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Single crystal superalloys are widely used in the manufacturing of turbine blades for aero-engines due to their superior performance at high temperatures. The directional solidification process is a key technology for producing single crystal turbine blades with excellent properties. In the directional solidification process, withdrawal rate is one of the critical parameters for microstructure formation and will ultimately determine the blade's properties. In this paper, the as-cast microstructures in the typical sections of a DD9 single crystal (SX) superalloy turbine blade were investigated with 3 mm/min and 5 mm/min withdrawal rates during the directional solidification process. With increased withdrawal rate, the dendrite morphologies tended to become more refined, and the secondary dendritic arms tended to be highly developed. The dendrite in the blade aerofoil section was more refined than that in the tenon section, given the same withdrawal rate. Additionally, with increasing withdrawal rates, the size and dispersity of the γ′ precipitates in the inter-dendritic (ID) regions and dendritic core (DC) tended to decrease; furthermore, the size distributions of the γ′ precipitates followed a normal distribution law. Compared with the ID regions, an almost 62% reduction in the average γ′ sizes was measured in the DC. Meanwhile, given the same withdrawal rate, at the blade's leading edge closest to the heater, the γ′ sizes in the aerofoil section (AS) were more refined than those in the tenon section (TS). As compared with the decreasing cross-sectional areas, the increased withdrawal rates clearly brought down the γ′ sizes. The sizes of the γ–γ′ eutectics decreased with increasing withdrawal rates, with the γ–γ′ eutectics showing both lamellar and rosette shapes. [ABSTRACT FROM AUTHOR]

To further understand the mechanism of freckle formation and explore effective control methods. The effects of withdrawal velocity and casting geometry structure on freckle formation were studied by a casting with a sudden reduction in the cross section in this research. Optical microscope (OM) and scanning electron microscope (SEM) were used to characterize the microstructural of freckles, and electron backscattered diffraction (EBSD) was used to measure the crystallographic orientation of the freckled area. Numerical simulation software ProCAST was used to analyze the morphology of the mushy zone during directional solidification. The results suggest that with the increase of the withdrawal velocity, primary dendrite arm spacing would be reduced and the permeability K of the mushy zone would decrease, which inhibits the formation of freckles. On the contrary, the sloping angle of the mushy zone would increase with the rise of the withdrawal velocity, which promotes the freckle formation. These inhibitory effect and promotion effect jointly influence the formation of freckles. The slanted convection channel result from the lateral heat loss makes it easier for freckles to occur in the higher isotherms of the mushy zone. Reduced size of the casting diminishes the volume of liquid phase in the mushy zone and reduces the tendency of freckle formation. A modified freckle formation criterion based on Rayleigh number considering the casting geometry and the morphology of the mushy zone is proposed according to this experimental and simulation results, and the original calculation methods of these parameters are given. [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]

A thorough TEM analysis has been carried out to study the dislocations cutting intoγ′ phase in a single-crystal superalloy during uniaxial tensile creep under high-temperature and low-stress conditions. It is proved that the a〈100〉 edge superdislocation originates from the interfacial a〈100〉 dislocations and moves into the γ′ phase by pure climbing. And the dissociation of the a〈100〉 superdislocation core into two a/2〈101〉 superpartial dislocations during uniaxial tensile creep has been identified by HRTEM method for the first time. [ABSTRACT FROM PUBLISHER]

The γ/γ′ interfaces are thought to play an important role in determining the mechanical properties in single-crystal nickel-based superalloys. In this article, interfacial width in DD6, one second-generation single-crystal superalloy containing 2 wt% Re, has been studied by means of scanning transmission electron microscopy. From an atomic resolution high angle annular dark field image, both compositional interfaces and order–disorder interfaces are studied, and it is first found that the interfacial width of chemical composition is same with that between the ordered and disordered phases. [ABSTRACT FROM PUBLISHER]

The glide of <110>/{111} superdislocations in Ni 3 Al becomes prevalent in tertiary creep and take a dominant role in influencing the creep resistance of Ni-based single crystal superalloys at the tertiary stage. Here we carried out atomistic simulations to investigate the mobility law of screw, 30° mixed, 60° mixed and edge <110> superdislocations in Ni 3 Al. Based on the Peierls-Nabarro model, we found that the distance of the dissociated anti-phase boundary (APB) and complex stacking fault (CSF) of <110> superdislocations increases with increasing the character angle (from screw to edge), keeping a good accordance with the analysis by anisotropic elastic theory. We further study the motion of the superdislocations at temperatures ranging from 300 K to 1000 K. The dislocation mobility is distinct to its character angle, i.e. , the edge-like (30° and 90°) superdislocations have a higher speed than the screw-like (0° and 60°) superdislocations. For all types of superdislocations, the drag coefficient increases linearly with increasing temperature in the phonon-drag regime, while behaves to be independent on temperature in the asymptotic regime. Finally, we explored the origin of mobility disparity between the edge-like and screw-like superdislocations in terms of energy landscape for the dislocation glide. We observed that the two 1 / 2 <110> dissociated superpartials move coordinately in edge-like superdislocations with no variation of APB ribbon width. However, the motion of two superpartials are uncorrelated in screw-like superdislocations with the expansion and reduction of APB, resulting in a relatively higher Peierls barrier. Our present work could help to understand the creep failure in Ni-based single crystal superalloys. • The equilibrium configurations of <110> superdislocations in Ni 3 Al are characterized using the Peierls-Nabarro model. • The distance of anti-phase boundary and complex stacking fault increases with increasing the character angle (from screw to edge). • Edge-like (30° and 90°) superdislocations have a relatively higher mobility than screw-like (0° and 60°) superdislocations. • The mobility disparity between the edge-like and screw-like superdislocations lies in their distinct glide manners. [ABSTRACT FROM AUTHOR]

The excellent mechanical properties of Ni-based superalloys rely on their unique two-phase microstructure. Here we probe the influence of interfacial dislocation network on strain-rate sensitivity in Ni-based single crystal superalloys. The interfacial dislocation could generate a weak long-range interaction to the matrix dislocations at low stress, and react with the incoming dislocation to form junctions at high stress. The weak interaction results in the dislocation pile-up around the interphase boundary, while the strong dislocation reaction makes the gradual loss of interface coherency. With the activation energies, we predict the dependence of strain-rate sensitivity on temperature in a regime with two bounds correlated with these two kinds of interactions. The predicted values match well with that measured in experiments at elevated temperatures. [Display omitted] [ABSTRACT FROM AUTHOR]

A second-generation nickel-based single crystal superalloys DD6 were creep tested in the [001] direction (within 15°) at 1100 °C/140 MPa. The specimen tested until rupture was investigated using scanning and transmission electron microscopy to determine the evolution of the dislocation behaviors during tertiary creep. It was found that the tertiary creep deformation was highly localized and inhomogeneous along the gauge length, and the types of superdislocations in γ′ rafts varied with the distances from the rupture surface, including individual screw dislocations, antiphase boundary-coupled dislocation pairs, and superlattice intrinsic stacking faults. It can be concluded that γ′ rafts shearing events occur in the following sequence with the evolution of tertiary creep: individual screw dislocations, antiphase boundary-coupled dislocation pairs, and superlattice intrinsic stacking faults. The origin of these transformations of superdislocation types and its influence on tertiary creep rate are discussed. It is proposed that at the microscopic level, a more reasonable explanation for the strain softening mechanism during the tertiary creep of nickel-based superalloys at high temperatures and low stresses is the emergence of new superdislocation types with higher mobility rather than the density rise of a single type of superdislocation produced during the later secondary creep stage. [ABSTRACT FROM AUTHOR]

The kinetical recrystallization behaviors over wide ranges of annealing temperature of 1100–1200 °C and annealing time range of 0.5–16 h of a second-generation single-crystal superalloy have been investigated by combined model simulation, TEM and EBSD techniques. The kinetics represented by Johnson–Mehl–Avrami and Arrhenius equations revealed its complicated diffusion-controlled mechanism. With increasing annealing temperature and time, the recrystallized grain size exhibited a steady increasing tendency, and gradually, the subgrain boundaries in the original matrix were consumed and replaced by high angle grain boundaries, leading to improved Schmid factor. The dominant fraction of twins in the resultant microstructure indicated its significant role in strain energy dissipation, facilitating the complete progress of recrystallization. [ABSTRACT FROM AUTHOR]

The mechanical properties of nickel-based single crystal superalloys deteriorate with increasing misorientation, thus the finished product rate of the casting of single crystal turbine airfoils may be reduced due to the formation of grain boundaries especially when the misorientation angle exceeds to some extent. To this day, evolution of the microstructures at the grain boundaries with misorientation and the relationship between the microstructures and the mechanical properties are still unclear. In this work a detailed characterization of the misorientation related microstructure at the grain boundary in DD6 single crystal superalloy has been carried out using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques; the elemental distribution at the grain boundaries has been analyzed by energy dispersive (EDS) X-ray mapping; and the effect of precipitation of μ phases at the grain boundary on the mechanical property has been evaluated by finite element calculation. It is shown that the proportion of γ phase at the grain boundaries decreases, while the proportion of γ′ phase at the grain boundaries increases with increasing misorientation; the μ phase is precipitated at the grain boundaries when the misorientation angle exceeds about 10° and thus it could lead to a dramatic deterioration of the mechanical properties, as well as that the enrichment of Re and W gradually disappears as the misorientation angle increases. All these factors may result in the degradation of the mechanical properties at the grain boundaries as the misorientation increases. Furthermore, the finite element calculation confirms that precipitation of μ phases at the grain boundary is responsible for the significant deterioration of the mechanical properties when the misorientation exceeds about 10°. This work provides a physical imaging of the microstructure for understanding the relationship between the mechanical properties and the misorientation of the grain boundaries in the single crystal superalloy. Based on our study we hope that we can offer some scientific basis for further improvement of the nickel-based single crystal superalloys. [ABSTRACT FROM AUTHOR]

The elemental vaporization and redistribution effect during sub-solvus recrystallization (1100 °C) of a second-generation single crystal superalloy DD6 containing 2 wt% Re has been investigated in this work. For fine polished specimens free from selective chemical erosion effect, accurate elemental energy dispersive spectroscopy (EDS) analysis was taken from the surface to the inner original matrix and across the boundary of the recrystallization reaction front. The results revealed that the vaporization induced elemental depletion of Ni and Al had occurred in the top surface layer. This would lead to the destabilization of the γ phase in the recrystallized aggregates, facilitating the final precipitation of refractory-rich P phase. [ABSTRACT FROM AUTHOR]

A semi-quantitative, macroscopic, phenomenon-based, thermo-elastic–plastic model was developed to predict the final plastic strains of single crystal nickel-based superalloys by considering their orthotropic mechanical properties. Various cases were considered and simulated to investigate the basic factors that influence the final plasticity. Thermo-mechanical numerical analysis was conducted to predict the recrystallization sites of simplified cored rods, with the results in good agreement with the experimental results. These hollowed rods with thin walls showed an increased propensity for recrystallization. The geometric features, especially stress concentration sites, are more significant to the induced plasticity than the material's orientation or shell/core materials. This paper also attempts to provide useful suggestions, such as introducing filets, to avoid causing plastic strains during the casting process that induce recrystallization. [ABSTRACT FROM AUTHOR]

Abstract: The reaction of LAlH2 (L = HC(CMeNAr)2, Ar = 2,6- i Pr2C6H3) (1) with cyclopentylboronic acid resulted in the boroxine-linked aluminum compound LAl[OB(C5H9)]2(μ-O) (2). However, the reaction of 1 with 3-hydroxymethylphenylboronic acid gave an eight-membered boroxine-linked dialuminum compound of composition [LAlOB(3-OHCH2C6H4)]2 (3) with a spiro center at each of the aluminum atoms. The two complexes were characterized by 1H NMR, IR spectroscopy, elemental analysis, and single crystal X-ray structural analysis. [Copyright &y& Elsevier]

Mitigating the fire hazards of epoxy resin (EP) and simultaneously improving mechanical properties have always been a goal pursued by polymeric scientists. Herein, we prepared a lithium-containing hepta-isobutyl-POSS (ibu-T 7 -Li-POSS). The chemical structure of ibu-T 7 -Li-POSS was well characterized. Just via tuning the ethanol solution concentration, we can obtain zero-dimensional (0D) spherical nanoparticles, two-dimensional (2D) ultrathin mono-layer nanoflakes, three-dimensional (3D) multiple-layered lamellae and hexagonal petal-shaped micron-size crystal based on ibu-T 7 -Li-POSS. Regular macroscopic octahedron single crystal of ibu-T 7 -Li-POSS was analisized by single crystal X-ray diffraction. The prepared ibu-T 7 -Li-POSS was incorporated into EP to improve the mechanical properties and enhance the fire safety. When the ibu-T 7 -Li-POSS content was 1 wt%, the flexural strength of EP-1 was enhanced from 81.8 MPa of EP to 110 MPa. The incorporation of ibu-T 7 -Li-POSS can apparently decrease the dielectric constant and loss of EP nanocomposites. Moreover, when 5 wt% of ibu-T 7 -Li-POSS was incorporated into EP, the reduction of peak of heat release rate (p-HRR), peak of smoke production rate (p-SPR) and peak of CO production rate (p-COP) reached to 34.1%, 38.6% and 40.6%, respectively. Therefore, our findings provide a considerable promotion for the facile preparation of low-dimensional environment-friendly nano-/micron-flame retardants based on metal-containing polyhedral oligomeric silsesquioxane (M-POSS) compounds. [Display omitted] • A novel metal-POSS (ibu-T 7 -Li-POSS) is firstly synthesized via one-pot method. • Controllable dimensional ibu-T 7 -Li-POSS nano-/micron-crystals are fabricated. • Thermal stability and mechanical properties of EP nanocomposites are improved by introducing ibu-T 7 -Li-POSS. • Ibu-T 7 -Li-POSS apparently reduces fire hazards of the formed EP nanocomposites. • The mechanism of catalytic charring of lithium for EP is illustrated in detail. [ABSTRACT FROM AUTHOR]