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Optical Metallography Copyright © 1986 ASM International® ASM Handbook, Volume 10: Materials Characterizations All rights reserved. R.E. Whan, editor, p 299-308 www.asminternational.org Optical Metallography M.R. Louthan, Jr., Department of Materials Engineering, Virginia Polytechnic Institute and State University General Uses Limitations • Imaging of topographic or microstructural features • Resolution limit: Approximately 1 tzm on polished and etched surfaces at magnifications • Limited depth of field (cannot focus on rough of 1 to 1500x surfaces) • Characterization of grain and phase structures and • Does not give direct chemical or crystallographic dimensions information about microstructural features Examples of Applications Estimated Analysis Time • Determination of fabrication and heat-treatment • 30 min to several hours per specimen, including history preparation • Determination of braze- and weld-joint integrity Capabilities of Related Techniques • Failure analysis • Characterization of the effects of processing on • Scantling electron microscopy: Provides better microstructure and properties resolution (higher magnifications); greater depth of field (can image rough surfaces); qualitative Samples elemental microanalysis • Form: Metals, ceramics, composites, and geologic • Electron probe x-ray microanalysis: Provides materials quantitative elemental microanalysis • Size: Dimensions ranging from 10 -5 to 10 -1 m • Transmission electron microscopy: Provides much • Preparation: Specimens are usually sectioned and better resolution (much higher magnifications) on mounted, ground, and polished to produce a flat, specially prepared specimens; semiquantitative scratch-free surface, then etched to reveal elemental microanalysis; crystallographic microstructural features of interest information on microstructural features Introduction proximately 50x or higher; macroscopy Because the macro- and microstructure of Optical metallography, one of three gen- (macrostructural examination), 50 x or metals and alloys often determine the behav- eral categories of metallography, entails ex- lower. ior of the material, characterization of the amination of materials using visible light to Optical microscopy and, occasionally, effects of composition, processing, service provide a magnified image of the micro- and SEM are used to characterize structure by conditions, and other such variables on the macrostructure. In scanning electron micros- revealing grain boundaries, phase bound- macro- and microstructure is frequently re- copy (SEM), the second category, the sur- aries, inclusion distribution, and evidence of quired. Typical structure-property relation- face of the specimen is bombarded with a mechanical deformation. Scanning electron ships that have been established using optical beam of electrons to provide information for microscopy is also used to characterize frac- metallography include: producing an image (see the article "Scan- ture surfaces, integrated circuits, corrosion ning Electron Microscopy" in this Volume). products, and other rough surfaces, espe- • A general increase in yield strength and Lastly, transmission electron microscopy cially when elemental microanalysis of small hardness of a metal with decreasing grain (TEM) consists of passing a beam of elec- features is desired. Transmission electron size trons through a very thin specimen and microscopy is used to examine dislocation • A general tendency for a decreased duc- analyzing the transmitted beam for structural arrangements or structures and other small tility with increasing inclusion content information (see the article "Analytical defects in metals and alloys. Second-phase • Correlations of weld penetration, heat- Transmission Electron Microscopy" in this particles not observable using optical metal- affected zone (HAZ) size, and weld- Volume). Microscopy (microstructural ex- lography can frequently be analyzed using defect density with the nature and char- amination) involves magnifications of ap- TEM. acter of the welding 300 / Metallographic Techniques • Evaluation of such surface treatments as Edition of Metals Handbook). The most Fig. 1 One method of mounting carburizing and induction hardening by widely used sectioning device is the abrasive the sample to retain flatness for determinations of the depth and micro- cutoff machine, ranging from units using metallographic examination structural characteristics of the hardened thin diamond-rimmed wafering blades to The mount can also be filled with ground glass, region those using wheels that are more than 1.5 pelletized AI203, or another hard material to • Correlations of fatigue crack growth rates mm (1/16 in.) thick, 30 to 45 cm (12 to 18 in.) maintain flatness. and fracture-toughness parameters with in diameter, containing silicon carbide parti- Test Specimen mounted in Bakelite such structural variables as inclusion con- cles. tent and distribution Heat is generated during abrasive cutting, Test specimen • Association of failure initiation sites with and the material just below the abraded microstructural inhomogeneities, such as surface is deformed. To minimize burning second-phase particles and deformation, a lubricant or coolant is • Correlations of anisotropic mechanical typically used. Wet cutting yields a flat ~_oo -- L behavior with elongated grains and/or relatively smooth surface. However, because preferred grain orientations of the abrasion associated with cutting, the structure of the metal or alloy is damaged to The microstructures of metals and alloys a depth of approximately 1 mm (0.04 in.). are determined by composition, solidifica- The exact depth of damage depends on the tion processes, and thermomechanical treat- type of cutoff wheel used, the cutting speed, ment. Therefore, these process variables de- and the hardness of the specimen. The harder termine the response of metals and alloys to the specimen, the shallower the depth of laboratory and service environments. Be- damage. This damaged layer must be re- cause of the relationships between structure moved by grinding. However, before the and properties, metallographic characteriza- specimen can be conveniently ground, it Hardened steel balls or chilled iron shot (typ) tion is used in materials specification, qual- often must be mounted. ity control, quality assurance, process con- Mounting facilitates handling of the trol, and failure analysis. specimen. A procedure that does not damage Optical metallography is applicable to the specimen should be selected. Because studies ranging from fundamental research to large specimens are generally more difficult production evaluations. This article will dis- to prepare than small ones, specimen size cuss use of optical methods to evaluate should be minimized. Standard or typical structure and to relate that structure to pro- specimen mounts are right circular cylinders cess conditions and/or material behavior. 25 to 50 mm (1 to 2 in.) in diameter. Detailed information on the principles and Mounting mediums should be compatible instrumentation of optical microscopy is with the specimen regarding hardness and ,- / 32 mm (1.25 in.) diam ] -, available in the article "Optical Micros- abrasion resistance. Two common mounting copy" in Volume 9 of the 9th Edition of Bakelite mounting materials are thermosetting phenolics, such Nickel plate, 0.05 mm Metals Handbook. as Bakelite, and thermoplastic materials, (0.002 in.) thick Specimen Preparation such as methyl methacrylate (Lucite). A Section A-A thermosetting polymer develops a rigid The first step in metallographic analysis is three-dimensional structure upon being to select a sample that is representative of the heated and held at 200 to 300 °C (390 to 570 material to be evaluated. This step is critical °F). A thermoplastic polymer softens when thermoplastic materials are relatively soft to the success of any subsequent study. The held at elevated temperatures. mounting materials, and the specimen in second, equally important step is to correctly Mounting involves placing the specimen such a mount must often be surrounded by a prepare a metallographic specimen. in a mold and surrounding it with the appro- hard material, for example, hardened steel The region of the sample that is of interest priate powders. The mold and its contents balls (Fig. 1). This material helps retain the must be sectioned from the component. For are then heated under pressure to the thermal edges of the sample by maintaining a flat example, if a failure occurred because a steel setting or the softening temperature. Once surface during grinding and polishing. Ad- pipe leaked during service, the metallo- the powder sets, thermosetting mounts can ditional information on mounting techniques graphic analysis would probably involve at be removed from the mold without lowering and materials is available in the article least three samples: one removed from the the temperature; thermoplastic mounts must "Mounting of Specimens" in Volume 9 of pipe such that a portion of the leak is be cooled to ambient temperature before the 9th Edition of Metals Handbook. contained in the sample, another removed removal. Mounting pressure or temperature Grinding is generally considered the near the leak, and a third taken far from the may alter the structure of low melting tem- most important step in specimen preparation. leak. Each of the samples would be mounted perature or soft and/or fragile specimens; Care must be taken to minimize mechanical to facilitate handling. Selected surfaces therefore, castable (cold-mounting)
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