Pure Grain Boundary Research Articles

Developing High-Performance Ceramics from Multicomponent Grain Boundary Entropy Design.

Grain boundary (GB) glassy phase often results in poor ceramic performances. Here, a Multicomponent Grain Boundary Entropy (MGBE) descriptor extracted from high-throughput first-principle calculations is proposed to capture the nature of high-entropy GB phases in ceramics. In a Si3N4 ceramic model system, MGBE is found to have a direct correlation with GB phase crystallinity, element segregation, and formation of pores. The predicted highest MGBE sintering additive combination (MgO-Y2O3-Er2O3-Yb2O3) leads to high-performance ceramics of homogenous microstructure and pure GB (YErYb)2Si3O3N4 phase without observable glassy film. Conversely, low MGBE additives result in a substantial amount of GB glassy phase, element segregation, and pore clusters. The MGBE descriptor can make a rapid screening of multicomponent sintering additives, offering a novel approach for rational designing of ceramics with targeted microstructure and performances.

Microstructure, grain boundary evolution and anisotropic Fe segregation in (0001) textured Ti thin films

The structure and chemistry of grain boundaries (GBs) are crucial in determining polycrystalline materials’ properties. Faceting and solute segregation to minimize the GB energy is a commonly observed phenomenon. In this paper, a deposition process to obtain pure tilt GBs in titanium (Ti) thin films is presented. By increasing the power density, a transition from polycrystalline film growth to a maze bicrystalline Ti film on SrTiO3 (001) substrate is triggered. All the GBs in the bicrystalline thin film are characterized to be Σ13 [0001] coincident site lattice (CSL) boundaries. The GB planes are seen to distinctly facet into symmetric {7¯520} and {134¯0} and asymmetric {101¯0} // {112¯0} segments of 20–50 nm length. Additionally, EDS reveals preferential segregation of iron (Fe) in every symmetric {7¯520} segment. Both the faceting and the segregation are explained by a difference in the CSL density between the facet planes. Furthermore, in the GB plane containing Fe segregation, atom probe tomography is used to experimentally determine the GB excess solute to be 1.25 atoms/nm2. In summary, the study reveals for the first time a methodology to obtain bicrystalline Ti thin films with strong faceting and an anisotropy in Fe segregation behaviour within the neighbouring GB facets.

Numerical and experimental characterization of twin transmission across grain boundaries along the forward and lateral directions

Pervasive deformation twinning and transmission events across grain boundaries (GBs) affect the strength and failure of hexagonal close-packed (HCP) magnesium. A three-dimensional twin can transmit along the twinning shear direction, η1 (forward), and along the direction perpendicular to both the twinning plane normal and the shear direction, λ (lateral). For the first time, phase-field calculations and electron backscatter diffraction (EBSD)-based statistical analysis are combined to investigate the effect of the twinned grain boundary characteristics on twin transmission (TT) along the forward and lateral directions. This combined analysis reveals that TT propensity decreases with increasing misorientation angle across the GB for both forward and lateral directions. Also, the TT is more favorable along the lateral than along the forward direction. Twin transmission seems harder across GBs with a misorientation axis closer to the twin λ-direction than the other directions (κ1 and η1). Further, the PF calculations reveal that, at the onset of a transmission process, the crystallography tends to be preserved in the case of lateral transmission, whereas, in the forward transmission case, the transmitted twin punches straight through the GBs and its morphology prevails. The EBSD analysis finds that pure forward and lateral transmissions do not occur often, yet reveals a preference for lateral propagation consistent with PF simulations. Further, the local twin transmission configurations observed in the actual material do not correspond to pure tilt or twist GBs, which are most commonly considered as model cases.

Shear-strain induced structural relaxation of Cu Σ3 [110](112) symmetric tilt grain boundary: The role of foreign atoms and temperature

Grain boundaries (GBs) relaxation is a promising and effective strategy to improving GB stability or stabilizing nanocrystalline metals. However, previous studies mainly focused on nanocrystalline pure metals and GB behaviors therein, without considering the role of foreign atoms such as impurity or alloying atoms in GB relaxation. In this work, the shear-strain induced structural relaxation of pure Cu Σ3 [110](112) symmetric tilt GBs (STGBs), and the effects of foreign elements (Fe and Ni) and temperature on the GB relaxation were investigated in detail by molecular dynamics method. The results show that shear strain can trigger the structural relaxation of pure, Fe- and Ni-containing Cu GBs by the emission of Shockley partial dislocations from Cu GBs. Both Fe and Ni have impediment effects on the shear-strain induced GB relaxation, though the content of Fe or Ni atom (0.00165 at.%) is quite low in the GB model. The temperature cannot trigger GB relaxation independently within the considered temperature range, but play a positive role in the shear-strain induced structural relaxations of pure, Fe- and Ni-containing Cu Σ3 [110](112) STGBs. Our work might gain new insights into the mechanically induced GB relaxation in nanocrystalline copper and could be beneficial for improving the stability of Cu GBs.

Hydrogen-enhanced intergranular failure of sulfur-doped nickel grain boundary: In situ electrochemical micro-cantilever bending vs. DFT

Intergranular failure of nickel (Ni) single grain boundaries (GBs) owing to the segregation of sulfur (S), hydrogen (H), and their co-segregation has been investigated by employing micro-cantilever bending tests and density functional theory (DFT) calculations. A pure Ni GB shows completely plastic behavior with no fracture observed in the experiments. Electrochemical H-charging of the sample with no S present in the GB leads to a crack formed at the notch tip, which propagates by means of the mixed plastic–brittle fracture mode. Cantilever testing of the H-charged GB with S results in a clear brittle fracture of the GB. The co-segregation of S and H shifts the sudden drop in the load–displacement curves to smaller values of displacement. This is explained by the combined effect of these elements on the work of separation of the selected GB leading to severely decreased GB cohesion.

Carbide effects on tensile deformation behavior of [001] symmetric tilt grain boundaries in bcc Fe

The mechanical performance of carbides on the grain boundary (GB) of ferritic steels is always concerned. In this paper, four symmetric tilt GBs in body-centered cubic Fe were studied under tensile tests at 0 and 300 K through atomistic simulations. The GBs contained various sized Cr23C6 precipitates. Stress–strain curves were obtained through the molecular dynamics simulations and the tensile strength varied with GB type. Centro-symmetry parameter, common neighbor and dislocation analyses were performed for defect characterization. GBs with precipitates showed lower tensile strength than pure GBs. As the number and size of carbides increased, the tensile strength decreased. Cavities were easily formed near dislocation loops with a Burgers vector parallel with loading direction during tensile deformation of pure GBs, while ruptures occurred around the precipitate in GBs with carbides. The simulations indicate that Cr23C6 carbides in Fe are a brittle phase where the dislocations and cracks nucleate.

Effects of H segregation on shear-coupled motion of 〈110〉 grain boundaries in α-fe

By using large-scale molecular dynamics (MD) simulations, the influence of solute hydrogen (H) segregation into several typical [11¯0] symmetric tilt grain boundaries (STGBs) on the shear response and coupled grain boundary (GB) migration of bicrystals in α-Fe is systematically examined. Depending on our geometric model of coupling for [11¯0] STGBs in body-centred cubic (BCC) metals, two different coupling branches (〈100〉 and 〈111〉) are predicted and further validated by the MD results. Our atomistic simulations show that solute H atoms impede coupled GB motion irrespective of the GB structure, which mainly stems from the fact that H considerably modifies the local atomic structures of the GBs. At high H concentrations, the response of GBs to shear deformation changes from GB coupling to pure GB sliding. In addition, it is found that H can facilitate the vacancy generation via enhancing the interaction of GB dislocations within the framework of the H-enhanced localized plasticity (HELP) mechanism. Although H-vacancy clusters are formed by solute H combining with nucleated vacancies, they cannot grow larger owing to the migration of GBs with extensive dislocation plasticity. This directly prevents the occurrence of H embrittlement (HE). These findings deepen our overall understanding of the role of solute H in GB-mediated plasticity process (GB migration) of metallic materials and provide a possible path to designing new materials with high resistance to HE.

Effect of P impurity on mechanical properties of NiAl Σ5 grain boundary: From perspectives of stress and energy**Project supported by the National Natural Science Foundation of China (Grant Nos. 11404396 and 51201181) and the Subject Construction Fund of Civil Aviation University of China (Grant No. 000032041102).

In this paper, we employ the first-principle total energy method to investigate the effect of P impurity on mechanical properties of NiAl grain boundary (GB). According to “energy”, the segregation of P atom in NiAlΣ5 GB reduces the cleavage energy and embrittlement potential, demonstrating that P impurity embrittles NiAlΣ5 GB. The first-principle computational tensile test is conducted to determine the theoretical tensile strength of NiAlΣ5 GB. It is demonstrated that the maximum ideal tensile strength of NiAlΣ5 GB with P atom segregation is 144.5 GPa, which is lower than that of the pure NiAlΣ5 GB (164.7 GPa). It is indicated that the segregation of P weakens the theoretical strength of NiAlΣ5 GB. The analysis of atomic configuration shows that the GB fracture is caused by the interfacial bond breaking. Moreover, P is identified to weaken the interactions between Al–Al bonds and enhance Ni–Ni bonds.

Effect of low-angle grain boundaries on morphology and variant selection of grain boundary allotriomorphs and Widmanstätten side-plates

Morphology and variant selection (VS) of grain boundary (GB) allotriomorphs and Widmanstätten side-plates of α phase in an α/β titanium alloy, Ti-6Al-4V (wt%), are investigated using a three-dimensional phase field model. The structures of low-angle GBs (misorientation θm ≤ 10°) are modeled as discrete dislocation networks using Frank-Bilby theory. It is shown that α allotriomorphs and side-plates compete with each other during precipitation and the final morphology and selected α variants exhibit a strong correlation with the GB dislocation structures. While the side-plate morphology is more preferred by a symmetrical tilt GB with θm ∼ 10°, it can also be induced by a pure twist GB with θm ≤ 5°. Quantitative analysis indicates that precipitate morphology and VS are determined by the interplay among (i) elastic interaction between a nucleating α precipitate and the GB dislocation networks, (ii) growth anisotropy determined by the relative inclination of the habit plane with respect to the GB dislocations, (iii) density of nucleation sites for the same variant and coalescence during growth, and (iv) spatial confinement from simultaneously nucleated neighboring α variants of dissimilar types.

The inverse hall–petch relation in nanocrystalline metals: A discrete dislocation dynamics analysis

When the grain size in polycrystalline materials is reduced to the nanometer length scale (nanocrystallinity), observations from experiments and atomistic simulations suggest that the yield strength decreases (softening) as the grain size is decreased. This is in contrast to the Hall–Petch relation observed in larger sized grains. We incorporated grain boundary (GB) sliding and dislocation emission from GB junctions into the classical DDD framework, and recovered the smaller is weaker relationship observed in nanocrystalline materials. This current model shows that the inverse Hall–Petch behavior can be obtained through a relief of stress buildup at GB junctions from GB sliding by emitting dislocations from the junctions. The yield stress is shown to vary with grain size, d, by a d1/2 relationship when grain sizes are very small. However, pure GB sliding alone without further plastic accomodation by dislocation emission is grain size independent.

First-principles study of Si and Mg segregation in grain boundaries in Al and Cu: application of local-energy decomposition

Segregation of Si and Mg at grain boundaries (GBs) in Al and Cu has been investigated using density-functional theory calculations combined with recently developed local-energy and local-stress schemes. The physics behind the impurity-segregation energy is effectively analyzed by the local-energy decomposition. For the \(\Sigma \)9 tilt and \(\Sigma \)5 twist GBs in Al and Cu, Si shows large segregation-energy gains only at tighter sites, where local configuration of remarkably short Si–Al or Si–Cu bonds with high charge densities of covalent-bonding features are formed, leading to the local-energy stabilization as the final-state effects. On the other hand, Mg shows large gains only at looser sites. For Mg in the Cu GBs, the formation of stable Mg–Cu bonds or Mg states at looser sites is the origin of the preferential segregation as the final-state effects. For Mg in the Al GBs, however, the local energies of Mg–Al bonds are not so stable at looser sites, while the instability of Al atoms at looser sites in pure GBs before substitution is the origin of the preferential segregation as the initial-state effects. The behaviors of Si and Mg in Al GBs are dominated by the difference in local sp bonding nature among Mg, Al and Si, while Si–Cu and Mg–Cu \(sp-d\) hybridization interactions dominate the behaviors of Si and Mg in Cu GBs.

Synergy of grain boundary sliding and shear-coupled migration process in nanocrystalline materials

An attempt has been made to illustrate a new physical mode of plastic deformation in nanocrystalline materials that synergizes two processes, i.e. grain boundary (GB) sliding and stress-driven shear-coupled GB migration. The latter process incorporates a rotational and a translational plastic flow, in which a normal migration is coupled with a shear. The energy change resulting from the above-mentioned mode was calculated, and showed that the new deformation mode was much more energetically favorable than both the pure GB sliding mode and the cooperative process of GB sliding and migration without a coupling shear. In addition, the new deformation mode considerably enhances the ductility of nanocrystalline materials compared with the other two above-mentioned processes.

On shear-coupled migration of grain boundaries in nanocrystalline materials

Shear-coupled migration of grain boundary (GB) was theoretically modeled as a generic mode of plastic deformation in nanocrystalline materials. The energy change due to this process that carried both rotational and translational plastic flow through the normal migration and the shear coupled to the said migration, respectively, was calculated. The results obtained revealed that the shear-coupled migration was energetically more favorable than the pure migration of a grain boundary. The translational deformation can turn the initially energetically unfavorable process into a favorable one, and it can decrease the critical stress required to initiate the process of migration by around 30%–80% as compared with that for pure GB migration.

Shear response of theΣ9⟨110⟩{221}symmetric tilt grain boundary in fcc metals studied by atomistic simulation methods

The shear response of the Sigma 9 {221} symmetric tilt grain boundary (GB) in three fcc metals Cu, Al, and Ni has been studied by atomistic simulation methods with the embedded atom method for interatomic potentials and with a bicrystal model. By applying an energy minimization procedure, it was found that there are two optimized structures of this particular GB at zero temperature for all the three metals studied. Shear of bicrystals at room temperature has been studied by the molecular-dynamics simulation method. Various kinds of structure evolution behavior have been found for this GB depending on the shear direction: (1) pure GB sliding; (2) GB atomic shuffling accompanied by lattice dislocation emission from the GB; and (3) GB migration coupled with GB sliding, namely, GB coupling motion. The GB coupling motions can differ in the direction and distance of the GB migration depending on the shear direction. An analysis with the aid of the coincidence site lattice theory indicates that the structure evolution behavior can be attributed to several elementary structure transformations inherent to this particular GB. A pair parameter (lambda, kappa) is proposed to describe the GB coupling motions.

Relation between grain growth and grain-boundary diffusion in a pure material by molecular dynamics simulations

Traditionally, due to the impurities inevitably present in real materials that limit the grain-boundary (GB) mobility, the processes of grain growth and GB diffusion are thought to involve similar activation barriers. However, recent molecular dynamics (MD) simulations of GB migration in bicrystals have suggested that, in pure materials, GB diffusion and GB migration involve distinct mechanisms and, hence, different activation barriers. Here we report MD simulations of grain growth in an impurity-free model nanocrystalline palladium microstructure containing only high-energy GBs. By contrast with the bicrystal simulations, we observe virtually identical activation energies for grain growth and GB diffusion. We discuss several mechanisms that might be responsible for this difference between the bicrystal and polycrystal results, including accommodation of the curvature-driven GB migration by the elimination of GB area and GB excess free volume and a possible finite mobility of the triple junctions. The qualitative agreement of our observations with the general experimental findings is remarkable given the absence of any impurities in our model system. This suggests that a fundamental, intrinsic process may be responsible for the similar activation energies for GB migration and GB diffusion in polycrystals. In our model system this process seems to involve the accommodation of the curvature-driven GB migration by GB diffusion to eliminate the GB area and related GB free volume at the triple junctions.

Molecular dynamics simulation of Y-doped Σ37 grain boundary in alumina

A molecular dynamics technique with a multi-atom potential was used to simulate the behavior of Y-doping in Σ37 grain boundary (GB) in α-Al 2O 3. Results on melting process, elastic constants, and GB diffusivity in pure α-Al 2O 3, Σ37 GB, and Y-doped Σ37 GB have been obtained and compared with available experimental data. Our simulation indicates that the pure GB pre-melts at a temperature of 1417 K before the bulk melts at 2105 K. When doped with Y, a melting nucleate around Y is formed and the pre-melting temperature is increased to 1536 K. The diffusivities of Al and O in the GB region have the same order of magnitude. In comparison, Y-doping decreases diffusivity and increases diffusion activation energy of both Al and O at the GB. Our results are consistent with the notion of “site-blocking” effect to explain the decrease in GB diffusivity with Y-doping. At low temperature (<1536 K), “site-blocking” is due to the larger size of Y ions, but at a high temperature (>1536 K), “site-blocking” is related to possible formation of nucleates surrounding the Y ions.

Modelling grain-boundary resistance in intergranular dislocation slip transmission

Abstract We investigate the resistance on the glide of lattice dislocations between adjacent crystal grains due to the presence of a grain boundary (GB). Applying a combination of molecular dynamics (MD) simulations and a line tension (LT) model we identify the geometrical parameters that are relevant in the description of this process. In the MD simulations we observe slip transmission of dislocation loops nucleated from a crack tip near a series of pure tilt GBs in Ni. The results are rationalized in terms of a LT model for the activation of a Frank-Read source in the presence of a GB. It is found that the slip transmission resistance is a function of only three variables: firstly, the ratio of resolved stress on the incoming slip system to that on the outgoing slip system, secondly, the magnitude of any residual Burgers vector content left in the GB and, thirdly, the angle between the traces of the incoming and outgoing slip planes in the GB plane. Comparison with the MD simulations and experimental da...

Characterization of three-dimensional grain boundary topography in a YBa2Cu3O7−d thin film bicrystal grown on a SrTiO3 substrate

The topography and crystallography of YBa2Cu3O7−d (YBCO) bicrystal films grown epitaxially on oriented SrTiO3 (STO) bicrystals have been characterized by scanning and transmission electron microscopies (SEM and TEM) and atomic force microscopy (AFM). The YBCO films were formed by laser ablation on melt-grown Σ13 STO bicrystals with a misorientation of 24° around the 〈001〉 tilt axis. In agreement with previous reports, TEM analysis revealed that the grain boundary in the film did not always follow the planar substrate grain boundary faithfully, but undulated about the average boundary plane. High resolution electron microscopy observations of the apparently complex undulating boundary structures could be explained as a result of an overlap between different orientation variants of the orthorhombic YBCO film. Cross correlation between SEM, AFM, and TEM imaging gave a clear evidence that an island growth mechanism is responsible for the observed grain boundary structure and morphology for which a schematic model is presented. It is seen that meandering of the YBCO grain boundary (GB) is necessarily coupled to a wide range of inclination of the GB plane in the z direction. The implications of this interfacial structure for the behavior of GB based Josephson junctions are discussed and compared to models proposed in the literature. It is also seen that inclination of the GB may be responsible for the poor correlation usually found in the literature between calculations and experimental curves of current density Jc versus the GB angle since the most elaborate models proposed up to now take into account only pure tilt GB plane facets, that is to say facets in the zone of the tilt axis. Moreover, such a GB structure may affect the interpretation of recent phase sensitive experiments done on bicrystal or tricrystal high Tc superconductors to determine the symmetry of the order parameter.

Creep cavity growth under interaction between lattice diffusion and grain-boundary diffusion

A model and a method of numerical analysis of the cavity growth in grain boundaries in metals due to lattice diffusion are proposed. The growth behavior simulated is compared to that due to grain-boundary (GB) diffusion. The simulation method is extended to analyze the growth under the interaction between lattice diffusion and GB diffusion. The growth rate calculated under the interaction is approximately equal to the linear sum of those due to the pure lattice diffusion and the pure GB diffusion.

A Molecular Dynamics Study of Cu Segregation to the Σ11 Grain Boundary In Al

AbstractWe present results of molecular dynamics (MD) simulation studies of Cu segregation to the Σ11 grain boundary (GB) in Al. The simulations were performed with Embedded Atom Method (EAM) potentials for Al and Al-Cu. The results predict that copper atoms tend to order along either side of the interface even at the pure symmetrical GB, forming alternating chains along the direction. The nucleation of the chains is driven by a change in the local atomic level stress induced by the pre-existing Cu atoms at the GB.