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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]

The freckle formation and evolution in directionally solidified single-crystal specimens with different thicknesses and abruptly expanding rectangular cross-section were investigated at different withdrawal rates. The microstructure was studied by optical microscope (OP) and scanning electron microscope (SEM), and the crystallographic orientation in freckle areas was characterized by electron backscattered diffraction (EBSD). The experimental results showed that the freckle was the most severe in specimens with a thickness of 10 mm at the same withdrawal rate, and the freckle chain was observed to form below an abruptly expanding cross-section ended at the expanding step. It was found that freckles formed exclusively on the edges of casting facing the central sprue of the shell mould. Findings reveal that freckles tend to form at the highest position of the mushy zone, mainly due to the thermosolutal convection and the radial flow caused by feeding behavior in the mushy zone. The freckle formation is pronounced at a low withdrawal rate, and can further generate stray grains. • Freckle formation is more pronounced at a low withdrawal rate of 1 mm/min. • Freckles form exclusively on the edges facing the central sprue of the shell mould. • Radial flow and feeding behavior play a major role in freckle position. • Freckle grains can evolve into stray grains at a low withdrawal rate of 1 mm/min. [ABSTRACT FROM AUTHOR]

In this paper, the effect of Y content on interface reaction and wettability between a nickel-base single crystal supperalloy melt and ceramic mould were studied by means of sessile drop tests. The microstructural characterization of the alloy-ceramic interface and the elemental compositions of reaction products were performed by scanning electron microscope (SEM) and electron probe micro-analyzer (EPMA), the phases of reaction products were identified by X-ray diffraction (XRD), and the surface analysis of reaction products was carried out by X-ray photoelectron spectroscopy (XPS). The results showed that Al and Hf in the alloy reacted with SiO 2 in the ceramic mould when Y content was lower than 0.011 wt%, and the reaction products were Al 2 O 3 and HfO 2 , with little change in the wetting angle. However, when Y content was higher than 0.017 wt%, Y and Al in the alloy reacted with SiO 2 in the ceramic mould, and the wetting angle decreased significantly. 0.017 wt% was the critical concentration of Y content that would cause interface reactions. The reaction products of 0.017 and 0.025 wt% Y alloys were Y 3 Al 2 (AlO 4) 3 , the reaction products of 0.1 wt% Y alloy were Y 3 Al 5 O 12 and Y 4 Al 2 O 9 , and the reaction products of 0.5 and 1.0 wt% Y alloys were double reaction layer composed of Y 2 O 3 and Y 3 Al 5 O 12. Si generated by the interface reactions could react with O 2 in the atmosphere to form SiO x during the cooling of the alloy-ceramic couples. • Y-containing interface reactions can obviously increase the wettability. • Al 2 O 3 and HfO 2 are formed at the interface when Y content is less than 0.011 wt%. • Different yttrium aluminate oxides are formed when Y content exceeds 0.017 wt%. • Si generated reacts with O 2 in the atmosphere to form SiO x during the cooling. [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]

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]

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]

The formation and distribution of freckles in the third generation single crystal superalloy DD9 were investigated by a designed stepwise specimen with suddenly reduced cross section in this paper. The microstructural characteristics of freckles were analyzed by optical microscope(OM), scanning electron microscope(SEM) and electron back scattering diffraction(EBSD). Numerical simulation was used to analyze the thermal field and the shape of the mushy zone of the specimens during the directional solidification. The results show that freckles are composed of disorderly orientated dendrite fragments and γ/γ′ eutectic. Freckles only appeared on the surface of the specimens and are prone to occur in the larger parts and edges of the specimens. The morphology of the isotherm has an important influence on the formation of freckles. The presence of lateral temperature gradient increases the instability of segregated liquid in the mushy zone near the shell mould wall and makes the freckles tend to form on the higher position of the isotherm. The reduction of the melt volume in the mushy zone combining with the decrease in the tilt of the isotherm suppress the formation of freckles. Based on the experimental results and simulation results, a modified Rayleigh-number based criterion for freckle formation is proposed. • A stepwise casting was designed to thoroughly study the formation of freckles. • Freckles are prone to occur in the higher position of the isotherms. • Lateral heat dissipation has an important effect on the formation of freckles. • Flatten the isotherm and decrease the melt volume can suppress freckle formation. • A modified Rayleigh-number based criterion for freckle formation was proposed. [ABSTRACT FROM AUTHOR]

The solidification characteristics of C700R-1 alloy was investigated by DSC analysis, in-situ observation, quenching experiment and Thermo-Calc calculation. The segregation behavior during solidification was analyzed by Scanning Electron Microscope and Electron Probe Microanalysis. At the initial stage of solidification, the volume fraction of liquid experienced a steep decrease, accompanied by a rapid formation of dendrites. The residual liquid solidified slowly due to segregation at the final stage. Al, Ni, W and Co elements segregated in the dendrite stems, while Ti, Nb and Mo were enriched in the interdendritic, in which MC carbides also precipitated. At the end of solidification, the segregation of Mo and Cr is further aggravated, resulting in extremely low solidus and formation of eutectic region. The study on the solidification characteristics of C700R-1 alloy demonstrates that aggravation of segregation leads to deterioration of the solidification conditions. • DSC, in situ, quenching experiment and Thermo-Calc calculation were applied. • L→γ at 1370 ℃; L→γ + MC at 1298 ℃; L→γ + μ at 1217 ℃. • The solidification interval is widened due to the addition of Ti, Nb, Mo and W. • The segregation of Mo and Cr leads to the formation of multiple eutectic phases. [ABSTRACT FROM AUTHOR]

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: 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]

The Inconel 718 is a promising Ni-based alloy for a wide range of high-temperature industrial applications. However, the radiative characteristics of Inconel 718 alloy change due to surface oxidation and cause large deviation in temperature field computation and radiation thermometry. Herein, the Inconel 718 alloy has been oxidized at different temperatures, ranging from 700 K to 1100 K, for different time (up to 100 h) to systematically study the influence of oxidation on radiative properties. The highly oxidized alloy samples have been prepared, whose oxidation degrees are more similar to the engineering applications. The normal spectral emittance of as-prepared and surface-oxidized Inconel 718 alloys has been measured in the range of 2–15 μm at different temperatures (400–1400 K). The spectral data have been integrated to analyze the effect of temperature on total emittance. In the case of as-prepared Inconel 718, the normal spectral emittance increased with decreasing wavelength, whereas the total emittance linearly increased with increasing temperature. By comparing the emittance data of oxidized and as-prepared samples, it is clear that the high-temperature oxidation improves the emittance of Inconel 718 alloy. The effect of oxidation on radiative characteristics of Inconel 718 can be divided into three stages: 1. the spectral emittance slightly increases in short wavelength range; 2. obvious emittance fluctuation occurs due to radiation interference; 3. emittance oscillations disappear and the spectral emittance curves shows more radiative characteristics of the main oxide (Cr 2 O 3). Moreover, the total emittance of Inconel 718 alloy, with compact oxide scales, decreased with increasing temperature, and during the 2nd stage, a knee point is observed on the total emittance curve near the Néel temperature of NiO. To unveil the mechanism of oxidation-induced emittance variations, scanning electron microscope (SEM) and energy disperse spectroscopy (EDS) are employed. The structure of a multilayered oxide scale is demonstrated and the process of oxidation is explained. According to the analysis, the oxidation degree determines the weight of influence of substrate alloy and surface oxides on radiative characteristics of oxidized alloy, and this is also the reason why the influence of oxidation on radiative characteristics shows staged. Moreover, NiO was detected on the surface of oxidized samples, and combined with the emittance results, it can be concluded that the magnetic ordering of oxides will influence the radiative behavior of Inconel 718 alloy. • The emittance characteristics of highly oxidized Inconel 718 are investigated. • The emittance of Inconel 718 alloy varies with the oxidation degree in three stages. • The magnetic ordering influences the emittance characteristics of the alloy. [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]

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]

The influence of boron on the oxidation resistance of conventionally cast U720Li alloy was investigated at 1130 °C and analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDX) and electron probe microanalysis (EPMA). It has been found that the boron-undoped and -doped alloys all obey a super-parabolic rate law, but the oxidation rate is obviously increased by the boron addition. These alloys all show non-uniform oxidation between dendritic and interdendritic areas due to the serious microsegregation, while the oxide scale morphology of the high-boron alloy is quite different from that of the boron-undoped alloy. For boron-doped alloys, in the initial stage of oxidation the outward diffusion of boron from borides formed in the interdendritic region results in the formation of discontinuous and non-uniform B-rich oxide layers beneath and inside the Cr 2 O 3 layer, while as the oxidation proceeds further boron can also transfer from the B-rich oxide to outer layers which eventually leads to disappearance of the B-rich oxide layers. The formation of B-rich oxide layers destroys the continuity of Cr 2 O 3 layer and decomposition of B-rich oxide layers makes Cr 2 O 3 layer more porous, which significantly reduce the protective property of the Cr 2 O 3 layer and in turn decrease the oxidation resistance. Moreover, the eutectic (γ + γ′) highly enriched in Ti, Al is prone to be oxidized and form a thicker scale than the other regions, and hence the significant increase of its precipitation with boron addition also contributes to increasing the oxidation rate. • Boron obviously worsens oxidation resistance of as-cast Ni-based U720Li alloy. • How the distribution of boron changes as oxidation proceeds has been revealed. • B-rich oxide is formed beneath and inside Cr 2 O 3 layer at initial oxidation stage. • As oxidation proceeds further the B-rich oxide is decomposed. • Effect of B on oxidation is due to formation and decomposition of B-rich oxide. [ABSTRACT FROM AUTHOR]