Metallographic specimens

Table IV summarizes the gold concentration in each object as determined by electron microprobe and neutron activation analysis (NAA). The electron microprobe has a lower sensitivity and, therefore, lower precision than NAA for gold. Despite this, however, the data from the two techniques are remarkably consistent when NAA data are obtained from a recently exposed internal surface of the object—i.e., a metallographic specimen.

Surface finish requirements for the Vickers test vary with the test load. Heavy load tests can be made on a 120 grit ground surface. At low loads increasingly finer surface preparation is required, approaching that for metallographic specimens, to permit accurate diamond indentation measurements. 

Figure 7.44 Prepared metallographic specimen F = fatigue fracture W=web ) = journal. (Reproduced from COM 1999 and 2005 with permission from the Metallurgy Society ofCIM)...

Wet fluorescent magnetic particle inspection of the remaining journal fillets on this crankshaft revealed the presence of a number of secondary cracks in the same orientation and position as the failure initiation site. A metallographic specimen, including the matching halves of the fracture from the initiation site in the fillet of main journal, was cut, mounted, polished and etched in 2% nital, in preparation for examination with an optical microscope. The polished and etched specimen is shown in Figure 7.44. 

In this case it may be shown that the fractional error in d is proportional to sin 4(f) tan = (90° — 0). With either of these extrapolation functions a fairly precise value of the lattice parameter can be obtained in addition, the back-reflection pinhole camera has the particular advantage that mounted metallographic specimens may be examined directly. This means that a parameter determination can be made on the same part of a specimen as that examined under the microscope. A dual examination of this kind is quite valuable in many problems, especially in the determination of phase diagrams. 

Figure 4. A net of intergranular cracks on the unetched microsection metallographic specimen near the external surface of the traverse, x 1500.

Figure 5.42 Metallographic specimen fatigue fracture (F), Web (W), and Journal (J). (Figure originally published in Reference 26. Reproduced with permission of the Canadian Institute of Mining, Metallurgy and Petroleum, www.cim.org.)...

Cain, F.M., Jr., 1974. Practical applications of cathodic vacuum etching. In McCall, J.L., Mueller, W.M. (Eds.), Metallographic Specimen Preparation. Plenum Press, New York, p. 207. 

Other methods are available and the reader is advised to explore them. A good outline of additional mounting methods and materials is given in Metallography Principles and Practice by G. Vander Voort, Ref 2, section 2.4, and ASTM E 3, Methods of Preparation of Metallographic Specimens, sections 7 through 9. 

E 3 Methods of Preparation of Metallographic Specimens G 1 Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens... 

Figure 10 Cross section of a metal with a shallow crater due to cathodic delamination of a coating. The coating was pulled off before the sample was embedded and the cross section was made. The black areas are formed by a crevice between the metal and the embedding. The white line parallel to the metal surface indicates the (exaggerated) formation of a crater. The cross section was embedded as a metallographic specimen and photographed with an optical microscope.

As a first approach to the understanding of the properties of a metal or alloy it is useful to prepare a metallographic specimen polished to a mirror fini.sh. This... 

Examination of the three metallographic specimens at a magnification of 800x showed the structure to be essentially the same on each specimen and to contain a fairly dense distribution of a veiy fine precipitate of magnesium silicide (Mg2Si) throughout the material. This constituent would be visible only if aluminum alloy 6061 had been heated to temperatures above about 175 °C (350 °F) or if it had been improperiy heat treated. 

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