Color Tint Etching

GUIDELINES

  • Best results occur when etching right after polishing, time can create a passive surface
  • Specimen preparation must remove all damage
  • Etch the specimen with a general-purpose etch, then repolish and tint etch for best results
  • Tint etching must be done by immersion, as swabbing will inhibit film formation
  • The film grows epitaxially with the underlying grain structure with little “etch” depth

TINT ETCHING REVEALS

  • Grains are colored according to their crystallographic orientation (film thickness is a function of crystal orientation)
  • Very sensitive to chemical inhomogenieties
  • Very sensitive to residual deformation
  • Etchants are phase specific, i.e., selective

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Color Metallography - Volume 9

THE USE OF COLOR in metallography has a long history, with color micrographs published over the past eighty-some years. A number of general articles (Ref 1–15) have been published reviewing methods and applications.
Natural color is of use in only a few classic metallographic applications. Prior to the development of wavelength-dispersive spectrometers and energy-dispersive spectrometers used on electron microprobe analyzers and scanning
electron microscopes, the color of inclusions using different illumination modes was part of the identification schemes used. However, natural color has limited applicability.

Color can be created by optical methods, such as with polarized light and differential interference contrast illumination. Polarized light examination is extremely useful for studying the structure of certain metals, without etching, that have noncubic crystal structures, such as beryllium, hafnium, -titanium, uranium, and zirconium.
In many cases, polarized light can be used with etched specimens, regardless of their crystal structure, to produce color. Differential interference contrast reveals height differences between constituents and the matrix, but in most cases, the color is of esthetic value only.

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Color Metallography from Microscopy Today

octahedride-grain_1-wsColor 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. However, with the growth of digital imaging, capturing color images is much simpler and cheaper. Also, printing images in color is inexpensive for in-house reports, and can be distributed cheaply on CDs, although reproduction in journals is still expensive. Color does have many advantages over black and white. First, the human eye is sensitive to only about forty shades of gray from white to black, but 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, they are usually selective to certain phases and this is valuable in quantitative microscopy. By George Vander Voort

(Courtesy of the Microscopy Society of America - www.microscopy.org) - Original Published by Microscopy Today, November 2005 - Volume 13, Number 6 - www.microscopy-today.com

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Color Metallography - Paper

Natural Color

  • Rare in Metals
  • Elemental Copper in Steel
  • Plated Metals (Cu on Fe, etc.)
  • Intermetallics (AuAl2)

Microstructure of a copper-infiltrated, porous powder metallurgy high-carbon steel (after heat treatment which
produced coarse martensite and retained austenite. The copper, which did not completely fill the pores (arrows),
exhibits its natural yellow color and twins can be seen (green arrows). The martensite was revealed with
Beraha’s 10/3 reagent, similar to Klemm’s I, and viewed with polarized light plus sensitive tint.

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resourcesThe articles and presentations that can be down-loaded from this web site are based upon work done by GFV while employed at Bethlehem Steel (1967-1983), Carpenter Technology (1983-1996), Buehler Ltd. (1996-2009) and Struers (2009-Present) and from the authors consulting work for companies such as, Latrobe Steel, Scot Forge, etc., and from his litigation work. GFV's bylined articles appearing in various issues of the ASM Handbook series have been listed here courtesy of ASM International, Materials Park, Ohio.

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