Metal Boundary Research Articles

Metallic High-Angle Grain Boundaries in Monolayer Polycrystalline WS2.

Metallic High-Angle Grain Boundaries in Monolayer Polycrystalline WS2.

The observation of structural multiplicity in Σ5(310) grain boundaries in FCC metals

Grain boundaries (GBs) have often been characteristically defined by the global minimum energy atomic structural configuration. However, less favourable thermodynamic structures may become significant to accommodate non-equilibrium crystalline transformations and defect interactions. This paper provides a concise overview of recent findings that demonstrate strong corroborative evidence matching the structural characteristics of a few metastable structures observed in previous studies of Σ5(310) GBs.

Segregation-induced changes in grain boundary cohesion and embrittlement in binary alloys

Grain boundary embrittlement occurs when a solute enriches at a grain boundary and lowers its cohesive energy. While grain boundary enrichment is often attributed to equilibrium segregation effects, most models of embrittlement consider either the energetics of decohesion or the equilibrium adsorption at the boundary, but not both phenomena together. We develop a model for the change in cohesive energy of a grain boundary of a pure metal upon introduction of solute under conditions of equilibrium segregation prior to fracture. A heuristic grain boundary cohesion map is presented to delineate whether a given solute–solvent pair will exhibit weakening or strengthening of grain boundaries. The analysis helps to clarify that grain boundary embrittlement requires a solute that will both lower the cohesive energy of the boundary and segregate in the first place. The map reasonably captures known metal–metal embrittling pairs.

Structural characterization of [formula omitted] irregular-shaped twinning boundary in hexagonal close-packed metals

In deformation twinning, the twinning boundary (TB) contour usually looks elliptical. Here, the 101¯2 TB contours are investigated in AZ31, Ti and Co by using transmission electron microscopy (TEM) and high-resolution TEM, suggesting a very irregular shape. The formation of a 101¯2 irregular-shaped TB is closely related to the interface defects and basal–prismatic interfaces, which place basal and prismatic planes face to face.

Vacuum Casimir energy densities and field divergences at boundaries

We consider and review the emergence of singular field fluctuations or energy densities at sharp boundaries or point-like field sources in the vacuum. The presence of singular energy densities of a field may be relevant from a conceptual point of view, because they contribute to the self-energy of the system. They could also generate significant gravitational effects. We first consider the case of the interface between a metallic boundary and the vacuum, and obtain the structure of the singular electric and magnetic energy densities at the interface through an appropriate limit from a dielectric to an ideal conductor. Then, we consider the case of a nondispersive and nondissipative point-like source of the electromagnetic field, described by its polarizability, and show that also in this case the electric and magnetic energy densities show a singular structure at the source position. We discuss how, in both cases, these singularities give an essential contribution to the electromagnetic self-energy of the system; moreover, they solve an apparent inconsistency between the space integral of the field energy density and the average value of the field Hamiltonian. The singular behavior we find is softened, or even eliminated, for boundaries fluctuating in space and for extended field sources. We discuss in detail the case in which a reflecting boundary is not fixed in space but is allowed to move around an equilibrium position, under the effect of quantum fluctuations of its position. Specifically, we consider the simple case of a 1D massless scalar field in a cavity with one fixed and one mobile wall described quantum-mechanically. We investigate how the possible motion of the wall changes the vacuum fluctuations and the energy density of the field, compared with the fixed-wall case. Also, we explicitly show how the fluctuating motion of the wall smears out the singular behaviour of the field energy density at the boundary.

Measurement of heat transfer coefficients at up to 25,500 g – A sensor test at a rotating free disk with complex telemetric instrumentation

Heat transfer coefficients (HTC) are important for the design of efficient energy machines as for instance gas turbines for industrial, power generation and propulsion applications. Published HTC measurement data considering geometries and operating conditions which are relevant for these applications are very rare or not available.This study presents an experimental investigation using a telemetric setup for the direct HTC measurement at metallic boundaries with gaseous flow. The rotor rig has a complex instrumentation using one of the in house developed telemetry systems. The experimental investigation relies on a recent measurement/sensor concept tested for the first time in the rotating frame with high centrifugal acceleration. The measurement setup has miniaturized sensor dimensions and low energy consumption. Therefore, the sensor concept is suitable for integration in a telemetry system as necessary for turbomachinery research. The sensor setup features good local resolution, short response time and a high resolution versus temperature and HTC.Measurements of the radial distribution of the heat transfer coefficient of a rotating free disk are presented. Additionally a comparison with correlations found in literature as well as a calibration of the HTC sensors for the test case forced convection at the flat plate is included.

Crossing grain boundaries in metals by slip bands, cleavage and fatigue cracks.

The size and the character (low and large angle, special boundaries, tilt and twist boundaries, twins) of the grain boundaries (GBs) in polycrystalline materials influence their strength and their fracture toughness. Recent studies devoted to nanocrystalline (NC) materials have shown a deviation from the Hall-Petch law. Special GBs formed by Σ3 twins in face-centred cubic metals are also known to have a strong effect on the mechanical behaviour of these metals, in particular their work-hardening rate. Grain orientation influences also crack path, the fracture toughness of body-centred cubic (BCC) metals and the fatigue crack growth rate of microstructurally short cracks. This paper deals both with slip transfer at GBs and with the interactions between propagating cracks with GBs. In the analysis of slip transfer, the emphasis is placed on twin boundaries (TBs) for which the dislocation reactions during slip transfer are analysed theoretically, experimentally and using the results of atomic molecular simulations published in the literature. It is shown that in a number of situations this transfer leads to a normal motion of the TB owing to the displacement of partial dislocations along the TB. This motion can generate a de-twinning effect observed in particular in NC metals. Crack propagation across GBs is also considered. It is shown that cleavage crack path behaviour in BCC metals is largely dependent on the twist component of the GBs. A mechanism for the propagation of these twisted cracks involving a segmentation of the crack front and the existence of intergranular parts is discussed and verified for a pressure vessel steel. A similar segmentation seems to occur for short fatigue cracks although, quite surprisingly, this crossing mechanism for fatigue cracks does not seem to have been examined in very much detail in the literature. Metallurgical methods used to improve the strength of the materials, via grain boundaries, are briefly discussed.

Grain boundary interfacial plasticity with incorporation of internal structure and energy

Modelling the behaviour of grain boundaries in polycrystalline metals using macroscopic continuum frameworks demands a multi-scale description of the underlying details of the structure and energy of grain boundaries. The objective in this work is the incorporation of a multi-scale atomistic-to-continuum approach of the initial grain boundary structure and energy into a grain boundary extended crystal plasticity framework in order to investigate the role of the grain boundary energetics on the macroscopic response. To this end, the methodology includes: (i) the generalisation of the atomistic-to-continuum results of the initial grain boundary structure and energy, (ii) an analytical analysis of the resulting grain boundary energetics in the continuum framework, (iii) the numerical implementation of the developed framework in the case of a periodic bicrystal subjected to simple shear deformation considering a symmetric tilt boundary system in the full misorientation range. This work provides a step forward towards the physically based continuum modelling of grain boundary interfacial plasticity.

Landau damping in a bounded magnetized plasma column

The damping decrement of Landau damping and the effect of thermal velocity on the frequency spectrum of a propagating wave in a bounded plasma column are investigated. The magnetized plasma column partially filling a cylindrical metallic tube is considered to be collisionless and non-degenerate. The Landau damping is due to the thermal motion of charge carriers and appears whenever the phase velocity of the plasma waves exceeds the thermal velocity of carriers. The analysis is based on a self-consistent kinetic theory and the solutions of the wave equation in a cylindrical plasma waveguide are presented using Vlasov and Maxwell equations. The hybrid mode dispersion equation for the cylindrical plasma waveguide is obtained through the application of appropriate boundary conditions to the plasma–vacuum interface. The dependence of Landau damping on plasma parameters and the effects of the metallic tube boundary on the dispersion characteristics of plasma and waveguide modes are investigated in detail through numerical calculations.

Research on the development mechanism: from two-sided multipactor to one-sided multipactor

Multipactor discharge always causes disastrous damage to a vacuum window in high power microwave system, which actually becomes a limiting factor for the output power of vacuum device. To explore the multipactor phenomenon of complicated pill-box window, the mulitpactor in normal field between the metal boundary and the window disk is studied. Through Monte Carlo (MC) simulations, the susceptive curve is fitted and analyzed. The secondary electrons' avalanche behavior under the normal RF field is discussed. It is noticed that the one-sided multipactor is excited within a very limited Vrf-fD region when two-sided multipactor is excited initially. The development condition from two-sided multipactor to the one-sided multipactor is proposed. Through analyzing and MC simulation, the condition is achieved. When the normal RF electric field can satisfy the phase focus conditions of one-sided multipactor, the two-sided multipactor will develop into one-sided multpactor and then reach a saturation value. Meanwhile, the initial effect of electrostatic field on one-sided multipactor is also discussed. On condition that two-sided multipactor can be excited, the number of secondary electrons can increase up to a saturation value when Edc0 is lower than the minimal saturate value of Edc. When Edc0 is lager than the minimal saturate value of Edc and in the Edc/Erf threshold of one-sided resonant multipactor, the number of secondary electrons can also increase to a saturate value. However, When Edc0 is lager than the minimal saturate value of Edc but beyond the Edc/Erf threshold of one-sided resonant multipactor, secondary electrons will be suppressed.

Evaluation of microstructure and texture across the welded interface of super duplex stainless steel and high strength low alloy steel

The evolution of microstructure and texture across the fusion boundary of a dual-layer super duplex stainless steel clad metal produced on a high strength low alloy steel substrate by gas tungsten arc welding process was examined by optical microscopy, X-ray diffraction, electron backscatter diffraction, and energy-dispersive X-ray spectroscopy. It was found that a martensitic band with occasional Type I and Type II boundaries separated the substrate's heat affected zone from the high austenite containing region of the clad metal. Due to the competitive growth, both the ferrite and austenite grains showed almost the same <001>║ND orientation near the fusion boundary. While the texture of the austenite was not as strong as that of the ferrite across the cladding layers, the results confirmed that the austenite daughter phase was formed with a close Kurdjumov–Sachs orientation relationship with respect to the parent ferrite phase. It was also found that although both the residual stress and reheating caused some microstructural transformation and texture modification to some parts of the cladding, the major factors affecting the texture were unidirectional solidification, competitive growth, and δ/γ orientation relationship. The only exception was related to the occasional highly deformed mostly austenite area adjacent to the fusion boundary, where partial recrystallization led to formation of some annealing twins. The findings suggested that the reheating which occurred during the deposition of the second layer generated higher ferrite content and produced some secondary austenite with dominant Widmanstätten morphology across the first cladding layer. Moreover, it imposed higher residual strain and also promoted limited recrystallization adjacent to the fusion boundary.

First-principles study of energy and atomic solubility of twinning-associated boundaries in hexagonal metals

Twinning-associated boundaries (TB), {101¯n} coherent twin boundaries (CTB) and the coherent basal–prismatic (CBP) boundary in six hexagonal metals (Cd, Zn, Mg, Zr, Ti and Be) are studied using first-principles density function theory, with the focus on the structural character of TB and the solute’s solubility at TB. Regarding the structure and energy of TB, the formation of TB is associated with the creation of an excess volume. All six metals show positive excess volume associated with (101¯1) and (101¯3) CTB, but the excess volume associated with (101¯2) CTB and CBP can be positive or negative, depending on the metal. The (101¯2) CTB has higher excess energy than (101¯1) and (101¯3) CTB for metals with c/a<8/3 , but lower for metals with c/a>8/3. More interestingly, CBP has lower excess energy than (101¯2) CTB for all metals. This is consistent with the recent finding concerning the pure-shuffle nucleation mechanism of (101¯2) twins. To understand solubility at TB, the solubility of solute atoms in Mg, Ti and Zr is calculated for solute positions in bulk, (101¯2) CTB and CBP boundaries. In general, solute atoms have better solubility at CTB and CBP than in bulk. Interestingly, the solubility of solute atoms changes linearly with normal strain at CBP, increasing with normal strain for solute atoms with a greater metallic radius than the matrix, and decreasing with normal strain for solute atoms with a smaller metallic radius than the matrix. This suggests that the distribution of solute atoms in bulk, CTB and CPB boundaries varies with stress state and, in turn, affects the mobility of TB.

Exact Eigenfrequencies in Concentric Prolate Spheroidal-Spherical Metallic Cavities

In this letter the eigenfrequencies in prolate spheroidal-spherical and spherical-spheroidal cavities are calculated. The metallic boundaries of the cavities are perfect conductors. The matrix equations are constructed using expansions connecting the spherical and the spheroidal eigenvectors. Then, by truncating the resulting matrices, the exact eigenfrequencies are obtained as the converged roots of determinantal equations. This method allows to study elongated spheroidal cavities that existing methods fail to handle. The method is validated by comparison with published results from the literature. Original results for spheroids having large eccentricities are given for various values of the parameters.

Interaction of dislocations with low angle tilt boundaries in fcc crystals

The interaction of dislocations with low angle tilt boundaries (LATB) in fcc metals was studied using discrete dislocation dynamics. A symmetrical LATB was constructed using glissile edge dislocations. Free glissile dislocations on different slip systems were allowed to react with the boundary to produce an array of binary junctions of the type collinear, Lomer, and two variants of the glissile junction glissile-a and glissile-b. The simulations were run at constant shear strain rate as computed on the slip system of the incoming dislocation. The transmission resistance was taken as the minimum resolved shear stress required to break the array of junctions. The transmission resistance was higher for collinear and glissile-a junctions, which had a 90° angle between glide plane of the incoming dislocation and the LATB plane, as compared to Lomer and glissile-b for which the angle was 35.2°. With increasing dislocation density within the boundary the transmission resistance increased for all interactions, with the increase being higher for collinear and glissile-a than for Lomer and glissile-b. The transmission resistance was found to be inversely related to the unreacted arm length of the incoming dislocation.

The reciprocal relationship of orientation dependence of the dislocation boundaries in body-centered cubic metals and face-centered cubic metals

Abstract The orientation dependence of deformation microstructure has been investigated in uniaxial deformed body-centered cubic molybdenum by electron backscatter diffraction and transmission electron microscopy. It has been found that the dislocation boundaries in both tensile and compressed molybdenum, similar to that in face-centered cubic metals, can be classified into three types: dislocation cells (Type 2), and extended planar boundaries parallel to (Type 1) or not parallel to (Type 3) a {110} trace. Furthermore, it shows a reciprocal relationship between body-centered cubic metals and face-centered cubic metals on the orientation dependence of the deformation microstructure. Type 3 grains have stress axes direction which is near the [110] corner in the unit triangle for BCC metals, whereas Type 3 grains have stress axes direction which is near the [111] corner for FCC metals. The analysis of Schmid factors shows that the dislocation structure depends on the slip systems.

Migration behaviors of boundaries in polycrystalline metals during grain growth process

New equations for grain growth process induced by migration of boundary ligaments and corners were deduced, in order to overcome the shortcoming of the conventional basic theory for normal grain growth in three dimensional polycrystals. The Kelvin's 14-sided polyhedron was assumed as basic shape of the grains. The evolutions of ligament density and corner density during grain growth as well as their influences on grain growth rate and migration activation enthalpy were analyzed. The grain growth process of an experimental Fe-3%Si alloy was simulated based on the new equations. It was indicated that the boundary migration driven by motion of ligaments and corners, especially the latter one, comes closer to the real grain growth process, and the shortcoming in conventional equation has been modified obviously. The evolution tendency of activation enthalpy for boundary migration calculated agrees with the actual observations. The new equations did not yet include the effects of second phase particles and solute atoms in three dimensional polycrystalline metals, and therefore could not describe the real process perfectly, which needs to be improved further.

Probing local strain at MX(2)-metal boundaries with surface plasmon-enhanced Raman scattering.

Interactions between metal and atomically thin two-dimensional (2D) materials can exhibit interesting physical behaviors that are of both fundamental interests and technological importance. In addition to forming a metal–semiconductor Schottky junction that is critical for electrical transport, metal deposited on 2D layered materials can also generate a local mechanical strain. We investigate the local strain at the boundaries between metal (Ag, Au) nanoparticles and MX2 (M = Mo, W; X = S) layers by exploiting the strong local field enhancement at the boundary in surface plasmon-enhanced Raman scattering (SERS). We show that the local mechanical strain splits both the in-plane vibration mode E2g(1) and the out-of-plane vibration mode A1g in monolayer MoS2, and activates the in-plane mode E1g that is normally forbidden in backscattering Raman process. In comparison, the effects of mechanical strain in thicker MoS2 layers are significantly weaker. We also observe that photoluminescence from the indirect bandgap transition (when the number of layers is ≥2) is quenched with the metal deposition, while a softened and broadened shoulder peak emerges close to the original direct-bandgap transition because of the mechanical strain. The strain at metal–MX2 boundaries, which locally modifies the electronic and phonon structures of MX2, can have important effects on electrical transport through the metal–MX2 contact.

Nature of high-angle grain boundaries in metals subjected to severe plastic deformation

The evolution of structural parameters of deformation fragments and dynamically recrystallized grains during severe plastic deformation in a Bridgman chamber as the number of complete revolutions in torsion at room temperature increases is analyzed using transmission electron microscopy and electron backscattered diffraction. It is found that the formation of a significant amount of high-angle grain boundaries in the structure is caused not by deformation fragmentation but by the low temperature dynamic recrystallization.

Brazing of copper to stainless steel with a low-silver-content brazing filler metal

The brazing of copper to stainless steel (SUS304 JIS) was performed using a low- silver-content brazing filler metal, Ag-50Cu, under an Ar gas atmosphere with a conventional furnace, owing to the potential economic benefits of using low-silver-content filler metals. The brazeability of the low-silver-content brazing filler metal to copper and SUS304 was investigated. A good joint was obtained, and a drastic dissolution reaction occurred at the copper side. Molten BAg8 penetrated along the crystal grain boundary of the copper base metal when BAg8 was used as the filler metal. This was caused by the dissolution of Ni from the stainless steel into the molten filler metal. Ag-50Cu, which was investigated in this work, can be used instead of BAg8 filler metal.

Nearly perfect absorption in a single-layer metallic grating with rectangular grooves on its front surface

A subwavelength metallic grating can support horizontal surface plasmons (HSPs) at its horizontal metallic boundaries and vertical cavity modes (CMs) inside its slits. It has been shown that the coupling between these two resonant modes can enhance the absorption of the transverse magnetic polarized wave with the maximum absorption up to 75 % (Roszkiewicz et al. in Opt Lett 37(18):3759–3761, 2012). In this work, we propose and analyze a modified grating structure and show that it is possible to raise the absorption to nearly 100 %. The modified structure is a freestanding metallic grating with rectangular grooves on its front surface which can change the distribution of the HSPs on the same side, while leave those on the other side unaffected. When the HSPs on the front surface are changed to some certain situations, all incident energy can be launched into the grating slits by the CMs resonance and then be prohibited from transmitting through the grating by the HSPs on the back surface. Therefore, in such a single-layer metallic grating structure, all of the incident energy is absorbed, i.e., nearly 100 % absorption is obtained.