Grain Boundary Atoms Research Articles

Energetics and structure of ⟨0 0 1⟩ tilt grain boundaries in SiC

We have developed a scheme, based on molecular dynamics, that allows finding minimum energy structures of grain boundaries (GBs) with relatively large cell of non-identical displacements. This scheme has been used to study symmetric ⟨0 0 1⟩ tilt GBs in cubic SiC. We analyze atomic configurations of dislocation cores found in low-angle GBs and we report structural units found in high-angle GBs. In contrast to what had been previously assumed we find that the lowest energy structures often do not favor perfect coordination of GB atoms and that most of the analyzed GBs contain 6- and 7-atom rings. We tested the applicability of existing empirical potentials to studies of high-symmetry GB structures in SiC and we found the Tersoff potential to be most appropriate. Knowledge of detailed atomic structures of GBs is essential for future studies of GB-controlled phenomena in SiC, such as diffusion of metallic fission product through this material or GB strengthening.

The Effect of Solute Atoms on Aluminum Grain Boundary Sliding at Elevated Temperature

Grain boundary sliding (GBS) is an important deformation mechanism for elevated temperature forming processes. Molecular dynamics simulations are used to investigate the effect of solute atoms in near grain boundaries (GBs) on the sliding of Al bicrystals at 750 K (477 °C). The threshold stress for GBS is computed for a variety of GBs with different structures and energies. Without solute atoms, low-energy GBs tend to exhibit significantly less sliding than high-energy GBs. Simulation results show that elements which tend to phase segregate from Al, such as Si, can enhance GBS in high-energy GBs by weakening Al bonds and by increasing atomic mobility. In comparison, intermetallic forming elements, such as Mg, will form immobile Mg-Al clusters, decrease diffusivity, and inhibit GBS.

Structural changes during annealing of submicro- and nanocrystalline aluminum alloys

The annealing-induced evolution of the structure and microhardness of submicro-and nanocrystalline Al—3% Mg and Al 1570 alloys produced by torsional severe plastic deformation are studied. Annealing of the Al-3% Mg alloy at 373–423 K and annealing of the Al 1570 alloy at 373–473 K are shown to result in the relaxation of internal stresses and subsequent normal grain growth. As the annealing temperature increases, the microhardness decreases. At higher temperatures (473 K for the Al—3% Mg alloy and 573 K for the Al 1570 alloy), anomalous grain growth takes place. This growth is accompanied by the appearance of numerous grains with a high dislocation density, a high concentration of impurity atoms in grain boundaries, and an increase in the microhardness. These effects are explained.

Cohesive and magnetic properties of grain boundaries in bcc Fe with Cr additions

Structural, cohesive, and magnetic properties of two symmetric $\ensuremath{\Sigma}3(111)$ and $\ensuremath{\Sigma}5(210)$ tilt grain boundaries (GBs) in pure bcc Fe and in dilute FeCr alloys are studied from first principles. Different concentrations and positions of Cr solute atoms are considered. We found that Cr atoms placed in the GB interstice enhance the cohesion by $0.5--1.2\text{ }\text{J}/{\text{m}}^{2}$. Substitutional Cr in the layers adjacent to the boundary shows anisotropic effect on the GB cohesion: it is neutral when placed in the (111) oriented Fe grains and enhances cohesion by $0.5\text{ }\text{J}/{\text{m}}^{2}$ when substituted in the boundary layer of the (210) grains. The strengthening effect of the Cr solute is dominated by the chemical component of the adhesive binding energy. Our calculations show that unlike the free iron surfaces, Cr impurities segregate to the boundaries of the Fe grains. The magnetic moments on GB atoms are substantially changed and their variation correlates with the corresponding relaxation pattern of the GB planes. The moments on Cr additions are two to four times enhanced in comparison with that in a Cr crystal and are antiparallel to the moments on the Fe atoms.

Effect of hydrogen annealing on L10 ordering transformation and magnetic properties of CoPt thin films on B4C underlayer

The influence of hydrogen annealing on the morphology, microstructure and magnetic properties of CoPt films has been investigated. After changing the annealing environment from vacuum to hydrogen, the coercivity of the CoPt films increased from 610 to 2100 Oe at 500 °C. The more complete phase transformation may be attributed to the dissolution of hydrogen atoms in the interstitial sites of the CoPt lattice. Moreover, the size of the CoPt grains is smaller and the distribution is much more uniform in the hydrogen-annealed samples. The morphology change may be due to the existence of hydrogen atoms in the CoPt grain boundary which can inhibit the growth and coalescence of the CoPt grains.

Atom Redistribution during co-Doped Amorphous Silicon Crystallization

Atom redistribution during crystallization of a B and P co-doped amorphous Si layer produced by Si and P chemical vapor co-deposition and B implantation has been investigated. The crystallization of the entire layer is quasi-instantaneous for annealing temperature greater than 650 °C. The crystallization rate is well reproduced by the Avrami-Johnson-Mehl-Kolmogorov model of transformation. The Avrami n is found equal to 4, which is corresponding to 3D bulk crystallization. Crystallization promotes a non-Fickian redistribution of B atoms, allowing for an abrupt interface between B-doped and B-undoped regions. After crystallization, B diffuses in the polycrystalline Si layer for concentrations lower than 1.5  1020 at cm3 via the type B kinetic regime. Crystallization has no significant (or detectable) influence on the P profile. For temperatures higher than 750 °C, P diffuses in the poly-Si layer towards the region of highest B concentration via the type B kinetic regime, leading to P uphill diffusion. This phenomenon can be simulated considering chemical interactions between B and P atoms in both grains and grain boundaries.

Phase-field modeling of gas bubbles and thermal conductivity evolution in nuclear fuels

A phase-field model was developed to simulate the accumulation and transport of fission products and the evolution of gas bubble microstructures in nuclear fuels. The model takes into account the generation of gas atoms and vacancies, and the elastic interaction between diffusive species and defects as well as the inhomogeneity of elasticity and diffusivity. The simulations show that gas bubble nucleation is much easier at grain boundaries than inside grains due to the trapping of gas atoms and the high mobility of vacancies and gas atoms in grain boundaries. Helium bubble formation at unstable vacancy clusters generated by irradiation depends on the mobilities of the vacancies and He, and the continuing supply of vacancies and He. The formation volume of the vacancy and He has a strong effect on the gas bubble nucleation at dislocations. The effective thermal conductivity strongly depends on the bubble volume fraction, but weakly on the morphology of the bubbles.

Anomalous evaporation behavior of ZnO powder milled mechanically under high-energy conditions

ZnO powder showed anomalous evaporation behavior after its mechanical milling treatment under high-energy conditions. The amount of generated vapor is about 10 times higher in the first 15 min of annealing at 1300 °C than that of unmilled ZnO powders. The strong ball impacts are responsible for the greatly enhanced evaporation ability. Low-energy ball milling involving shearing actions and rare weak impacts leads only to a small evaporation rate enhancement. The possible explanation of the high evaporation rate of the heavily milled material is the existence of large fraction of weakly bonded atoms in grain boundaries, surface defects and strained areas.

Analyses of tensile deformation of nanocrystalline α-Fe 2O 3+ fcc-Al composites using molecular dynamics simulations

The tensile deformation of nanocrystalline α-Fe 2O 3+ fcc-Al composites at room temperature is analyzed using molecular dynamics (MD) simulations. The analyses focus on the effects of variations in grain size and phase volume fraction on strength. For comparison purposes, nanostructures of different phase volume fractions at each grain size are given the same grain morphologies and the same grain orientation distribution. Calculations show that the effects of the fraction of grain boundary (GB) atoms and the electrostatic forces between atoms on deformation are strongly correlated with the volume fractions of the Al and Fe 2O 3 phases. In the case of nanocrystalline Al where electrostatic forces are absent, dislocation emission initiates primarily from high-angle GBs. For the composites, dislocations emits from both low-angle and high-angle GBs due to the electrostatic effect of Al–Fe 2O 3 interfaces. The effect of the interfaces is stronger in structures with smaller average grain sizes primarily because of the higher fractions of atoms in interfaces at smaller grain sizes. At all grain sizes, the strength of the composite lies between those of the corresponding nanocrystalline Al and Fe 2O 3 structures. Inverse Hall–Petch (H–P) relations are observed for all structures analyzed due to the fact that GB sliding is the dominant deformation mechanism. The slopes of the inverse H–P relations are strongly influenced by the fraction of GB atoms, atoms associated with defects, and the volume fractions of the Al and Fe 2O 3 phases.

Diffusion of He interstitials in grain boundaries in α-Fe

Helium diffusion in metals is a complex process due to its very low solubility in solids and its ability to be trapped by vacancy type defects or impurities. The preferential positions and predominant migration mechanisms of He atoms depend on temperature, as well as the presence of other intrinsic or irradiation induced defects that can act as traps for He. This work presents results of a systematic molecular dynamics study of the diffusion mechanisms of He atoms in grain boundaries in α-Fe. Two grain boundaries, Σ11〈1 1 0〉 {3 2 3} and Σ3〈1 1 0〉 {1 1 2}, were used for the current investigations. The low-temperature (about 0 K) equilibrium structures of these grain boundaries were determined using standard molecular dynamics relaxation techniques, with a flexible border condition. The migration of He atoms were followed for 1–14 ns, at temperatures between 600 and 1200 K. The diffusion coefficient of He atoms using the mean square displacements of He atoms, and the effective migration energies were determined. We found that He atoms diffuse rapidly in the Σ11 grain boundary, where the binding energy of a He atom to the boundary is high. He migration is primarily one-dimensional along specific directions, but a few directional changes were observed at higher temperatures. In the Σ3 grain boundary, where the He binding energy is low, He atoms migrate one-dimensionally at low temperature, two-dimensionally at intermediate temperature and three-dimensionally at higher temperature. The different activation energies and diffusion mechanisms in these two representative grain boundaries suggests that the atomic structures of the grain boundaries play an important role in the diffusivity of He.

The Effect of Triple Capping Layer ( Ti ∕ Ni ∕ TiN ) on the Electrical and Structural Properties of Nickel Monosilicide

The effects of a triple capping layer on the electrical and structural properties of nickel monosilicide (NiSi) have been investigated as a function of rapid thermal annealing temperature. It is shown that the samples with the triple capping layer produce lower sheet resistances than the samples with double or single (TiN) capping layers across the whole annealing temperature range. Scanning transmission electron microscopy results show that, after annealing, interfacial layers consisting of Ni, Ti, and Si elements are formed in the samples with the triple and double capping layers. It is further shown that the triple-capped samples are more thermally stable than the double- and single-capped samples. This could be attributed to the segregation of Ti atoms in grain boundaries of NiSi film, which reduces grain boundary energy. The simple model is presented to explain the influence of Ti reaction flux on the surface morphology and the interface uniformity between the silicide and Si substrate.

The role of grain boundaries in the mechanism of plasma immersion hydrogenation of nanocrystalline magnesium films

Abstract In this paper, attention in focused on the nanostructured magnesium films for hydrogen storage. It is shown that 2 μm thick Mg film is transformed into MgH2 film under high-flux and fluence hydrogen plasma immersion ion implantation at 450 K for 15 min. All hydrogen desorbs at temperature about 530 K, which corresponds to the decomposition of MgH2 → Mg + H2↑. The macroscopic and microscopic observations show that magnesium film undergoes a high deformation and restructuring during hydrogenation–dehydrogenation reaction. The suggested hydrogenation model is based upon the incorporation of excess of hydrogen atoms in grain boundaries of nanocrystalline Mg film driven by the increase in surface chemical potential associated with the implantation flux. The results provide new aspects of hydriding of thin nanocrystalline film materials under highly non-equalibrium conditions on the surface.

Temper embrittlement dynamics induced by non-equilibrium segregation of phosphorus in steel 12Cr1MoV

Abstract Temper embrittlement dynamics were experimentally studied in an industrial steel 12Cr1MoV. Results showed that both the concentration of phosphorus atoms in grain boundaries and the degree of embrittlement reaches a maximum at the critical time. It can be satisfactorily elucidated by the temper embrittlement mechanism of non-equilibrium grain boundary segregation caused by cooling from solution temperature to isothermal holding temperature. An embrittlement prediction method for steel in service is proposed based on this mechanism.

Grain-size dependence of the flow stress of Cu from millimeters to nanometers

Data in the literature on the effect of grain size (d) from millimeters to nanometers on the flow stress of Cu are evaluated. Three grain-size regimes are identified: regime I, d>∼10−6 m; regime II, d ≈10−8 to 10−6 m; and regime III, d<∼10−8 m. Grain-size hardening occurs in regimes I and II; grain-size softening occurs in regime III. The deformation structure in regime I consists of dislocation cells; in regime II, the dislocations are mostly restricted to their slip planes; in regime III, computer simulations indicate that dislocations are absent and that deformation occurs by the shearing of grain-boundary atoms. The transition from regime I to II occurs when the dislocation cell size becomes larger than the grain size, and the transition from regime II to III occurs when the dislocation spacing due to elastic interactions becomes larger than the grain size. The rate-controlling mechanism in regime I is concluded to be the intersection of dislocations; in regime II, it is proposed to be grain-boundary shear promoted by the pileup of dislocations; in regime III, it appears to be grain-boundary shear by the applied stress alone.

Diffusion in nanocrystalline materials

The problem of calculating the effective diffusivity in nanocrystalline materials is addressed. Two simple two-dimensional grain patterns are constructed and several equations are derived from the Hart–Mortlock and Maxwell–Garnett equations. Monte Carlo computer simulation is used to test the validity of the various equations. It is shown that one of the derived equations provides an excellent description of the effective diffusivities up to a volume fraction of atoms in grain boundaries of 40%.

The structural phase transition and mechanism of abnormal temperature dependence of conductivity in ZnTe:Cu polycrystalline thin films

The structure and electrical transport of ZnTe polycrystalline thin films have been studied as a function of doping concentration and post-grown annealing temperature. For undoped ZnTe thin films, the electrical transport characterizes as commonly expected thermal activation of carriers with activation energy 0.22 eV, and the structure is the cubic phase with a (1 1 1)-preferred orientation as shown by X-ray diffraction (XRD) measurement. For copper doped ZnTe thin films an abnormal temperature dependence of conductivity has been observed. The conductivity measurement with temperature cycles at top values of 144, 170, 217, and 260 °C, has been performed and the conductivity activation energy has been calculated. For copper doped ZnTe thin films, the structural phase transition, i.e. cubic→hexagonal→cubic, is observed by XRD measurement with the increase of annealing temperatures. With increasing temperature, a part of the copper atoms in grain boundaries can diffuse into grains and some of copper atoms in grains are thermally ionized and then inhabit the Zn sites of ZnTe lattice in grains, which cause the structural phase transition of ZnTe films and, the variation in grain boundary barrier. The effects of the variation in both grain boundary barrier and average carrier concentration on the changes in conductivity with temperature are discussed taking into consideration of the grain boundary trapping theory. Then the abnormal electrical transport behavior in copper doped films is explained.

Molecular Dynamics Study of Structure and Mechanical Properties of Grain Boundaries in Nanocrystalline Materials.

Grain boundary (GB) structures of Al nanopolycrystalline (NPC) materials and their mechanical properties are studied by using molecular dynamics simulation. An embedded atom method (Mishin et al. 1999) is adopted to describe the interatomic interaction. First, bicrystal models are studied to evaluate the GB energies, then three different NPC models are examined to investigate the dependency of grain size, and a computational tensile test of a NPC model is carried out. In the bicrystal simulation, the result is compared with the effective medium theory, experiment and abinitio calculation and it corresponds well with them. Concerning NPC models, the atoms located in GBs are distinguished and the average site potential energy and the average specific volume over the atoms in GBs are calculated. The relation between these values and misorientation angles shows that there is a structural difference between low angle and high angle GBs and the border is about 20 degree. In the simulation of tensile test, the result shows the GB sliding is closely related to not only Schmid factor of inclination of GB plane but the GB character, i.e. it is observed at high-energy and low-density GB. Migrations of low angle GB that is not necessarily easy in coarse-grain polycrystals, are often observed. It is concluded that the mechanical properties and deformation mechanisms of NPC materials are closely related to the microscopic GB character and that the computer simulation play an important role as an indispensable tool to estimate GB character and to design the NPC materials.

Low-frequency vibrational properties of nanocrystalline materials.

The vibrational density of states (VDOS) of bulk nanocrystalline Ni and Cu (model) samples with grain diameters between 5 and 12 nm are derived from molecular-dynamics simulations. The results show an enhancement of the density of states at low and high energies. Because of large system sizes and a decomposition of the VDOS into grain and grain-boundary components, the low-frequency region can be investigated for the first time. It is found that the anomalous increase of the VDOS is mainly caused by the high number of grain-boundary atoms and that a power-law behavior of the low-frequency grain-boundary VDOS exists, which suggests a reduced dimensionality effect.

Thermal stability of nano-RuAl produced by mechanical alloying

The thermal stability of as-milled nano-RuAl has been studied by isothermal annealing at high temperatures. All three kinds of structural evolutions in as-milled powders upon high temperature exposure, namely reordering, strain relaxation and grain growth, show signs of stagnation. The total quantity of impurities, mainly 15 at.% Fe has been analyzed as being dissolved substitutionally in RuAl and also segregated to grain boundaries. Upon grain growth, lattice diffusion and segregation of impurity atoms in grain boundaries have been verified by the lattice parameter variation. The incremental apparent activation energy for grain growth at different temperatures is related to the accumulation of impurities in grain boundaries. Reordering and strain relaxation processes that accompany grain growth could consume a certain part of the driving force for grain growth.

Structural relaxation within the grain boundaries of nanocrystalline Ni3Fe

Abstract Results are reported from small-angle neutron scattering (SANS) and Mossbauer spectrometry measurements on four Ni3Fe materials: nanocrystalline material prepared by mechanical attrition, nanocrystalline material after an exposure to 4K, nanocrystalline material after a heat treatment of 100°C for 10 days, and a control sample composed of large crystals. Scattering from the nanocrystalline materials after thermal treatments showed changes in the intensity and the slope of both the nuclear and the magnetic scattering profiles of the SANS measurements. The Mossbauer spectra from the nanocrystalline samples showed changes in the hyperfine fields associated with grain-boundary atoms. The changes induced by cryogenic exposure and annealing at 100°C were somewhat different but were consistent with a sharper density gradient between the crystalline region and the grain-boundary region. Some grain-boundary atoms in mechanically attrited Ni3Fe are in energetically unfavourable positions, and move into new...