显微结构的量化处理及组织分析的教程 Lecture Notes 1.docx

Quantification of Microstructure and TextureIntroductionQuantification of Microstructure and Texture1. IntroductionWhat is a Microstructure?The microstructure of a metal or alloy is the arrangement of its structure on a very small scale; 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 commonly used method is optical microscopy, but for finer scales electron microscopy may also be used. A relatively new technique is x-ray 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 x-ray 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: http:/www.doitpoms.ac.uk/tlplib/optical-microscopy/index.php0.5-1 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 (e.g. in fractography). More details may be found at: /CMRAcfem/semoptic.htm and /PiN/rdg/elmicr/elmicr.shtml1-20 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: /CMRAcfem/temoptic.htm and .uk/tem/15 (2 with high resolution)2DScanned Probe MicroscopyIn 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: /jwcross/spm/2-5 lateral, 0.1 vertical3D surface profileX Ray (Computed) TomographyA series of x-ray 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: /skymto.html#Intro1 m3DAtom Probe MicroscopySingle atoms on the surface of a sharp needle specimen are ionised by field evaporation. These ions are projected toward a detector, and their position recorded. With time, a profile through the depth may also be built up. Only very small volumes possible More details may be found at: http:/www.materials.ox.ac.uk/fim/whatis3dap.html and http:/www.nims.go.jp/apfim/apfim.htmlAtomic3DCoarse pearlite in slow cooled Fe, C 0.8 (wt%) (optical microscopy)Martensite laths in Fe, C 0.9 (wt%) water quenched from 800C (SEM)As-cast aluminium alloy (cross polarised light optical microscopy, sample etched using Barkers reagent; 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)Nickel-based superalloy (SEM)Fracture surface of Al, Si 1.2, Mg 0.4 (wt%) (AA6016), showing microvoid coalescence (SEM) Figure 1 Examples of various microstructures in different metallic systems. Images from DoITPoMS, University of Cambridge (http:/www.doitpoms.ac.uk/index.html)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 processing-microstructure-properties links) and industry (quality control). The table below shows a short list of examples of such properties.PropertyInfluenced byEffectYield StrengthGrain SizeUnder the Hall-Petch 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 composite) 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 (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 composition 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 kinetics of such processes.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- LocationAll 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.TerminologyCertain terminology is well established in the field of Quantitative Metallography (which is also sometimes called Stereology).PNumber of point features / test pointsLLength of linear features / test lineAFlat area of intercepted features / test areaSCurved surface / interface areaVVolume of 3D features / test volumeNNumber of featuresThese symbols can be combined, e.g. 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 CourseMore detail on the topics covered in this course can be found in: Higginson and Sellars, Quantitative Metallography, Maney, 2003 Underwood, Quantitative Stereology, Addison-Wesley, 1970 De Hoff and Rhines, Quantitative metallography, McGraw - Hill 1968 Brandon and Kaplan, Microstructural Characterisation of Materials, Wiley, 2008 Randle and Engler, Introduction to Texture Analysis, CRC Press, 2000R Goodall, October 20104 Quantification of Microstructure and TextureSample Preparation Techniques for Optical MicroscopyQuantification of Microstructure and Texture2. Sample Preparation Techniques for Optical MicroscopyMost 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.SectioningThe 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 sawFigure 2 A diamond blade low speed cutting sawFigure 3 Electric Discharge Machining (EDM) equipmentElectrically 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 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 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.MountingOnce 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; 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 20-40 minutes is common). 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 150C, and may use either a thermosetting plastic, e.g. phenolic resin, or a thermosoftening plastic e.g. 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; these are typically two component 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 non-flat 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 comfortable 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; smaller numbers mean coarser SiC particles. A typical starting paper would be 120 or 240 grit, followed by intermediate papers such as 320, 400, 600 or 800 grit. The final step is normally on 1200 grit paper, although 2500 and 4000 grit grades are available, and are used in some specialist polishing methods. Between each stage the specimen should be washed to prevent transport of coarse particles to a higher grade spoiling the grinding. An ultrasonic bath can also be used but is not necessary.When grinding manually, moderate pressure is normally sufficient; 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 90 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, scratch-free 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 oil-based (each will give