Grain Boundaries

The present document is a progress report describing the work accomplished to date during the second year of our four-year grant (February 15, 1990--February 14, 1994) to study grain boundaries. The research was focused on the following three major efforts: Study of the atomic structure of grain boundaries by means of x-ray diffraction, transmission electron microscopy and computer modeling; study of short-circuit diffusion along grain boundaries; and development of a Thin-film Deposition/Bonding Apparatus for the manufacture of high purity bicrystals

Fast Diffusion Process in Quenched hcp Dilute Solid 3^3He-4^4He Mixture

The study of phase structure of dilute $^3$He - $^4$He solid mixture of different quality is performed by spin echo NMR technique. The diffusion coefficient is determined for each coexistent phase. Two diffusion processes are observed in rapidly quenched (non-equilibrium) hcp samples: the first process has a diffusion coefficient corresponding to hcp phase, the second one has huge diffusion coefficient corresponding to liquid phase. That is evidence of liquid-like inclusions formation during fast crystal growing. It is established that these inclusions disappear in equilibrium crystals after careful annealing.Comment: 7 pages, 3 figures, QFS200

Interatomic potentials for atomistic simulations of the Ti-Al system

Semi-empirical interatomic potentials have been developed for Al, alpha-Ti, and gamma-TiAl within the embedded atomic method (EAM) by fitting to a large database of experimental as well as ab-initio data. The ab-initio calculations were performed by the linear augmented plane wave (LAPW) method within the density functional theory to obtain the equations of state for a number of crystal structures of the Ti-Al system. Some of the calculated LAPW energies were used for fitting the potentials while others for examining their quality. The potentials correctly predict the equilibrium crystal structures of the phases and accurately reproduce their basic lattice properties. The potentials are applied to calculate the energies of point defects, surfaces, planar faults in the equilibrium structures. Unlike earlier EAM potentials for the Ti-Al system, the proposed potentials provide reasonable description of the lattice thermal expansion, demonstrating their usefulness in the molecular dynamics or Monte Carlo studies at high temperatures. The energy along the tetragonal deformation path (Bain transformation) in gamma-TiAl calculated with the EAM potential is in a fairly good agreement with LAPW calculations. Equilibrium point defect concentrations in gamma-TiAl are studied using the EAM potential. It is found that antisite defects strongly dominate over vacancies at all compositions around stoichiometry, indicating that gamm-TiAl is an antisite disorder compound in agreement with experimental data.Comment: 46 pages, 6 figures (Physical Review B, in press

Kinetic processes at grain boundaries. Progress report, 15 August 1979-14 August 1980

A broad investigation was made of kinetic processes at grain boundaries and the relationship between these kinetic processes and the boundary structure. The approach was both experimental and theoretical. Extensive use was made of high resolution experimental methods of investigating grain boundaries in specimens containing boundaries of controlled geometry. Computer simulation was also employed. Elements of the following projects were completed: a study of intrinsic and extrinsic secondary grain boundary dislocation structure in (001) high angle twist boundaries in MgO; a study of grain boundary dislocations in plane matching grain boundaries; an analysis and review of high angle grain boundaries as sources or sinks for point defects; an analysis and review of grain boundary structure in metals and ceramic oxides; and simulation of the structure of vacancies in high angle grain boundaries. Progress was made: in the development of a model for diffusion induced grain boundary migration; and the determination of the mechanism for grain boundary diffusion in metals

STRUCTURE AND PROPERTIES OF POINT DEFECTS IN GRAIN BOUNDARIES IN METALS

The atomistic structures of a number of single vacancies and interstitials at a number of sites in the cores of a number of grain boundaries in a number of metals were calculated [1,2] by the method of molecular statics. These point defects were found to exist in all cases as bona fide point defects. In some cases binding energies to the boundaries were also determined. The thermally activated jumping of single vacancies and interstitials in a Σ=5 tilt boundary in BCC iron was also studied by means of molecular dynamics [2,3]. The vacancies executed relatively rapid jumps between a variety of sites in the boundary core to produce diffusive displacements mainly along the tilt axis. The interstitial became tightly bound to a particular site in the boundary and was unable to jump thermally amongst other sites. All of these results were considered along with other information in the literature, and it was concluded [4] that grain boundary self-diffusion in this boundary (and very likely most boundaries in metals) occurs by a vacancy exchange mechanism

High angle grain boundaries as sources or sinks for point defects

A secondary grain boundary dislocation climb model for high angle grain boundaries as sources/sinks for point defects is described in the light of recent advances in our knowledge of grain boundary structure. Experimental results are reviewed and are then compared with the expected behavior of the proposed model. Reasonably good consistency is found at the level of our present understanding of the subject. However, several gaps in our present knowledge still exist, and these are identified and discussed briefly

Secondary grain-boundary dislocations in (001) twist boundaries in MgO. II. Extrinsic structures

Extrinsic secondary grain boundary dislocation (GBD) structures were observed by weak beam transmission electron microscopy in a variety of (001) twist boundaries in MgO. These structures were derived from segments of lattice dislocations embedded in the boundaries and could be interpreted as the result of the decomposition of the lattice dislocations into extrinsic GBDs and the subsequent interaction of the product GBDs with the intrinsic boundary structure. The results demonstrate that lattice dislocations in MgO are attracted to grain boundaries over a wide range of conditions and tend to remain embedded in the boundaries as extrinsic GBD structures

ON THE SEARCH FOR EXPERIMENTALLY OBSERVED GRAIN BOUNDARY PHASE TRANSITIONS

The phase space for a heterogeneous system containing a grain boundary involves a relatively large number of variables (i.e., at least six plus the number of components), and it is therefore conceptually possible to induce a large variety of grain boundary phase transitions by selectively varying these parameters. Despite this, a review of the literature reveals that there have been virtually no clear-cut experimental observations of transitions reported in which the boundary structure has been observed as a function of time under well defined conditions. In current work, we are searching for roughening/faceting transitions and melting transitions for boundaries in Al by hot stage transmission electron microscopy. A clear example of a reversible roughening/faceting transition has been found. No evidence for melting has been found for temperatures as high as 0.96 Tm (by monitoring GBD core delocalization in several special boundaries with Σ≤ 13) or 0.999 Tm (by observing the local diffraction contrast at general boundaries in polycrystalline specimens)

Effect of Grain Boundary Structure on Grain Boundary Diffusivities in the Au/Ag System

Grain boundary chemical diffusivities for a series of symmetric (001) tilt boundaries in the Au/Ag system were measured by the surface accumulation method using newly developed thin-film multi-crystal specimens, in which the grain boundaries feeding the accumulation surface were all of the same type. Possible effects due to segregation at the grain boundaries and surfaces were avoided. CSL boundaries of low-{Sigma} (i.e., 5, 13, 17, 25) and also more general boundaries with tilt angles between the low-{Sigma} orientations were selected. The diffusivities were found to vary monotonically with tilt angle (i.e., no cusps at low-{Sigma}'s were found) in a manner consistent with the Structural Unit model. 8 refs., 7 figs

STRUCTURAL UNIT/GRAIN BOUNDARY DISLOCATION MODEL FOR TWIST BOUNDARIES IN CUBIC CRYSTALS

The systematics of [001] twist boundary structure is presented formally in terms of a structural unit/grain boundary dislocation hierarchical model and the earlier model of Sutton is generalized. By comparison with experimental observation and atomistic calculation using pair-potential models the physical significance of the individual members of the hierarchy is determined. Comparison with experiment indicates a strong type primary relaxation for θ ≤ 36.9° and a significant secondary relaxation near Σ5 which must result from "oblique" perturbations in the array of primary GBD's. On the other hand, comparison with available calculated results indicates a strong type primary relaxation at low angles but a progressively weaker relaxation at higher angles. Also, no evidence is found for any significant secondary relaxations when at least one pair potential is employed. However, very recent studies indicate stronger secondary relaxations with other potentials, and this, in future work, should lead to better agreement between calculations and experiment