【正文】 ose to its surface. Electric discharge occurs between the wire and the sample and some of the sample material is removed as a result. The slow movement of the electrode and the progressive removal of material by further discharges results in cutting. Although in the region if the cut the material is heated, the depth of the material affected by this cutting method is very low, and it is particularly suitable for hard materials that would otherwise be difficult to cut. Mounting Once a piece of material of the correct size has been obtained it is normally necessary to mount it in another material to facilitate handling and polishing, and to protect the specimen from damage. There are two broad classes of mounting methods。 (2 197。 with high resolution) 2D Scanned Probe Microscopy In this family of techniques, a probe is scanned across the surface of a specimen. By measuring the interaction of this probe with the specimen surface as a function of its position, an image of the surface can be built up. More details may be found at: 25 197。 hot and cold mounting. Hot mounting Mounting 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。m alumina slurry can also be used. Before using a finer polishing wheel the specimen should be washed thoroughly with Quantification of Microstructure and TextureSample Preparation Techniques for Optical Microscopy R Goodall, October 2022 9 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. Etching Depending 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 sites such as grain boundaries. This results in a surface relief that enables different crystal orientations, grain boundaries, phases and precipitates to be easily distinguished, as shown diagrammatically in Figure 11. Figure 11 – Schematic diagram of how a grain boundary is revealed in the microscope after etching. Following the etching process there may be numerous small pits present on the surface. These etch pits are caused by localised chemical attack, and in most cases they do not represent features of the microstructure, although they may occur preferentially in regions of high local disorder, for example where there is a high concentration of dislocations. If the specimen is over etched, ie. etched for too long, these pits tend to grow, and obscure the main features to be observed. Should this occur, the specimen must be repolished (starting from around the 6181。 one of them will focus separately, as many people have slightly different focus in each eye. The image should be brought into focus in the non adjustable eyepiece first using the fine focus knob, and then the other eyepiece adjusted so that it is also in focus. 7) If a higher magnification is required the appropriate lens can be moved into place, and the image can be refocused using the fine focus knob. When using higher magnification lenses, the coarse knob should not be used for focussing, due to the risk of crashing the lens in to the specimen. 8) If different areas need to be imaged, many microscopes have mechanical controls that allow repositioning of the specimen under the lens. If the specimen has not been mounted flat, making such movements at high magnification brings the risk of crashing the lens into the sample. Figure 13 – A reflected light microscope Transmission Light Microscopy In transmission light microscopy the sample is illuminated and imaged from opposite sides, and will therefore only be effective on thin slices of transparent materials, which are usually mounted on a glass slide. The same protocol given above can be used, and a transmitted light microscope is shown in Figure 14. Quantification of Microstructure and TextureSample Preparation Techniques for Op