High-energy Grain Boundaries Research Articles

What's so special about nanocrystalline semiconductors?

Nanocrystalline semiconductors display unique features compared to coarse-grained microstructures and even to their monocrystalline counterparts. We contend that such peculiarities are due to: (1) the extremely large fraction of atoms located at Grain Boundaries (GBs) and (2) the 'character distribution' of GBs, which are mostly high-energy, random interfaces. Initially, we study the structure of random GBs in nanocrystalline semiconductors by means of large-scale Molecular Dynamics (MD) simulations. Subsequently, the atomic structure and electronic properties of some typical high-energy GBs in Si- and C-based nanostructures are characterised by means of a semi-empirical tight-binding Hamiltonian. We show that relevant properties of nanocrystalline semiconductors containing a large fraction of high-energy GBs are quite distinct with respect to those of coarse-grained and bulk semiconductors.

Atomic mechanisms of grain boundary diffusion: Low versus high temperatures

We analyze recent results of atomistic computer simulations of grain boundary (GB) diffusion in metals. At temperatures well below the bulk melting point T m GB diffusion occurs by random walk of individual vacancies and self-interstitials. Both defects are equal participants in the diffusion process and can move by a large variety of diffusion mechanisms, many of which are collective transitions. GB diffusion coefficients can be computed by kinetic Monte Carlo simulations. At high temperatures, the presence of large concentrations of point defects is likely to alter the diffusion mechanisms. Molecular dynamics simulations of GB structure and diffusion in copper reveal a continuous GB premelting in close vicinity of T m . However, diffusion in high-energy GBs becomes almost independent of the GB structure (“universal”) at temperatures well below T m . This behavior can be tentatively explained in terms of heterophase fluctuations from the solid to the liquid phase. The exact diffusion mechanisms in the presence of heterophase fluctuations are yet to be established.

Stress-induced surface damage and grain boundary characteristics of sputtered and electroplated copper thin films

The morphology of the stress-induced surface damage and its relationship with the microstructure in electroplated and sputtered copper thin films is discussed. After annealing at 435 °C for 1 h, two types of surface damage were observed. In some films, grooves along the grain boundaries were formed, whereas in other films, voids at the grain boundary triple junctions were observed. In films of similar thickness, the triple junction voids were deeper than the grain boundary grooves. It was found that the high energy grain boundaries (HEGBs) or their triple junctions are the sites where damages are generated by thermal stress. The area ratio of the HEGBs to the surface area per grain, which is a function of both the grain size ( d) and the film thickness ( h), as well as the fraction of HEGB ( f), determines the morphological equilibrium between the two types of damage. It is suggested that, in general, a microstructural parameter, d/ hf, can be used to predict the damage morphology in Cu films.

Grain-boundary diffusion creep in nanocrystalline palladium by molecular-dynamics simulation

Molecular-dynamics (MD) simulations of fully three-dimensional (3D), model nanocrystalline face-centered cubic metal microstructures are used to study grain-boundary (GB) diffusion creep, one mechanism considered to contribute to the deformation of nanocrystalline materials. To overcome the well-known limitations associated with the relatively short time interval used in our MD simulation (typically <10 −8 s), our simulations are performed at elevated temperatures where the distinct effects of GB diffusion are clearly identifiable. In order to prevent grain growth and thus to enable steady-state diffusion creep to be observed, our input microstructures were tailored to (1) have a uniform grain shape and a uniform grain size of nm dimensions and (2) contain only high-energy GBs which are known to exhibit rather fast, liquid-like self-diffusion. Our simulations reveal that under relatively high tensile stresses these microstructures, indeed, exhibit steady-state diffusion creep that is homogeneous, with a strain rate that agrees quantitatively with that given by the Coble-creep formula. The grain-size scaling of the Coble creep is found to decrease from d −3 to d −2 when the grain diameter becomes of the order of the GB width. For the first time a direct observation of the grain-boundary sliding as an accommodation mechanism for the Coble creep, known as Lifshitz sliding, is reported.

Goss Texture Development in Fe–Si Steels

Goss texture development in silicon steels has been studied through EBSP measurements and various computer simulations and calculations. The results of these studies suggest the possible role of high energy grain boundaries (HEGB) in the abnormal growth of Goss grains. The Goss orientation has a fraction of HEGBs that is higher than any other commonly observed orientations in the primary recrystallized silicon steels. The HEGBs have high GB diffusion coefficients which cause rapid coarsening of precipitates on these HEGBs and release them earlier, at the time when other GBs are still pinned. A difference in the mobility between the HEGBs and the other GBs favours the abnormal growth of Goss grains. The Monte-Carlo methods that have been developed and used to validate this assumption have generated abnormally growing Goss grains. The experimentally observed grain boundary character distributions (GBCD) around the growing Goss grains have been reproduced in simulation by assuming high mobility to HEGBs. Apart from the high mobility differences between different GBs, the importance of the fraction of GBs with high mobility around growing Goss grains is realized.

Importance of fractions of highly mobile boundaries in abnormal growth of Goss grains

Abstract The grain boundaries (GB) that surround growing grains play a major role in deciding the character of grain growth during annealing. In this work, various computer experiments have been performed to ascertain that high energy boundaries are responsible for abnormal grain growth (AGG). An initial computer specimen with half a million grains represents the experimentally observed orientation distribution function (ODF). By assigning high mobility to high energy GBs, growth of grains with various orientations has been analyzed. The computer simulated grain boundary character distribution (GBCD) around the growing Goss grain is compared with the experimentally measured GBCDs. The results of the computer experiments support our previous work that assumes that the high energy boundaries are responsible for AGG in Fe–3%Si steels. The importance of fractions of mobile GBs on the development of Goss texture is discussed.

Nickel and selenium grain boundary solute diffusion and segregation in silver

For the first time, the grain boundary (GB) solute diffusion of Ni and Se in Ag polycrystals was investigated systematically by the radiotracer serial sectioning technique using 63Ni and 75Se isotopes. Measurements in the temperature ranges 589–989 K for Ni and 371–877 K for Se were carried out in Harrison's type B regime at high temperatures and in type C regime at low temperatures. In the B regime, the product sδD GB was determined ( s being the solute GB segregation factor, δ the GB width, D GB the GB diffusion coefficient), while in the C-regime D GB values were measured directly. The solid solubility of both solutes, Ni and Se, in Ag is very small. The equilibrium phase diagrams, however, are quite different: Se forms an intermetallic compound whereas Ni does not form any compound with Ag, which indicates dominating Se–Ag and Ni–Ni bonds, respectively. Comparing the solute D GB values with C regime measurements of GB self-diffusion in Ag, a drastic decrease of the Ni diffusivity and an increased activation enthalpy of Ni diffusion, H GB Ni, in Ag GBs is observed. This behaviour is discussed in terms of repulsive Ni-vacancy interactions and decreased jump frequencies of the Ni atoms which are mainly located at low energy positions in the GB. The measured D GB values of Se, on the other hand, are comparatively close to GB self-diffusion, whereas H GB Se is again larger than H GB Ag of GB self-diffusion. This behaviour is explained in terms of the formation of “embryos” of two-dimensional phases at high energy GB sites. Combining the obtained sδD GB and D GB values and assuming δ=0.5 nm, the GB segregation factors and segregation enthalpies of Ni ( H s Ni=−39.7 kJ/mol) and Se ( H s Se=−21.4 kJ/mol) in Ag were evaluated.

On grain boundary character distribution in materials with cubic structure

From analysis of numerous experimental data on grain boundary (GB) statistics in polycrystals it has been established that certain groups of materials with cubic structure reveal similar GB character distributions (GBCD) (distribution of GBs by reciprocal density of coincidence sites ∑). It has been shown that GBCD can be described with an empirical low with different parameters for various groups. Several criteria for classification of materials by these groups (the stacking fault energy value, hierarchy of GB energies and mechanism of replacement of high-energy GBs with low-energy ones) have been considered. It has been found that peculiarities of electronic structure of materials are correlated with the classification proposed.

Electronic bonding characteristics of hydrogen in bcc iron: Part II. Grain boundaries

Electronic structure calculations were carried out for bcc iron grain boundaries (GB) with or without hydrogen, using the self-consistent Discrete Variational embedded cluster method within the first-principles local density formalism. Bonding characteristics were mainly investigated. Simple rigid body translations perpendicular to the GB plane were used for estimation of relaxed GB geometry. Analysis of bond order summation over the GB shows considerable volume expansion normal to the GB plane of a dense 23(111) twist/tilt GB and some compression for the rather open 23(110) twist configuration. These results are discussed in the context of atomistic simulations which suggest that higher energy GB's generally have larger volume expansion normal to the GB plane. H in a 23(111) GB reduces Fe-Fe bonding strength by —3% within a 0.25 nm spherical volume around the H site, associated with reduction of the 4s and 4p occupancy of the nearest neighbor Fe. Since these orbitals contribute mainly to metallic bonding, the action of H atoms as an embrittlement inducer can be understood.

The effect of local texture on crack propagation in FeAl ordered alloy

The ordered Fe-Al alloys have received considerable attention during the last few years, due to their possible high temperature structural use. B2 iron aluminide is attractive also from a density consideration and might be a promising matrix material for fiber reinforce composite systems. However, poor ductility prevents potential applications of this material. Iron aluminides exhibit preferentially intergranular fracture at room temperature which indicates the importance of grain boundary (GB) properties for mechanical behavior of these polycrystals. It is well known that the type of GB strongly affects its cohesion energy, which leads to preferential crack propagation along high energy GBs, whereas, the low energy GBs are resistant to the fracture. The effect of GB parameters on fracture behavior was an impetus for the study of GB characteristics in the ordered alloys. In these works, an enhanced plasticity is associated with the higher portion of small angle and low-[Sigma] GBs in polycrystal. Obviously, these data evidence the possibility of GB design, which can be used in addition to the alloying to improve the plasticity of these materials. This paper aims to study the GB characteristics (misorientation parameters and spatial distribution of different type GBs) in Fe-Al alloy to elucidate the microstructuralmore » aspects of fracture behavior of B2 FeAl.« less