Application of convergent-beam electron diffraction to the determination of grain boundary structures

Convergent-beam electron diffraction (CBED) has been applied in grain boundary structure determination, both for horizontal and vertical boundaries in thin specimens. First the relation between bicrystal symmetry and CBED pattern symmetry is developed for the two types of boundaries. These results are subsequently employed for a structure determination of twin boundaries in Au and Si. The CBED patterns from twinned bicrystals obtained experimentally are compared with computer-simulated patterns, for the case of a horizontal twin boundary in Au and Si. Finally the symmetry of CBED patterns from vertical twin boundaries in Si is analysed and found to depend on whether a static incident beam or a rocking-beam geometry is employed.La diffraction en faisceau convergent (DFC) a été appliquée à la détermination des structures de joints de grains placés horizontalement et verticalement dans une lame mince. D'abord, on établit la relation entre symétrie du bicristal et symétrie du diagramme DFC pour les deux types de joints. Les résultats sont ensuite appliqués pour déterminer la structure de joints de macle dans Au et Si. Les diagrammes DFC de bicristaux maclés sont comparés avec des diagrammes calculés pour un joint de macle horizontal dans Au et Si. Finalement, la symétrie des diagrammes DFC pour des joints de macle verticaux dans Si est analysée ; elle varie selon que l'on emploie un faisceau incident statique ou oscillant

Statistical thermodynamics of vacancies in binary alloys

Applied Science

TEM DETERMINATION OF INCOHERENT TWIN-BOUNDARY STRUCTURE IN L12 ORDERED ALLOYS

The structure of incoherent Σ=3 twin boundaries in the L12 ordered alloy Cu3Au is investigated employing TEM. The boundary planes are found to vary about ten degrees from a {112} plane. Employing the TEM method of α-fringes common diffraction vectors are used for determining the rigid-body translation between the two crystals at the boundary. No dilatation has been found at a (754) incoherent twin boundary within experimental error. Four grain boundary areas having different translations that preserve the density in the boundary are demonstrated, separated from each other by an antiphase boundary on one side of the boundary plane

THE INTERACTION OF DISLOCATIONS AND A COHERENT TWIN BOUNDARY IN ORDERED Cu3AU AS STUDIED BY IN-SITU DEFORMATION IN A TEM

In order to investigate the microstructural behaviour of grain boundaries in L12-ordered Cu3Au during plastic deformation, in-situ deformation experiments were performed in a transmission electron microscope. The dislocation structure at a coherent twin boundary (CTB) before and after straining the specimen using a single-tilt straining holder is analysed in a double-tilt holder. During the in-situ deformation specific slip systems were active in both crystals from which dislocation transmission across the CTB was expected. However, the majority of the moving superdislocations are absorbed into the boundary, and change the boundary structure locally from a symmetrical structure into an asymmetrical one and vice versa. The implication is that grain-boundary fracture might occur if larger strains are applied to the specimen for which experimental evidence is given

APPLICATION OF CONVERGENT BEAM ELECTRON DIFFRACTION TO GRAIN BOUNDARY STRUCTURE DETERMINATION

Nous avons utilisé la technique de la diffraction des électrons faisceau convergent pour étudier la structure des joints de grains. L'influence de la position d'un joint de macle, situé parallèlement à la surface de l'échantillon, sur les clichés de diffraction, est discutée. Aussi quelques résultats obtenus pour un joint de macle dans le silicium parallèle au faisceau d'électrons sont rapportés.Convergent beam electron diffraction has been applied for determining grain boundary structures, both for horizontal as well as vertical boundaries in thin specimens. The influence of boundary location on the CBED pattern for a horizontal twin boundary is discussed, and some preliminary experiments on the pattern symmetry of vertical twin boundaries in Si are presented

TEM INVESTIGATION OF GRAIN BOUNDARIES IN POLYCRYSTALLINE SILICON

Une étude par microscopie électronique en transmission de la structure de joints de grains dans du silicium polycristallin est présentée, pour deux types de joints voisins des désorientations de coïncidence Σ = 7 et Σ = 9. Dans le joint Σ = 7, un réseau de dislocations secondaires existe ainsi qu'une série de marches. Dans le joint Σ = 9, nous avons trouvé un système de franges α (causé par une translation des deux grains le long du plan du joint) et un réseau de dislocations secondaires dont la structure dépend des conditions de diffraction.The structure of two near-coincidence boundaries (close to a Σ = 7 and a Σ = 9 misorientation) occurring in commercial poly-silicon has been examined in TEM. Both the structure of secondary dislocation networks as well as the occurrence of α-fringes (indicating a relative translation of the crystals along the boundary) were investigated. The near Σ = 7 boundary is found to contain a simple network in addition to a regular array of steps. The near Σ = 9 boundary exhibits α-fringes and a dislocation network which appears as a different structure for different diffraction conditions