Materials Characterization & Testing

Color Metallography

May 16, 2013
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Color has historically seen limited use in metallography, mainly due to the cost of film and prints and the difficulty and cost of reproducing images in publications. With the growth of digital imaging, however, capturing color images is much simpler and cheaper.



Fig. 1.  Tegramin grinder/polisher and automated system for adding lubricants and abrasives.


 

Color does have many advantages over black and white. First, the human eye is sensitive to only about 40 shades of gray from white to black, but it is sensitive to a vast number of colors. Tint etchants reveal features in the microstructure that often cannot be revealed using standard black-and-white etchants. Color etchants are sensitive to crystallographic orientation and can reveal if the grains have a random or a preferred crystallographic texture. They are also very sensitive to variations in composition and residual deformation. Further, color etchants are usually selective to certain phases, and this is valuable in quantitative microscopy.



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Fig. 2.  (left) Grain structure of pure ruthenium that was hot worked and viewed in polarized light.  Fig. 3.  (right) Microstructure of Spangold (Au-19Cu-5Al) that was polished and cycled through the shape-memory effect to produce martensite. This is best seen by viewing the as-polished, non-etched surface with Nomarski differential interference contrast illumination, as it vividly reveals the surface upheaval due to the shear reaction at the free surface. The magnification bars are 100 and 20 µm, respectively.

Introduction

The use of color in metallography has a long history with color micrographs published over the past 80-some years. But to do good work in color does require the specimens to be prepared as damage free as possible. Since the major source for preparation-induced damage is the sectioning step, it is obvious that special attention be applied to this process. This means that production cutting devices should not be used to produce the plane to be polished. Instead, choose the least damaging technique, such as a precision saw (e.g., the Accutom) or the Secotom, which allows you to cut somewhat larger specimens. A laboratory abrasive cutter may also be used – always with the correct blade for the material being sectioned. If the edges are critical, mount with a high-quality resin that yields good edge retention, such as DuroFast.

Because we have minimized surface damage in cutting, grinding can commence with a relatively fine particle-size abrasive, depending upon the hardness of the metal/alloy and/or the metal-removal rate in grinding. For some metals or alloys, one grinding step is adequate before going to diamond polishing abrasives. For others, two steps might be needed. Polishing is generally accomplished with two diamond steps and one final polishing abrasive, either colloidal silica or alumina. But the method must be properly planned and executed. During polishing, the cloth surface must be uniformly covered with abrasive and lubricant. If it dries, the surface will be smeared and deformed, and color etch results will be poor. To achieve this, set the platen to rotate counterclockwise and the head to rotate clockwise at a speed between 30 and 90 RPM, depending upon the metal/alloy. The new Tegramin grinder/polisher is ideal for such work (Fig. 1).

Examples of natural color in metals are rare. Gold and copper exhibit yellow color under bright-field illumination. Color can be produced using optical methods such as polarized light (Fig. 2) and differential interference contrast illumination (Fig. 3). The microstructure of metals with non-cubic crystal structures can be examined without etching using polarized light, but color is not always observed. The specimen must be prepared completely free of residual damage for color to be observed, and even then, some non-cubic metals still exhibit little color. However, many metals and alloys can be etched with reagents that deposit an interference film on the surface that creates color in bright-field illumination. If it is difficult to grow such a film to the point where the color response is excellent, the color can be enhanced by examination with polarized light, perhaps aided with a sensitive tint filter (also called a lambda plate or first-order red filter).



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Figs. 4 and 5.  FCC twinned grain structure of cartridge brass, Cu-30% Zn, after cold reduction by 50% and full annealing, tint etched with Klemm’s I (left) and Klemm’s III (right) reagents and viewed with polarized light plus sensitive tint. Magnification bars are 200 µm long.

Color Tint Etching

Many metals etched with standard reagents to reveal the grain boundaries often yield only a high percentage of the boundaries rather than all of them. Color tint etchants, however, reveal the grain structure completely. In the case of metals with annealing twins, such as copper, it can be very difficult to rate the grain size when a standard etchant reveals only a portion of the grain and twin boundaries. In fact, it can be quite difficult to make a precise measurement of the grain size, even manually with such a specimen, because distinguishing between grain and twin boundaries (the latter must be ignored in the measurement) is not always simple. With a color-etched microstructure, however, it is relatively easy to separate grain boundaries from twin boundaries, at least manually. Further, the films grow as a function of crystal orientation. Therefore, one can detect any preferred crystallographic orientation by the narrowness of the color range present. If a wide range of colors is present in a random pattern, the crystal orientation is random. If a narrow range of colors is present in the grains, then a preferred orientation is present. Tint etch compositions are given in references 1 and 2.



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Fig. 6.  (left) Pearlitic plessite in the Arispe meteorite revealed with Beraha’s CdS complex tint etch.  Fig. 7.  (right) Ferrite preferentially colored in 312 stainless steel weld metal using Beraha’s BII reagent. The magnification bars are 25 and 100 µm long, respectively.

 



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Fig. 8.  (left) FCC twinned grain structure of heading-quality Custom Flo 302 stainless steel revealed using Beraha’s BII reagent.  Fig. 9.  (right) Martensite (brown) and upper bainite (blue/white) formed in isothermally treated (1525°F – 1000°F – 60s in a salt pot – water quenched), partially transformed 5160 alloy steel using 10% sodium metabisulfite. Both images were viewed with polarized light plus a sensitive tint filter. The magnification bars are 100 and 10 µm long, respectively.

 

Beraha also developed four etchants based upon sulfamic acid (a weak organic acid) that have not been used much, even though they are quite useful, reliable and easy to employ. The sulfamic acid-based reagents are applicable to cast iron, low-carbon and alloy steels, tool steels and martensitic stainless steels. Figure 12 shows an example. Beraha also developed tint etchants that deposit elemental selenium on the surface of steels and cast iron (Fig. 13), nickel-based alloys and copper-based alloys (Fig. 14).

Erika Weck developed a number of tint etchants and utilized many of them in her research. Several were developed to color aluminum (Fig. 15) or titanium alloys (Fig. 16). In each case, it is easier to develop good color with the cast alloys than with the wrought alloys. Details on these etchants can be found in the references.



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Fig. 10.   (left) Retained austenite (brown) and plate martensite (blue) revealed in as-cast NiHard cast iron with Beraha’s CdS reagent. The large white (non-colored) phase is cementite. Fig. 11.  (right) Microstructure of wrought, partially recrystallized Cu-3% Co tint etched with Beraha’s PbS reagent.

Conclusions

The examples shown have demonstrated the great value of color and tint etching for examining microstructures of metals. Reagents exist to develop color with most commercial alloy systems. The examples clearly demonstrate the value of these reagents in fully revealing the grain structure, even for the most difficult-to-etch specimens. Further, they are selective in nature, which can be quite useful for quantitative metallographic studies. Tint etchants reveal segregation very clearly, and either EDS or WDS can be performed on a tint-etched surface without any problems from the interference surface layer. IH

For more information: Contact George F. Vander Voort, Vander Voort Consulting L.L.C., Consultant – Struers Inc., 24766 Detroit Rd., Westlake, OH 44145; tel: 847-623-7648; e-mail: georgevandervoort@yahoo.com; web: www.struers.com and www.georgevandervoort.com.



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Fig. 12.  (left) Twinned FCC grain structure in Fe-39% Ni revealed by Beraha’s sulfamic acid reagent. The image was viewed with polarized light plus a sensitive tint filter.  Fig. 13.  (right) Cementite in chill-cast hypoeutectic gray iron colored using Beraha’s selenic-acid reagent for cast iron. The magnification bars are 100 and 50 µm long, respectively

 



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Fig. 14. (left) Twinned FCC alpha phase and beta phase (mottled and outlined) in Cu-40% Zn revealed using Beraha’s selenic acid reagent for copper alloys. The magnification bar is 20 µm long. Fig. 15. (right) Chemical heterogeneity in the cast grain structure of a semi-solid thixocast billet of A357 aluminum using Weck’s reagent for Al alloys (200X, bright field).

 



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Fig. 16. Equiaxed grain structure of annealed CP Ti (ASTM F67, Grade 4) etched with modified Weck’s reagent. The magnification bar is 100 µm long.

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