Microstructural observation

Composite samples are sectioned with a diamond saw and mounted in cold curing epoxy resin. Because of their porous nature, the composites are infiltrated under vacuum and subsequently cured under pressure in order to force the mounting resin into the pores. Mounted samples are ground flat on 240 grit silicon carbide paper, finely ground with a 9 pm oil-based diamond slurry and finally polished with a 1 pm diamond slurry and a 50 nm silica suspension. 

The polished samples are sputtered with a thin layer of gold for analysis in a scanning electron microscope (SEM), a Jeol JSM 35c fitted with a link AN 10000 energy-dispersive X-ray spectrometer (EDS). The fractured surfaces and polished sections through fractured specimens can also be prepared and analysed in this manner. SEM analysis may reveal a non-uniform fibre distribution in the composite. In composites sintered at different temperatures, cracking in the matrix phase and residual porosity can be identified and the filler particles are discernible. The EDS indicates the higher particles and the matrix constituents. 

XRD spectra of composites sintered at different temperatures are obtained by using a defractometer. Samples sintered at a particular temperature indicate the structural behaviour as either amorphous or crystalline in nature. Such indication is of immense help to researchers for improving the properties of the resultant materials by optimizing the rate of sintering temperature. 

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Fig. 20.49 Schematic illustration of some of the ferritic/pearlitic microstructures observed in hypo-eutectoid steels after various heat treatments...

Fig. 20.50 Some of the microstructures observed during the tempering of martensite...

While for the dienes we have seen much information can be obtained from simple one unit active polymer chain models, this is less helpful in the case of vinyl monomers where microstructure depends on the relationship between two or more monomer units in the chain and orientation of the incoming monomer. Little work has been done in this field (18). A mechanism has, however, been proposed to explain the changes in microstructure observed in poly ot-methyl st yrene formed by lithium catalysis in THF which does... 

According to the nature of Initiator, the respective living end percentages varies. With Na+ as counter ion, the (1,4) living ends are favored. This is shown by the differences of microstructures observed for the fractions (1) of PAB14 and PAB15 (likewise with fraction (2)). 

FIG. 13.2 Schematic illustration of the relation between the interparticle forces and the corresponding microstructure observed in dense, monodisperse colloids. (Adapted with permission from D. R. Ulrich, Chem. and Eng. News, 28-35, January 1, 1990.)... 

Peters, H., and Bokhorst, R. (2000) Microstructure observations of turbulent mixing in a partially mixed estuary. Part I Dissipation rate. J. Phys. Oceanogr. 30, 1232-1244. 

Flexible foams are three-dimensional agglomerations of gas bubbles separated from each other by thin sections of polyurethanes and polyureas. The microstructures observed in TDI- and MDI-based flexible foams are different. In TDI foams monodentate urea segments form after 40% conversion, followed by a bidentate urea phase, which is insoluble in the soft segment. As the foam cures, annealing of the precipitated discontinuous urea phase... 

The main goal in material science is to provide behaviour laws, i.e. to be able to predict the material properties under given conditions (mechanical, electrical, environmental conditions, temperature, etc.). This requires relating microscopic parameters and local mechanisms to macroscopic behaviours, as there is no other way to express such behaviour laws based on chemical-physical parameters. In other words, the study of materials requires a large part of microstructural observation and analysis. 

Post-mortem analysis has also been carried out. Microstructural observation at the cross-section showed that significant grain growth, recrystallization and plastic deformation due to thermal stress occurred. Surface analysis indicated that the deuterium concentration was less than 0.1%... 

Since then, TEM has been used to study dislocation microstructures in a wide range of naturally and experimentally deformed minerals and rocks. In general, the aim of the experimental studies is to determine the deformation mechanisms by relating the evolution of the observed mi-crostructures to the macroscopic deformational behavior observed under varying conditions of temperature, confining pressure, chemical environment, strain-rate, stress, and total strain, and then to use this knowledge to interpret the microstructures observed in naturally deformed specimens and hence to determine their deformational history. 

In this section, the basic dislocation processes involved in the progressive deformation of a crystalline solid are discussed briefly to provide background for the detailed discussion of the deformation microstructures observed by TEM in specific minerals to follow. Particular attention is given to relating the nucleation, glide, climb, multiplication, and interaction of dislocations to the various stages of the creep and stress-strain curves. More discussion can be found in the texts referred to in Section 9.1. 

Figure 9.22. Dislocation microstructures observed at a microcrack-ladder in a single crystal of natural quartz experimentally deformed under conditions of high water fugadty (Mn)04 buffer). In all micrographs, the electron beam is parallel to [1210] and g = 10ll. The loading direction [lOTO] and [0001] are marked.

The trace of the planar central zone of the microcrack-ladder is marked by the presence of negative crystals and inclusions. This characteristic indicates that microcrack-ladders must be related to healed longitudinal fractures. The formation of unhealed microcracks normal to the loading direction (i.e., the rungs of the ladders) can be understood as follows. Undulatory extinction and dislocation microstructures observed within the microcrack-ladders indicate that plasticity is restricted to a narrow zone ( 100 / m wide) within the otherwise-undeformed host crystal. Therefore, the host crystal responded elastically while the load was applied the elastic strain A/// being about 0.5 percent. Now, in the plastic zone of the... 

Crystals deformed at a constant strain-rate (e = 10 s ) with a confining pressure of 300 MPa and 400 C in an orientation expected to activate the (100) [010] slip system, developed numerous microtwins in (100) and some dislocations that were not fully characterized. However, interesting dislocations and associated faults were observed in specimens scratched on a (110) surface. Figure 9.32 is typical of the dislocation microstructures observed in these specimens and shows segments of dislocation loops bounding planar defects on (100). 

The previous sections have been mostly concerned with the dislocations and microstructures observed in single crystals deformed to various strains under known experimental conditions. In some minerals, notably quartz and olivine, the macroscopic deformational behavior, as revealed by the creep and stress-strain curves, can be understood in terms of the micro-structural evolution during deformation and, furthermore, certain quantifiable characteristics of the microstructure correlate with the imposed... 

Thus, because of these empirical correlations, it may be possible, at least in principle, to estimate quantitatively the stress, temperature, and perhaps the strain-rate of a naturally deformed rock from measurements of dislocation density, subgrain size, and dynamically recrystallized grain size, together with Burgers vector determinations. However, these estimates will be questionable unless certain conditions are fulfilled. Some of the more important of these conditions will now be discussed before considering specific examples of the application of microstructural observations to tectonic problems. 

A transmission electron micrograph taken at a magnification of x 10,000 presents information from a specimen volume of about 10 m x 10 >tm x 0.1 m = 10" cm. It is obvious, therefore, that the microstructure observed in a region of this size will not be characteristic of the bulk specimen... 

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