ISSN:0022-2720    

DOI:10.1111/j.1365-2818.1995.tb03650.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

SUMMARYWe examine some of the assumptions underlying the standard optical equation as used for modelling high‐resolution annular darkfield (Z‐contrast) images of thin specimens. The validity of the approximations (i) that scattering occurs as spherical waves (atoms are point potentials) and (ii) that the depth of focus is infinite is found to be resolution‐dependent.The correction to the optical equation for non‐spherical wave scattering is found to be important when the probe size is of the order of 0·1 nm or smaller, and does not vanish in the ideal case of a large annular detector with no hole. Depth of focus becomes an important consideration at 100 kV for probes smaller than about 0·2 nm when the specimen thickness exceeds 10 nm. Both corrections can significantly degrade the effective point‐to‐point resolution of high‐resolution Z‐contrast images relative to that predicted by the standard

ISSN:0022-2720    

DOI:10.1111/j.1365-2818.1995.tb03651.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

SUMMARYInternal interfaces determine the macroscopic mechanical and electronic properties of materials. Their investigation is therefore essential for an understanding of the relationship between microstructure and the materials properties. Besides the atomic structure, the bonding and electronic structure on an atomic scale are of interest. This information is contained in the interface‐sensitive component of the electron energy‐loss near‐edge structure (ELNES). It can be extracted by the spatial difference method. Such spectra have already been interpreted with respect to the bonding and the local atomic environment of the atoms at the interface. Here we assess critically the application of the spatial difference for the investigation of internal interfaces. The method is subjective because a suitable scaling factor has to be chosen. As a guideline we propose to minimize the number of turning points in the difference spectrum. With this hypothesis we successfully determine the ELNES arising from atoms at two internal interfaces between a metal and a ceramic. This reveals qualitatively which type of bonding is present at the interface and quantitatively how many atoms are involved

ISSN:0022-2720    

DOI:10.1111/j.1365-2818.1995.tb03652.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

SUMMARYTo extract quantitative electronic structure information from electron energy‐loss near‐edge structure typically necessitates the separation of spectra into linearly independent components. For this to be accomplished, certain conditions need to be met concerning the localization of chemical bonding and the linear scaling of fine structure with density of states. The dependence of the processing parameters, such as window widths and positions, of the commonly used ‘two‐window intensity‐ratio’ method to determine the distribution of π and σ bonding of diamond‐like carbon films is investigated. It is shown that, although the selection of the integration windows can lead to very large variations in the resultant bonding fractions, the placement of two small windows centred on the edge structures of interest does permit a robust and reliable estimate of bonding. It is demonstrated that the sp3variations can be reproducibly monitored with a relative accuracy of about ± 5%, and an absolute concentration given with an accuracy of ± 13% for foils of thickness at least up to twice the inelast

ISSN:0022-2720    

DOI:10.1111/j.1365-2818.1995.tb03653.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

SUMMARYCluster‐size dependence of alloying behaviour in nanometre‐sized atom clusters has been studied by transmission electron microscopy, using clusters in the Au‐Al system, which forms several intermetallic compounds in the phase diagram. In gold clusters of approximately 6–12 nm mean size, rapid dissolution of aluminium atoms took place and Au2Al or AuAl2clusters were formed. When the size of gold clusters was increased to approximately 40 nm, no such rapid spontaneous alloying occurred. These observations suggest that there exists a critical size of initial gold clusters below which aluminide clusters are successfully formed by spontaneous a

ISSN:0022-2720    

DOI:10.1111/j.1365-2818.1995.tb03654.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

SUMMARYElectron holography is leaving its state of infancy and is rapidly becoming a tool for everyday use. This is due to the increasing availability of field‐emission electron microscopes as well as the increasing use of slow‐scan CCD cameras, which provide the linear image recording so essential to electron holography. At the same time, the continuing boost in performance of desktop computers both in hardware and in software has facilitated the handling of many of the complex reconstruction and processing tasks of electron holography. We describe here the software package ‘HoloWorks©’, recently developed at Oak Ridge National Laboratories. This package is designed for the processing of electron holograms stressing easy‐to‐use functions and is a plug‐in module for Gatan's software Digi

ISSN:0022-2720    

DOI:10.1111/j.1365-2818.1995.tb03655.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

SUMMARYTi50Pd(50‐x)Crxβ2alloys (B2, CsCl‐type crystal structure) have been investigated as part of an overall study of the systematics of phase transformation behaviour and also for the commercial applicability of the shape memory effect at elevated temperatures. The phase transformations are unique in the sense that electron diffraction and high‐resolution transmission electron microscopy of Ti50Pd(50‐x)Crxalloys withxbetween 6 and 10 at.% reveal {110}B2modulated structures with periodicities between 3 and 4·5 planes that vary linearly with composition. Satellites along certain* lying at incommensurate positions are observed in electron diffraction patterns and are consistent with transverse lattice displacements. High‐resolution transmission electron microscopy (HRTEM) imaging usually reveals sinusoidal displacements; however, a few images appear ‘banded’ at three‐ and four‐plane intervals suggesting the possibility that three‐ and four‐plane commensurate packets combine to give the overall incommensurate satellite spacing. With the use of a newly developed technique for quantifying atomic displacements from HRTEM images, modulated phases in Ti50Pd43Cr7and Ti50Pd42Cr8alloys, where atoms are shifted from the cubic lattice sites by a transverse lattice displacement wave, have been studied. The real space displacements of the modulated structures were determined and compared to truly incommensurate and varying three‐ and four‐plane wave packet combinations by examination of the goodness of fit as assessed by the correlation coefficient for a least‐squares fit. In the cases of the Ti50Pd43Cr7and Ti50Pd42Cr8alloys, the incommensurate sine wave exhibited much better matching than the commensurate packets, indicating that these modulation

ISSN:0022-2720    

DOI:10.1111/j.1365-2818.1995.tb03656.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

ISSN:0022-2720    

DOI:10.1111/j.1365-2818.1995.tb03657.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

SUMMARYIt is generally assumed that amorphous magnetic alloys respond to field annealing by a process of local directional ordering which leaves the amorphous structure intact. We have made a comparative microstructural study of field‐annealed Co95‐xFe5(BSi)xamorphous alloys using transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) with thin sections parallel to the ribbon surface. Field annealing response was measured from anisotropy in low‐field hysteresis loops. These alloys show appreciable surface crystallization for annealing as much as 80 K below the bulk crystallization temperature. The surface crystallization proceeds by a known mechanism (selective oxidation) to which we have added a more detailed understanding. Three steps are involved: (1) formation of an amorphous borosilicate surface oxide layer during annealing; (2) depletion of glass stabilizing elements (boron and silicon) from the underlying amorphous metal substrate; (3) primary crystallization of the destabilized, metal‐enriched subsurface layer to an fee or hep cobalt‐rich phase. Striking differences in the microstructural morphology were revealed for different glass former ratios B/Si. For high B/Si ratios, the surface crystallites are predominantly fee Co and show a high density of oxygen faults. For low B/Si ratios, the surface crystallites are predominantly hep Co and almost free of faults. Response to field annealing is proportional to the B/Si ratio and correlates with the presence of oxygen faults in surface crystallites. Electron diffraction and microprobe analysis indicate that the surface oxide in silicon‐rich alloys is a dense silica glass which appears to be an effective diffusion barrier to oxygen. The surface oxide in boron‐rich alloys is a more porous oxide richer in B2O3. These observations appear to be related to those from perminvar alloys where oxygen was found to be necessary for field annealing to

ISSN:0022-2720    

DOI:10.1111/j.1365-2818.1995.tb03658.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

SUMMARYSingle crystalline α‐SiC films and Au(target metal)/α‐SiC(substrate) bilayer films were irradiated with 2‐MeV electrons in an ultrahigh‐voltage electron microscope (UHVEM) to form,in situ, pure amorphous SiC (a‐SiC) and gold‐implanted a‐SiC. respectively. In the latter case, sample films were set so that the electron beam was incident on the gold layer. Differences in the bonding state between pure a‐SiC and gold‐implanted a‐SiC were studied by Auger valence electron spectroscopy (AES). In pure a‐SiC. the carbon valence state was markedly different from that of α‐SiC but the silicon valence state was similar to that of α‐SiC. In gold‐implanted α‐SiC. both the carbon and the silicon valence states were different from those of α‐SiC. It is suggested that a high density of molecularized carbon clusters are formed in the am

ISSN:0022-2720    

DOI:10.1111/j.1365-2818.1995.tb03659.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY