Grain Boundary Self-diffusion Research Articles

Self-diffusion in < 100 > symmetrical tilt grain boundaries in tungsten: Molecular dynamics simulation

The structure and energy of grain boundaries, the energies of point defects formation in them and grain-boundary self-diffusion coefficients in 18 < 100 > symmetrical tilt grain boundaries in tungsten have been determined by atomistic simulation. The dependences of grain boundary energies, defect formation energies in them and activation energy of grain-boundary self-diffusion on the misorientation angle have been calculated. A structural phase transformation has been discovered in Σ5(310) and Σ5(210) boundaries, which affected the diffusion properties of these grain boundaries. The calculations performed are compared with the available experimental data on self-diffusion in grain boundaries of tungsten.

Calculation of GB Energies and Grain-Boundary Self-diffusion in Nickel and Verification of Borisov Relations for Various Symmetric Tilt Grain Boundaries

Calculation of GB Energies and Grain-Boundary Self-diffusion in Nickel and Verification of Borisov Relations for Various Symmetric Tilt Grain Boundaries

Atomistic Modeling of Symmetric and Asymmetric Σ5 $$\left\langle {{\text{001}}} \right\rangle $$ Tilt Grain Boundaries in Niobium: Structure, Energy, Point Defects, and Grain-Boundary Self-Diffusion

Atomistic Modeling of Symmetric and Asymmetric Σ5 $$\left\langle {{\text{001}}} \right\rangle $$ Tilt Grain Boundaries in Niobium: Structure, Energy, Point Defects, and Grain-Boundary Self-Diffusion

Atomistic Simulation of Symmetric and Asymmetric Tilt Grain Boundaries 5 &lt;001&gt; in Niobium: Structure, Energy, Point Defects, Grain Boundary Self-Diffusion

Symmetric and three asymmetric tilt grain boundaries Ʃ 5 in niobium have been studied by computer simulation methods. The structure and energies of the boundaries under consideration, as well as the energies of the formation of point defects in them, are calculated by the method of molecular-static modeling. The dependences of the formation energies of point defects on the distance from the plane of the grain boundary are analyzed. Using the method of molecular dynamics, the coefficients of grain-boundary self-diffusion for the considered boundaries are calculated.

Simulation of structure of special tilt boundary and grainboundary self-diffusion in Ti

Simulation of structure of special tilt boundary and grainboundary self-diffusion in Ti

Atomistic Simulation of Grain Boundaries in Niobium: Structure, Energy, Point Defects and Grain-Boundary Self-Diffusion

Atomistic Simulation of Grain Boundaries in Niobium: Structure, Energy, Point Defects and Grain-Boundary Self-Diffusion

An Atomistic Simulation of Special Tilt Boundaries in α-Ti: Structure, Energy, Point Defects, and Grain-Boundary Self-Diffusion

An Atomistic Simulation of Special Tilt Boundaries in α-Ti: Structure, Energy, Point Defects, and Grain-Boundary Self-Diffusion

Atomistic study of grain-boundary segregation and grain-boundary diffusion in Al-Mg alloys

Mg grain boundary (GB) segregation and GB diffusion can impact the processing and properties of Al-Mg alloys. Yet, Mg GB diffusion in Al has not been measured experimentally or predicted by simulations. We apply atomistic computer simulations to predict the amount and the free energy of Mg GB segregation, and the impact of segregation on GB diffusion of both alloy components. At low temperatures, Mg atoms segregated to a tilt GB form clusters with highly anisotropic shapes. Mg diffuses in Al GBs slower than Al itself, and both components diffuse slowly in comparison with Al GB self-diffusion. Thus, Mg segregation significantly reduces the rate of mass transport along GBs in Al-Mg alloys. The reduced atomic mobility can be responsible for the improved stability of the microstructure at elevated temperatures.

Effect of light elements impurity atoms on grain boundary diffusion in FCC metals: a molecular dynamics simulation

Effect of carbon and oxygen impurity atoms on diffusion along the tilt grain boundaries with &lt;100&gt; and &lt;111&gt; misorientation axis in metals with FCC lattice was studied by mean of molecular dynamics method. Ni, Ag, and Al were considered as metals. Interactions of metal atoms with each other were described by many-particle Clery-Rosato potentials constructed within the framework of tight binding model. To describe interactions of atoms of light elements impurities with metal atoms and atoms of impurities with each other, Morse pair potentials were used. According to obtained results, impurities in most cases lead to an increase in self-diffusion coefficient along the grain boundaries, which is caused by deformation of crystal lattice near the impurity atoms. Therefore, additional distortions and free volume are formed along the boundaries. It is more expressed for carbon impurities. Moreover, with an increase in concentration of carbon in the metal, an increase in coefficient of grain-boundary self-diffusion was observed first, and then a decrease followed. This behavior is explained by formation of aggregates of carbon atoms at grain boundary, which leads to partial blocking of the boundary. Oxygen atoms had smaller effect on diffusion along the grain boundaries, which is apparently explained by absence of a tendency to form aggregates and lesser deformation of crystal lattice around impurity. The greatest effect of impurities on self-diffusion along the grain boundaries among the examined metals was observed for nickel. Nickel has the smallest lattice parameter, impurity atoms deform its lattice around itself more than aluminum and silver, and therefore they create relatively more lattice distortions in it and additional free volume along the grain boundaries, which lead to an increase in diffusion permeability. Diffusion coefficients along the high-angle boundaries with misorientation angle of 30° turned out to be approximately two times higher than along low-angle boundaries with a misorientation angle of 7°. Diffusion along the &lt;100&gt; grain boundaries flowed more intensively than along the &lt;111&gt; boundaries.

Atomistic Study of Grain-Boundary Segregation and Grain-Boundary Diffusion in Al-Mg Alloys

Mg grain boundary (GB) segregation and GB diffusion can impact the processing and properties of Al-Mg alloys. Yet, Mg GB diffusion in Al has not been measured experimentally or predicted by simulations. We apply atomistic computer simulations to predict the amount and the free energy of Mg GB segregation, and the impact of segregation on GB diffusion of both alloy components. At low temperatures, Mg atoms segregated to a tilt GB form clusters with highly anisotropic shapes. Mg diffuses in Al GBs slower than Al itself, and both components diffuse slowly in comparison with Al GB self-diffusion. Thus, Mg segregation significantly reduces the rate of mass transport along GBs in Al-Mg alloys. The reduced atomic mobility can be responsible for the improved stability of the microstructure at elevated temperatures.

Role of loading direction on compressive deformation behavior of extruded ZK60 alloy plate in a wide range of temperature

The plastic flow behavior of extruded Mg-6Zn-0.6Zr (ZK60) alloy plate in T5 treated condition was characterized in the extrusion direction (ED) and along plate normal (ND) in the temperature range of 25 °C–500 °C, with a view to study the effect of loading direction. Cylindrical specimens were compressed parallel to ED and along ND, and flow curves and specimen shapes were recorded. The flow behavior at temperatures >200 °C in the strain rate range 0.0003–10 s−1 was analyzed using the approach of processing map. The plate texture could be described as basal planes parallel to the plate surface with <101¯0> as ED. The ED specimens deformed at temperatures <175 °C showed limited plastic strain leading to shear cracks, which were attributed to the restriction of basal slip and activation of limited non-basal slip. The ND specimens showed more plastic flow due to the occurrence of {101l} slip in <112¯3> direction in the absence of both basal and prismatic slip. The processing map for the ED specimens exhibited three domains in the ranges: (1) 230 °C–375 °C and 0.0003 s−1–0.002 s−1, (2) 410 °C–500 °C and 0.0003 s−1–0.008 s−1, and (3) 370 °C–430 °C and 2 s−1–10 s−1. In Domains 1 and 3, dynamic recrystallization (DRX) occurred due to prismatic slip and recovery by lattice-diffusion controlled climb, and first-order pyramidal slip and recovery by grain-boundary self-diffusion controlled climb, respectively. In the second domain, DRX occurred by second-order pyramidal slip and cross slip, which was followed by grain boundary sliding leading to superplasticity. The processing map for ND specimens exhibited an additional domain at lower temperatures and strain rates (225 °C–350 °C and 0.0003 s−1–0.1 s−1) which was due to easy occurrence of {101l} <112¯3> slip with lattice self-diffusion controlled climb. The map for ND orientation was free from flow instability regime unlike that for ED and this was the result of extensive occurrence of slip on {101l} planes in <112¯3> direction.

Model of grain-boundary self-diffusion in α- and β-phases of titanium and zirconium

A model of the grain-boundary self-diffusion process in metals undergoing phase transitions in the solid state is proposed. The model is based on the ideas and approaches of the theory of nonequilibrium grain boundaries. It is shown that the range of application of basic relations of this theory can be extended, and they can be used to calculate the parameters of grain-boundary self-diffusion in high-temperature and low-temperature phases of metals with phase transition. Based on the constructed model, activation energies of grainboundary self-diffusion in titanium and zirconium are calculated, and their anomalously low values in the low-temperature phase are explained. The calculated activation energies of grain-boundary self-diffusion are in good agreement with experimental data.

Anisotropy of self-diffusion in forsterite grain boundaries derived from molecular dynamics simulations

Diffusion rates and associated deformation behaviour in olivine have been subjected to many studies, due to the major abundance of this mineral group in the Earth’s upper mantle. However, grain boundary (GB) transport studies yield controversial results. The relation between transport rate, energy, and geometry of individual GBs is the key to understand transport in aggregates with lattice preferred orientation that favours the presence and/or alignment of specific GBs over random ones in an undeformed rock. In this contribution, we perform classical molecular dynamics simulations of a series of symmetric and one asymmetric tilt GBs of $$\hbox {Mg}_2\hbox {SiO}_4$$ forsterite, ranging from 9.58° to 90° in misorientation and varying surface termination. Our emphasis lies on unravelling structural characteristics of high- and low-angle grain boundaries and how the atomic structure influences grain boundary excess volume and self-diffusion processes. To obtain diffusion rates for different GB geometries, we equilibrate the respective systems at ambient pressure and temperatures from 1900 to 2200 K and trace their evolution for run durations of at least 1000 ps. We then calculate the mean square displacement of the different atomic species within the GB interface to estimate self-diffusion coefficients in the individual systems. Grain boundary diffusion coefficients for Mg, Si and O range from $$10^{-18}$$ to $$10^{-21}\,\hbox {m}^3$$ /s, falling in line with extrapolations from lower temperature experimental data. Our data indicate that higher GB excess volumes enable faster diffusion within the GB. Finally, we discuss two types of transport mechanisms that may be distinguished in low- and high-angle GBs.

Molecular dynamics of ionic self-diffusion at an MgO grain boundary

The characterization of self-diffusion in MgO grain boundaries is a materials science problem of general interest, being relevant to the stability and reactivity of MgO layers in artificial nanostructures as well as to the understanding of mass transport and morphological evolution in polycrystalline metal oxides which are employed in many technological applications. In addition, atomic transport in MgO is a key factor to describe the rheology of the Earth’s lower mantle. In this work, we tackle the problem using a classical molecular dynamics model and finite-temperature simulations. To this purpose, we first design a stable grain boundary structure, which is meant to be representative of general internal interfaces in nanocrystalline MgO. The Mg and O self-diffusion coefficients along this grain boundary are then determined as a function of temperature by calculating the mean-square ionic displacement in the boundary region. Two different diffusion regimes at low and high temperature are identified, allowing to obtain the relevant activation enthalpies for migration from the temperature dependance of the diffusion coefficients. Our results prove that Mg diffusion along MgO grain boundaries is sufficiently fast to explain the recently reported development of MgO hollow structures during repeated hydrogen sorption cycles in Mg/MgO nanoparticles.

Self-Diffusion in Grain Boundaries and Dislocation Pipes in Al, Fe, and Ni and Application to AlN Precipitation in Steel

Diffusion along microstructural defects, such as grain boundaries or dislocation pipes, is significantly faster than diffusion through an undisturbed crystal. The ratio of diffusion enhancement is 3-4 orders of magnitude close to the melting point and reaches up to several ten orders of magnitude close to room temperature. An assessment of literature shows a large scatter in the available data and emphasizes the need for representative mean values. Applying a least mean square fit to selected experimental information delivers temperature-dependent functions for the ratio of grain boundary and dislocation pipe to bulk diffusion, respectively. We demonstrate that application of the attained results in a computational framework for the kinetics of precipitation makes the predictive simulation possible for the evolution of particles located at dislocations and grain boundaries.

Atomistic understanding of diffusion kinetics in nanocrystals from molecular dynamics simulations

Understanding the grain size effect on diffusion in nanocrystals has been hampered by the difficulty of measuring diffusion directly in experiments. Here large-scale atomistic modeling is applied to understand the diffusion kinetics in nanocrystals. Enhanced short-circuit diffusivity is revealed to be controlled by the rule of mixtures for grain-boundary diffusion and lattice diffusion, which can be accurately described by the Maxwell-Garnett equation instead of the commonly thought Hart equation, and the thermodynamics of pure grain-boundary self-diffusion is not remarkably affected by varying grain size. Experimentally comparable Arrhenius parameters with atomic detail validate our results. We also propose a free-volume diffusion mechanism considering negative activation entropy and small activation volume. These help provide a fundamental understanding of how the activation parameters depend on size and the structure-property relationship of nanostructured materials from a physical viewpoint.

A molecular-dynamics simulation of grain-boundary diffusion of niobium and experimental investigation of its recrystallization in a niobium-copper system

A model of high-angle general-type grain boundaries is developed and grain-boundary diffusion of niobium in a Nb–Cu system is investigated using the method of molecular dynamics. The results of predictions of the effect of a copper impurity on grain-boundary self-diffusion of niobium are compared with the experimental data on diffusion annealing of multi-layered composites containing Cu/Nb boundaries. Recrystallization of polycrystalline niobium is investigated under the conditions of grain-boundary diffusion of copper.

Grain boundary width, energy and self-diffusion in nickel: Effect of material purity

The effect of material purity on grain boundary (GB) width and energy is investigated by GB self-diffusion measurements in Ni. The effect of matrix purity is examined for the first time for the low temperature C regime of GB diffusion. Grain boundary self-diffusion is measured in Ni of 99.99wt.% purity in both B and C kinetic regimes, and the diffusional GB width δ was determined as δ=0.6nm. This result is in good agreement with previous measurements in 99.999wt.% Ni. Thorough analysis of all available results on GB width in different coarse-grained and nanograined materials suggests that the diffusional GB width is independent of temperature, material purity and material nature and can be taken as 0.5nm in undeformed materials. Grain boundary self-diffusion strongly depends on the amount of residual impurities, and the effect is marginal for less pure materials, Dgb99.999≫Dgb99.99≈Dgb99.6 at low temperatures in Ni of the indicated purity levels. The effective dependence of the GB energy and the self-diffusion coefficient on grain size in a material of a given purity level is evaluated within the framework of a newly developed model.

Role of grain-boundary diffusion in the grain growth in nanocrystalline nickel

The grain growth in a nanocrystalline nickel during nonisothermal annealing is studied by differential scanning calorimetry (DSC) and transmission electron microscopy. Nanocrystalline nickel is prepared by electrodeposition at a pulsed voltage. Its nonisothermal annealing is accompanied by anomalous grain growth; at a temperature corresponding to a DSC peak, a bimodal grain structure forms. The processing of DSC signals in terms of the Avrami formalism permits the grain growth activation energy to be determined, which is found to be close to the activation energy of grain-boundary self-diffusion. The anomalous grain growth creates conditions such that grain-boundary diffusion is a controlling stage of the process.

Equations of diffusion in nonequilibrium grain boundaries

Equations are derived which describe self-diffusion and interdiffusion in nonequilibrium grain boundaries. The influence of the degree of the nonequilibrium state of the boundaries on the development of the process of grain-boundary diffusion is examined. The kinetics of the variation of the nonequilibrium excess volume due to the diffusional mass transfer along grain boundaries is analyzed.