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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: This paper aims to identify the optimal forming angle for the selective laser melting (SLM) process and evaluate the mechanical properties of the SLM-formed GH3536 alloy in the aero-engine field. Design/methodology/approach: Forming the samples with optimized parameters and analyzing the microstructure and properties of the block samples in different forming angles with scanning electron microscope, XRD, etc. so as to analyze and reveal the laws and mechanism of the block samples in different forming angles by SLM. Findings: There are few cracks on the construction surface of SLM formed samples, and the microstructure shows columnar subgrains and cellular subgrains. The segregation of metal elements was not observed in the microstructure. The pattern shows strong texture strength on the (111) crystal plane. In the sample, the tensile strength of 60° sample is the highest, the plasticity of 90° forming sample is the best, the comprehensive property of 45° sample is the best and the fracture mode is plastic fracture. The comprehensive performance of the part is the best under the forming angle of 45°. To ensure the part size, performance and support structure processing, additional dimensions are added to the part structure. Originality/value: In this paper, how to make samples with different forming angles is described. Combined with the standard of forged GH3536 alloy, the microstructure and properties of the samples are analyzed, and the optimal forming angle is obtained. [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]

In the present study, the grinding experiment of second-generation nickel-based single-crystal superalloy DD5 was carried out under different grinding parameters. The grinding force was recorded during the grinding process, and it was found that it decreased with increasing grinding speed and increased with feed speed. The microstructure evolution of ground subsurface was obtained by optical microscope (OM) and scanning electron microscope (SEM), and the elemental distribution of γ/γ' phases was investigated by energy dispersion spectrum (EDS). The results show that there are two layers different from the bulk material beneath the ground surface: (i) a white layer (WL) with no obvious structural features under limited observation scale and (ii) a severe deformed layer (SDL) with the elongated and rotated γ' phase and the narrowed γ channel. Elements segregation behavior exists in both the white layer and severe deformed layer. The grinding parameters have a great influence on the thickness of the white layer, which is due to the elemental diffusion behavior caused by intensive thermo-mechanical load. There is work hardening in the white layer, and the hardening degree aggravates with the increase in cutting speed and feed speed. [ABSTRACT FROM AUTHOR]

In this study, the effect of composite shear flow casting (CSFC) on microstructure and hot deformation process of Ni-based superalloy Udimet720Li has been investigated. Udimet720Li superalloy was produced by using CSFC and conventional casting methods, followed with the same isothermal forging process. The microstructures of both CSFC and conventionally solidified (CS) samples were characterized using optical microscope, scanning electron microscope, energy dispersive X-ray spectroscopy and electron backscatter diffraction. Results show that the CSFC sample owns excellent microstructural uniformity with a reduced area fraction of segregation regions, leading to a wider processing window for subsequent deformation, compared to the CS sample. Besides, the deformation resistance and the thermal workability of CSFC sample are significantly improved because of the homogeneous microstructure, resulting in the uniform deformation during subsequent isothermal forging. • Ni-based superalloy Udimet720Li was produced by using composite shear flow casting. • The microstructure evolution of Udimet720Li was investigated. • The fracture mechanism of forged microstructure was analyzed. • The CSFC-processed Udimet720Li has homogeneous microstructure and superior deformation workability. [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]

Nickel-based superalloys are used for turbine component parts in the power industry, aircraft engines, and in the marine sector. One such alloy that is available for these purposes is Inconel 738LC (IN738LC) nickel-based superalloy. Therefore, it is pertinent to have good structural and thermal stabilities for high-temperature applications, since these characteristics are important for its performance. In this study, spark plasma sintering technology was used to fabricate IN738LC nickel-based superalloy by using sintering temperature in the range of between 900 and 1200 °C. The fabricated products were characterized by using the scanning electron microscope and X-ray diffraction. There was the formation of the γ′(Ni3(Ti, Aℓ)) intermetallic phase in the microstructure, which contributed to the material (Inconel 738LC) property. The solid solution elements present are chromium, cobalt, tungsten, and tantalum; these equally contribute to the structural stability of IN738LC. The density, hardness, and the predicted yield strength increased with increasing sintering temperature, while the porosity decreased with increasing sintering temperature. The thermal conductivity, which is an important thermophysical property of IN738LC, enables the evaluation of the usefulness and service life of IN738LC superalloy at high-temperature applications, was also examined. The thermal conductivity was measured by using a laser flash method in the temperature range of 100–600 °C. For the sintered alloy, the thermal conductivity was determined as a function of the thermal diffusivity, specific heat capacity, and density. Analysis showed inflection pattern with increase in temperature on the sample fabricated at 900, 1000, and 1100 °C, ; a monotonic increase with increase in temperature was observed on the sample fabricated at 1200°C, ; the inflections are associated with the γ′ precipitates dissolution and redistribution. [ABSTRACT FROM AUTHOR]

The significance of grain refinement in improving the resistance to high temperature oxidation and corrosion was studied. An as-cast nickelbased superalloy used for manufacturing blisks and vanes in advanced gas turbines was used in the present work. A Yb-doped fibre laser having 1080 µm wavelength operated in continuous wave mode was used to laser treat the surface of the test material to modify the microstructure across the treated depth. The laser beam scan parameters were optimized through pilot experiments and the surface is treated in parallel tracks having 30% overlap. The cross-section of the treated samples was studied to understand the volume fraction of grain refinement. A finely refined microstructure was observed across the treated depth and their morphology varies with the different combination of laser power and scan speed. The combination of higher laser power with reduced interaction time imparts finely refined equiaxed grain structure near the top surface and columnar grains having reduced primary arm spacing near the bottom of the treated depth. This imparts higher surface hardness near the surface which reduces linearly towards the treated depth. The effect of laser surface treatment in improving the oxidation and corrosion resistance was analysed. The samples were tested at 1100 and 800°C in a muffle furnace to study the high temperature oxidation and corrosion behaviour of the nickel-based superalloy base and treated surfaces. For high temperature corrosion testing the samples were coated with a thin film of 40%Na2SO4+ 50%NaCl+10%V2SO4 salt to accelerate the corrosion. The weight of the samples was monitored at a regular interval. The oxidized and corroded samples were examined using scanning electron microscope (SEM). The results revealed that the laser treated surface favour the formation of stable oxide layers at high temperature and resists the surface from further oxidation and the refinement of grains resist the corrosion compared with the coarse microstructure of the nickel-based superalloy base substrate. [ABSTRACT FROM AUTHOR]

The effect of strain rate on microstructure evolution of a Ni-Co-base superalloy during superplasticity was examined in this study. The experiments were conducted at 1050 °C with various strain rates. The strain rate had profound impacts on flow behavior and elongation that were connected to the characteristics of dynamic recrystallization (DRX). A comparatively high strain rate promoted the occurrence of DRX, and the elongation of >1335% prior to failure was obtained at 0.01/s. Scanning electron microscope (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) measurements were used to investigate the deformed microstructures. A 0.01/s strain rate accelerated the recovery and dissolution within the primary γ'-phase, resulting in a reduction in its volume fraction and size, which enhanced the DRX process of the γ-matrix. The primary γ'-phase partly retarded the DRX process of γ-matrix grains, which was expedited by twin boundaries. Regardless of strain rate, discontinuous-DRX (DDRX) dominated the superplastic deformation. Meanwhile, continuous-DRX (CDRX) occurrence increased slightly at 0.1/s, which had a minor effect during the superplastic deformation of the Ni-Co-base superalloy. • The flow behavior during hot deformation is complex and significantly influenced by strain rate. • The complicity of flow behavior affects the microstructure evolution and then the mechanical properties of superalloys. • Microstructure evolution and the DRX process are significantly affected by the strain rate during superplasticity. [ABSTRACT FROM AUTHOR]

Effect of heat treatment on the recovery of microstructure and mechanical properties of a service-exposed IN939 superalloy was studied. Four-stage heat treatment was performed on the service-exposed alloy. The microstructures of the service-exposed and rejuvenated alloys were examined by optical and scanning electron microscopes. The hardness, tension, and stress-rupture tests were carried out to characterise the mechanical properties. The results showed that the heat treatment could rejuvenate the microstructure of the alloy that was deteriorated during the service at high temperatures. Decomposed MC carbides were transformed to new fine carbides, continuous M23C6 carbides were dissolved and new discontinuous carbides along the grain boundaries were regenerated and, finally, the fine γ׳ particles were reformed. The microstructural recovery resulted in an increase in the hardness and ultimate tensile strength of the alloy. The results also showed that the creep behavior of the alloy at a testing temperature of 850°C could be improved by the heat treatment. [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]

High-temperature creep tests of a novel Super304H steel under 650 °C/195 MPa were conducted and the evolutions of microstructure and property with creep time of the material were investigated by using optical microscope, scanning electron microscope, micro Vickers hardness tester and electrochemical workstation. The results show that M23C6 carbides precipitated along grain boundaries of austenite matrix in a chain distribution and then got coarsened with the increase of creep time. Creep cavities started to form near the surface when the steel was crept for 2500 h. Afterward creep cavities increased, developed, interconnected and finally formed micro cracks along grain boundaries till fracture at the time of 4578 h. The hardness of the steel increased dramatically at the early stage of creeping and reached a high level at 500 h, and then kept a stable state at the succedent stage till fracture. Intergranular corrosion susceptibility of the steel increased first and then declined gradually, indicating the occurrence of sensitization -- desensitization process of the steel during creeping. [ABSTRACT FROM AUTHOR]

In this work, individual influences of Al, Fe and Ni on the microstructure, wear resistance and mechanical properties of high-entropy alloys (Al x CrFe y Ni z Ti 0.2) were studied. Microstructures and phase constituents of the alloys with different values of x, y and z were characterized via Scanning Electron Microscope, Energy Disperse Spectroscopy, and X-ray diffraction. Mechanical properties and wear resistances of the alloys were evaluated, including hardness, micro-hardness, and resistance to abrasive wear. It was shown that with increasing Al content, the FCC phase was replaced by disordered BCC (A2) and ordered BCC (B2) phases, leading to an increase in micro-hardness by ∼73% and a decrease in wear volume loss by ∼78%. Fe also increased the fraction of A2 phase, but reduced hardness by ∼41% and led to a considerable decrease in wear resistance. In contrast, Ni helped form a FCC phase, resulting in a decrease in hardness by ∼35%, and an increase in wear volume loss by ∼73%. Abrasive wear appeared to be the dominant wear mechanism, while the formed oxide scale played a role in improving the wear resistance. Efforts were made to correlate the changes in hardness and wear resistance with changes in microstructure. Among all specimens, AlCrFeNiTi 0.2 and Al 0.5 CrNiTi 0.2 reached the highest wear resistance. The results obtained help shed light on the development of advanced wear-resistant high-entropy alloys via the favorable addition of Al. • Influences of Al, Fe and Ni on microstructure, hardness and wear of AlxCrFeyNizTi0.2 high-entropy alloy were investigated. • Increasing Al content led to a significant increase in hardness by 73% and a decrease in wear volume loss by 78%. • Increasing Fe content lowered hardness and consequently increased the wear volume loss. • Varying Ni of low fraction had little effect on wear of the alloy, which markedly increased as Ni fraction rose to 1.25. [ABSTRACT FROM AUTHOR]

In order to demonstrate the transformation relation of two primary Al 13 Mn 4 Si 8 and Al 15 Mn 3 Si 2 phases during solidification of Al-12Si-4Cu-2Mn (in wt%) alloy, verification experiments through controlling cooling rate of solidification and quenching at different stages during solidification were designed and the microstructure was characterized by optical microscope, scanning electron microscope and transmission electron microscope. The cooling rate during solidification plays a decisive role in the formation of primary Mn-rich phases. Fast cooling rate favors the slender rod-like Al 13 Mn 4 Si 8 phase and slow cooling rate leads to the formation of dendritic Al 15 Mn 3 Si 2 phase. A great number of symbiotic complexes, constituted with inner Al 13 Mn 4 Si 8 and outer Al 15 Mn 3 Si 2 phases, are found in the transition area of the surface and center of the casting with a middle cooling rate. Microstructure observation of the liquid quenching samples demonstrates that the first phase formed during solidification must be 2D lath-like Al 13 Mn 4 Si 8 phase whether with high cooling rate or with slow cooling rate. And 3D dendritic Al 15 Mn 3 Si 2 phase is transformed from 2D Al 13 Mn 4 Si 8 by a peritectic reaction: L+ Al 13 Mn 4 Si 8 →Al l5 Mn 3 Si 2 , which is dependent of the cooling rate during solidification. A schematic model with double nose curves was established to exhibit the influence of cooling rate on the peritectic reaction. With sufficient high cooling rate, this peritectic reaction will be suppressed completely. When the cooling rate is slow enough, the peritectic reaction will proceed completely. However, in case of middle rate, the peritectic reaction will happen but incompletely. [ABSTRACT FROM AUTHOR]

The paper presents the results of the structure and mechanical tests of the low-alloy steel 14MoV6-3 which shows internal damage after long-term creep service. The tests of mechanical properties, including: tensile and yield strength at room temperature, yield strength at elevated temperature (+540 °C) and brittle transition temperature, were carried out on longitudinal samples in accordance with the requirements of standards for elements pressure equipment. The internal damage of the tested material was also classified depending on the degree of wear, based on the principles set out in the industry specification. It was shown that the strength properties of the tested material from pipelines operated under primary steam conditions decrease with increasing service time. Microscopic tests (Scanning Electron Microscope SEM) revealed that the cause of damage to steel with a ferrite-bainite structure after long-term operation is the cavitation cracking mechanism. The study of the microstructure was also carried out by means of the backscattered electron diffraction method EBSD using the IQ, KAM and GOS functions. These parameters are very important in terms of analysis of the occurring structural changes in the fatigue and creep processes. They allow to determine the deformed part of the substructure subjected to these processes. • Degree of wear of Cr–Mo–V steels under various work cycles was analyzed. • Creep and low-cycle fatigue are the main degradation mechanisms of this materials. • Bainite structure degradation and coagulation of carbide precipitates were observed. • The cavitation fracture mechanism is the main cause of structural degradation. • IQ, KAM and GOS obtained by EBSD determine the degree of deformation of the material. [ABSTRACT FROM AUTHOR]

Powder bed fusion using a laser beam (PBF-LB) is a promising additive manufacturing (AM) technology, used to produce components with complex geometry and design freedom. The parts are built in a layer-by-layer fashion during PBF-LB, and changing the processing parameters within different layers can lead to the modification of microstructures and finally a great impact on the mechanical properties. Thus, PBF-LB provides the possibility for the customization of material properties by changing the microstructures of layers using different processing parameters. In this investigation, a twin PBF-LB system, i.e., with a low-power laser beam and a high-power laser beam in a machine, is used to produce fine and coarse microstructures using the Inconel 718 (IN718) alloy, respectively. The fine and coarse microstructures are alternately printed to form different heterogeneous structures. Correspondingly, different mechanical properties are achieved. The microstructures are analyzed using optical microscope (OM), scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and numerical calculation. The defects generated by these 2 laser beams are simultaneously discussed. This study facilitates the tailoring of alloy properties using the PBF-LB technology to meet the users' needs of multiple service conditions. • IN718 heterogeneous structures are built by PBF-LB using 2 different laser systems. • A fine microstructure is formed using the low-power laser beam. • The high-power laser beam tends to produce the samples with coarse grains. • The different sizes of microstructures are ascribed to the different cooling rates. • The heterogeneous structures have effects on the mechanical properties. [ABSTRACT FROM AUTHOR]

In this study, an advanced powder metallurgy (PM) Ni based superalloy was successfully diffusion bonded. Microstructure of the joints was characterized by scanning electron microscope, electron backscatter diffraction and transmission electron microscopy; mechanical properties of the joints were evaluated by tensile test. When bonded without interlayer, many microvoids and a planar bond line were found at the interface, resulting in the low ultimate tensile strength (UTS, 1071 MPa) and elongation (EL, 3.4%) of the joint. Besides, weakly bonded areas appeared at the smooth fracture surface. When 5 μm-thick-Ni foil was used as the interlayer, microvoids were still observed at the joint. In addition, a carbide depleted zone and massive γ′ precipitates occurred at the interface. Tensile test showed that the UTS (1266 MPa) and EL (8.2%) of the joint increased slightly, but discontinuous weakly bonded areas and microvoids were observed on the fracture surface. As 2 μm-thick-Ni coating was used, a soundly bonded joint without microvoid was obtained. However, the carbide depleted zone and γ′ precipitates still existed at the interface. Tensile test showed that the UTS (1386 MPa) and EL (12.8%) of the joint increased dramatically, and plenty of dimples appeared on the fracture surface. • Microvoids were found on joints bonded without interlayer and with 5 μm Ni foil. • Microvoids were eliminated when 2 μm Ni coating was used as the interlayer. • γ′ and carbide depleted zones appeared on joints bonded with Ni foil and coating. • Joint bonded with 2 μm Ni coating showed highest strength and ductile fracture mode. [ABSTRACT FROM AUTHOR]

In this paper, laser-arc hybrid welding of 10 mm thick low-thermal-expansion superalloy GH909 components was carried out to obtain a joint with good performance. This investigation was conducted using an optical microscope, scanning electron microscope, energy diffraction spectrum and other methodologies. The results showed that weld joints with a desirable wineglass-shaped weld profile can be obtained employing appropriate process parameters. The different grains in between the upper central seam and the bottom seam were associated with the temperature gradient, the pool’s flow and the welding thermal cycle. MC-type carbides and eutectic phases (γ+Laves) were produced at grain boundaries due to the component segregation during the welding process. In addition, γ′ strengthening phase presented in the interior of grains, which kept a coherent relationship with the matrix. The lowest hardness value occurred in the weld center, which indicated that it was the weakest section in the whole joint. The average tensile strength of the joints reached to 632.90 MPa, nearly 76.84% of the base metal. The fracture analysis revealed that the fracture mode of the joint was ductile fracture and the main reason for joint failure was as a result of the occurrence of porosities produced in the weld during the welding process. [ABSTRACT FROM AUTHOR]

After a standard heat treatment, the morphological evolution of γ′ precipitates in the directionally solidified Ni-base superalloy M4706 with time is investigated during thermal exposure at 800 °C. In contrast to previous results, it is found that as the average particle size increases from 241 to 368 nm with increasing aging time up to 10,000 h, the shape of γ′ precipitates gradually changes from cuboidal to sphere-like. X-ray diffraction experiments reveal that the constrained γ/γ′ lattice misfits of M4706 fall into the range of −0.226% to −0.223% between 800 and 850 °C, whereas the value is −0.529% at 1050 °C. Based on the experimental results, the morphological transition of γ′ precipitates is discussed. [ABSTRACT FROM AUTHOR]

The two-phase microstructural evolution and the remained fractions of γ′ were investigated by scanning electron microscope (SEM) for a series of γ′-richen single crystal superalloys from the IC alloys system. At alloys IC31 and IC32, high levels of the dissolution of γ′ cause the remarkable damages of two-phase microstructure, especially when the fraction drops to less than 45%, the γ′ phase becomes sphere significantly and the two-phase interface distorts fairly. And the relationship between different degrading stages and the remained fractions are discussed systemically. The dissolution temperatures, Tsol, of IC alloys determined by differential scanning calorimetry (DSC) are further considered. It is found that there is a good linear relationship between the Tsol and the remained fraction of the γ′ at 1200°C and the slide keeps fixed as about 0·5%/°C. Meanwhile, the remained fraction of the γ′ seems to play an important role on the mechanical properties, and a method with Tsol used to estimate the properties at 1200°C is proposed. [ABSTRACT FROM AUTHOR]

Nickel based single crystal superalloy DD5 was brazed with the AWS classification filler metal BNi-2 at brazing temperature of 1323-1423 K for holding 10-240 min. The effect of the brazing temperature and brazing time on the microstructure of the brazed joint was investigated by scanning electron microscope (SEM) and electron probe microanalyser (EPMA). The result indicates that the joints exhibit sound bonding without any defect and crack. Three distinct regions can be identified in the joint: a diffusion affect zone composed of γ-γ′ phase and fine M3B2 precipitates, an interfacial bonding zone solidified isothermally at the brazing temperature, and braze alloy zone formed by the solidification of surplus molten filler alloy during cooling. The braze alloy zone consists of γ+γ′ eutectic phase, chromium boride, and ternary eutectic phase of γ-nickel, nickel boride and nickel silicide. With extending brazing time, the thickness of interfacial bonding zone and diffusion affect zone increased while brittle compounds reduced. The bonding mechanism was discussed with reference to the microstructure evolution of the joint. [ABSTRACT FROM AUTHOR]

This paper reports the high temperature corrosion of two Ni-base superalloys: a newly developed alloy, SCA425+, and the extensively used IN792. The composition of the two materials is quite similar, but SCA425+ contains more Cr and Al (17.1 and 10 at.% compared to 13.9 and 7.4 at.% in IN792). The results from exposures at 900 °C in SO 2 (3,000 ppm) + O 2 (69 vol%) + H 2O (31 vol%) mixed gas for 260 h using 65 h/cycle are compared with those obtained from tests in laboratory air. The microstructure of the formed oxide scales was studied using several techniques, such as XRD, SEM, FIB, EDX, STEM and XPS. It is shown that in IN792 severe internal oxidation takes place after both types of exposures. On the other hand, the newly developed SCA425+ has the tendency to form an alumina layer proving that it has more potential to be used in the aggressive environments. Surprisingly the mass gains for SCA425+ alloy exposed in SO 2-rich environment are lower than in laboratory air. The reason for this behavior is discussed. [ABSTRACT FROM AUTHOR]

In this study, Alloy 247LC samples were built with different laser powder bed fusion (L-PBF) process parameters. The samples were then subjected to solution heat treatment at 1260 °C for 2 h. The grain size of all the samples increased significantly after the heat treatment. The relationship between the process parameters and grain size of the samples was investigated by performing a design of experiment analysis. The results indicated that the laser power was the most significant process parameter that influenced the grain height and aspect ratio. The laser power also significantly influenced the grain width. The as-built and as-built + heat-treated samples with high, medium, and low energy densities were characterized using a field emission gun scanning electron microscope equipped with an electron backscatter diffraction detector. The micrographs revealed that the cells present in the as-built samples disappeared after the heat treatment. Isolated cases of twinning were observed in the grains of the as-built + heat-treated samples. The disappearance of cells, increase in the grain size, and appearance of twins suggested that recrystallization occurred in the alloy after the heat treatment. The occurrence of recrystallization was confirmed by analyzing the grain orientation spread of the alloy, which was lower and more predominantly <1° in the as-built + heat-treated conditions than in the as-built conditions. The microhardness of the as-built + heat-treated samples were high which was plausible because γ' precipitates were observed in the samples. However, the L-PBF process parameters had a very low correlation with the microhardness of the as-built + heat-treated samples. • The process parameter employed will affect the material condition obtained after heat-treatment. • The material condition that resisted cracking during the laser powder bed fusion process produced large grain size. • The heating rate required to avoid cracking during post processing heat-treatment was calculated. [ABSTRACT FROM AUTHOR]

Abstract: The paper presents results of simple, Pt/Pd- and Pt-modified aluminide coatings'' cyclic oxidation-induced degradation analysis. The coatings were deposited by Pt and Pt+Pd electroplating, followed by vapor phase aluminizing at 1050°C. Cyclic oxidation tests were performed at 1100°C in 23hcycles. Microstructural and phase analysis conducted using XRD, SEM, EDS and EBSD methods revealed that the oxide layers that formed on the coatings were composed of three distinctive types of oxides growing according to a specific pattern which is described. The oxide layer that formed on the simple aluminide coating exhibited low adhesion in comparison to Pt- and Pt,Pd-modified aluminide coatings which managed to maintain an adherent oxide layer that contained higher amount of desirable α-alumina. The Al-depleted βNiAl grains remained much larger in the modified aluminide coatings, even after failure. What is more, stripes characteristic for martensitic transformation were discovered in the β phase grains in all coatings. [Copyright &y& Elsevier]

Abstract: Two-nanostructure intermetallics (nanoporous film and nanocube) on single crystal nickel-based superalloys were fabricated by electrochemical dealloying in an aqueous electrolyte containing 1wt% (NH4)2SO4 and 1wt% citric acid solution respectively. Electrochemical properties of two phases in single crystal nickel-based superalloys are different, and it has been utilized to dissolve preferentially γ matrix phase and γ′ phase remains passive. The electrochemical corrosion behavior of a Ni-based superalloy after high temperature creep was investigated using an EG&G PAR M273 potentiostat. The results indicated that the microstructure after high temperature creep determined the morphology of the resultant γ′ phase intermetallics after dealloying. The morphologies and structure of nanoporous film and nanocube were characterized using a JSM-6700 (Japan) field emission scanning electron microscope (FE-SEM) and JEM-3010 high resolution transmission electron microscopy (HRTEM). [Copyright &y& Elsevier]

Abstract: The thermit reaction sequence of MoSi2 formation from Mo and Si powder mixtures was investigated by SEM using quenching techniques. It was observed that the mechanism of MoSi2 formation depends on the Mo particle size. In the case of small particles, the final structure consists of equiaxed MoSi2 grains. The large particles, however, cannot be completely dissolved and remain unreacted surrounded by thin solid-state diffusional layers of Mo5Si3 and Mo3Si. [Copyright &y& Elsevier]

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]

Abstract: Dual beam focused ion beam-scanning electron microscopes are well suited for characterizing micron and sub-micron size microstructural features in three dimensions via serial-sectioning procedures. Importantly, these commercially-available instruments can be used to collect morphological, crystallographic, and chemical information throughout a serial-sectioning experiment. Selected examples are shown to demonstrate these capabilities. [Copyright &y& Elsevier]

Abstract: Platinum-based alloys are being developed which have microstructures that are analogous to the γ/γ′ microstructure of the nickel-based superalloys. These Pt-based superalloys have the potential to substitute Ni-based superalloys for even higher temperature applications. The Pt–Cr–Ru system has been studied experimentally as part of this project. The alloys were studied in the as-cast condition, using scanning electron microscope (SEM) and X-ray diffraction (XRD). A solidification projection and liquidus surface were constructed. The major phases were (Ru), (Pt) and ∼CrPt, with ∼Cr2Ru (D8b), ∼Cr3Ru (A15) and ∼Cr3Pt (A15) being minor in extent. Both the tetragonal (L10) and cubic (A1) forms of ∼CrPt were observed. Two invariant reactions were identified in the Pt–Cr–Ru system. [Copyright &y& Elsevier]

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]