Color Metallography

Abstract

The use of color in metallography has a long history with color micrographs published over the past eighty-some years. 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, as in dark field illumination, polarized light and differential interference contrast illumination. 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). There are a number of electrolytic etching reagents that can be used to produce color. Second-phase constituents can be colored and viewed with bright field. Anodizing aluminum specimens with Barker's reagent, or similar solutions, does not produce an interference film, as color is not observed in bright field. This procedure produces fine etch pitting on the surface. The grain structure can be seen in black and white in polarized light, and in color if a sensitive tint plate is added. Heat tinting is a more universal procedure for growing an interference film on the surface of a metallic specimen. Also, the Pepperhoff interference film method, where a dielectric film with a high refractive index is produced by vapor deposition onto the specimen surface to a thickness that produces interference effects, is universally applicable. This technique makes features with little differences in reflectivity, or very low reflectivity, visible in color.There are some standard chemical etchants that produce gray-scale images in bright field, but when viewed with polarized light and sensitive tint, produce color images. There are two main examples of this behavior. First, if an etchant produces grain contrast effects, rather than a "flat etch" (only phase boundaries, grain boundaries and/or twin boundaries are visible), a color image will be observed when polarized light and sensitive tint is used. Second, any fine lamellar structure, such as exhibited by all eutectoid transformation products, will exhibit color when viewed with polarized light and sensitive tint. But, the main way to yield color micrographs is the use of "tint" etchants, also called "stain" etchants.Color is more useful than black and white because the human eye is sensitive to only about 40 shades of gray from white to black, but a myriad of colors with variations in hue, saturation and intensity. Metals with non-cubic crystal structures are often difficult to etch and obtain a co...

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