Metal Boundary Research Articles

Effect of grain boundaries on the preferential orientation distribution of θ′ precipitates in stress-aged Al–2Cu alloy bicrystals

Grain boundaries in metals would introduce extra nucleation energy to promote the growth of θ′ precipitate during ageing process. The influence of grain boundaries on the distribution behavior of two θ′ precipitates variants (010) and (001) were studied in Al–2Cu alloys bicrystals. Ageing conditions without external stress and with 40 MPa and 60 MPa stresses were conducted to study the effect of grain boundaries on the oriented distribution of two θ′ precipitates variants. The hardness differences between the near grain boundaries area and the single crystal of the bicrystal samples were tested to understand the relationship between microstructures and mechanical properties. Results of transmission electron microscopy (TEM) images showed that the stress-orienting effect on precipitates was inhibited by the grain boundary. TEM images and quantitatively measurement of two θ′ precipitates variants indicated that grain boundaries inhibited the orienting effect of external stresses on the distribution of precipitates. Especially in 60 MPa stress-aged sample, θ′ precipitates near the grain boundary distribute homogeneously and have the same density as in single crystal areas. Hardness difference in stress-aged sample was smaller compared to the stress-free-aged sample. External stresses during ageing also narrowed the PFZ width near grain boundaries. The anisotropy of precipitation strengthening contribution of two θ′ variants were reduced due to the extra energy provided from grain boundaries. The compressive yield strength of Al–2Cu alloys bicrystal increased with the applied stresses during ageing. This work provides new insight into solving the anisotropy problem during stress ageing based on the effect of grain boundaries.

Atomic structure observations and reaction dynamics simulations on triple phase boundaries in solid-oxide fuel cells

The triple phase boundary (TPB) of metal, oxide, and gas phases in the anode of solid oxide fuel cells plays an important role in determining their performance. Here we explore the TPB structures from two aspects: atomic-resolution microscopy observation and reaction dynamics simulation. Experimentally, two distinct structures are found with different contact angles of metal/oxide interfaces, metal surfaces, and pore opening sizes, which have not previously been adopted in simulations. Reaction dynamics simulations are performed using realistic models for the hydrogen oxidation reaction (HOR) at the TPB, based on extensive development of reactive force field parameters. As a result, the activity of different structures towards HOR is clarified, and a higher activity is obtained on the TPB with smaller pore opening size. Three HOR pathways are identified: two types of hydrogen diffusion processes, and one type of oxygen migration process which is a new pathway.

Synergy of elastic strain energy and electron wind force on thin film grain growth at room temperature

Grain growth in polycrystalline materials is commonly achieved with thermal annealing. Here, we explore an alternate athermal route through a synergy of electrical current and tensile strain at or near room temperature. Our experimental approach involves passing electrical current in 100 nm thick freestanding palladium films with about 5 nm initial grain size. The resulting Joule heating increases the temperature up to 470 K (homologous temperature of 0.25) at the middle section of the specimen. The massive heatsinks at the two ends of the specimen constrain them at the room temperature. At current density of 7 × 105 A/cm2, in-situ transmission electron microscopy shows more than two orders of magnitude grain growth in the high temperature regions and little growth at the room temperature regions. However, application of elastic strain (about 0.1%) dramatically increased the grain size to about 100 nm in a few seconds at the room temperature regions. Our finding suggests that the synergy of elastic strain energy and electrical current density may achieve grain growth in metallic materials even at the room temperature. Molecular dynamics simulation of this phenomenon reveals that the externally applied strain is localized at the grain boundaries in nanocrystalline metals, which promotes the effects of electron wind force on the grain boundary atoms. We conclude that a synergy of two or more stimuli can achieve grain growth at room or even lower temperatures.

Non-Hermitian boundary and interface states in nonreciprocal higher-order topological metals and electrical circuits

Non-Hermitian skin-edge states emerge only at one edge in one-dimensional nonreciprocal chains, where all states are localized at the edge irrespective of eigenvalues. The bulk topological number is the winding number associated with the complex energy spectrum, which is well defined for metals. We study non-Hermitian nonreciprocal systems in higher dimensions, and propose to realize them with the use of electric diode circuits. We first investigate one-dimensional interface states between two domains carrying different topological numbers, where all states are localized at the interface. They are a generalization of the skin-edge states. Then we generalize them into higher dimensions. We show that there emerge a rich variety of boundary states and interface states including surface, line and point states in three-dimensional systems. They emerge at boundaries of several domains carrying different topological numbers. The resulting systems are the first-order, second-order and third-order topological metals. Such states may well be observed by measuring the two-point impedance in diode circuits.

PATTERNS OF MASS TRANSFER BETWEEN THE LADLE BATH AND SURFACE SLAG

The influence specific reagent consumption and the degree assimilation of sulfur desulphurizers on the loss pig iron with coils in the cover slag, which is leaking from the surface melt after completion bucket refining pig iron, is investigated. According to the results analysis industrial data in the conditions bucket pig iron desulphurisation at PJSC «Dneprovsky Metallurgical Plant» it was established that the iron content in the slags, which are rolled after treatment, fluctuates in the range of 60-78%. It is determined that, regardless of the place sampling slag in the height its layer, in the latter is not less than 25% cast iron corks different fractions. The regularities mass exchange between slag and metal bath are determined in conditions change thickness of cover slag and its physical and chemical properties. The classification mechanisms for attracting droplets metal to the amount slag, including by passing the slag - metal into the surface section, with the removal waste gas – metal flow and the draining slag part to the metal bath by circulation flows is proposed.The method determining the size slag – metal gas bubbles floating on the boundary is proposed. The gas bubbles flowing to the upper horizons bath, on the surface which undergoes a reaction formation refractory magnesium sulfide, are surrounded by a film of a metallic melt. The collapse gas bubbles, surrounded by a metallic film, leads to the formation droplets of iron, which are driven by the flow metal into a layer slag. The peculiarities formation and maintenance cast iron cores with surface slag are investigated. It is shown that the droplets cast in the slag in turn coalesce and, provided that they reach a critical size (3-5 mm), when the force gravity becomes more than the force surface tension, settle down into the lower layers slag to the metal boundary, and under conditions sufficient exposure – return to the metal bath. In order to study the features tightening the cover slag to the volume a metal bath from the interfacial slag-metal boundary, grappling with metal droplets, the development mass transfer processes, including the use a mixture reagents of a rotary submersible lance, a series low-temperature experiments using a full factor experiment on a model heavy-duty flood bucket. The factors influence include: the speed submersible lance, the height layer of cover slag, the flow gas. The mathematical models are obtained, which are suitable for determining the degree influence investigated factors on the depth dragging slag to a metal bath. The defining factors influence are as follows: at a rotational speed submersible lance from 0 to 90 rpm. – the speed lance and the thickness layer of cover slag; from 90 to 120 rpm. – thickness layer of cover slag; from 120 to 240 rpm. – speed of lance rotation.

Structural Vacancy Model of Grain Boundaries

A structural vacancy model of tilt grain boundaries in metals has been developed. For construction of the stable structure of the boundary, the initial pattern was chosen according to the CSL model. The introduction of additional atoms and vacancies into the boundary region and shifting of atoms by the interatomic forces stabilize its structure. The criterion of a stable structure is the grain-boundary energy. The comparison of two main approaches to the stabilization of the grain structure demonstrated that changing the number of atoms at the boundary is more energetically advantageous than the relative shift of grains. The stability of the structure obtained has been studied under the shear stress. In the model developed, atomic structures obtained with pair and many-body potentials have been compared. The comparative analysis has shown that the grain-boundary structure does not depend on the choice of potential; atomic positions differ by less than 0.1 A, which is 2.5% of the lattice parameter. The atomic structure is in agreement with experimental images of grain boundaries.

Analysis of Weld Cold Cracking In Excavator Bucket Adapter

This paper presents a failure analysis on welding of bucket adapter for an excavator that made of low carbon steel with Ideal Diameter (DI) value 341 and Carbon Equivalent 0.73. The bucket component is stated OK as the final inspection, but four months later found cracked and even it has not been used yet. Adapter mounted to bucket through CO2 gas shielded Gas Metal Arc Welding (GMAW) process, filler metal MG-50 (AWS A5.18 ER70S-G), pre-heating and post-heating. Crack occurred on the boundary of weld metal and adapter casting. The metallurgical examination was conducted in the crack origin and the adapter casting. Nondestructive test (NDT) covered visual and magnetic test (MT), while destructive test (DT) covered fractography, mechanical properties, and Electron Probe Micro Analyzer (EPMA). The investigation resulted in that crack initiated from slag inclusion in the weld metal due to insufficient cleaning on weld joint. The higher hardness of martensite structure in the heat affected zone (HAZ) of adapter indicates high cooling rate that gen erates high residual stress. High stress that caused by insufficient post heating triggered severe crack propagation. By its characteristic, crack is called the cold cracking, delayed cracking, or hydrogen-induced cracking (HIC).

Effect of carbon and oxygen impurity atoms on the migration rate of tilt boundaries in fcc metals: a molecular dynamics simulation

Effect of carbon and oxygen impurity atoms on the migration rate of tilt boundaries in fcc metals: a molecular dynamics simulation

Investigation of grain boundary and orientation effects in polycrystalline metals by a dislocation-based crystal plasticity model

Polycrystalline metal with well-designed grain structure exhibits different strength and ductility compared with the conventional macro scale materials. In the present study, the mechanisms of strain hardening and inhomogeneous deformation in polycrystals are analyzed by considering the interaction between dislocations and grain boundaries. Firstly, a dislocation density based crystal plasticity model is developed and then used to quantitatively study the grain size and orientation effects in polycrystals. With the decrease of grain size, the accumulation and interaction of dislocations are promoted in the grain, resulting in back-stress hardening. Simultaneously, two opposite effects of the inhomogeneity in grain orientation are obtained by evaluating the local Schmidt factors in each grain and their standard deviation. The findings highlight the effect of grain boundary on the strength and provide quantitative insight into the effect of grain orientation on ductility.

Helium irradiation of Y2O3-Fe bilayer system

Y2O3/Fe bilayer systems and associated interfaces with different crystallographic orientations were implanted with helium. High resolution scanning transmission electron microscopy (HRSTEM) analysis showed that implanted helium formed larger sized bubbles at the interface than within the individual film layers. The bubbles at the interface were located on the oxide side of the interface and only “wetted” a small area of the interface. In contrast, bubbles nucleating at grain boundaries within the iron layer were observed to symmetrically form across the grain boundaries, suggesting remarkable differences in helium bubble nucleation between metal/oxide interfaces and grain boundaries in metals.

Dislocation absorption and transmutation at [formula omitted] twin boundaries in deformation of magnesium

How matrix dislocations, i.e. basal, prismatic and pyramidal, interact with {101¯2}101¯1¯ twin boundaries in hexagonal close-packed metals has been discussed extensively in the literature. However, so far no systematic investigation has been reported. In this work, we performed atomistic simulations to study interaction between matrix dislocations in pure Mg with a {101¯2} twin boundary. Our results show that for the basal and the prismatic slip, when the Burgers vector is parallel to the zone axis of the twins, a matrix basal dislocation can be transmuted to a twin prismatic dislocation and vice versa. However, when the Burgers vector of the matrix dislocation is non-parallel to the zone axis, no transmutation occurs and the dislocation is absorbed by the twin boundary which acts as a dislocation sink. For a matrix pyramidal dislocation, the dislocation is absorbed by the twin boundary and no transmutation occurs either. It appears that if the product dislocation is a real slip system, transmutation may occur during twin-slip interaction. Otherwise the matrix dislocation will be shredded by atomic shuffling and then absorbed by the twin boundary. If the core structure of the product dislocation is complex, transmutation may not occur as well and dislocation absorption will occur. Lattice correspondence in deformation twinning was applied in explaining the interaction mechanisms. Our results can be well correlated to macroscopic experimental observations which show twin-slip interaction only contributes negligibly to work hardening in deformation of hcp metals.

An efficient Monte Carlo algorithm for determining the minimum energy structures of metallic grain boundaries

Sampling minimum energy grain boundary (GB) structures in the five-dimensional crystallographic phase space can provide much-needed insight into how GB crystallography affects various interfacial properties. However, the complexity and number of parameters involved often limits the extent of this exploration to a small set of interfaces. In this article, we present a fast Monte Carlo scheme for generating zero-Kelvin, low energy GB structures in the five-dimensional crystallographic phase space. The Monte Carlo trial moves include removal and insertion of atoms in the GB region, which create a diverse set of GB configurations and result in a rapid convergence to the low energy structure. We have validated the robustness of this approach by simulating over 1184 tilt, twist, and mixed character GBs in both fcc (Aluminum and Nickel) and bcc (α-Iron) metallic systems.

To the Question of Forming Geometrically Necessary Disclinations in Triple Junctions of Grain Boundaries in Metals

The Bollman and King models are tested by means of molecular dynamics simulation for the formation of geometrically necessary disclinations in triple junctions of grain boundaries in metals. It is shown that the stresses arising in a triple junction due to the non-multiple lengths of low-angle tilt boundaries to the distance between grain boundary dislocations is not compensated for mainly by the formation of an additional disclination in the junction (the King model) but by the bending of one or several grain boundaries, accompanied by the displacement of grain boundary dislocations. A triple junction of the Bollman U-type (containing a geometrically necessary disclination) is not formed at the conjugation of tilt boundaries with common misorientation along the junction or at the conjugation of mixed-type boundaries.

Dislocation Content of Non-Classical Twin Boundaries in Hexagonal Metals

Dislocation Content of Non-Classical Twin Boundaries in Hexagonal Metals

Observation of Cu-rich grain boundary nanoparticles and complexions in Cu/Ti-doped alumina

Alumina powders were processed with 7.8 wt% CuO and post-sintering reduction heat-treatments were applied with the aim of producing metallic grain boundary complexions. The influence of titanium co-doping was also studied. Grain boundaries were characterized by aberration-corrected scanning transmission electron microscopy. Copper grain boundary enrichment was achieved, but only in the titanium doped samples; the segregated copper exhibited a Cu2+ valency state, the segregated titanium exhibited Ti3+ or Ti4+ valency states, and metallic Cu nanoparticles were also observed. The crystallographic orientation of the grain boundary plane had a marked influence on concentration and oxidation state of the segregating species.

Understanding the anomalous thermal behavior of Σ3 grain boundaries in a variety of FCC metals

We present a case study of the complex temperature dependence of grain boundary mobility. The same general incoherent twin boundary in different FCC metals is found to display antithermal, thermal, and mixed mobility during molecular dynamics synthetic driving force simulations. A recently developed energy metric known as the generalized interfacial fault energy (GIFE) surface is used to show that twin boundaries moving in an antithermal manner have a lower energetic barrier to motion than twin boundaries moving in a thermally activated manner. Predicting the temperature dependence of grain boundary motion with GIFE curves stands to accelerate research in grain boundary science.

Observation of the Recrystallization Process in Pure Nb and Zr Using Positron Lifetime Spectroscopy and XRD Techniques

Results of research on the recrystallization of highly deformed Nb and Zr with the use of positron lifetime spectroscopy and XRD measurements are presented here. A significant decrease in positron lifetime during annealing of deformed Nb and Zr samples is observed, which is clearly correlated with changes in the pole figures and reduction of the crystal lattice strain of the tested samples. This indicates their recrystallization, that is, nucleation and growth of new, defect free grains. Application of the positron diffusion trapping model to description of the obtained dependencies allows us to determine the activation energy of migration of grain boundary in both metals, which is 2.26 ± 0.4 eV and 1.33 ± 0.6 eV for Nb and Zr, respectively.

Slip transmission of high angle grain boundaries in body-centered cubic metals: Micropillar compression of pure Ta single and bi-crystals

Here we seek to probe and understand the mechanical behavior of grain boundaries in pure Ta as a model bcc material using micropillar compression experiments. Three high angle grain boundaries are chosen with varying crystal orientations. Multiple bi-crystal pillars are prepared containing a single, nearly vertical grain boundary in the approximate center of the pillar and compared against their single crystalline pillar counterparts. The main phenomenon of interest was slip transmission or strain transfer in the bi-crystals which was considered to occur when slip traces aligned across the grain boundary. This occurred in two of the three bi-crystals. These observations were compared against two slip transmission factors, m'=cos(ψ)cos(κ) and LRB=cos(θ)cos(κ), where ψ,θ,andκ are the angles between the slip vectors, slip plane normals, and the intersection of the slip planes with the grain boundary from the slip systems on either side of the grain boundary. Additionally, transmission was compared against the stress-strain response and overall deformation (i.e., sheared boundary) of the bi-crystal. The transmission factors exhibited a consistent behavior with slip transmission occurring for high transmission factors and not occurring for low transmission factors. For example, slip transmission occurred for m'≥ 0.85 and did not occur for m'≤ 0.46. The engineering stress-strain response and overall deformation behavior did not show correlations with the presence or absence of slip transmission. High angle boundaries in bcc metals are shown to represent a diverse set of responses in bi-crystalline micropillar compression experiments.

T-Type Equivalent Circuit of On-Chip Microstrip Line With Magnetic Film-Type Noise Suppressor

This paper discusses $R$ , $L$ , $G$ , and $C$ parameters of an on-chip transmission line with a magnetic film for developing the design rule of a film-type electromagnetic noise suppressor. A microstrip line (MSL) is fabricated using a regular silicon process, and a Co85Zr3Nb12(Co–Zr–Nb) film is deposited on the MSL. The target frequency of the noise suppression and the passband are set to 5 GHz and a several hundred MHz range, respectively. The T-type equivalent circuit model is converted from the measured S-parameter. In the passband frequency range, the resistance value of the MSL with the magnetic film is almost the same as that without the film. The measured resistance successfully peaks at $34~\Omega $ around the 5.4 GHz target frequency based on the ferromagnetic resonance and Joule losses. Furthermore, the inductance and the capacitance with the magnetic film are 1.3 and 2.5 times higher, respectively, than without the film because of the high permeability and metallic boundary of the magnetic film. Therefore, these results demonstrate the great usefulness of the approach for developing a standardized design method for a magnetic film-type noise suppressor.

Features of Uniaxial and Biaxial Deformation of the Contact Boundary of Metals during Explosion Welding

X-ray diffraction and scanning electron microscopy are used to study the nature of deformation of the contact boundary (weld seam) between dissimilar metals (Cu–Ti and Cu–Al) formed during explosion welding. Under biaxial dynamic loading, a significant plastic deformation of the contact zone and its heating up to higher temperatures are shown to occur. This is confirmed by the formation of intermetallic compounds in the weld seam. The level of plastic deformation of the weld zone can be estimated on the basis of the crystallization temperature of intermetallic compounds.