Papers by Keyword: HAADF

Abstract: Microstructure of Ni_50-xCo_xMn_35.5In_14.5 (x=0, 3, 5) melt-spun ribbons was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The typical layered microstructure consisting of oriented columnar grains and colonies of fine equi-axed grains was observed in the cross section of ribbons. The crystallographic structure of ribbons varied with the content of Co in alloys what affected of their M_s temperature. For the x=0 the single phase of monoclinic 14M modulated martensite was observed, but for x=3 and 5, a two phase structure of L2₁ austenite and monoclinic 14M or orthorhombic 10M modulated martensite were identified. Different temperature range of martensitic transformations were explained basing on valence electron concentration per atom e/a versus M_s relationship.

Abstract: (Zn,Mn)Te nanowires were grown via vapor-liquid-solid mode as test structures for spintronic applications. The structural and chemical properties of the nanowires were inspected by transmission electron microscopy. The nanowires contain much less stacking faults compared to ZnTe nanowires. This high structural perfection can be attributed to a rough liquid-solid interface as found by high-resolution transmission electron microscopy. The composition of the nanowires and, in particular, the Mn distribution is homogeneous. A ZnO cover layer forms after the growth of the nanowires.

Abstract: Precipitates in Al-Mg-Si-(Cu) alloys all contain a similar hexagonal arrangement of Si-atoms. Precipitates come and go but their inner Si ordering appears to vary little throughout the precipitation process. In order to improve understanding of precipitation and the related material properties, it is becoming increasingly clear that this includes a good understanding of the hexagonal Si-network, its relation to the precipitates and the Al matrix. Previous studies have revealed that adding Cu atoms to the ternary system, causes the Si network to twist slightly in the matrix about its hexagonal axis, favoring different precipitates. Here we investigate these two rotations. It is shown they can be viewed as a mirror of the network itself about a {310} Al plane. Since precipitates are coherent, the Si-network with its triangular arrangements of Si must also match a fourfold arrangement of Al on the {100} planes. Sets of Al lattice positions exist which can approximate the tree-fold Si symmetry, according to the experimentally observed orientations, and one or more large super-cells can be found having near fit in both lattices. The mirror plane is a main plane in one such super-cell. We show that the mirror leaves every seventh node of the network unchanged, thus defining a smaller hexagonal super-cell in the network, similar to the B’ or Q’/Q phase, where corners are invariant, but where the Si contents is flipped.

Abstract: By means of a combination of high-resolution electron microscopy (HREM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) techniques, we have directly revealed that periodic arrangements in different manners for flattened hexagons constructed with atom columnar clusters can form two Al-Ni-Rh crystalline approximant phases. In contrast to periodic arrangements of flattened hexagons, configurations and distributions of various defects in these structurally-complicated alloy phases have been examined and their structural characteristics discussed. HREM observations clearly show that structural defects in Al-Ni-Rh crystalline approximants are of phason type and they are correlated with incorrect arrangements of atom columnar clusters. The distribution of high density planar defects can destroy the long-range periodicity in at least one direction in the pseudo decagonal symmetry plane. By means of the HAADF-STEM imaging technique, the existence of ill-formed atom columnar clusters in the core area of a linear defect, which is usually not visible in HREM observations, has been clearly revealed.

Abstract: By means of high resolution transmission electron microscopy (HREM) and high-angle annular dark-field image technique (HAADF), morphological, structural and compositional characteristics of the precipitates in the Mg-4Y-3Nd alloy aged at 200°C for different periods of time have been studied. On the basis of HREM observations, an atomic structural model for the β’-precipitate with an orthorhombic unit cell has been proposed. The characteristic distribution of the precipitates which are rich in rare-earth elements (Y, Nd) has been clearly revealed by the HAADF imaging technique.

Abstract: Various techniques in conventional transmission electron microscopy (CTEM) and scanning transmission electron microscopy (STEM) are applied to characterize comprehensively the microstructures of the nano-composite materials, including Cu2O quantum dots deposited on multiwall carbon nanotubes (CNTs) and Fe particles encapsulated in carbon nanohorns (CNHs) as two studying cases.

Abstract: Niobium is a strong carbide forming element which is often used in microalloyed steels to control the grain size during thermomechanical treatments and to provide strengthening through precipitation processes. A detailed microscopic investigation is one of the keys for understanding the first stages of the precipitation sequence, thus Transmission Electron Microscopy (TEM) is required. The main difficulty of TEM studies is due to the nanometre scale dimensions of the particles, which makes their detection, structural and chemical characterization delicate. Model Fe- (Nb0.06%,C0.05%) and Fe-(Nb0.05%,C0.03%,N0.03%) ferritic alloys subjected to isothermal annealing treatments have been investigated. High Resolution TEM (HRTEM) and conventional TEM (CTEM) were used to characterise the morphology, nature and location of precipitates. Volume fraction measurements and a statistical approach to the determination of precipitate size histograms have been investigated using Energy Filtered TEM (EFTEM) and High Angle Annular Dark Field (HAADF) imaging. Chemical compositions were quantified by Electron Energy Loss Spectroscopy (EELS). The evolution of precipitate composition with time and temperature is compared with previous simulations obtained from new thermodynamic models based on equilibrium boundary conditions.