10 Metallography

10 Metallography Metallography is the branch of science dealing with the study of the consititution and structure of metals and alloys, its control through processing, and its influence on properties and behavior. Its original implementation was limited by the resolution of the reflected light microscope used to study specimens. This limitation has been overcome by the development of transmission and scanning electron microscopes (TEM and SEM). The analysis of x-rays generated by the interaction of electron beams with atoms at or near the surface, with wavelength- or energy- dispersive spectrometers (WDS, EDS) with the SEM or the electron microprobe analyzer (EMPA), has added quantitative determination of local compositions, e.g., of intermediate phases, to the deductions based upon observations. Introduction of metrological and stereological methods, and the development of computer-aided image analyzers, permits measurement of microstructural features. Crystallographic data can be obtained using classic x- ray diffraction methods using a diffractometer, or diffraction analysis can be performed with the TEM using selected area or convergent-beam electron diffraction (SAD and CBED) techniques, and more recently with the SEM with the orientation-imaging (EBSD) procedure. There is a wide variety of very sophisticated electron or ion devices that can be utilized to characterize surfaces and interfaces, but these devices are generally restricted in availability due to their high cost. Conventional light-optical techniques are still the most widely used and are capable of providing information needed to solve most problems. Examination by light optical microscopy (LOM) should always be performed before use of electron metallographic instruments. LOM image contrast mechanisms are different than EM imaging modes. Natural color can not be seen with EM devices. Microstructures are easier to study at low magnification with the LOM than with the SEM. The LOM examination may indicate the need for SEM or TEM analysis and determine the locations for such work. Interpretation of LOM examination results is enhanced and reinforced by the use of electron metallographic techniques. The SEM has become ubiquitous in the metallographic laboratory. 10.1 Macroscopic examination For examination of large-scale features, known as macrostructure152 because it is visible with the unaided eye, disks are cut from cast or wrought products (in the case of wrought products, usually before extensive hot deformation is performed). The disks must be representative of the product and are usually taken from prescribed test locations. Mechanical sawing or abrasive sectioning is used to obtain the disk, which may be ground to various surface finish levels, depending upon the nature of the detail that must be observed. The disks are cleaned and hot acid etched to reveal the solidification structure, deformation structure, segregation, soundness, etc. Disks may also be subjected to contact printing methods, such as sulfur printing (see 152, ASTM E 1180 and ISO 4968). The required disk is obtained by sawing, abrasive sectioning or machining with adequate cooling and lubrication, and is normally finished by grinding. Due to the size of these disks, machine shop grinders are used. In the case of a small section, laboratory practices may be utilized. Grinding on abrasive cloth or paper, such as an ―endless belt grinder‖, is often used, but the platen beneath the belt must be kept flat. For most macroexamination work, a ground surface is adequate. In a few cases, chiefly dependant upon the nature of the examination and the desired etchant, a polished surface is required. Conventional metallographic procedures are adequate. The main problem is obtaining flatness over a section area that is rather large compared to standard metallographic specimens. But, with modern grinder/polisher devices, this can be readily accomplished. Capturing images of macroetched components can be quite difficult due to the need for obtaining proper uniform illumination, especially if the surface is as-polished, as might be the case when documenting porosity, cracks or other voids in sections. Although film-based documentation is becoming less common, compared to digital imaging, film is still preferred at this time in failure analysis work involving litigations. Regardless of the technology used, illumination still must be uniform. With an as-polished surface, ―hot spots‖ and reflections are a major problem that can be overcome by a variety of ―tricks‖ utilizing vertical illumination and passing light through translucent material. Etching reagents for macroscopic work are given in many national and international standards, such as ASTM E 340, and text books152. The commonly used reagents are listed in Table 10.1. Directions for sulfur-printing are given in ISO 4968 and ASTM E 1180. This technique is used to show the distribution of sulfur in steel and is also described in Table 10.1. 10.2 Microscopic examination Metallographic specimens are normally prepared for examination with the light microscope by cutting out the piece to be examined (preferably not more than about 3 cm diam.) using a laboratory abrasive cutoff machine or a precision saw. Sometimes specimens are cut in the shop or in the field using more aggressive methods, such as power hacksaws, dry abrasive cutting, and even by flame cutting. These techniques introduce a great deal of damage into the structure adjacent to the cut. It is generally best to re-section the specimen with a laboratory abrasive cutoff machine rather than to try to grind through the damaged layer. In many cases, the specimen is encapsulated in a polymeric compound, either a compression mounting resin or a castable resin. Larger specimens of uniform shape may be prepared without mounting but the edge retention may not be adequate for examination above 100X. The specimens are then subjected to at least one grinding step and one or more diamond abrasive steps, followed by one or more polishing steps with other abrasives, such as alumina or colloidal silica. But, the steps must be designed to remove the damage from sectioning. However, each abrasive does produce damage proportional to its particle size. So, each step must remove the damage from the previous step so that at the final step the damage depth is so thin that etching will remove it. At the same time, the preparation procedure must keep the specimen surface flat and other problems (e.g., pull out, drag, smear, embedding, etc.) are prevented. The surface must be more than just reflective in nature if the true microstructure is to be revealed. For some purposes, e.g., the study of slip processes involving individual dislocations using transmission electron microscopical studies of fine structure, and microindentation hardness testing under light loads, electropolishing has been considered in the past to be almost indispensable. In the later case, this is certainly unnecessary today with proper mechanical preparation methods. Preparation of specimens for replica work requires a proper metallographic preparation procedure, but this can be done mechanically. Preparation of TEM thin foils normally employs jet-electropolishing (or similar alternative procedures) to perforate the specimen. But, this is the final step after initial mechanical grinding. Electropolishing does have certain advantages, but also disadvantages. It is best for single phase metals, particularly for relatively pure metals. In alloys containing two or more phases, the rate of electropolishing varies for each phase and flat surfaces may be difficult to obtain. Even for single phase metals, the surface is often wavy rather than flat which makes high magnification examination difficult or impossible. Electropolishing solutions are often quite aggressive chemically, and may be dangerous, even potentially explosive under certain operating conditions, or under careless operating conditions. Table 10.1 ETCHING REAGENTS FOR MACROSCOPIC EXAMINATION Material Reagent* Remarks A. Aluminium base 1. Aluminium and (a) Concentrated Keller's Reagent Can be diluted with up to 50ml water, its alloys Nitric acid (1.40) 100 ml Hydrochloric acid (1.19) 50 ml Hydrofluoric acid (40%) 1 ½ ml (b) Nitric acid (1.40) 30 ml Widely applicable, but very vigorous Hydrochloric acid (1.19) 30 ml 2% conc. Hydrofluoric acid 30 ml (c) Tucker's Reagent Use fresh Nitric acid (1.40) 15 ml Hydrochloric acid (1.19) 45 ml Hydrofluoric acid (40%) 15 ml Water 25 ml (d) 10% sodium hydroxide Use at 60-70C in water 2. Unalloyed Aluminium (e) Flick's Reagent and Al-Cn alloys Hydrochloric acid 15 ml Wash in warm water after etching and clear by Hydrofluoric acid 10 ml dipping in concentrated nitric acid Water 90 ml 3. Aluminium – silicon (f) Hume-Rothery's Reagent For high-silicon alloys, Fine polish undesirable. Cupric chloride 15 g Immerse specimen. 5-10s, remove, and brush Water 100 ml away deposited copper or remove it with 50% nitric acid in water 4. Aluminium – copper (g) Keller's Reagent More frequently used as micro-etch 2 ½ % nitric acid (1.40) 1 ½ % hydrochloric acid (1.19) ½ % hydrofluoric acid (40%) Rem. water 5. Aluminium – (h) 5% cupric chloride Clear surface with strong nitric acid magnesium 3% nitric acid (1.40) Rem. water 6. Aluminium – (g) Keller's Reagent (as above) copper – silicon (i) Nitric acid (1.40) 15 ml Hydrochloric acid (1.19) 10 ml Hydrofluoric acid (40%) 5 ml Water 70ml 7. Aluminium–copper– (j) Zeerleder's Reagent magnesium–nickel Hydrochloric acid (1.19) 20 ml Nitric acid (1.40) 15 ml Hydrofluoric acid (40%) 5 ml Water 60 ml *Acids are concentrated, unless otherwise indicated, e.g. with specific gravity. B. Copper base 1. Copper and (a) Alcoholic ferric chloride copper alloys Ethyl alcohol 96 ml Avoid use of water for washing or staining generally Ferric chloride (anhydrous) 59 g may result. Use alcohol or acetone instead. Hydrochloric acid (1.19) 2 ml Grain contrast (b) Acid aqueous ferric chloride Ferric chloride 25 g (a) and (b) require moderately high standard of Hydrochloric acid (1.40) 25ml surface finish Water 100 ml (c) Concentrated nitric acid (1.40) 50 ml A rapid etch, suitable for roughly prepared Nitric acid (1.40) 50 ml surfaces.

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