【正文】 e surface of Al, Si , Mg (wt%) (AA6016), showing microvoid coalescence (SEM) Figure 1 – Examples of various microstructures in different metallic systems. Images from DoITPoMS, University of Cambridge ()Each of the methods of examining a microstructure will need some sort of sample preparation, which in practice means that we are normally looking at a 2D section through the material, and attempting to infer the 3D structure. Figure 2 indicates why a certain amount of care must be taken when quantifying data obtained from such measurements.Figure 2 – Schematic diagram indicating how the angle of a microstructure relative to a 2D section through a material can influence the apparent size of features.Why do Microstructural Features need to be Quantified?The spatial, size and geometrical distribution of microstructural features affect numerous mechanical and physical properties of the material, and can therefore be very important to quantify, both for research (to understand processingmicrostructureproperties links) and industry (quality control). The table below shows a short list of examples of such properties.PropertyInfluenced by…EffectYield StrengthGrain SizeUnder the HallPetch relationship, a smaller grain size gives a higher yield strength as dislocations pile up at grain boundaries more rapidly.Volume FractionIf a reinforcement phase has been added (as in, for example a metal matrix posite) the amount present will affect the degree of reinforcement. The same is true in multiphase materials where one phase has a higher yield stress.Dislocation DensityA material with a higher dislocation density (one that has been work hardened) will have a higher yield stress than a material with a relatively low dislocation density.Fracture ToughnessGrain Size and ShapeA microstructure with small interlocking grains will have a higher resistance to crack propagation than one with large grains, as the crack is forced to take a more tortuous path.ConductivityVolume FractionWhere phases of different thermal or electrical conductivity are present, their volume fractions will affect the conductivity of the material as a whole.Magnetic PropertiesGrain OrientationSteels 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 magnetic 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。 vertical3D surface profileX Ray (Computed) TomographyA series of xray images of the specimen are taken at different angles. Computer software is then used to calculate the 3D distribution of structure and phases that will give rise to these 2D images. For high resolution a synchrotron is required. More details may be found at: 1 181。m2DScanning Electron MicroscopyA beam of electrons is scanned back and forth over the specimen, and the response at each point is detected and used to modulate the grey level of the corresponding pixel in the image that is built up. This method has a good depth of field, and is therefore good for topographic surfaces (. in fractography). More details may be found at: ://120 nm2DTransmission Electron MicroscopyThe whole area of interest is illuminated by a beam of electrons. Local variations mean that in some places electrons will be transmitted more easily than elsewhere, giving rise to contrast. A thin foil sample is required. More details may be found at: ://15 197。 an ensemble of volume features, area features, line features and point features. These features may be, for example, grains, grain boundaries, twins, inclusions and second phases, and all will be affected by the history of the material.Some examples of microstructures of metallic systems are shown in Figure 1.How can Microstructures be Observed?There are various techniques that allow the observation of microstructural features. As such features are generally very small, in almost all cases some form of magnification is required. The most monly used method is optical microscopy, but for finer scales electron microscopy may also be used. A relatively new technique is xray tomography, which can give a 3D image of a sample, but the equipment required to do this at small resolutions (a synchrotron as an intense xray source) is difficult to get access to.MethodDescriptionResolutionImage Optical MicroscopyLight rays magnified by passing through lenses can be used to image a specimen either after reflection from its surface or transmission through it if it is transparent. More information can be found at: 181。 lateral, 197。s reagent。 Underwood, Quantitative Stereology, AddisonWesley, 1970 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。 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 alumina slurry can also be used. Before us