Oriented specimens metal

Alloy and temper Temperature Filler metal Specimen orientation Yield strength Kq Valid Kic max... 

Based on TEM studies of supported metal catalysts, several workers have concluded that their catalysts were made of two-dimensional discs or rafts , where virtually all atoms are at the particle surface. However, sample tilting experiments in TEM have shown that great care should be exercised in the interpretation of TEM images of small particles (<2 nm in size), since phase contrast effects may dominate and variations in the particle contrast with specimen orientation can occur as a result of amplitude contrast effects (Treacy and Howie 1980). Sample tilting is therefore necessary to ensure correct interpretations of TEM images of metal-particle catalysts. This will be discussed further in the following sections. 

The various orientations of the crystallites of a metal specimen show up clearly, and it can be shown that the electron emission depends on the degree of covering with adsorbed atoms, which itself depends on orientation and on temperature (285-287). 

Specimens that contain materials with very different ion-milling rates, such as metallic multilayers grown on silicon substrates, often tend to form bridges of material across the perforated area. Ion-milling at very low angles of incidence ( 1-2°) in a direction parallel to the interface can sometimes be used to overcome or at least alleviate these bridging problems. Finally, it should be noted that the use of a crystalline substrate such as silicon provides a convenient reference material for specimen orientation purposes in the TEM. Examination of the substrate EDP can be used to ensure that the substrate normal is aligned exactly perpendicular to the electron beam direction. The thin-film microstructure can then be easily determined. 

The level of chemical information that can be gleaned from XPS and ToF-SIMS is potentially much greater than with analytical TEM but at the expense of spatial resolution. Rather than using a metallographic cross-section one must adopt a plan view specimen orientation, or something close to this geometry. One approach that has been used with a degree of success is to remove the metal sub-... 

The resistance curves for base metal specimens of the TL orientation at three temperatures are shown in Fig. 3. The values of //c decreased from 255 to 120 to 50 kJ m, as test temperature was lowered from 295 to 76 to 4 K. The slopes of the resistance curves also decreased progressively as temperature was lowered. Therefore, less energy was required for both the initiation and propagation of cryogenic fractures. Despite the adverse temperature effects, unstable linear-elastic fractures were never observed in this study the Kic(J) data in Table II are estimates based on Eq. (3). 

ASTM G 30, Practice for Making and Using U-Bend Stress-Corrosion Test Specimens— This describes methods for constructing specimens for evaluating SCC of metals. This standard includes specimen orientation, dimensions. 

In order to obtain a specimen oriented in the plane perpendicular to the roll plane and whose size corresponded in any degree to that of a standard-type specimen material of considerable thickness was required. Reynolds Metals supplied us with a piece of 3 /2-in.-thick 5083 production plate. 

It may be felt that the initiation of a stress-corrosion test involves no more than bringing the environment into contact with the specimen in which a stress is generated, but the order in which these steps are carried out may influence the results obtained, as may certain other actions at the start of the test. Thus, in outdoor exposure tests the time of the year at which the test is initiated can have a marked effect upon the time to failure as can the orientation of the specimen, i.e. according to whether the tension surface in bend specimens is horizontal upwards or downwards or at some other angle. But even in laboratory tests, the time at which the stress is applied in relation to the time at which the specimen is exposed to the environment may influence results. Figure 8.100 shows the effects of exposure for 3 h at the applied stress before the solution was introduced to the cell, upon the failure of a magnesium alloy immersed in a chromate-chloride solution. Clearly such prior creep extends the lifetime of specimens and raises the threshold stress very considerably and since other metals are known to be strain-rate sensitive in their cracking response, it is likely that the type of result apparent in Fig. 8.100 is more widely applicable. 

Many investigations of small particles or of other materials may involve the collection and analysis of diffraction patterns from very large numbers of individual specimen regions. For small metal particles, for example, it may not be sufficient to obtain diffraction patterns from just a few particles unless there is reason to believe that all particles are of the same composition, structure, orientation and size or unless these parameters are not of interest. More commonly, it is of interest to obtain statistics on the variability of these parameters. The collection of such... 

In drawn metal wires the fibre axis is usually not a crystal axis. The problem of the determination of crystal orientation in such specimens (and in rolled metal sheets), though closely related to those dealt with here, is outside the scope of this book. (The unit cell dimensions, and indeed the complete structures of such crystals, are usually known, and the problems that arise are questions of correlation of physical properties with orientation.) See Schmid and Boas, 1935 Orowan, 1942. 

In most studies of surface chemistry, it is common practice to devote a small fraction of the total effort to the preparation of the surface and then much effort to an elaborate measurement of rates and other surface properties. Obviously, the measurement is no better than the preparation of the surface, and if the surface is contaminated, strained, or has an unknown crystal orientation, the measurements can have very little meaning. Furthermore, since metals as ordinarily used consist of minute crystals randomly arranged, measurements made on such specimens are composite quantities and tell little about the reactivity of a particular type of structure. It is therefore highly important that the preparation of the catalyst specimen be carried out with great care, and this is generally a tedious undertaking. 

It must be noted that most studies on metal-doped TiCh are oriented toward the photo-oxidation of environmental pollutants (e.g., 4-nitrophenol,299 4-chloro-phenol300) rather than toward the photoelectrolysis of water (but see Section 6.4 of this Chapter). Other aspects of metal doping include the effect of UV radiation of Ag-doped Ti02 specimens,301 302 plasma treatment,303 and thermal treatment.304 Metal doping by ion implantation of I1O2 has been discussed.91,305... 

There is a large variety of specimen holders in use, each suited to some particular purpose. The simplest consists of a fixed post to which the specimen is attached with wax or plasticine. A more elaborate holder is required when it is necessary to set a crystal in some particular orientation relative to the x-ray beam. In this case, a three-circle goniometer is used (Fig. 5-7) it has three mutually perpendicular axes of rotation, two horizontal and one vertical, and is so constructed that the crystal, cemented to the tip of the short metal rod at the top, is not displaced in space by any of the three possible rotations. 

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