【正文】 rge grains, as the crack is forced to take a more tortuous path. Conductivity Volume Fraction Where phases of different thermal or electrical conductivity are present, their volume fractions will affect the conductivity of the material as a whole. Magic Properties Grain Orientation Steels for transformer cores are made with a very large (~cm) grain size and with a preferential orientation of {110}001 (Goss texture), which increases the magic flux density in the plane of the strip and reduces losses in service. Examination of the microstructure of an unknown material can also allow us to make deductions concerning its position and processing route。 for example, processes such as rolling and extrusion will tend to produce microstructures with elongated grains, as shown in Figure 3. Furthermore, when there is a microstructural change (such as during recrystallisation or a phase transformation), measurement of the extent of the transformation at different times can allow data on the rate to be gathered, which can be important when exploring the kiics of such processes. Quantification of Microstructure and Texture Introduction R Goodall, October 2022 4 Figure 3 – The microstructure of extruded aluminium, showing grains that are elongated in the direction of extrusion (the horizontal direction in the image). What can be Measured? As referred to above, a microstructure will consist of volume features, area features, line features and point features, which will have associated with them: Size Shape Volume Surface area Curvature Location All of these can be measured for the features on an image. An important point that will be returned to throughout the course is that fact that real microstructures will show statistical variations in these features. We therefore need to make a sufficiently large number of measurements, and need tools to allow us to quantify the measured parameters in a statistically meaningful way. Terminology Certain terminology is well established in the field of Quantitative Metallography (which is also sometimes called Stereology). P Number of point features / test points L Length of linear features / test line A Flat area of intercepted features / test area S Curved surface / interface area V Volume of 3D features / test volume N Number of features These symbols can be bined, . to give SV – Surface / interface per unit volume, VV – Volume of one phase in total volume (= volume fraction) or LA – Length of linear features 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 Course More detail on the topics covered in this course can be found in: ? Higginson and Sellars, Quantitative Metallography, Maney, 2022 ? Underwood, Quantitative Stereology, AddisonWesley, 1970 ? De Hoff and Rhines, Quantitative metallography, McGraw Hill 1968 ? Brandon and Kaplan, Microstructural Characterisation of Materials, Wiley, 2022 ? Randle and Engler, Introduction to Texture Analysis, CRC Press, 2022 Quantification of Microstructure and TextureSample Preparation Techniques for Optical Microscopy R Goodall, October 2022 5 Quantification of Microstructure and Texture 2. Sample Preparation Techniques for Optical Microscopy Most images we will look at of metallic materials are the result of optical microscopy of prepared samples, and metallography is simply the name given to the systematic method used to examine the structure of materials. This lecture covers the standard techniques used in the preparation of samples for optical microscopy. It should be noted that, although we will concentrate on metals here, these techniques work equally well for ceramics, polymers, semiconductors, etc. Sectioning The first step in the process of producing a specimen suitable for observation in the microscope is to obtain a suitably sized piece, which may need to be cut to the right size. When cutting a specimen from a larger piece of material, care must be taken to ensure that it is representative of the features found in the larger sample, or that it contains all the information required to investigate a feature of interest. For this reason, even when the artefact is sufficiently small to permit metallographic preparation, it may still be advantageous to section it so that potentially unrepresentative surface regions are not examined. Many cutting methods can be used to remove part of an artefact for metallographic preparation, but in certain circumstances the selection may be restricted due to the effect this may have on the microstructure. Abrasive cutting, for example using equipment such as that in Figure 1, may introduce a lot of damage and deformation into a specimen, and the high speed can cause heating of the sample which could alter the microstructure. For these reasons, low speed cutting processes are often used, such as the diamond saw shown in Figure 2. Figure 1 – An SiC blade abrasive saw Figure 2 – A diamond blade low speed cutting saw Figure 3 – Electric Discharge Machining (EDM) equipment Electrically conductive specimens may also be cut using Electric Discharge Machining (EDM), sometimes called spark cutting, Figure 3. In this method the sample is submerged in a dielectric fluid Quantification of Microstructure and TextureSample Preparation Techniques for Optical Microscopy R Goodall, October 2022 6 and an electrode (often in the form of a wire) is brought close 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