Publications (13)10.99 Total impact
- [Show abstract] [Hide abstract] ABSTRACT: The primary Al phase crystallizes preferentially on TiAl3 fine precipitate during the solidification process of hot-dip Zn-11 mass%Al-3 mass%Mg-0.2 mass%Si coating, which leads to the formation of fine dendrite structures by the small amounts of Ti added to the coating bath. TiAl3 is thought to act as heterogeneous nucleation sites of the primary Al phase. Detailed TEM observation of the sectioned specimen by a micro-sampling unit equipped with an FIB system revealed that the TiAl3 fine precipitate is not a single crystal but polycrystals. However, a crystallographic relationship exists between the TiAl3 phase and the surrounding Al phase, namely, TiAl3||Al, TiAl3||Al and TiAl3||Al. High-resolution TEM observation of the interface indicates a good coincident relationship between the Al matrix and the precipitate. Therefore, the primary Al phase nucleated heterogeneously on the surface of the TiAl3 phase. Furthermore, detailed TEM observation revealed a monotectoid reaction by the primary Al phase to the extremely fine Al and Zn phases.
- [Show abstract] [Hide abstract] ABSTRACT: In this paper, advantage and measurement principle of energy-filtered transmission electron microscopy (EF-TEM), which can measure the elemental distribution on local area in a material, are outlined. The energy-filtering technique by EF-TEM is applied to the visualization of fine precipitates TiN in a low-alloy steel. Through the observations of the energy-filtered images of TiN using Ti-M edge in electron energyloss spectra, TiN in alpha-Fe matrix is clearly observed. As a result, it is clarified that this technique is quite useful to evaluate the spatial distribmien of TiN in the low-alloy steel. Furthermore, the advantage of the energy filtering technique using M edge for visualization of nano-precipitates containing transition metals such as titanium is discussed.
- [Show abstract] [Hide abstract] ABSTRACT: Thickness measurements of diamond-like carbon (DLC) films by electron energy-loss spectroscopy (EELS) and electron holography are discussed. In order to evaluate the thickness by EELS and electron holography, the mean free path for inelastic scattering and mean inner potential of DLC films were determined precisely, respectively. It is found that both the mean free path for inelastic electron scattering and the mean inner potential are sensitive to the preparation methods, namely the density of DLC films. The present work has demonstrated that thickness measurement by EELS is available to DLC films thicker than 20 nm, while electron holography can be applied to thinner films ( approximately 5 nm). Furthermore, close relations are observed between the density of DLC films and the energy-loss spectra.
- [Show abstract] [Hide abstract] ABSTRACT: A focused ion beam milling is a relatively new technique that has been developed for an efficient method to prepare a transmission electron microscopy specimen for such observations as the cross-section of individual structures. Especially, its selective thinning ability for a specific area is far superior to conventional methods such as an electropolishing or an Ar ion milling for metals and steels. However, the high energy Ga ion beam fabrication often causes a problematic damage to a sample with different manner, depending on the microstructure or the composition of materials. In the present study, the focused ion beam milling equipped with micro-sampling technique is mainly examined to prepare a thin foil specimen for a low melting Sn metal. The advantage of the micro-sampling method is to decrease the milling time, resulting in minimization of sample damage effects. Under such fabrication conditions, some artificial precipitation of a Cu-Sn alloy in a Sn layer of a tinning plate not including Cu atoms has been found. This precipitation is observed whenever the specimen is mounted on the Cu support holder in the micro-sampling method. Using an analytical transmission electron microscope, the Cu-Sn alloy is identified to be a Cu6Sn5 phase. Since the Cu(6)sn(5) phase is a type of peritectoid reaction indicated by the phase diagram, it is considered that a Cu atom or a Cu cluster with a high energy are firstly sputtered from the support holder, then reacted to the Sn layer. When a Mo support holder was used in the micro-sampling method, such an artificial precipitation was not induced in the Sn layer. The precipitation phenomenon is discussed on a view point of the difference in the sputtering ratio and reaction behavior of Cu and Mo atoms for Sn metal.
- [Show abstract] [Hide abstract] ABSTRACT: To evaluate the accuracy of measurements using stereoscopic observation employing a transmission electron microscope (TEM), the thickness (∼0.6 μm) of platelet-type α-Fe2O3 particles was investigated by stereoscopic observation using TEMs and imaging plates. The accuracy of these measurements was evaluated by comparing ts with tg, where ts and tg were the thicknesses determined by stereoscopic observation and geometric calculation taking the particle's shape into account, respectively. The standard deviation of the difference between ts and tg decreases with increased tilt angle up to 20°. The minimum standard deviation of the difference was evaluated to be 4.8% at a tilt angle of 20°. From the present study, it was found that a particle's shape, such as thickness on a sub-micrometer scale, can be evaluated by stereoscopic observation using a TEM set at the highest accuracy at a tilt angle of about 20°.
- [Show abstract] [Hide abstract] ABSTRACT: The effects of accelerating voltage and collection angle on the mean free path for all inelastic electron scattering (lambdap), which is an important parameter for determining specimen thickness by using electron energy-loss spectroscopy, were investigated with crystalline Si and amorphous SiO2. First, thickness of Si film was measured with the convergent-beam electron diffraction method, while thickness of SiO2 particles was estimated from their spherical shape. Then from electron energy-loss spectra, lambdap was evaluated for Si film and SiO2 particles by changing the accelerating voltage (100 to approximately 300 kV) and the collection angle for the scattered electrons. Under the condition of no objective aperture, lambdap for Si film and SiO2 particles was found to increase with the increase of accelerating voltage and to take values of 180+/-6 nm (Si) and 247+/-8 nm (SiO2) at 300 kV. Also, it was found that lambdap in both cases decreases drastically with the increase of collection angle in the range smaller than 25 mrad, while it tends to take a constant value at the collection angle larger than 25 mrad at 200 kV.
- [Show abstract] [Hide abstract] ABSTRACT: Characteristic intensity distribution of diffuse scattering in III‐V alloy semiconductor GaAs0.5Sb0.5 epitaxially grown was observed by the energy‐filtered electron diffraction method with  incidence. The diffuse scattering situates at the one‐third positions between the fundamental reflections extending parallel to the q002 direction in the reciprocal space. A high‐resolution electron microscope image shows weak contrast modulation corresponding to the diffuse scattering. The image processed with the Fourier transform by selecting the diffuse scattering and a fundamental reflection shows small regions consisting of bright dots being elongated along the (111) planes and aligning on the (002) planes, which are considered to result from the ordering of As and Sb during the growth process. The effect of including the fundamental reflection for imaging the ordered regions in the image processing method is also discussed. Finally, based on the results obtained by energy‐filtered electron diffraction and high‐resolution electron microscopy, a simple structure model for the short‐range ordered structure in GaAs0.5Sb0.5 is proposed.
- [Show abstract] [Hide abstract] ABSTRACT: Recent progress of electron microscopy on phase transformations of alloys, especially energy-filtered electron microscopy, was reviewed mainly on the basis of the recent works by the authors. In the energy-filtered electron diffraction patterns of both Cu72.5Pd27.5 and Ti50Ni48Fe2 alloys, weak diffuse scattering which resulted from the diffusional and displacive phase transformations, respectively, was clearly observed by virtue of the background subtraction. In a short-range ordered state of a Cu72.5Pd27.5 alloy, through the careful evaluation of the dynamical diffraction effect on the diffuse scattering, short-range order parameters were evaluated quantitatively. On the other hand, in the premartensitic state of a Ti50Ni48Fe2 alloy, the morphology and growth process of microdomains less than 5 nm with a single transverse type of atomic displacement were clarified by energy-filtered electron diffraction and in situ dark-field electron microscopy. Furthermore, through in situ electron energy-loss spectroscopy on L2,3 core edges of a Ti50Ni48Fe2 alloy, the change in the electronic structure due to the martensitic transformation was analyzed. Thus, it is clearly demonstrated that advanced electron microscopy utilizing electron energy-loss spectroscopy is effective to make clear the change in both crystal and electronic structures associated with phase transformations in alloys.
- [Show abstract] [Hide abstract] ABSTRACT: To determine the mean free path (λp) for inelastic electron scattering of a silicon crystal at an accelerating voltage of 200 kV precise measurements of the specimen thickness were carried out by convergent-beam electron diffraction (CBED) technique and electron energy-loss spectroscopy (EELS) using an analytical electron microscope. Through the evaluation of the specimen thickness by using CBED technique, it was found that precise measurements in the case of 400 reflection with convergent beam angle of 12 mrad were performed in the condition that the specimen thickness was set to be in the range of 100 to 500 nm. By subtracting the background in CBED patterns with an energy filter, minima and maxima of the intensity profile of the symmetric fringes in the diffraction disks were clearly observed. Consequently, it was clarified that the maximum thickness measurable for the 400 diffraction disks increased from 400 nm to 670 nm. Using the precise specimen thickness evaluated by CBED technique, λp with the collection semiangle 157 mrad was determined to be about 147±5 nm through EELS.
- [Show abstract] [Hide abstract] ABSTRACT: The accuracy of electron energy-loss spectroscopy (EELS) and electron holography on thickness measurement of amorphous SiO2 was discussed. Since the SiO2 particles investigated in this work have a spherical shape, local thickness along the incident electron beam can easily be evaluated. Thus, from EELS, the mean free path of inelastic scattering was determined to be 178±4 nm at 200 kV. It is considered that thickness measurement is limited to amorphous SiO2 film thicker than about 20 nm with EELS. On the other hand, from phase shift in the electron hologram, the mean inner potential was evaluated to be 11.5±0.3 V. It is suggested that the thickness measurement is possible up to a few nanometers with electron holography. Thus, it is pointed out that the accurate thickness measurement is possible for a thinner region with electron holography than EELS.
- [Show abstract] [Hide abstract] ABSTRACT: Digital electron microscopy has been developed and applied to the structure analysis of advanced materials such as semiconductors and alloys. First of all, quantitative high-resolution electron microscopy was carried out on a Z-type faulted dipole in GaAs with the through-focus method. Through the quantitative analysis of the high-resolution images, the atomic displacement around the stacking fault was accurately evaluated. In the analysis of electron diffraction patterns of a Cu0.725Pd0.275 alloy, an energy filter was utilized to obtain electron diffraction patterns with a small background removing the inelastically scattered electrons. From the analysis of diffuse scattering of the Cu0.725Pd0.275 alloy, the short-range order parameters were quantitatively evaluated. Finally, it is pointed out that, based on the digital data of electron microscope images, the construction of the data base such as “EMILIA” (Electron Microscope Image Library and Archive: http://asma7.iamp.tohoku.ac.jp/EMILIA) is quite important for the future research of advanced materials characterization.
- [Show abstract] [Hide abstract] ABSTRACT: Diffuse scattering associated with a short-range ordered (SRO) state in a Cu-27.5 at%Pd alloy was quantitatively analyzed using energy-filtered transmission electron microscopy. By subtracting the background in electron diffraction with the energy filter, diffuse scattering splitting in two and fourfold was clearly observed at around 100, 110 and their equivalent positions in reciprocal space. Through the analysis of the electron diffraction patterns, it was clarified that the intensity distributions of the diffuse scattering did not change considerably with the crystal thickness, while those of the fundamental reflections remarkably changed. The intensity ratio of the total diffuse scattering to the total electron intensity increased monotonously with the increase of the crystal thickness. Based on these results, the two-dimensional SRO parameters of this alloy were quantitatively evaluated from the energy-filtered electron diffraction patterns.
- [Show abstract] [Hide abstract] ABSTRACT: Based on the priniple of energy-filtered transmission electron microscopy (EF- TEM), characterization technique for the elemental distribution on local area in materials are discussed. The study for visualization of nano-precipitates in a low- alloy steel including a small amount of titanium by using EF-TEM has been also introduced. Through the study, it was clarified that the measurement of elemental distribution using EF-TEM is quite useful to visualize nano-precipitates containing transition metals such as titanium in low-alloy steels.
Sendai-shi, Miyagi, Japan
- • Institute of Multidisciplinary Research for Advanced Materials (IMRAM)
- • Institute for Materials Research