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Refractory high-entropy alloys are considered as potential structural materials for elevated temperature applications. To obtain refractory high-entropy alloys with high strength, different amounts of Si were added into the NbMoTiVW refractory high-entropy alloys. The effects of Si on the phase constitution, microstructure characteristics and mechanical properties of NbMoTiVWSix alloys were investigated. Results show that when the addition of Si is 0, 0.025 and 0.05 (molar ratio), the alloys are consisted of primary BCC and secondary BCC in the intergranular area. When the addition of Si is increased to 0.075 and 0.1, eutectic structure including silicide phase and secondary BCC phase is formed. The primary BCC phase shows dendritic morphology, and is refined by adding Si. The volume fraction of intergranular area is increased from 12.22% to 18.13% when the addition of Si increases from 0 to 0.1. The ultimate compressive strength of the NbMoTiVW alloy is improved from 2,242 MPa to 2,532 MPa. Its yield strength is also improved by the addition of Si, and the yield strength of NbMoTiVWSi0.1 reaches maximum of 2,298 MPa. However, the fracture strain of the alloy is decreased from 15.31% to 12.02%. The fracture mechanism of the alloys is changed from mixed fracture of ductile and quasi-cleavage to cleavage fracture with increasing of Si. The strengthening of alloys is attributed to the refinement of primary BCC phase, volume fraction increment of secondary BCC phase, and formation of eutectic structure by addition of Si. [ABSTRACT FROM AUTHOR]

The effects of Ag, Co, and Ge additions on microstructure and mechanical properties of Be-35Al (wt.%) alloys fabricated by investment casting were studied. The results reveal that the trace metals 1.5wt.% Ag, 0.7wt.% Co, and 0.8wt.% Ge additions do not change the nucleation temperature of Be phase. However, the nucleation temperature of the Al phase decreases from 642 °C to 630 °C by DSC due to the Ge addition. The strength of the alloys sharply increases due to the dissolution of the Ag and Ge solutes into the Al phase and the Co into the Be phase characterized by SEM and EDS. Obviously, the strength of Be-Al-Ag-Co-Ge alloy is improved by the solution strengthening. Furthermore, a few Ag3Al particles contribute to the strength of the Al phase. Be-Fe-Al ternary intermetallic compounds which can effectively reduce the negative effect of an impurity element Fe on the mechanical properties of Be-Al alloys are also found by XRD and EDS. [ABSTRACT FROM AUTHOR]

No-bake resin-bonded sand is commonly used in casting production. However, its air pollution is relatively serious, especially in the molding and pouring process. For this reason, it is necessary to study the gas evolution characteristics of no-bake resin-bonded sand from room temperature to high temperatures, and not only the amount of gaseous products, but also the composition of the gaseous products. No-bake furan resin-bonded sand (#1), phenolic urethane no-bake resin-bonded sand (#2), and alkaline phenolic no-bake resin-bonded sand (#3) are the three most common no-bake resin-bonded sands in casting. The gas evolution volume and rate of these three no-bake resin-bonded sands were studied. Thermogravimetry-mass spectrometer (TG-MS), headspace-gas chromatography/mass spectrometer (HS-GC/MS), and pyrolysis-gas chromatography/mass spectrometer (PY-GC/MS) were used to measure the composition of the gaseous products emitted from binders at room temperature and high temperatures. The differences between formaldehyde, heterocyclic aromatic compounds (HAC), monocyclic aromatic hydrocarbons (MAH), and polycyclic aromatic hydrocarbons (PAHs) gaseous products from the three types of no-bake resin-bonded sands during the molding and casting process were compared. From the perspective of environmental protection, alkaline phenolic no-bake resin-bonded sand and no-bake furan resin-bonded sand are better than phenolic urethane no-bake resin-bonded sand. [ABSTRACT FROM AUTHOR]

The healing behavior of shrinkage cavity inside the cast Ti6Al4V alloy during hot isostatic pressing (HIP) was investigated experimentally by interrupted hot isostatic pressing tests. The X-ray micro computed tomography was used to record the morphology changes before and after hot isostatic pressing. The two-dimensional geometry obtained by the microCT scan was used in simulation to study the evolution of the real shrinkage cavity during hot isostatic pressing. Shrinkage cavities, shrinkage porosity and small gas pores can be effectively eliminated under proper HIP conditions. The two-dimensional morphology in the simulation results agrees well with the experimental results. This study reveals that plastic deformation, creep and diffusion are the main mechanisms of cavity closure during hot isostatic pressing. In addition, the simplified elliptical pores with aspect ratios at different positions were used to replace the real pores to further study the factors affecting the position of dimples after HIP by simulation. It is found that the position of the dimples mainly depends on the aspect ratio of the elliptical pore and the distance between the pore surface and the external surface of the geometric model. [ABSTRACT FROM AUTHOR]

This paper extends the covariant derivative under curved coordinate systems in 3D Euclid space. Based on the axiom of the covariant form invariability, the classical covariant derivative that can only act on components is extended to the generalized covariant derivative that can act on any geometric quantity including base vectors, vectors and tensors. Under the axiom, the algebra structure of the generalized covariant derivative is proved to be covariant differential ring. Based on the powerful operation capabilities and simple analytical properties of the generalized covariant derivative, the tensor analysis in curved coordinate systems is simplified to a large extent. [ABSTRACT FROM AUTHOR]

This paper extends the classical covariant derivative to the generalized covariant derivative on curved surfaces. The basement for the extension is similar to the previous paper, i.e., the axiom of the covariant form invariability. Based on the generalized covariant derivative, a covariant differential transformation group with orthogonal duality is set up. Through such orthogonal duality, tensor analysis on curved surfaces is simplified intensively. Under the covariant differential transformation group, the differential invariabilities and integral invariabilities are constructed on curved surfaces. [ABSTRACT FROM AUTHOR]

This paper further extends the generalized covariant derivative from the first covariant derivative to the second one on curved surfaces. Through the linear transformation between the first generalized covariant derivative and the second one, the second covariant differential transformation group is set up. Under this transformation group, the second class of differential invariants and integral invariants on curved surfaces is made clear. Besides, the symmetric structure of the tensor analysis on curved surfaces are revealed. [ABSTRACT FROM AUTHOR]

Recent experiments and molecule dynamics simulations have shown that adhesion droplets on conical surfaces may move spontaneously and directionally. Besides, this spontaneous and directional motion is independent of the hydrophilicity and hydrophobicity of the conical surfaces. Aimed at this important phenomenon, a general theoretical explanation is provided from the viewpoint of the geometrization of micro/nano mechanics on curved surfaces. In the extrinsic mechanics on micro/nano soft curved surfaces, we disclose that the curvatures and their extrinsic gradients form the driving forces on the curved spaces. This paper focuses on the intrinsic mechanics on micro/nano hard curved surfaces and the experiment on the spontaneous and directional motion. Based on the pair potentials of particles, the interactions between an isolated particle and a micro/nano hard curved surface are studied, and the geometric foundation for the interactions between the particle and the hard curved surface is analyzed. The following results are derived: (a) Whatever the exponents in the pair potentials may be, the potential of the particle/hard curved surface is always of the unified curvature form, i.e., the potential is always a unified function of the mean curvature and the Gaussian curvature of the curved surface. (b) On the basis of the curvature-based potential, the geometrization of the micro/nano mechanics on hard curved surfaces may be realized. (c) Similar to the extrinsic mechanics on micro/nano soft curved surfaces, in the intrinsic mechanics on micro/nano hard curved surfaces, the curvatures and their intrinsic gradients form the driving forces on the curved spaces. In other words, either on soft curved surfaces or hard curved surfaces and either in the extrinsic mechanics or the intrinsic mechanics, the curvatures and their gradients are all essential factors for the driving forces on the curved spaces. (d) The direction of the driving force induced by the hard curved surface is independent of the hydrophilicity and hydrophobicity of the curved surface, explaining the experimental phenomenon of the spontaneous and directional motion. [ABSTRACT FROM AUTHOR]

Three-phase line tensions may become crucial in the adhesion of micro-nano or small droplets on solid planes. In this paper we study for the first time the nonlinear effects in adhesion spanning the full range of physically possible parameters of surface tension, line tension, and droplet size. It is shown that the nonlinear adhesion solution spaces can be characterized into four regions. Within each region the adhesion behaves essentially the same. Especially, inside the characteristic regions with violent nonlinearities, the co-existence of multiple adhesion states for given materials is disclosed. Besides, two common fixed points in the solution space are revealed. These new results are consistent with numerical analysis and experimental observations reported in the literatures. [ABSTRACT FROM AUTHOR]

The stability and shapes of domains with different bending rigidities in lipid membranes are investigated. These domains can be formed from the inclusion of an impurity in a lipid membrane or from the phase separation within the membrane. We show that, for weak line tensions, surface tensions and finite spontaneous curvatures, an equilibrium phase of protruding circular domains or striped domains may be obtained. We also predict a possible phase transition between the investigated morphologies. [ABSTRACT FROM AUTHOR]