Secondary Grain Boundary Dislocations Research Articles

Dislocation emission at junctions between Σ=3 grain boundaries in gold thin films

The interaction of dislocations with grain boundary junctions plays an important role during plastic deformation and stress relaxation in polycrystalline thin films. In the present work, arrays of secondary grain boundary dislocations (SGBDs) and their behavior at junctions between orthogonal Σ=3{111} and Σ=3{112} grain boundaries in Au thin films have been studied using room temperature and in situ transmission electron microscopy (TEM). Through diffraction contrast experiments, we find that these dislocations have Burgers vectors of the type a/6 112 . In situ TEM experiments conducted at elevated temperature show that the arrays of SGBDs on {111} twin planes originate in the {112} boundaries where they accommodate a small rotational misorientation from the exact coincident-site-lattice (CSL) orientation. We propose that the discontinuous distribution of SGBDs in the {112} boundary produces a climb stress that drives the dislocation motion. As the dislocations in the grain boundary increase their separation, the climb stress and the misorientation between grains is reduced. To test the plausibility of this explanation, we consider the balance between the reduction in energy due to motion in response to the climb stress and the increase in energy due to the lengthening of dislocation line on the {111} twin plane.

High-resolution and conventional electron microscopy study of a Σ = 3, [101]{121} twin grain boundary in molybdenum

Abstract Complete structural analysis of a Mo bicrystal containing a Σ = 3, [101](121)A grain boundary (GB) was achieved by means of transmission electron microscopy from the microscopic to the nanoscopic level. Secondary GB dislocations compensating a small deviation from the exact Σ = 3 concidence were observed and their character was determined. Rigid-body displacements (RBDs) were determined directly from high-resolution electron microscopy images using samples with surface normal parallel and perpendicular to the tilt axis. Measured values of RBDs are compared with recent theoretical calculations.

Effect of Grain Boundary Dislocations on 9R Stacking Errors at an Incoherent Twin Boundary in Copper

We consider the nature of faults in the dissociated 9R phase at an incoherent twin boundary in copper. Analysis of the dislocation content of a five-layer break in the normal ABC/BCA/CAB stacking sequence of the 9R shows that this defect is associated with a secondary grain boundary dislocation (SGBD) of Burgers vector a/6[211]. Atomistic simulations of SGBDs at the incoherent twin interface show that the sign of the Burgers vector is important to determining the core structure of the dislocation. The resulting atomistic structures and the five-layer width of the fault can be directly understood by analyzing the set of twinning dislocations that make up the boundary. Atomistic calculations show further, in agreement with the experimental observations, that a shear parallel to the interface causes an increase in the fault width that accompanies the growth of the 9R phase.

A high-resolution electron microscopy study of secondary dislocations in Σ = 3, [ī10]—(ī1) grain boundaries of aluminium

Abstract The atomic structure of secondary dislocations at Σ = 3, [ī10]—(ī1) grain boundaries in pure aluminium have been studied by high-resolution electron microscopy. In defect-free regions, the grain boundary coincident-site lattice is continuous across the interface but several different secondary grain-boundary dislocations are observed in the vicinity of interplanar steps.

Grain boundary kinetics—I. In situ observations of coupled grain boundary dislocation motion, crystal translation and boundary displacement

Extensive stress-induced climb-glide motion of secondary grain boundary dislocations (SGBDs) in nearΣ 5 grain boundaries in gold was directly observed and video recorded by means of hot-stage in situ transmission electron microscopy. The passage of appreciable numbers of SGBDs along the boundary ( N) caused a translation of the abutting crystals with respect to one another ( T) and a lateral displacement of the boundary with respect to each crystal ( D i ). Quantitative measurements of N, T, and D i , for a number of boundaries provided unambiguous evidence that these three processes were coupled. The observed T's and D i 's were in agreement with a geometrical model involving the N's, the Burgers vectors of the moving SGBDs, and the heights of the steps which the SGBDs were expected to produce in the boundary plane. Most of the SGBDs whose motion produced the measured translations and displacements were intrinsic in nature; a minor fraction appeared to be extrinsic. The deduced SGBD-step configurations tended to be those associated with the smaller step heights possible within the framework of the Σ5 CSL. Operating sources and sinks for moving SGBDs were directly observed in some of the boundaries.

Grain boundary kinetics—II. In situ observations of the role of grain boundary dislocations in high-angle boundary migration

The capillary-induced motion of near-Σ5 grain boundaries in Au was directly observed and video-recorded by means of hot-stage in situ transmission electron microscopy. The observed boundary motion differed from the boundary “displacement” caused by the stress-induced passage of secondary grain boundary dislocation (SGBD)-steps along the boundary which was studied in Part I of this work in the following respects. It occurred in a jerky fashion, no translation of one abutting crystal with respect to the other was observed (proving that the motion occurred by the growth of one crystal at the expense of the other), and quantative evidence proved that SGBDs were not involved. These results ruled out the SGBD-Step Model for the observed motion which was identified as common long-range boundary migration. In situ observations of the migration of more general high-angle boundaries in polycrystalline Al films revealed similar jerky motion and an apparent lack of any significant contribution to migration by observed SGBDs. Although it was impossible to rule out absolutely the SGBD-Step Model for these boundaries because all possible SGBDs could not be imaged, its applicability seems highly unlikely. On the basis of these results, it is suggested that the migration of high-angle boundaries, in general, occurs mainly by the local conservative shuffling of atoms across the boundary. These shuffles take place either at pure steps or at other favorable sites, the details of which depend upon the boundary type.

Study of the Structure and Chemistry of Point, line and Planar Imperfections Via Field-Ion and Atom-Probe Field-Ion Microscopy

We first list, in catalogue form, a number of research subjects which have utilized the fieldion microscope (FIM) and atom-probe field-ion microscope (APFIM) techniques in their solution. Then we present the results of a combined transmission electron microscopy (TEM) and APFIM study of a grain boundary (GB) in a Mo-5.4 at.% Re alloy, which had been annealed in bulk form for 35 hours at 1273 K to induce Re segregation. A GB with an orientation within ≈0.4° of Σ = 9 was studied employing TEM and analyzed in detail using Bollmann's 0-Lattice theory and Frank's formula. A set of secondary GB dislocations was observed with a spacing of 11.4 nm. The APFIM measurements -- on this same GB -- indicate that it has a Re concentration of ≈9.8 at.%; this value is 1.75 times greater than the matrix's measured concentration of 5.6±0.9 at.% Re. Thus this research constitutes direct and quantitative experimental evidence for solute-atom segregation to a high-angle grain boundary with a relatively high degree of coincidence ( ≈Σ1 = 9 ). These results are consistent with our Monte Carlo simulations of high coincidence twist boundaries and a Σ = 5 tilt boundary in Pt-1.0 at.% Au alloys which show that solute-atom segregation occurs mainly to the dislocation cores. The experimental and simulated values of the enhancement factor are approximately the same.

In-Situ TEM study of the role of secondary grain boundary dislo-cations in grain boundary migration and atom source/sink action

Over the past several years, a wealth of experimental evidence concerning the structure of simple grain boundaries (GBs) in metals has accumulated which suggests that the GBD/SU description of GB structure is applicable, at least at some level, to GBs in general. According to this model, high-angle GBs contain intrinsic arrays of grain boundary dislocations (GBDs) possessing Burgers vectors (b) which are translation vectors of an appropriate DSC-lattice, arranged in a network which is consistent with Frank's formula.A special property of secondary GBDs (SGBDs), i.e., GBDs possessing bs which are not lattice vectors of either of the adjoining crystals, is that a step in the boundary plane is often associated with their core.

Secondary grain boundary dislocation structures in gold and silver 〈001〉 twist boundaries revisited

Abstract A search was made for the presence of localized secondary grain-boundary dislocations (SGBDs) in selected [001] twist boundaries in gold and silver using transmission electron microscopy Screw SGBD networks were detected in near-Σ5, -Σ13, -Σ17, -Σ25 and -Σ29 boundaries in gold and in near-Σ-5 boundaries in silver. The Σ25 and Σ29 networks were observed for the first time and extend the range of SGBD structures observed in [001] twist boundaries in gold beyond that found in earlier work. The observation of Σ5 SGBDs in silver contradicts an earlier claim that these SGBDs are delocalized in this metal. Extensive tests were carried out to verify that all of the observed SGBD images were indeed due to localized dislocations and were not moire fringe patterns.

Secondary grain-boundary dislocation nodes at the junction of three grains

Abstract It is shown, from the determination of the Burgers vectors of secondary grain-boundary dislocations in arrays in near ∑27b, ∑9 and ∑3 grain boundaries meeting at a triple junction, that the Burgers vectors balance to zero at nodes along the triple line. Thus, when treated independently, the secondary grain-boundary dislocations obey a nodal balance in the same way as that predicted theoretically for the total Burgers vector content at a triple line. In addition it is shown, for the example considered, that there is no significant imbalance in the rigid-body displacements across the three boundaries at the triple line.

Dissociated grain boundary dislocations in some incoherent twin boundaries

It is shown that partial grain boundary dislocations (GBDs) form two of the three independent arrays of secondary GBDs in incoherent Σ3 boundary facets on {421}/{1085} planes in a f.c.c. Cu+6at%Si ...

Hexagonal arrays of grain-boundary dislocations in coherent twin boundaries

Abstract Hexagonal arrays of secondary grain-boundary dislocations have been observed in several near-Σ3 coherent twin boundaries in polycrystalline specimens of a f.c.c. Cu + 6 at.% Si alloy. The Burgers vectors of the dislocations in the networks have been determined and the steps associated with them are discussed.

TEM Study of Secondary Grain Boundary Dislocations in Near-∑13, Near-∑17 and Near-∑29 [001] Twist Boundaries in Gold

In 1970, an extensive search by transmission electron microscopy (TEM) for evidence of ordered structure in high-angle [001] twist boundaries helped to establish the credibility of the DSC/CSL description of grain boundary structure. In this work, square grids of line contrast were found in boundaries for which the twist misorientation (Θ) was very near the special Σ5+ Σ13 and Σ17 Θ. The lines ran parallel to the primitive translation vectors (b(1) and b(2)) of the appropriate low-Σ DSC lattice, and their spacing correlated well with the spacing predicted by Frank's formula for dislocations with Burgers vectors b(1) and b(2). The images were interpreted as secondary grain boundary dislocation (SGBD) networks. Only for the near-Σ5 case was g•b analysis carried out to show that the line contrast was characteristic of b= 1/10 <310> type screw SGBD's.

The crystallography of annealing twins in alumina ceramics

The crystallography of basal and rhombohedral annealing twins in polycrystalline alumina ceramics has been analysed by selected area electron diffraction, and secondary grain boundary dislocations (SGBDs) within those boundaries have been analysed. The twins arise from misorientations of the high coincidence fraction ∑, and the SGBDs have Burgers vectors from the displacement shift complete lattice (DSCL). The basal twin is unique to the rhombohedral system. A O lattice calculation is used to show that the rhomhohedral twin is similar to the ∑ = 7 〈210〉 boundary in hexagonal metals.

Direct observation of grain-boundary dislocations in the FeCo alloy

An extensive transmission electron microscopy study has been made of the structure of high and low angle grain boundaries in both ordered and disordered FeCo alloys which were electrochemically thinned from the bulk material. The grain boundaries observed were determined to be of the primary coincidence type and were found to contain numerous secondary grain-boundary dislocations. These secondary dislocations were either of the true grain-boundary types or else crystal lattice dislocations. The secondary grain-boundary dislocations were presumed to arise from the migration of the grain boundaries during grain growth and were frequently found to dissociate into crystal lattice dislocations arranged in the form of low angle boundaries within one of the adjacent grains.