High-angle Twist Boundaries Research Articles

Role of atomic structure on grain boundary-defect interactions in Cu

We investigate the role that the atomic structure of grain boundaries in Cu has in the interaction with point defects produced during irradiation. We focus on three aspects of boundary-defect interaction: how defects interact with pristine boundaries, how boundaries modify defect production during collision cascades, and how defects interact with damaged boundaries. We find that there are generic features common to most boundaries, including biased absorption of interstitials over vacancies during collision cascades and strong interactions with vacancies for interstitial-loaded boundaries. However, we find that the magnitude of these behaviors depends strongly on the atomic structure of the boundary. In particular, the biased absorption is much stronger for a high-angle twist boundary and smallest for a more general twist-asymmetric tilt boundary. Further, the strength of boundary-defect interactions is also sensitive to the boundary structure. We conclude that the sink strength of grain boundaries for interacting with point defects is not an intrinsic property of the boundary but rather depends on the irradiation conditions through the absorbed defect content at the boundary.

Plasticity of metal wires in torsion: Molecular dynamics and dislocation dynamics simulations

The orientation dependent plasticity in metal nanowires is investigated using molecular dynamics and dislocation dynamics simulations. Molecular dynamics simulations show that the orientation of single crystal metal wires controls the mechanisms of plastic deformation. For wires oriented along 〈 1 1 0 〉 , dislocations nucleate along the axis of the wire, making the deformation homogeneous. These wires also maintain most of their strength after yield. In contrast, wires oriented along 〈 1 1 1 〉 and 〈 1 0 0 〉 directions deform through the formation of twist boundaries and tend not to recover when high angle twist boundaries are formed. The stability of the dislocation structures observed in molecular dynamics simulations are investigated using analytical and dislocation dynamics models.

Order and structural units in simulations of twist grain boundaries in silicon at absolute zero

In contrast to previous results of simulations of the ground-state structures of high-angle twist grain boundaries (GB's) in silicon at absolute zero, in which amorphous intergranular films were found, recent experimental results imply structural order in some high-angle twist boundaries. With a novel protocol for simulating twist GB's, which allows the number of atoms at the boundary to vary, we have found new, lower energy, ordered structures. We have also found structural units common to many of these boundaries, which is further evidence that these boundary structures are not arbitrary or disordered. We give a detailed exposition of the results for five boundaries and conclude that there is structural order present in all of them.

On the relationship between the mechanical and the thermal instabilities of crystalline lattices

Numerical simulations have been employed to explore the melting of pure face-centered-cubic (fcc) metals under superheating conditions. For each chemical species considered, simulations were carried out on two different configurations. The first configuration consisted of two perfect semi-crystals rotated in order to obtain a high-angle twist boundary. The presence of an interface between the semi-crystals permitted to estimate the thermodynamic melting point for the tight-binding interatomic potential employed in calculations. The second configuration consisted, instead, of a perfect crystalline bulk. In this case, the crystalline lattice was slowly heated and the solid brought within the metastable region of existence beyond the thermodynamic melting point. The static long-range order parameter was employed to quantify the degree of disorder during the gradual heating and reveal the sudden collapse of crystalline structures at the equilibrium melting point or at the limit of superheating. The effects of gradual heating on system volume and elastic properties were also accurately monitored. The volume at which the continuous elastic softening should induce the mechanical failure of the crystalline structure was found to approximately correspond to the volume attained by the molten metal at the equilibrium melting temperature. In addition, a relationship was found between the volume reached by crystals at equilibrium melting and at the limit of superheating. Calculations revealed that melting occurs via a proliferation of defects in the bulk of crystalline lattices. Such defects, mainly vacancy-interstitial pairs, tend to form extended string-like clusters. Their density at equilibrium melting as well as at the limit of superheating was approximately the same for all the fcc metals considered.

Segregation Effects at a High-Angle Twist Boundary in ZnO

Ab initio density functional plane-wave pseudopotential calculations were performed for a Σ = 7 (θ = 21.79°) [0001] twist boundary in ZnO with and without the presence of Sb impurities. The segregation energies revealed a significant driving force for segregation and it was shown that the formation of an Sb monolayer was favoured. Decreased coordination in the boundary core suggested a trend towards the formation of an intergranular phase. The impurity states caused by the monolayer were located within the band gap and higher in energy relative to the state produced by a single impurity. Charge transfer to the Sb monolayer was observed indicating a possible enhancement of the grain boundary potential barrier.

Electromagnetic properties of high angle [001] twist grain boundaries in Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+δ/ bicrystals

Detailed measurement of electrical resistivity and V-I curves as a function of temperature and magnetic field up to 9 T were performed on several superconducting Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+/spl delta// (Bi-2212) bicrystals with synthetic high angle [001] twist grain boundaries. We found that the resistive onset temperature and field (at level of 10 nV) for all of the measured high angle twist boundaries were similar to those for their constituent single crystals. Furthermore, these boundaries were able to carry critical current as high as the single crystals. Our results suggested absence of weak-link behavior in the high angle [001] twist grain boundaries of Bi-2212 bicrystals.

Growth Kinetics of Grain-Boundary Induced Melting: A Molecular-Dynamics Study

AbstractThe growth kinetics of melting nucleated at a high-angle twist boundary in silicon are investigated using molecular dynamics. Melting is found to be a two-stage process. In the first stage order is lost within a single plane at the interface and the density of the solid increases to that of the liquid. In the second stage the atomic coordination changes and an isotropic liquid is formed.

Grain Boundaries of MgO Bicrystals

Bicrystals with (110) tilt boundaries and (100) twist boundaries were prepared by joining two single crystals of Mgo, using a graphite heating element at 2480°C without pressure. The joined boundaries were observed with an optical microscope and TEM. The evaporation/condensation mechanism was operative in joining. The boundary energies were measured at 1500°C by the thermal grooving method. The development of boundary grooves was controlled by surface diffusion. The ratio of the boundary energy to the surface energy (σb/σs) for small angle (110) tilt boundaries below 17° varies according to the Read-Shockley model, which suggests the boundary being constructed by edge dislocation arrays. TEM observation suggested that the small angle twist boundaries below 10° were formed by screw dislocation arrays. The (110) tilt boundaries had low σb/σs at about 40°, 70° and 130° which correspond to twin boundaries of (221), (111) and (113), respectively. σb/σs for high angle boundaries at 1500°C was around 0.4, lower than that of (100) tilt boundaries. Twist boundaries above 10° had also low energies at 22°, 29° and 37° which correspond to Σ of 13, 17 and 15 of the CSL theory, respectively. Hence, it is concluded that the high angle twist boundaries on (100) are explained by the CSL theory.

Effect of interatomic potential on the calculated energy and structure of high-angle coincident site grain boundaries—II. (100) Twist boundaries in Cu, Ag and Au

Earlier calculations on (100) high-angle twist boundaries in Al are extended to Cu, Ag and Au. Recent pseudopotentials of Dagens together with three empirical potentials for Cu are applied. Boundaries with values of Σ, the inverse density of coincidence sites, ranging between 5 and 73 are considered. In contrast to our earlier results for the Al pseudopotential of Dagens et al., the relaxed grain-boundary energy, E Σ , is found to increase smoothly as function of the twist angle, θ, for all six potentials considered here; i.e. “cusps” in the E Σ ( θ) curve are not observed. Rather similar boundary structures are obtained for the noble-metals; these differ drastically from those derived by means of the pseudopotential for Al. From a comparison with (100) twist boundaries in ionic crystals with NaCl structure it is concluded that also in the f.c.c. metals a volume expansion at the interface which increases smoothly with θ should be present. Interfacial structures determined without consideration of this expansion are hence concluded to be systematically in error. The article concludes with a summary of the properties of (100) twist boundaries in the f.c.c. and NaCl structures which are thought to be valid independent of the detailed form of the interatomic potential chosen.

Observation of intrinsic and extrinsic secondary grain boundary dislocations in [001] high angle twist boundaries in MgO

The report describes a systematic electron microscope investigation of possible GBD structures in (001) high angle twist boundaries in MgO using bicrystals of controlled geometry. Boundaries of this type were selected because they are relatively easy to prepare, and because GBD structures have been extensively observed and studied (3,4,7) in boundaries of this type in the metal gold.

The detection of the periodic structure of high-angle twist boundaries

Abstract An electron diffraction technique was developed to study the structure of high-angle grain boundaries. Diffraction patterns were obtained from the grain boundary region, and it was demonstrated for the first time that high-anglo 〈001〉 twist boundaries have a periodic nature over the entire range of possible misorientations. Relatively strong grain boundary reflections corresponding to reflections from the 0-lattice wore obtained from all boundaries studied. These results indicate that atomic relaxations occur around the 0-lattice points and those relaxations appear to be qualitatively similar to those which are well known to occur in low-angle twist boundaries.

The detection of the periodic structure of high-angle twist boundaries

The detection of the periodic structure of high-angle twist boundaries

Direct observation of grain boundary dislocation climb in ion-irradiated gold bicrystals

Abstract Thin-film gold bicrystals containing (001) low-angle twist boundaries, with twist angles in the range θ = 7-14°, and high-angle twist boundaries, with θ ≃ 37°, were prepared by welding evaporated films together face-to-face. All the boundaries contained regular orthogonal arrays of intrinsic screw grain boundary dislocations (GBD's) as well as more randomly occurring extrinsic edge GBD's having Burgers vectors essentially perpendicular to the boundary. The speciments were irradiated on one face with 20 kev gold ions producing excess self-interstitial atoms which diffused to the extrinsic GBD's causing them to climb in the boundary. The extrinsic GBD climb, in turn, caused the boundaries to act as overall interstitial sinks. The climb motion was observed directly by transmission electron microscopy. The intrinsic screw GBD's in the high-angle boundaries (and probably also in the low-angle boundaries) remained unaffected during the process, but possibly they acted as short circuiting pipes which de...

High-angle twist in GaSe

Crystals of GaSe grown by transport reaction with iodine were found to contain high-angle twist boundaries perpendicular to their c axis. A simple model is proposed which affords the twist boundary energy in layer structures such as GaSe and MoS2 to be calculated numerically. The relative minima in the angular dependence of the calculated boundary energies coincide with the experimentally observed series of discrete twist angles in GaSe. The proposed model confirms the validity of Bollmann's geometrical criterion for the stability of high-angle grain boundaries in the special case of layer structures. It is suggested that the model may also lead to a better understanding of the formation and the orientation of the nuclei in epitaxial crystal growth.