Aging behavior of Cu–Ti–Al alloy observed by transmission electron microscopy

Aging behavior of Cu–3 at.%Ti–4 at.%Al alloy at 723 K has been examined from mechanical, electrical, and microstructural points of view. Compared with binary Cu–3 at.%Ti alloy, the electrical conductivity improved six times to about 6%IACS (international annealed copper standard); whereas the peak hardness decreased from 280 to 180 Hv. The major strengthening phase is the tetragonal α-Cu₄Ti, which forms not via spinodal decomposition but based on the nucleation and growth mechanism. The precipitates grow in the c direction of the tetragonal phase, which lies along one of the axes of the matrix fcc Cu phase. This growth mode minimizes the strain energy arising from the lattice mismatch of about 2% between the matrix and precipitate; and results in a square rod shape, which reaches about 50 nm in length after 100 h anneal. Another precipitating phase is AlCu₂Ti (D0₃, Strukturbericht notation), with the major habit plane close to {110} of the fcc Cu matrix. The orientation relationship was not definitely determined, but it was found that the angle between the 100 and 110 poles of the matrix and precipitates, respectively, is about 5°, while the angle between the two 001 axes being about 7°. It was suggested that the formation of this ternary phase reduced the solute Ti concentration, leading to the decrease in the resistivity.     

Structural studies on γ-MnO2 by transmission electron microscopy

Single-crystal electron diffraction patterns were obtained on five specimens of -MnO₂: one natural, two electrolytical (EMD) and two chemical (CMD) samples. EMDs are best described by the orthorhombic structure proposed by De Wolff which is derived from the ramsdellite structure. A CMD prepared from MnCO₃ fits the hexagonal cell of -MnO₂. Flaky grains from the natural sample and fibres from a CMD prepared from Mn(NO₃)₂ are hexagonal with a new cell:a 0.494,c 0.539 nm. No simple relation between chemical composition, morphology and structure could be found.     

Scanning tunnelling microscopy and transmission electron microscopy study of ultrafine structure and surface relief of bainite in a Cu–Zn–Al alloy

The fine structure of bainite (1 plate) in a Cu–25.9 wt% Zn–4.0 wt% Al–0.1 wt% Re alloy has been studied by scanning tunnelling microscopy (STM) and transmission electron microscopy (TEM). The experimental results demonstrated that bainite plates are composed of subplates, and the subplates are composed of subunits. STM investigation shows that the surface relief with a bainite plate is composed of groups of small reliefs which correspond to subplates and subunits. The relief arising from the formation of bainite is tent like, which is different from that with martensite. Ledges exist on the broad faces of the subunit, indicating that it grows by a ledgewise mechanism. Three types of nucleation of the subunits were observed under TEM: face to face, edge to edge and edge to face. Based on the experimental results concerning the ultrafine structure and surface relief accompanying bainite, the sympathetic nucleation–ledgewise growth mechanism of bainite is proposed.     

Crystal interface and high-resolution electron microscopy—the best partner

Several contributions of HRTEM on the interface science are reviewed in chronological order. The first contribution of HRTEM is the observation of gold (113)S°11 boundary, giving experimental proof of the CSL model. An observation of the asymmetric (112)S°3 boundary follows. A SiC grain boundary is effectively assessed not by the density of CSL point but the number of dangling bonds in the boundary. A ZnO/Pd interface provides an example that a misfit dislocation does not necessarily accommodate the lattice mismatch. Segregated interface shows characteristic HRTEM image contrast, suggesting change in atomic bonding. An atomic height step in the semiconductor hetero interface is observed by the Chemical Lattice Image technique. In the diamond grain boundary a dangling bond may not elevate the boundary energy, being contradictory of the least dangling bond rule. Super–high resolution of the HVHRTEM enable us to determine atomic species in the grain boundary. Combined use of HRTEM and EELSE allows us to discuss the correlation between atomic structure and nature of the corresponding interface. It is not exaggeration to say that modern interface science does not exist witout HRTEM. On the other hand, many complicated interfaces found by HRTEM remained as unaswered questions. An innovative structural model is requested to appear on the scene.     

Visualizing biointerfaces in three dimensions: electron tomography of the bone–hydroxyapatite interface

A positive interaction between human bone tissue and synthetics is crucial for the success of bone-regenerative materials. A greater understanding of the mechanisms governing bone-bonding is often gained via visualization of the bone–implant interface. Interfaces to bone have long been imaged with light, X-rays and electrons. Most of these techniques, however, only provide low-resolution or two-dimensional information. With the advances in modern day transmission electron microscopy, including new hardware and increased software computational speeds, the high-resolution visualization and analysis of three-dimensional structures is possible via electron tomography. We report, for the first time, a three-dimensional reconstruction of the interface between human bone and a hydroxyapatite implant using Z-contrast electron tomography. Viewing this structure in three dimensions enabled us to observe the nanometre differences in the orientation of hydroxyapatite crystals precipitated on the implant surface in vivo versus those in the collagen matrix of bone. Insight into the morphology of biointerfaces is considerably enhanced with three-dimensional techniques. In this regard, electron tomography may revolutionize the approach to high-resolution biointerface characterization.     

Chemical thinning of III–V compound semiconductors for transmission electron microscopy

A vapour jet etching method and suitable equipment are tested in order to thin monocrystalline and polycrystalline material of GaAs, InP and InAs for transmission electron microscopy. The reactive vapour jet consists of air, bromine and methanol. The thinning procedure is monitored by an optical microscope. Large thin areas could be obtained using a bubbler temperature of 60° C, 20% bromine in methanol, a distance between nozzle and sample surface of 1.5 mm and a nozzle mouth diameter of 0.7 mm. During pre-etching the vapour flow rate was 0.71 min^–1, and 0.31 min^–1 was realized during final etching up to perforation. The etching times ranged between 1 to 2 min with an initial sample thickness of 350 to 365 m. Large thin areas have only been obtained using an excentric position of the nozzle with respect to the sample centre in combination with a manual rotation of the specimen holder.     

Observations and analyses of microstructural defects for chemical vapour deposited β-SiC by transmission electron microscopy

A transparent silicon carbide was grown by a cold-wall chemical vapour deposition (CVD) system using CH₃SiCl₃ and hydrogen gas mixtures on to a graphite substrate. Transmission electron microscopy was used to observe microstructural defects of CVD -SiC, usingg.b = 0,±1/3 and theg.R _F = integer criterion, quantitative information on defects can be obtained. Theb was identified as 1/6 [11¯2] for Shockley partial dislocations andR _F as 1/3[¯111], 1/3 [1¯11], 1/3 [11¯1] for stacking faults. Twin planes were identified as (11¯1), (¯11¯1) in this study. Stacking faults and twins always existed on {111}. Subgrains were surrounded by dislocation networks and full of microtwins. Other defects, such as dislocation loops and dislocation dipoles, were also observed.     

Study of the interfacial reactions between a bioactive apatite–mullite glass–ceramic coating and titanium substrates using high angle annular dark field transmission electron microscopy

Glass of generic composition SiO₂ · Al₂O₃ · P₂O₅ · CaO · CaF₂ will crystallise predominantly to apatite and mullite upon heat-treatment. Such ceramics are bioactive, osseoconductive, and have a high resistance to fracture. As a result, they are under investigation for use as biomedical device coatings, and in particular for orthopaedic implants. Previous work has shown that the material can be successfully enamelled to titanium with an interfacial reaction zone produced during heat treatment. The present study uses high angle annular dark field transmission electron microscopy (HAADF-TEM) to conduct a detailed examination of this region. Results show evidence of complex interfacial reactions following the diffusion of titanium into an intermediate layer and the production of titanium silicides and titanium phosphides. These results confirm previously hypothesised mechanisms for the bonding of silicate bioceramics with titanium alloys.     

Inhibition of scale growth on 20Cr–25Ni–Nb stabilized stainless steel by yttrium ion implantation revealed by analytical electron microscopy

Abstract

Yttrium ion implantation significantly improves the oxidation behaviour of the 20Cr–25Ni–Nb stabilised stainless steel in carbon dioxide at 750–900°C by the incorporation of this reactive element within the scale. The exact location and chemical state of the yttrium ion were established by transmission and scanning transmission electron microscopy. The implanted yttrium atoms are incorporated as Y₂O₃ grains and as ionic segregants along the oxide grain boundaries within the outer spinel layer. of the scale next to the inner Cr₂O₃ layer. As a consequence of the yttrium segregation, the cation diffusion along the oxide grain boundaries, which is responsible for continuing scale growth, becomes energetically less favourable. Scale growth is inhibited to an extent according to the proportion of grain boundaries so affected. The improved spallation resistance could derive from the yttria grains and yttrium segregants inhibiting scale failure, the prelude to decohesion, by increasing the energy for through scale crack initiation and propagation.

MST/932     

Influence of surface oxidation on transmission electron microscopy characterization of Fe–Ga alloys

Fe–Ga alloys are rapidly oxidized when exposed in air, forming both amorphous and crystalline surface oxides. These oxides hinder the observation of the ordered phases of B2 and D0₃ in Fe–Ga alloys by dark-field imaging and high-resolution imaging of transmission electron microscopy (TEM) techniques. Proper imaging techniques and reduction of surface oxides are necessary to obtain representative microstructural features to the bulk alloys by TEM.     

Nanofabrication by advanced electron microscopy using intense and focused beam∗

Abstract

The nanogrowth and nanofabrication of solid substances using an intense and focused electron beam are reviewed in terms of the application of scanning and transmission electron microscopy (SEM, TEM and STEM) to control the size, position and structure of nanomaterials. The first example discussed is the growth of freestanding nanotrees on insulator substrates by TEM. The growth process of the nanotrees was observed in situ and analyzed by high-resolution TEM (HRTEM) and was mainly controlled by the intensity of the electron beam. The second example is position- and size-controlled nanofabrication by STEM using a focused electron beam. The diameters of the nanostructures grown ranged from 4 to 20 nm depending on the size of the electron beam. Magnetic nanostructures were also obtained using an iron-containing precursor gas, Fe(CO)₅. The freestanding iron nanoantennas were examined by electron holography. The magnetic field was observed to leak from the nanostructure body which appeared to act as a ‘nanomagnet’. The third example described is the effect of a vacuum on the size and growth process of fabricated nanodots containing W in an ultrahigh-vacuum field-emission TEM (UHV-FE-TEM). The size of the dots can be controlled by changing the dose of electrons and the partial pressure of the precursor. The smallest particle size obtained was about 1.5 nm in diameter, which is the smallest size reported using this method. Finally, the importance of a smaller probe and a higher electron-beam current with atomic resolution is emphasized and an attempt to develop an ultrahigh-vacuum spherical aberration corrected STEM (Cs-corrected STEM) at NIMS is reported.     

Field effect in a graphene oxide transistor for proton and electron–hole conductivities

Proton (wet atmosphere) and electron (reduced graphene oxide) conductivities can be observed in graphene oxide films. The field effect in a graphene oxide transistor for different conductivity types has been discovered and investigated.     

Amorphisation and phase transformations in mechanically alloyed Ti–Al powders: electron microscopy investigation

Abstract

The microstructural evolution during mechanical alloying of Ti and Al powders has been investigated by X-ray diffraction, scanning electron microscopy (SEM)–energy dispersive spectroscopy, and transmission electron microscopy (TEM). Observations by SEM showed a progressive change of the powders' morphology as a function of milling time. Observations by TEM, performed on a sample milled for 20 h, revealed the simultaneous occurrence of amorphous zones and nanocrystalline domains. The observed amorphous phase is not the final milling product. After 34 h of milling it was possible to identify by TEM fcc (a=0·41 nm) nanocrystalline zones, with a mean size of about 10 nm. By irradiating the powder milled for 20 h with high density electron beams, a sudden in situ crystallisation took place. The crystallite (fcc with a=0·41 nm) size was between 0·1 and 0·5 μm.

MST/1281     

In situ transmission electron microscopy observation of microstructure and phase evolution in a SnO₂ nanowire during lithium intercalation

Recently we have reported structural transformation features of SnO(2) upon initial charging using a configuration that leads to the sequential lithiation of SnO(2) nanowire from one end to the other (Huang et al. Science2010, 330, 1515). A key question to be addressed is the lithiation behavior of the nanowire when it is fully soaked into the electrolyte (Chiang Science2010, 330, 1485). This Letter documents the structural characteristics of SnO(2) upon initial charging based on a battery assembled with a single nanowire anode, which is fully soaked (immersed) into an ionic liquid based electrolyte using in situ transmission electron microscopy. It has been observed that following the initial charging the nanowire retained a wire shape, although highly distorted. The originally straight wire is characterized by a zigzag structure following the phase transformation, indicating that during the phase transformation of SnO(2) + Li ? Li(x)Sn + Li(y)O, the nanowire was subjected to severe deformation, as similarly observed for the case when the SnO(2) was charged sequentially from one end to the other. Transmission electron microscopy imaging revealed that the Li(x)Sn phase possesses a spherical morphology and is embedded into the amorphous Li(y)O matrix, indicating a simultaneous partitioning and coarsening of Li(x)Sn through Sn and Li diffusion in the amorphous matrix accompanied the phase transformation. The presently observed composite configuration gives detailed information on the structural change and how this change takes place on nanometer scale.     

Thermochromism of vanadium–titanium oxide prepared from peroxovanadate and peroxotitanate

Vanadium–titanium complex oxides were prepared through a new synthetic route from peroxovanadate and peroxotitanate solutions, and their thermochromism were investigated. Reflectance spectra for V_0.48Ti_0.52O₂ pellet showed a reversible reflectivity increase in the wavelength region of 700–1,000 nm above 45 °C. The reflectivity was increased gradually from 45 °C to ca. 80 °C unlike the conventional VO₂ which shows an abrupt increase at 67 °C. The mechanism was discussed using powder XRD patterns, SEM images and element distribution maps.     

An experimental refinement of solid–solution relationship in Ca-α-Sialon ceramics by analytical electron microscopy

Local dopant compositions within individual α-Sialon grains were measured by analytical electron microscopy (AEM) in hot-pressed Ca _x Si_12−3x Al_3x O _x N_16−x (x = 0.3–1.4) ceramics. The reduction of local x values from the nominal dopant compositions is about 40% in general, and it reaches 60% for the end member (x = 1.4) which contains inclusions of AlN-based 21R phase. This results exhibit stronger departures from x than the previous report of 30% dopants missing in α-Sialon phase by electron probe micro-analysis (EPMA) [J Eur Ceram Soc 19:1637, 1999]. Amorphous films of ~1 nm thick were commonly found at grain boundaries (GBs), which could only take a small fraction of undetected dopants while the film composition exhibits a quite different behavior. The general presence of GB films can rationalize the discrepancy between AEM and EPMA results by their differences in probe size and detection geometry, while the much larger gap in the end member suggests the existence of Ca-rich glasses in the intergranular regions. By excluding this end member, a linear relation between dopant solution and lattice expansion is restored in α-Sialon structure, which leads to 20 and 80% increases of the expansion coefficients from those given in the previous and original reports, respectively. This study not only demonstrated the necessity of solubility study in ceramics by AEM refinement, but also opens a new front to correlate the solution behavior with the intergranular glass/amorphous structures, both were regarded so far as largely independent.     

Hydrogen content in titanium and a titanium–zirconium alloy after acid etching

Dental implant alloys made from titanium and zirconium are known for their high mechanical strength, fracture toughness and corrosion resistance in comparison with commercially pure titanium. The aim of the study was to investigate possible differences in the surface chemistry and/or surface topography of titanium and titanium–zirconium surfaces after sand blasting and acid etching. The two surfaces were compared by X-ray photoelectron spectroscopy, secondary ion mass spectroscopy, scanning electron microscopy and profilometry.The 1.9 times greater surface hydrogen concentration of titanium zirconium compared to titanium was found to be the major difference between the two materials. Zirconium appeared to enhance hydride formation on titanium alloys when etched in acid. Surface topography revealed significant differences on the micro and nanoscale. Surface roughness was increased significantly (p < 0.01) on the titanium–zirconium alloy. High-resolution images showed nanostructures only present on titanium zirconium.