Grain Boundary Atoms Research Articles

Unveiling annealing texture formation and static recrystallization kinetics of hot-rolled Mg-Al-Zn-Mn-Ca alloy

The development of Mg-Al-Zn-Mn-Ca series alloys provides a potential prospect to achieve high strength and formability at room temperature (RT). The formation of elliptical annular texture is treated as a crucial factor for the enhanced RT formability. However, the origin of such an elliptical annular texture formation has been rarely reported. Herein, we unveiled the formation and evolution of elliptical annular texture in the hot-rolled Mg-1.6Al-0.8Zn-0.4Mn-0.5Ca (AZMX1100, wt.%) alloy after annealing at different temperatures for 1 h, and its static recrystallization (SRX) kinetics in given annealing temperature for different time. The results revealed that the formation of elliptical annular texture in the hot-rolled AZMX1100 alloy after annealing was derived from nucleation-oriented SRX mechanism, which took place in 200−300 °C, induced by cracked chain-shaped Al2Ca phases, contraction twins, intersections of double twins, intersections of double twins and grain boundaries and non-basal slips. On further annealing from 300−450 °C, the grains with 45°–70° transverse direction (TD) preferentially grew, which made elliptical annular texture extended along the TD. Based on the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model, Avrami exponent n value was estimated to be 0.68–1.02, attributed to non-random SRX nucleation, giving rise to the lower activation energy QR of nucleation of ∼74.24 kJ/mol. Since the co-segregation of Al, Zn and Ca atoms in grain boundaries created a strong interaction of solutes and grain boundaries, the hot-rolled AZMX1100 alloy exhibited the higher activation energy Qg (∼115.48 kJ/mol) of grain growth.

Влияние зернограничной сегрегации на диффузию атомов в границах зерен в системах на основе меди

The influence of the segregation energy on the diffusion of second-component atoms in copper is studied by molecular statics and dynamics methods. A number of modified potential is considered. The segregation energy of atoms in a grain boundary is calculated. The number of second-component atoms involved in a diffusion process is found to decrease because of desorption, which leads to a decrease in the grainboundary diffusion coefficient.

Study of Grain Boundary Self-Diffusion in Iron with Different Atomistic Models

We studied grain boundary (GB) self-diffusion in body-centered cubic iron using ab initio calculations and molecular dynamics simulations with various interatomic potentials. A combination of different models allowed us to determine the principal characteristics of self-diffusion along different types of GBs. In particular, we found that atomic self-diffusion in symmetric tilt GBs is mostly driven by self-interstitial atoms. In contrast, in general GBs atoms diffuse predominantly via an exchange mechanism that does not involve a particular defect but is similar to diffusion in a liquid. Most observed mechanisms lead to a significant enhancement of self-diffusion along GBs as compared to diffusion in the bulk. The results of simulations are verified by comparison with available experimental data.

Influence of the segregation of 3d transition metal solutes on the elastic modulus of a tilt grain boundary in bcc Fe: ab initio local analysis

We investigated the elastic modulus changes normal to the interface of a coincidence tilt grain boundary (GB) in bcc Fe by the substitutional segregation of 3d transition metal (TM) solutes. For the $$\Sigma $$11(332) GB, we obtained cell-averaged elastic moduli by applying small tensile and compressive strains to the GB supercell normal to the interface with and without the solute segregation, where local strains or local responses at GB atoms and segregated 3d-TM atoms were analyzed. We observed that the segregations of V to the looser site and Co to the tighter site remarkably enhance the elastic modulus normal to the GB, while the segregations of Sc and Cu remarkably reduce it. The strengthening by V and Co is originated from the strong bonding between solute and neighboring Fe atoms at the GB interface, as represented by the changes of local elastic moduli and local densities of states.

A Simple Approach to Atomic Structure Characterization for Machine Learning of Grain Boundary Structure-Property Models

Grain boundaries (GBs) have a significant influence on the properties of crystalline materials. Machine learning approaches present an attractive route to develop atomic structure-property models for GBs because of the complexity of their structure. However, the application of such techniques requires an appropriate descriptor of the atomic structure. Unfortunately, common crystal structure identification techniques cannot be applied to characterize the structure of the vast majority of GB atoms (50-98% are classified as other). This suggests a critical need for atomic structure descriptors capable of identifying arbitrary atomic environments. In this work we present a simple procedure that facilitates the identification of arbitrary atomic structures present in GBs. We apply this approach to characterize the atomic structure of the 388 GBs from the Olmsted data set (Olmsted 2009). We show how this approach facilitates visualization of GB atomic structures in a way that reveals important structural information. We test the recently proposed hypothesis that Σ3 GBs contain facets of the GBs that form the corners of the corresponding GB plane fundamental zone. Finally, we briefly demonstrate how the structure descriptors resulting from our approach can be used as inputs to machine learning approaches for the development of atomic structure-property models for GBs.

Achieving Ultrahigh Hardness in Electrodeposited Nanograined Ni-Based Binary Alloys.

Annealing hardening has recently been found in nanograined (ng) metals and alloys, which is ascribed to the promotion of grain boundary (GB) stability through GB relaxation and solute atom GB segregation. Annealing hardening is of great significance in extremely fine ng metals since it allows the hardness to keep increasing with a decreasing grain size which would otherwise be softened. Consequently, to synthesize extremely fine ng metals with a stable structure is crucial in achieving an ultrahigh hardness in ng metals. In the present work, direct current electrodeposition was employed to synthesize extremely fine ng Ni-Mo and Ni-P alloys with a grain size of down to a few nanometers. It is demonstrated that the grain size of the as-synthesized extremely fine ng Ni-Mo and Ni-P alloys can be as small as about 3 nm with a homogeneous structure and chemical composition. Grain size strongly depends upon the content of solute atoms (Mo and P). Most importantly, appropriate annealing induces significant hardening as high as 11 GPa in both ng Ni-Mo and Ni-P alloys, while the peak hardening temperature achieved in ng Ni-Mo is much higher than that in ng Ni-P. Electrodeposition is efficient in the synthesis of ultrahard bulk metals or coatings.

Structure and shear response of tilt grain boundary in titanium nitride

The equilibrated structures and shear responses of <001> titanium nitride (TiN) symmetric tilt grain boundaries (GBs) are investigated using molecular dynamics simulation. The equilibrated GBs are only composed of the kite-shaped structures connected with edge dislocation cores. From the shear response analysis of TiN GBs, we find that there exist two major deformation modes, i.e., GB sliding and shear deformation coupled with GB motion (shear-coupling). GB sliding may finally lead to the GB fracture and the shuffling of GB atoms. The shear-coupling could be further divided into the ideal and non-ideal ones. For the ideal shear-coupling, the shear deformation coupling with GB motion can be exactly described by a factor solely dependent on the GB tilt angle. However, it is not if the additional shear deformations caused by GB sliding or vacancy emission occur during the shear-coupling. Based on the geometrical analysis of atomic deformation mechanism in GB, we propose two theoretical coupling factors to describe the shear-coupling with the additional GB shear deformations. In addition, the deformation mode of GBs could be changed by elevating the temperatures and adding vacancies into GBs.

First-principles investigation of grain boundary morphology effects on helium solutions in tungsten

Helium solutions at eight symmetric tilt grain boundaries (GBs) in tungsten (W) have been investigated through first-principles calculations. The GBs are constructed by the coincidence site lattice (CSL) model and all interstitial sites at GBs are identified with convex deltahedra. For each GB, helium atom prefers to dissolve in interstitial sites with the lowest charge density. Interactions between the helium atom and the GBs atoms are rather localized for all eight GBs studied here, and the helium solution explicitly relates to the local environment of the interstitial site. It is found out that the helium solution energies decrease as the Voronoi volumes of interstitial sites increase, and they can be quantitatively determined by helium solution energy in tungsten clusters Wn. Based on this quantitative relationship, it is easy to estimate the helium solution energy in various interstitial sites in tungsten without massive first-principles calculations. Our result provides a sound theoretical guide to design favorite grain boundaries to suppress helium segregations at GBs.

Effect of Atomic Complexes Formation in Grain Boundaries on Grain Boundary Diffusion

The peculiarities of grain boundary diffusion in Cu connected with the effect of atomic pairs formation in grain boundaries (GB) were studied using the molecular dynamics (MD) simulation. In present study Cu GB selfdiffusion was simulated with the use of semi-empirical potential. Besides, the ‘heterodiffusion’ simulation was performed with the artificially addеd energy of interaction (E) between identical atoms in arbitrary chosen pairs. To obtain reliable data on the mean square displacements (MSD) the simulation cell, consisted about three hundreds thousands atoms and two symmetrical GBs Σ5 (001)(012), was used. 70 pairs of identical Cu atoms in GBs, bonded into pairs, were chosen as initial state. Energy of interaction was varied between 0 and - 0.5eV/atomThe results obtained for selfdiffusion are in a good agreement with experimental results and other results of computer simulation. Two main effects for heterodiffusion are under discussion. The first is atomic exchange between GB zone and adjacent lattice zone, where the mobility of the atoms decreases significantly. As a result, the MSD decrease. Another effect is connected with attraction between the “marked” atoms, which leads to formation of relatively stable complexes and the MSD also decreases. The results obtained involve also dependence the number of the stable pairs on time and temperature and show the possibility of pairs to condense into ternary, quarterly and more numerous complexes.

A method for the construction of initial structures for molecular dynamics simulations of nanocrystals with nonequilibrium grain boundaries containing extrinsic dislocations

A method for the construction of initial atomic models of nanocrystals with extrinsic grain boundary dislocations (EGBDs) in grain boundaries (GBs) for molecular dynamics (MD) simulations is developed. The method is realized for f.c.c. nanocrystals with columnar grains having common crystallographic axis [112] parallel to the column axis and thus divided only by [112] tilt GBs. This system is convenient for studies of interactions between GBs and lattice dislocations, since each grain can be deformed by edge dislocations of only one slip system, which have lines parallel to the [112] axis. In order to introduce extrinsic dislocations to the boundaries of a selected grain, its contour is assumed to be strained by a given shear strain  so that a contour of a freely sheared grain is formed. This contour is filled in by atoms of a f.c.c. lattice with [112] direction parallel to the column axis and then the grain thus formed is subjected to an elastic shear strain -. This results in a deformed grain having the original shape, on the boundaries of which precursors of EGBDs are formed. In order to prevent these precursors from spontaneous annihilation during MD relaxation, one can temporarily fix GB atoms, or apply a proper external stress, or do both. A case study is carried out using two different protocols of MD relaxation to determine atomic structures and energies of nonequilibrium GBs.

Investigation on Sb-doped induced Cu(InGa)Se2 films grain growth by sputtering process with Se-free annealing

In this study, the influences of doping with antimony sulfide (Sb2S3) and metallic Sb on the properties of Cu(InGa)Se2 (CIGS) films and devices have been investigated. The incorporation of metallic Sb in CIGS films has little effect on grain growth, while the incorporation of Sb2S3 could significantly increase CIGS grain size and alter the preferential orientation of CIGS films. Photoluminescence spectra reveal that the pre-deposited Sb2S3 is beneficial to reduce or passivate defects in CIGS layer. Furthermore, the effects of annealing temperature on grain size and phase structure of Sb-doped CIGS films have been explored and Sb distribution profiles in CIGS film are elucidated. These results suggest that Sb2S3 could take effects on enhancing grain growth beyond a certain critical temperature when the generated antimony selenide (Sb2Se3), rather than Sb2S3, decomposed into some volatile phases. These volatile phases could facilitate the diffusion of the atoms in grain boundaries when they pass through from the bottom to outside of CIGS films. Finally, the efficiency of solar cells increases from 8.4% for un-doped sample to 11.9% for Sb2S3-doped sample. This study provides a new approach for fabricating CIGS thin film with large grains and a potential way for non-hazardous mass production application.

The effect of morphology of the long-period stacking ordered phase on mechanical properties of the Mg-7Gd-3Y-1Nd-1Zn-0.5Zr (wt.%) alloy

The effect of morphology of the long-period stacking ordered (LPSO) phase on mechanical properties of the Mg-7Gd-3Y-1Nd-1Zn-0.5Zr (wt.%) alloy have been studied. The results reveal that the block-like 14H-LPSO phases in and near grain boundaries can be formed during heat treatment at high temperature, but low temperatures is conducive to form the needle-like 14H-LPSO phases within α-Mg grains. During heat treatment at fixed temperature for different time, the preferential nucleation sites of the 14H-LPSO phases are located in grain boundaries, then the14H-LPSO phases grow into the α-Mg matrix in the form of needle-like, the needle-like 14H-LPSO phases gradually decrease its length after growing a certain stage, at last the needle-like 14H-LPSO phases disappear completely. The decomposition of the block-like 14H-LPSO phases is found during furnace cooling after homogenization. After decomposition of the block-like 14H-LPSO phases, the rapid diffusion of Zn atoms into α-Mg matrix leads to form the needle-like 14H-LPSO phases within α-Mg matrix, but the retention of RE atoms in grain boundaries results in forming the Mg-RE particles. After extrusion and subsequent ageing, it is found that the alloy with the needle-like 14H-LPSO phases exhibit superior elongation than the alloy without the needle-like 14H-LPSO phase. The precipitates of peak-aged alloys are studied by TEM. The <112−0>α demonstrates that the stacking faults can be observed, the serious lattice distortion appears near the stacking fault. The [0001] zone axis shows that the β′ and a new rod phase are found, and the enrichment of RE atoms are detected in the two phase by HADDF-STEM. It seems to be that the rod phase can be subdivided into a plurality of mutually parallel structure unit.

Segregation of solute atoms (Y, Nb, Ta, Mo and W) in ZrB2 grain boundaries and their effects on grain boundary strengths: A first-principles investigation

ZrB2 based ultra-high temperature ceramics (UHTCs) exhibit a unique combination of excellent properties that makes them promising candidates for applications in extreme environments. Evaluating the correlation between microscopic defects and macroscopic performance of these materials is crucial for the design of UHTCs. The present work deals with a first-principles investigation on segregations of solute atoms (Y, Nb, Ta, Mo and W) in ZrB2 grain boundaries and their influences on grain boundary strengths. Opposite segregation tendency between Y and Nb, Ta, Mo or W is obtained, where Y prefers sites with long M-B bonds, while Nb, Ta, Mo or W prefers sites with short M-B bonds. The short equilibrium M-B (M = Nb, Ta, Mo or W) bonds induce local contractions around grain boundaries, which in turn strengthens grain boundaries remarkably, thereby enhances the mechanical properties of ZrB2 at evaluated temperatures. In contrast, segregation of Y poisons grain boundaries due to local expansions induced by long Y-B bonds, which will deteriorate the performance of ZrB2 based UHTCs. The results provide useful guidelines for the design of ZrB2 based UHTCs, since grain boundaries play a key role in determining high temperature mechanical properties.

On the ultimate tensile strength of tantalum

Strain rate, temperature, and microstructure play a significant role in the mechanical response of materials. Using non-equilibrium molecular dynamics simulations, we characterize the ductile tensile failure of a model body-centered cubic metal, tantalum, over six orders of magnitude in strain rate. Molecular dynamics calculations combined with reported experimental measurements show power-law kinetic relationships that vary as a function of dominant defect mechanism and grain size. The maximum sustained tensile stress, or spall strength, increases with increasing strain rate, before ultimately saturating at ultra-high strain rates, i.e. those approaching or exceeding the Debye frequency. The upper limit of tensile strength can be well estimated by the cohesive energy, or the energy required to separate atoms from one another. At strain rates below the Debye frequency, the spall strength of nanocrystalline Ta is less than single crystalline tantalum. This occurs in part due to the decreased flow stress of the grain boundaries; stress concentrations at grain boundaries that arise due to compatibility requirements; and the growing fraction of grain-boundary atoms as grain size is decreased into the nanocrystalline regime. In the present cases, voids nucleate at defect structures present in the microstructure. The exact makeup and distribution of defects is controlled by the initial microstructure and the plastic deformation during both compression and expansion, where grain boundaries and grain orientation play critical roles.

Plastic and fracture behaviour of nanocrystalline binary Al alloys with grain boundary segregation

The paper studies the stress-strain and fracture behaviour of nanocrystalline (NC) pure Al and NC binary Al-X alloys (X can be Fe, Co, Ti, Mg or Pb) with grain boundary (GB) segregation during their tensile deformation at room temperature via molecular dynamics simulation. The computational cell used for the modeling contains nano-sized grains of Al majority of which has the high-angle type GBs. The binary alloys were obtained through the substitution of Al atoms in GBs by atoms of the alloying elements. Stress-strain curves of the considered materials were calculated, and their microstructure evolution was analyzed. It was found that GB segregations can significantly alter the deformation behaviour of NC Al. The NC pure Al and two alloys, Al with Fe and Al with Mg, undergo the intergranular fracture which is noticeable already at ~ 8 % strain, while the other alloys do not demonstrate any failure process up to 40 % deformation. The main crack growth mechanism is the formation of nano-voids at GBs and triple junctions followed by their coalescence at higher applied stresses. The obtained results demonstrate that GB segregation of Co can have a positive effect on both plasticity and strength of NC Al, and Ti atoms in GBs can result in its improved ductility.

Atomistic investigation of Cr influence on primary radiation damage in Fe-12 at.% Cr grain boundaries

In this paper, we investigate the influence of Cr on the primary radiation damage in Fe-12 at.% Cr with different atomic grain boundaries (GBs). Four different GB structures, two twists and two symmetric tilt boundaries are selected as the model structures. The primary radiation damage near each GB in α-Fe and Fe-12 at.% Cr is simulated using Molecular Dynamics for 9 keV primary knock-on atoms with velocity vectors perpendicular to the GB plane. In agreement with previous works, the results indicate that the atomic GBs are biased toward interstitials and due to the reduction of ‘in-cascade’ interstitial-vacancy annihilation rates, vacancies accumulate in the bulk grains. The minimum defect production occurs when the overlap between cascade center and GB plane is maximum; in contrast, the number of residual defects in the bulk (vacancies and interstitials) increases when the overlap decreases. Moreover, we find that the presence of Cr hardly affects the number of residual defects in the grain interiors, and causes a Cr-enrichment in the surviving self-interstitial atoms in bulk during relaxation of the primary cascades—also in agreement with previous studies. Further, in order to study the effect of 12 at.% Cr on the energetic and kinetic properties of vacancies near the atomic GBs, we calculate formation energies and diffusion barriers of defects using Molecular Static and climbing-Nudged Elastic Band methods. The results reveal that the vacancies energetically and kinetically tend to form and cluster around the GB plane due to the substantial reduction of their formation energies and migration barriers in layers close to the GB center and are immobile on the simulated time frame (~ps).

Effect of grain boundary segregation on the deformation mechanisms and mechanical properties of nanocrystalline binary aluminum alloys

The deformation mechanisms of nanocrystalline (NC) pure Al and NC Al–Co and Al–Mg binary alloys are studied via molecular dynamics simulation. The alloying elements are either segregated to grain boundaries (GB) or distributed randomly as solute. It is revealed that a shear deformation of the pure Al is associated with the GB sliding (GBS) and simultaneous their migration (GBM). GB segregations can significantly alter the mechanisms of plastic deformation. Mg atoms in GBs of the Al–Mg alloy lead to GBS which is accompanied with GBM and a grain growth, while the deformation process of the corresponding alloy with the random distribution of Mg is close to that for pure Al. Unlike Mg, GB segregations of Co atoms detain both GBS and GBM and result in a higher strength of the Al–Co alloy. On the contrary, the strength of the alloy with the Co atoms distributed randomly is very low due to the structure amorphisation leading to the ease of plastic flow. The details of the GBS and GBM processes are further studied for tilt bi-crystals of Al and Al–Co and Al–Mg systems with the alloying atoms being either segregated to the GB or dissolved. It is found that the results for the bi-crystals are in line with those for the NC materials. Overall, GB segregation can strongly influence the response of NC alloys to thermomechanical treatment by affecting such very important mechanisms of plastic deformation in NC metallic materials as GBS and GBM.

The formation of H bubbles at small-angle tilt grain boundaries in W films.

The accumulation of H at the small-angle tilt grain boundary (GB) in the W(001) surface is investigated, on the basis of the first-principles calculations. By exploring the solution and diffusion behaviors of H at the GB, we find that the small-angle GB can capture the H atoms nearby, serving as a nucleation site of H bubbles. With the increasing number of trapped H atoms, the GB expands gradually, and the GBs can be unripped with an areal density of H up to 5.0 × 1015 H atoms per cm2, leading to the formation of H bubbles. Moreover, H2 molecules are observed, when the areal density of H atoms in GB is over 6.6 × 1015 atoms per cm2. According to our calculations, we propose a possible formation mechanism of H bubbles observed in the experiment, which is valuable for improving the service performance of W as a plasma-facing material in nuclear fusion reactors.

Effect of Grain Boundary Segregation on Shear Deformation of Nanocrystalline Binary Aluminum Alloys at Room Temperature

The paper studies deformation mechanisms of nanocrystalline (NC) pure Al and its binary alloys with various distributions of an alloying element which can be Co or Mg via molecular dynamics simulations. It is revealed that a shear deformation of the pure Al is associated with the grain boundary sliding (GBS) and their simultaneous migration. Mg atoms in grain boundaries (GBs) of an Al-Mg alloy lead to GBS which does not accompany with a grain growth, while the deformation process of the corresponding alloy with a random distribution of Mg is close to that for the pure Al. Unlike Mg, GB segregations of Co atoms detain both GBS and GB migration and result in high strength of an Al-Co alloy. On the contrary, the strength of the alloy with the Co atoms distributed randomly is very low due to the structure amorphisation leading to the ease of plastic flow.

An investigation into the influence of grain boundary misorientation on the tensile strength of SiC bicrystals

ABSTRACTIn this work, molecular dynamics (MD) and Car-Parrinello molecular dynamics (CPMD) simulation-based analyses are performed to understand the influence of grain boundary (GB) misorientation on the tensile strength of SiC bicrystals. The tensile strength is governed by the changes in electron density and bond strength of atoms in GBs. An investigation of dislocation activity during mechanical deformation shows that the extent of the propagation of dislocations across the bicrystal grains is directly proportional to the extent of GB misorientation. An analytical relation that predicts the tensile strength as a function of GB misorientation is developed.