular M 6 C carbides in the crystal, but the solid solution strengthening effect of the matrix provides the strength. The worst strength and elongation occur after 10000 h aging, when the dissolution of granular M 6 C and secondary precipitation of M 6 C carbides are most abundant, interfacial cavity formation of granular M 6 C carbides and brittle fracture of M 6 C carbide particles, and the growth of non-co-grained secondary M 6 C carbides with the matrix allow more microscopic cavities to appear during high-temperature tension, reducing the strength of Haynes 230 and plasticity. • Phase precipitation behavior of Haynes 230 was investigated during long-term aging at 700 ℃. • Precipitation processes of secondary lamellar-like M 6 C carbide were clarified. • Dissolution of primary granular M 6 C carbides was discussed. • Variation in mechanical properties is divided into several stages. • The relationship between precipitation processes and mechanical properties was established. [ABSTRACT FROM AUTHOR]

GH4169 Ni-based superalloy was prepared by wire arc additive manufacturing (WAAM) in this study. Similar to the microstructure of other additive manufactured superalloys, the additively manufactured GH4169 alloy suffered from a high-volume fraction of Laves phase, which is detrimental to the alloy's mechanical properties. To solve this problem, two different post heat-treatment approaches were selected to improve the tensile properties at both room and elevated temperatures, which are the one with solution treatment temperature at 950 °C and the modified one with solution treatment at 1150 °C, respectively. Detailed phase analysis during different heat treatment processes was conducted using both scanning and transmission electron microscopes. Comparative tensile tests were also conducted at both room temperature and 650 °C to analyze the role of the different microstructures on the deformation responses of the prepared samples. By increasing the solution treatment temperature from 950° to 1150°C, the volume fraction of Nb-rich Laves phase dramatically decreased, and the previously Laves-phase-trapped Nb contributed to the formation of the strengthening γ″ phase. As a result, the tensile strength of this modified treated alloy is significantly improved at both room and elevated temperatures. Additionally, due to lower volume fraction of the Laves phase, the elongation of the modified treated alloy at room temperature is improved by ∼35%. These findings provided new insights into the post-heat treatment approaches of the WAAM Ni-based superalloy. • Wire arc additive manufactured GH4169 Ni-based superalloy and post-heat treatment. • Post-treatment induced microstructure evolution from μm to nm by electron microscopes. • Discussion of the treatment-microstructure-mechanical property relationship of GH4169. • Industrial heat treatment cannot avoid the effects from the detrimental Laves phase. • Modified heat treatment improves both room and high temperature tensile properties. [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]

Abstract: Composite coatings, made of nano-Al2O3/cobalt-based alloy, produced by a 5-kW CO2 laser on Ni-based superalloy were investigated. The coatings were examined to reveal their microstructure using optical microscope, scanning electron microscope, transmission electron microscope and X-ray diffraction instrument. The results showed that some equilibrium or non-equilibrium phases, such as γ-Co, Cr23C6, CoAl2O4, Al2O3 and ε-Co existed in the coatings. Fine and short dendritic microstructure and columnar to equiaxed transition occurred by adding nano-Al2O3 particle. With the increase of nano-materials (1%, mass fraction), fully equiaxed crystallization appeared. These were contributed to that nano-Al2O3 particles acted as new nucleation site and rapid solidification of the melted pool. The results also showed inhomogeneous dispersion of nano-Al2O3 that resulted in the formation of ε-Co phase in the coatings. The sub-microstructure of the clad was stacking fault. The mechanism of formation of equiaxed grains was also analyzed. [Copyright &y& Elsevier]

The oxidation behavior of the nickel superalloy Inconel 740H was studied at 750 °C for 100, 250, 500, 1000, and 2000 h in a steam atmosphere. Microstructure observations were performed using scanning electron microscopes and scanning-transmission electron microscope. The phase identification of existing oxidation products was conducted by electron diffraction in transmission electron microscope. The obtained results showed that the microstructure of Inconel 740H was stable during the oxidation process. The kinetic data showed that the superalloy has the ability to form protective oxide layers that are characterized by good adhesion and no tendency to spallation during the test. The oxidation products were mainly composed of external and internal oxides mainly at grain boundaries. The oxides in the external layer were Cr2O3, MnTiO3,, and α-Al2O3 after 2000 h of oxidation. Internal oxides were α-Al2O3 and TiO2. The occurrence of discontinuities in the internal oxidation zone was also observed after 500 h of test. It was found that the thickness of the internal oxidation zone was greater than the thickness of the external oxide layer, which proves the strong tendency of the superalloy to form internal oxides after oxidation in the steam atmosphere. [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]

Medium alloyed as-cast nickel base superalloy was subjected to two different cooling rates (fast and slow) after homogenization which resulted in mono-modal and bi-modal distribution of gamma prime (γ′) precipitates. Both the materials were subjected to high temperature deformation over a range of temperatures and strain rates. Irrespective of the strain rate used, the material subjected to fast cooling exhibited high compressive yield strength and flow softening characteristics when compared to the slow cooled material in the sub-solvus regime. Detailed microstructural investigation carried out using scanning and transmission electron microscope to ascertain the micro-mechanism revealed varying precipitate – dislocation interaction mechanism in both the materials. Higher flow softening and cracking in the fast cooled material is attributed to glide plane softening due to planar slip and the presence of fine network of carbides along the grain boundaries respectively. In the super-solvus regime, the flow stress of both the materials was comparable and both the materials exhibited discontinuous dynamic recrystallization (DDRX). Further, the slow cooled material exhibited lower critical strain for DDRX initiation and also better recrystallization kinetics irrespective of the processing conditions. Higher recrystallization kinetics in the slow cooled material is attributed to the concurrent particle simulated nucleation (PSN) at the coarse MC type carbide boundaries. • Effect of cooling rate on microstructure evolution and its effect on high temperature deformation behaviour evaluated. • Varying dislocation – precipitate interaction mechanisms captured and reported. • Softening & cracking in the fast cooled material is attributed to planar slip, glide plane softening & grain boundary carbides. • Homogenous deformation in slow cooled material is attributed to wavy slip. • Higher DRX kinetics in slow cooled material is ascribed to concurrent particle stimulated nucleation. [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]

Precipitate evolutions of 2.25Cr-1Mo heat-resistant steel during creep, under stress ranges of 30–300 MPa at 500–650 °C, were investigated by using X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM) equipped with energy dispersive spectroscopy (EDS). As-tempered microstructure was composed of ferrite and pearlite. Major precipitates in the pearlite region were identified as M 3 C, M 7 C 3 and M 23 C 6 (M = Fe, Cr, Mo) carbides. It was found that the precipitation sequence was changed according to the creep exposure temperatures and times. The composition of the carbide was shifted to the Cr-rich side as the creep exposure time at 550 °C or lower increases without changing the ferrite and pearlite structures. On the other hand, M 2 C was formed in an early stage of creep at 600 °C. The amount of M 2 C was decreased and M 6 C was newly precipitated after a long time creep exposure at 600 °C or higher. The amount of M 3 C decreased with increase in exposure time and fully disappeared even after a very short time creep exposure at temperatures higher than 600 °C. The overall precipitation sequence in the 2.25Cr-1Mo steel during creep was found to be M 3 C, M 7 C 3 , M 23 C 6 → M 3 C, M 7 C 3 , M 23 C 6 , M 2 C → M 7 C 3 , M 23 C 6 , M 6 C. It was considered that the slope change in Larson-Miller Parameter (LMP) plot was attributed to the formation of Mo-rich M 2 C and dissolution of M 7 C 3 precipitates during creep in the 2.25Cr-1Mo steel. • Precipitation behaviors during creep in 2.25Cr-1Mo steel was investigated. • Precipitation sequences were M 3 C, M 7 C 3 , M 23 C 6 → M 3 C, M 7 C 3 , M 23 C 6 , M 2 C → M 7 C 3 , M 23 C 6 , M 6 C. • Slope change in creep strength with rupture time was related to formation of M 2 C. [ABSTRACT FROM AUTHOR]

The effects of solute element content and cooling rate on the morphology of Al6Mn phase in suction casting Al–Mn alloys were investigated by transmission electron microscope, scanning electron microscope, and X-ray diffractometer. Results show that Al6Mn dendrite morphology with different degrees of development can occur in the microstructure of as-cast Al–Mn alloys. For the Al–4 wt.% Mn alloy, there are small amounts of block Al6Mn crystals at the center of sample, while we see a block Al6Mn phase and a feathery Al6Mn phase in the sample of Al–6 wt.% Mn alloy. Moreover, the block Al6Mn phases in the Al–8 wt.% Mn alloy disappear, and only snowflake-like Al6Mn phases play a dominant role in the microstructure. However, with an increase in Mn content to 10 wt.%, more dendritic trunks are formed, and secondary dendrite arms are degraded more seriously due to the formation of an icosahedral quasicrystal in suction casting. In addition to the effect of Mn content on Al6Mn morphology, with the increase in cooling rate from the center to the edge of samples, the outline diameter of equiaxed dendrite decreases. The evolution of Al6Mn dendrite morphology and the formation of quasicrystal are further 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]