For years, I received a calendar from a company that sells metallurgical lab equipment. The monthly photos were generally in color, artistically selected, metallographic cross sections of metal parts or metal research specimens. One of my employees knew the basic procedure for microstructure evaluation, including the preparation of polished and etched metallographic cross sections. But I was using black-and-white Polaroid film at the time, and he didn't realize that microstructural features could be in other hues.
I don't remember how the discussion came up, but he said that he thought that someone put the colors in with a software product. I had a hard time explaining that, in this case, the colors were not "colorized" effects as I described for scanning electron microphotos. Nor were they due to differences in angle of reflection, as in the pyrite examples I showed in an earlier post. The different colors in adjoining crystals are due to translucent or transparent films of sulfides, oxides or other substances that are grown, either electrochemically or by heating the sample in air, on mirror-smooth polished surfaces. Please see Figure 1, which is a section from a Lurisran dagger handle from Metallography and Microstructure of Ancient and Historic Metals by David A. Scott. (©1991, The J. Paul Getty Trust. All rights reserved.)
George Vander Voort, my fellow Industrial Heating blogger, is internationally known for his scientific and artistic work with color micrographs. Figure 2 is from George's beautiful and informative website. It shows a twinned austenitic grain structure of solution-annealed, wrought Hadfield manganese steel (Fe - 1.12% C - 12.7% Mn - 0.31% Si) tint etched with Beraha’s sulfamic acid reagent. To get the particular colors that we see here, we must view with polarized light plus sensitive tint. Achieving this artistic effect is not something that every metallographer knows how to do! The original photo was taken at 100X. The viewing mode and optics of the microscope contribute to the color appearance, as does the way the films are grown on the polished surface. The process works as follows.
The classical way to reveal a microstructure in commercial metals is to make a cross section. If the part is small, it may be possible to include both surface layers and core layers in a single specimen. Since the common sizes for metallographic cross sections made in commercial mounting presses are 1-1.5 inches (25-38 mm) in diameter, larger parts may require sampling of the layers of interest. Once the part is cut to expose the cross section of interest, it is usually mounted in hard plastic to make it easier to polish to a mirror flat finish. Note that there's a lot more to this process than I am explaining. The preparation aspects of metallography (or materialography as applied to nonmetallic materials) are a science and art unto itself. The metal section embedded in plastic, generally called a "mount," is then subjected to a series of grinding and polishing steps, ending with a layer of nominally undeformed material with a mirror finish.
We’ll finish our discussion next time in part 2.