【正文】 o the size of the field of view. The simplest way of achieving this is to measure the size of the field of view at a low magnification, and then scale the size appropriately as the magnification is increased. The field of view can be measured approximately by looking at a ruler under the lowest magnification lens. Accuracy can be improved by using a graticule (Figure 15). A graticule is a slide with a very fine grating which, if metric, will usually measure 1mm across, and is divided into 100 segments, . each segment is 10 181。m diamond polish step) and the process repeated.EtchantsThe most mon technique for etching is selective chemical etching and numerous formulations have been developed for different materials, and can be found in the literature. Common etchants often have three ponents:? A corrosive agent (frequently acids)? A modifier (such as an alcohol, glycerin, etc.)? An oxidizer (such as hydrogen peroxide, Fe3+, etc.)The table below gives some mon etchants.EtchantCompositionApplicationConditionsCommentsKeller’s Etch190 ml Distilled water5 ml Nitric acid3 ml Hydrochloric acid2 ml Hydrofluoric acidAl and Al alloysImmersion1030 secondsOnly use fresh etchantNital100 ml Ethanol110 ml Nitric acidSteel and Ni alloysImmersion or swab. Seconds to minutesPicral100 ml Ethanol24 grams Picric acidSteelImmersion or swab. Seconds to minutesExplosive when dry or crystallineKroll’s Reagent92 ml Distilled water6 ml Nitric acid2 ml Hydrofluoric acidTitanium15 secondsFerric ChlorideSaturated aqueous soln. of ferric chloride with a few drops of hydrochloric acidStainless Steel or CuSwab. Wipe over surface a few timesWash in alcohol and dry after exposureImaging in the MicroscopeReflected Light MicroscopyIn reflected light microscopy, the specimen is illuminated and imaged from the same side. This technique is monly used for metallic specimens. When placing the polished specimen in the microscope the surface to be examined optically should be perfectly flat and level. If not, then as the viewing area is moved across the surface it will pass in and out of focus and it will make it difficult to have the whole of the field of view in focus. Provided the polished surface is planar, this problem can be avoided using a specimen levelling press, Figure 12. The specimen is placed on a blob of plasticene on a microscope slide, and pressed to make it level. A small piece of paper or cloth covers the surface of the specimen to avoid scratching.Figure 12 – A specimen levelling pressOnce the specimen had been mounted on the slide, it can be imaged. A typical reflected light microscope is shown in Figure 13, and standard protocol for using any optical microscope is given below:1) Place the mounted specimen on the stage. The area to be viewed should be positioned approximately central on the stage, beneath the objective lens.2) If the microscope is binocular (has two eyepieces) adjust the eyepieces so they are the right distance apart.3) Switch on the illumination, starting at a low power setting and gradually increase until the desired light level is reached. This can best be judged by increasing the power until a clear spot of light is seen on the specimen surface.4) Use the lowest magnification objective lens to coarsely focus the microscope. To avoid “crashing” the lens into the surface of the specimen start by bringing the lens close to the surface looking from the side. Then, looking down the microscope, focus by pulling the lens away from the sample surface. 5) Looking through the eyepieces, use the fine focus knob to bring the image into sharp focus.6) On a binocular microscope, the eyepieces can now be further adjusted。 between papers. This way it is easy to see scratches that remain from the previous paper. When all of the scratches from the previous step have been removed, it is safe to proceed to the next paper.PolishingOnce the surface to be observed has been revealed by grinding, it is polished to a reflective, scratchfree finish. This is performed on rotating wheels, with the option of automatic attachments, such as that shown in Figure 9. On such machines, the speed of wheel rotation and the pressure applied to the specimen can be varied and both will have an effect on the quality of the polish produced.Figure 9 – An automatic polishing machineThe discs used in the machine are covered with soft cloth impregnated with abrasive diamond particles and a lubricant which can be water, alcohol or oilbased (each will give slightly different polishing behaviour and interaction with the specimen). There are two stages to polishing: a coarser polish of 303 181。C, and may use either a thermosetting plastic, . phenolic resin, or a thermosoftening plastic . acrylic resin (phenolics tend to be low cost, and acrylics have good clarity). These resins can sometimes have special additions to make them electrically conductive, which is important to prevent charging if the specimen is to be imaged in the SEM, or if an elecropolishing step is required later.Figure 4 – An example of a hot mounting pressCold MountingIn some circumstances, hot mounting may not be suitable. This can be because the specimen would be affected by the heat, or that the specimen contains fine pores that need a low viscosity resin to be filled. It is important to impregnate porous materials with resin before polishing to prevent grit, polishing media or etchant being trapped in the pores, and to preserve the open structure of the material. In these cases cold mounting resins are used。 De Hoff and Rhines, Quantitative metallography, McGraw Hill 1968 interference in the oxide layer produces colours which depend on grain orientation)Annealing twins in Cu 70, Zn 30 (wt%) (optical microscopy after a ferric chloride etch)Nickelbased superalloy (SEM)Fractur