Metallography and Sample Preparation PDF

Metallography and Sample Preparation Metallography Metallography is the study of the microstructure of all types of metals and their metallic alloys. It can be more precisely defined as the scientific discipline of observing and determining the chemical and atomic structure and spatial distr...

Metallography and Sample Preparation Metallography Metallography is the study of the microstructure of all types of metals and their metallic alloys. It can be more precisely defined as the scientific discipline of observing and determining the chemical and atomic structure and spatial distribution of the constituents, inclusions or phases in metal alloys. In other words, Metallography is a branch of materials science which relates to the constitution and structure, and their relation to the properties, of metals and alloys. Pearlitic grey cast iron Ductile iron with spheroidal graphite What is Microstructure? The structure of a suitable prepared specimen as revealed by a microscope. 3  Sample Preparation In order to investigate structure we should prepare samples. Preparation of metallographic specimens generally requires five major operations: 1.Sectioning 2. Mounting 5. Etching 3. Grinding 4. Polishing 4 Sample Preparation A well-prepared metallographic specimen :  Represents sample.  Sectioned, ground and polished so as to minimize disturbed or flowed surface metal caused by mechanical deformation, and thus to allow the true microstructure to be revealed by etching.  Free from polishing scratches and pits and liquid staining.  Flat enough to permit examination at high magnification. 5  Sectioning  Many metallographic specimens are used for process control.  Important uses of metallography include examination of defects that appear in finished or partly finished products and studies of parts that have failed in service.  Investigations for these purposes usually require that the specimen be broken from a large mass of material, and often involve more than one sectioning operation.  Many metallographic studies require more than one specimen.  Failed parts may best be studied by selecting a specimen that intersects the origin of the failure, if the origin can be identified on the surface. Depending on the type of failure, it may be necessary to take several specimen from that area of the failure and from adjacent areas. 6 Osmangazi Bridge (Gulf of Izmit) 7  Sectioning Sectioning Methods  Crushing  Cutting  Sawing  Abrasive cutting Cutting the Sample  The first thing to remember when cutting samples, is to preserve the sample axes orientation. Cut the sample in such a manner that important sample directions, like the Rolling Sectional Direction, Transverse Direction and Sample Normal are not lost. View  The second thing to remember that the cutting process must not damage or change the sample as this would lead to Cutting erroneous results 8  Wire saw with an endless loop. Low speed dimond saw 9  Mounting of Specimens  The primary purpose of mounting specimens is for convenience in handling specimens of difficult shapes or sizes during the subsequent steps of preparation and examination.  A secondary purpose is to protect and preserve extreme edges or surfaces defects during preparation.  Specimens also may require mounting to accommodate various types of automatic devices used in laboratories or to facilitate placement on the microscope stage.  An added benefit of mounting is the ease with which a mounted specimen can be identified by name, alloy number, or laboratory code number for storage by scribing the surface of the mount without damage to the specimen.  Small specimens generally require mounting so that the specimen is supported in a stable medium for grinding and polishing. The medium chosen can be either a cold curing resin or a hot mounting compound. 10 The mounting operation accomplishes three important functions: (1) it protects the specimen edge and maintains the integrity of a materials surface features. (2) fills voids in porous materials and (3) improves handling of irregular shaped samples, especially for automated specimen preparation without damage to the specimen. 11 Mounting Methods  The method of mounting should not injure the microstructure of the specimen.  Mechanical deformation and the heat are the most likely sources of injurious effects. The mounting medium and the specimen should be compatible with respect to hardness and abrasion resistance. A great difference in hardness or abrasion resistance between mounting media and specimen promotes differential polishing characteristics, relief, and poor edge preservation.  The mounting medium should be chemically resistant to the polishing and etching solutions required for the development of the microstructure of the specimen. 12 Clamp Mounting  Clamps are used most often for mounting thin sheets of metal when preparing metallographic cross sections.  Several specimens can be clamped conveniently in sandwich form.  The hardness of the clamp should be approximate or exceed the hardness of the specimen.  The clamp plates are cut longer and wider than specimens to be clamped. 13 Compression (Hot) Mounting  Compression mounting involves molding around the specimen by heat and pressure such molding materials as bakelite, diallyl phthalate resins, and acrylic resins.  Bakelite and diallylic resins are thermosetting, and acrlyic resins are thermoplastic. Both thermosetting and thermoplastic materials require heat and pressure during the molding cycle. Cold Mounting  Materials for cold mounting are classified as polyesters, epoxides and acrylics. Polyesters are transparent, epoxides are almost transparent and straw color; acrylics are opaque.  Cold mounting requires no pressure and little heat, and is a means of mounting large numbers of specimens more rapidly than by compression mounting. 14 15 Table 1 Typical properties of thermosetting molding resins Resin Heat Coefficient Abrasion Polishing Transparency Chemical Molding conditions of thermal rate, rate, resistance distortion Temperatu temperature expansion μm/min(b) μm/min(c) Pressure Tim in./in. re e, °C(a) °C MPa MPa psi min °C °F 135- 275- 17- 2500 5-12 140 285 3.0-4.5 × 100 2.9 Opaque Attacked Bakelite 170 340 29 - 10-5 by strong (wood- 4200 acids and filled) alkalies Diallyl 140- 285- 17- 2500 6-12 150 300 3.5 × 10-5 190 0.8 Opaque Attacked phthalate 160 320 21 - by strong (asbestos- 3000 acids and filled) alkalies Source: Ref 1 (a) Determined by method ASTM D 648. (b) Specimen 100 mm2 (0.15 in.2) in area abraded on slightly worn 600-grit silicon carbide under load of 100 g at rubbing speed of 105 mm/min (4 × 103 in./min). (c) 25-mm (1-in.) diam mount on a wheel rotating at 250 rpm covered with synthetic suede cloth and charged with 4 to 8 μm diamond paste. 16 Table 2 Typical properties of thermoplastic molding resins Resin Transpare Heat Coefficient Abrasio Polishing Chemical Molding conditions ncy distortion of thermal n rate, rate, resistance Heating Cooling temperatu expansion, μm/mi μm/min( re(a) in./in. °C n(b) c) Temperatu Pressure Tim Temperature Pressure re Time °C °F MPa psi e °C °F MPa psi Not resistant Methyl 140- 285- 17- 2500- 6 75-85 165- max ma 6-7 Water, 65 150 5-9 × 10-5... 7.5 to strong methac 165 330 29 4200 185 x white to acids and rylate clear some solvents, especially ethanol Polystyr 140- 285- 185- 17 2500 5 85 max... 6... 65 150............ ene 165 330 212 Polyvin Light Not resistant yl 220 430 27 4000.................. brown, 75 165 6-8 × 18-5 20 1.1 to strong formal clear acids Polyvin Resistant to 120- 250- yl 0.7 100 nil 60 140 27 400... Opaque 60 140 5-18 × 10-5 45 1.3 most acids 160 320 chloride 0 and alkalies Source: Ref 1 (a) Determined by method ASTM D 648. (b) Specimen 100 mm2 (0.15 in.) in area abraded on a slightly worn 600-grit silicon carbide paper under load of 100 g at rubbing speed of 105 mm/min. (c) 25-mm (1-in.) diam mount on a wheel rotating at 250 rpm covered with a synthetic suede cloth and charged with 4-8 μm diamond paste. 17  Grinding Grinding is a most important operation in specimen preparation. Grinding is accomplished by abrading the specimen surface through a sequence of operations using progressively finer abrasive grit. Grit sizes from 40 mesh through 150 mesh are usually regarded as coarse abrasives and grit sizes from 180 mesh through 600 mesh as fine abrasives. During grinding the operator has the opportunity of minimizing mechanical surface damage that must be removed by subsequent polishing operations. Even if sectioning is done in a careless manner, resulting is severe surface damage, the damage can be elimenated by prolonged grinding. Sectional View Grinding 18 Cutting Grinding  The purpose of grinding is to lessen the depth of deformed metal to the point where the last vestiges of damage can be removed by series of polishing steps.  The scratch depth and the depth of cold worked metal underneath the scratches decrease with decreasing particle size of abrasive.  It is imperative that each grinding steps completely remove the deformed metal produced by the previous step. To ensure the complete elimination of the previous grinding scratches found by visual inspection, the direction of grinding must be changed 45 to 90 degrees between successive grit sizes.  In addition, microscopic examination of the various ground surfaces during the grinding sequence may be worthwhile in evaluating the effect of grinding. Each ground surface should have scratches that are clean-cut and uniform in size, with no evidence of previous grinding scratches. 19  Grinding Most grinding of metallographic specimen is performed by manually holding the specimen with its surface against a grinding material. To establish and maintain a flat surface over the entire area being ground, the operator must apply equal pressure on both sides of the specimen and avoid any rocking motion that will produce a convex surface. Specimens should be cleaned after each grinding steps to avoid any carryover of abrasive particles to the next step. The grinding abrasives commonly used in the preparation of specimens are silicon carbide (SiC), aluminium oxide (Al2O3), emery (Al2O3 -Fe3O4), diamond particles, etc. Usually are generally bonded to paper or cloth backing material of various weights in the form of sheets and disks. 20 Grinding 21  Grinding The series of photos below shows the progression of the specimen when ground with progressively finer paper. Copper specimen ground with 180 grit paper Copper specimen ground with 400 grit paper Copper specimen ground with 800 grit paper Copper specimen ground with 1200 grit paper 22  Polishing  Polishing is the final step in production a surface that is flat, scratch free, and mirror like in appearance.  Such a surface is necessary for subsequent accurate metallographic interpretation, both qualitative and quantitative.  Before final polishing is started, the surface condition should be at least as good that obtained by grinding with a 1200-grit abrasive. Sectional View Grinding Polishing 23 Cutting Polishing Copper specimen polished to 6 micron level Copper specimen polished to 1 micron level 24  Etching  Although certain information may be obtained from as-polished specimens, the microstructure is usually visible only after etching.  Only features which exhibit a significant difference in reflectivity (10% or greater) can be viewed without etching.  This is true of microstructural features with strong color differences or with large differences in hardness. Cracks, pores, pits and nonmetallic inclusions may be observed in the as-polished condition.  In most cases, a polished specimen will not exhibit its microstructure because incident light is uniformly reflected. 25 Etching  The purpose of etching is to optically enhance microstructural features such as grain size and phase features.  Etching selectively alters these microstructural features based on composition, stress or crystal structure.  The most common technique for etching is selective chemical etching and numerous formulations have been used over the years. Other techniques such as molten salt, electrolytic, thermal and plasma etching have also found specialized applications. 26 Etching Etched copper specimen Over etched copper specimen 27 Specimen Storage When polished and etched specimens are need to be stored for long periods of time, they must be protected from atmospheric corrosion. Desiccators and vacuum desiccators are the most common means of specimen storage. 28 Materials Characterization Depending on the nature of the material being investigated, a suite of techniques may be utilized to assess its structure and properties. Whereas some techniques are qualitative, such as providing an image of a surface, others yield quantitative information such as the relative concentrations of atoms that comprise the aterial. Recent technological advances have allowed materials scientists to accomplish something that was once thought to be impossible: to obtain actual two- dimensional/three dimensional images of atomic positions in a solid, in real time. It should be noted that the sensitivity of quantitiative techniques also continues to be improved, with techniques now being able to easily measure parts per trillion (ppt) concentrations of impurities in a bulk sample. 29 Materials Characterization Optical Microscopy The microstructure of a material is related directly to its physical, chemical, and mechanical properties as they are influenced by processing and/or the environment. Among the numerous investigative techniques used to study materials, optical microscopy, with its several diverse variations, is important to the researcher and/or materials engineer for obtaining information concerning the structural state of a material. Information gained using optical microscopy is complementary to other techniques and provides unique information to assess the microstructure of the sample. 30 Materials Characterization Material characterization gives information needed about the structure and composition of materials. Microscopy is a category of characterization techniques which probe and map the surface and sub-surface structure of a material. These techniques can use photons, electrons, ions or physical cantilever probes to gather data about a sample's structure on a range of length scales. 31 Scanning Electron Microscopy The scanning electron microscope (SEM) is a type of electron microscope that images the sample surface by scanning it with a high-energy beam of electrons in a raster scan pattern. The electrons interact with the atoms that make up the sample producing signals that contain information about the sample's surface topography, composition and other properties such as electrical conductivity. 32 SEM – X-ray Analysis (eds) 33 Scanning Tunneling Microscopy A scanning tunneling microscope (STM) is a powerful instrument for imaging surfaces at the atomic level. Its development in 1981 earned its inventors, Gerd Binnig and Heinrich Rohrer (at IBM Zürich), the Nobel Prize in Physics in 1986. For an STM, good resolution is considered to be 0.1 nm lateral resolution and 0.01 nm depth resolution. With this Resolution, individual atoms within materials are routinely imaged and manipulated. The STM can be used not only in ultra high vacuum but also in air, water, and various other liquid or gas ambients. 34 Atomic Force Microscopy (AFM) Atomic or near-atomic resolution images of topography of conductors, semiconductors and insulators. 35 X-ray Photoelectron Spectroscopy (XPS) X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopic technique that measures the elemental composition, empirical formula, chemical state and electronic state of the elements that exist within a material. XPS spectra are obtained by irradiating a material with a beam of X-rays while simultaneously measuring the kinetic energy and number of electrons that escape from the top 1 to 10 nm of the material being analyzed. XPS requires ultra high vacuum (UHV) conditions. 36 X-ray Diffraction (XRD) Powder XRD (X-ray Diffraction) is perhaps the most widely used x- ray diffraction technique for characterizing materials. As the name suggests, the sample is usually in a powdery form, consisting of fine grains of single crystalline material to be studied. The technique is used also widely for studying particles in liquid suspensions or polycrystalline solids (bulk or thin film materials). 37