【正文】 es per unit area. In this format, the quantity represented by the main text letter is divided by the quantity represented by the subscript letter.Further Reading for the Whole CourseMore detail on the topics covered in this course can be found in: Underwood, Quantitative Stereology, AddisonWesley, 1970 Brandon and Kaplan, Microstructural Characterisation of Materials, Wiley, 2008 hot and cold mounting.Hot mountingMounting materials are often polymers, and so the possibility exists to embed a specimen by melting a polymer and squeezing it around it. To do this special machines exist that can apply the correct cycles of heat and pressure with good reproducibility and at relatively high speeds (a typical cycle time of 2040 minutes is mon). Because of this, hot mounting is the method used for the majority of specimens. An example of a hot mounting machine is shown in figure 4.In practice, hot mounting normally occurs at a temperature of about 150176。 these are typically two ponent systems (aresin and a hardener) such as epoxy or acrylic. Typical curing times range from minutes to hours with the faster curing resins producing higher exothermic temperature which causes the mounting material to shrink away from the edge during curing. Although this procedure can be carried out with nothing more advanced than a mould (see Figure 6 below), vacuum mounting systems exist to help with the infiltration of resin into porous samples, Figure 5.Figure 5 – A vacuum mounting system for cold mounting of porous specimensEdge MountingWhen the edge of a thin sample is required to be viewed, special plastic or metal clips can be used to hold it vertically during the mounting process. These are shown in Figure 6, along with some examples of moulds that can be used for cold mounting.Figure 6 – Edge mounting clips and moulds for cold mountingShape of Mounted SpecimensA mounted specimen should have a thickness of about half its diameter in order to prevent the creation of a bevel during grinding and polishing (a nonflat surface). The edges of the specimen may also be rounded off with coarse grinding paper. On the upper side this will make the specimen more fortable to hold, and on the lower side it will help to reduce wear on grinding papers and polishing cloths. For some automatic polishing machines it is important to have the right diameter, which is often 30 mm. A schematic diagram of a specimen is shown in Figure 7.Figure 7 – A schematic diagram of a mounted specimenGrindingA grinding step is usually necessary to make the specimen flat (removing marks left from sectioning), and to remove the surface layers that would still show evidence of damage from sectioning in the microstructure. This is normally done using SiC paper under flowing water to wash away material removed, and can either be in the form of strips on static equipment or as discs on a rotating wheel (see Figure 8).Figure 8 – A grinding setup with SiC paper strips and a grinding wheel with an SiC disc in use.The normal procedure is to start with a coarse grade of SiC paper (large SiC particles) which removes material rapidly, but leaves large scratches, and then to progress to finer papers, each of which removes material less rapidly, but leaves a smoother surface. SiC papers are graded according to “grit” size, a number corresponding to the number of grains per square inch。 heavy pressure is not required as it can cause further damage to the microstructure of the sample (rather than removing the damage induced by the sectioning process, as we are trying to do with this step), and also increases the risk of the specimen catching on the paper and being thrown off the wheel. The times required at each step can also be surprisingly short (often less than a minute). A useful way to tell is to rotate the specimen by 90176。m at a relatively fast rotational speed (typically with diamond particles 6 microns in diameter which should remove the scratches produced from the finest grinding stage, and a speed of 300 rpm), and a finer polish 1181。m alumina slurry can also be used. Before using a finer polishing wheel the specimen should be washed thoroughly with warm soapy water followed by alcohol to prevent contamination of the disc. An ultrasonic bath may also be used.To obtain even finer polishing, or for particularly difficult specimen, electropolishing may be used. In this method the specimen is made the anode in a suitable solution, and the conditions adjusted so that peaks on the surface dissolve faster than the troughs, thus tending to smooth the surface out. More information and details of specific electropolishing solutions and conditions can be found in the Smithells Metals Reference Book. Poorly adapted polishing techniques can lead to the generation of artefacts in the samples. Two examples of such artefacts are shown in Figure 10. Figure 10 – Artefacts caused by use of the incorrect polishing technique. The image on the left shows scratches which may persist in the sample as it has not been ground for enough time at each of the steps, or may indicate contamination of the polishing wheel with grit from a previous grade. The image on the right shows embedded polishing particles which result from the use of too high pressure during polishing.EtchingDepending on the sample, an etching step after polishing may be necessary. The purpose of etching is to optically enhance microstructural features such as grain size and phases. Etching is essentially a controlled corrosion process, and selectively alters the surface based on position, stress, or crystal structure, and this created contrast in the microscope due to differences in topography or reflectivity of the different phases or features. For example, in a pure material, an etchant (a reagent that chemically attacks the material being examined) will preferentially etch high energy site