Fivefold Twins Research Articles

HREM study of the microstructure in nanocrystalline materials

High resolution electron microscopy studies of nanocrystalline Pd and (Fe 0.99, Mo 0.01) 78Si 9B 13 indicate that the atomic structures of the grain boundaries consist of both ordered and disordered regions. The microstructure in ordered regions of the grain boundaries shows curved and little distorted appearance. In some areas, however, nanovoids and imperfect or disordered regions are also observed at the grain boundaries. The defects in the grains, such as five-fold twins, edge and 60° dislocations, are also observed. The nature of these defects is similar to those normally observed in conventional coarse-grained polycrystals.

From amorphous to nanocrystalline germanium— the role of twinning studied by high resolution electron microscopy

The structure of amorphous thin films of germanium obtained by physical vapor deposition at temperatures well below one half of the melting temperature has been studies by high resolution electron microscopy. Crystallization of the films was achieved during deposition by appropriate heating of the substrate or after deposition by annealing at elevated temperature, respectively. Therewith, very small crystallites extending from nearly 3 to about 25 nm in size are formed, at which twinning is frequently observed. In particular, multiply-twinned structures with five-fold twin junctions occur at various stages of the crystallization. The temperature at which the crystallization starts can be markedly lowered, and the crystallinity of the films obtained can be substantially increased by capping the crystalline substrates with amorphous carbon layers. A combination of substrate capping and post-deposition annealing leads to almost completely crystalline films which exhibit a heavily twinned structure.

Crystalline particles with multiply twinned structure in amorphous films of germanium

Crystalline particles of germanium grown in amorphous dilms during the vapour deposition of germanium on NaCl substrates have been studied by high resolution electron microscopy. The particles are shown to exhibit a heavily twinned structure which is characterized by mutually oriented fivefold twins accompanied by additional parallel twin lamellae and segments of subgrain boundaries. The particle surface has a rimose outline because of re-entrant faces at the outer terminations of twin boundaries. Growth and structure of the particles are discussed in terms of the nucleation of paracrystalline clusters as well as of twinning processes during growth. A mechanism is proposed by which interlinked multiply twinned structures may originate from crossing of twin lamellae because of internal elastic strains.

Atomic structure and energy of grain boundaries in silicon, germanium and diamond

Abstract We have used Stillinger-Weber and Tersoffpotentials to calculate and simulate the atomic structures of ⟨011⟩ tilt boundaries with Σ = 1+2n2 (n = 1,2,3,4) in silicon, germanium and diamond lying in (111), (122), (133) and (144) planes with corresponding angles of 70.53°, 38.94°, 26.53° and 20.05° respectively. Both Stillinger-Weber and Tersoff potentials gave consistent values of grain boundary energies for silicon. The energies for diamond were considerably higher (about a factor of 6–7) than the corresponding values for silicon and germanium. The calculated values of the grain boundary period are in good agreement with coincidence site lattice periods. The boundaries with tilt angles θ< 20.05° can be represented by arrays of (a/2)⟨110⟩{001} dislocations with no dangling bonds. High-resolution electron microscopy results on the atomie structures of Σ=3, fivefold twins, and Σ = 9 are discussed. The high energy Σ = 9 boundary (second order twin) has been shown to split into two Iow energy Σ = 3 (first-order or primary twin) boundaries.

Transformation of the two-dimensional decagonal quasicrystal to one-dimensional quasicrystals: A phason strain analysis.

The transformation of the two-dimensional (2D) decagonal quasicrystal to various 1D quasicrystals has been studied from the viewpoint of linear phason strain theory and compared with experiments. This theory accounts well for the shifts of electron-diffraction spots during this 2D-1D quasicrystalline transformation. In some cases five 1D quasicrystals can grow along the five twofold directions of the 2D decagonal quasicrystal as fivefold twins, and their composite electron-diffraction pattern has been simulated.

Icosahedral and decagonal quasicrystals, crystalline phases, and multiple twins in rapidly solidified Al13Cr4Si4

Both the icosahedral and decagonal phases have been found in Al13Cr4Si4 melt-spun ribbons. A definite orientation relationship existing between these two phases is determined by SAD which is the same as that obtained previously by Schaefer and Bendersky. During the icosahedral quasicrystal-crystal transformation in this Al−Cr−Si alloy, the Al13Cr4Si4 crystalline phase forms at low temperature and an unknown phase at high temperature. Both crystalline phases have definite orientation relationships with the icosahedral phase. The five-fold rotational twins of Al13Cr4Si4 around the [110] direction formed from the icosahedral phase follow the group-subgroup relationship. The unknown phase has a hexagonal lattice,a=0.73 andc=1.62nm.

QUASI-CRYSTALLINE TO CRYSTALLINE TRANSFORMATION IN (Ti1−xVx)2Ni ALLOYS

The transformation of quasicrystal in (Ti1−xVx)2Ni alloys, x=0.0–0.3, to the equilibrium Ti2Ni and TiNi phases on heating in-situ in an electron microscope has been studied. Twins occurred frequently in the fcc Ti2Ni across {111} planes and fivefold twins sometimes can be obtained. The electron diffraction patterns of fivefold twins and quasicrystal are similar but distinctly different. A parallel orientation relationship existed between the fcc Ti2Ni and the ordered bcc TiNi.