By George Vander Voort

Color Metallography

There are virtually no cases of naturally occurring color in meteoritic microstructures.  Color can be introduced using DIC but it has no physical significance or value.  Polarized light may produce some color effects with graphite and certain mineral phases but these are not commonly observed.  Crossed polarized light, in some instances, can produce color effects in coarse martensite within taenite wedges, as shown in Figure 1.  This is a high magnification micrograph of coarse martensite in Odessa using nearly crossed polarized light and a sensitive tint plate (which has colored the residual taenite magenta).

The best approach to create meaningful color micrographic images is to “tint” etch the specimen, sometimes after performing a light etch with picral or nital.  A tint etch [1] produces a stable film on the specimen surface whose thickness depends upon the composition (and variation in composition) of the phase, its crystal orientation, the presence of residual strain, and etch time.  Tint etchants are classified as being anodic, cathodic or complex, depending on the nature of the film that forms.  Anodic tint etchants, the most common type, produce films over the matrix phase, such as kamacite in meteorites.  Cathodic tint etchants produce films over the second phase constituents, such as cohenite.  Complex tint etchants will produce films over both matrix and second phases (but not necessarily over all types of second phases present).

Tint etchants are aqueous or alcoholic solutions that are balanced to produce a thin film on the specimen surface, typically 40 to 500 nm thick, of an oxide, sulfide, complex molybdate, chromate or elemental selenium.  Colors are developed by interference in the same manner as films produced by heat tinting or by vacuum deposition of a dielectric compound with a high index of refraction (the Pepperhoff method).  The film thickness controls coloration.  As the thickness increases, at a certain thickness interference begins and colors are produced, beginning with yellow, then green, red, violet, blue and silver.  Both first order and second order colors can be observed as the film grows.

With the cathodic tint etchants, the film only forms over the second-phase particles and the films are usually relatively constant in thickness with etch time.  In some cases, the color is rather constant which suggests that it grows to some specific thickness and then stops.  With the complex tint etchants, the same colors may, or may not, be observed only with certain second phases.  The matrix phase may be colored according to crystal orientation and some grains may have the same color as some of the second phases.  One must be careful in interpreting phases with the complex reagents.  Interpretation is easier with the anodic and cathodic tint etchants.

Experiments have shown that certain tint etchants that work well with carbon and alloy steels do not color meteorites.  This is due to the compositional differences between meteorites and ordinary steels.  For example, Klemm’s I is a very popular anodic tint etch that will color ferrite in carbon and low-alloy steels.  However, it is difficult to color kamacite (ferrite) in most meteorites because of their high nickel content. The lower Ni hexahedrites can be colored using Klemm’s I, but it is easier to color them if you double the potassium metabisulfite content, or use Klemm’s II or III.  Beraha’s sodium molybdate cathodic tint etch will color cementite (Fe₃C) in carbon and low- alloy steels, even if some manganese or chromium is dissolved in the cementite (replacing some of the iron).  However, it will not color cohenite (Fe,Ni,Co)₃C, in meteorites.  What works and what doesn’t work, can only be determined by experimentation.

Specimen preparation prior to tint etching must be of the absolute highest quality, otherwise tint etch results will be very poor.  A polished specimen that appears to be scratch free when viewed with bright field may exhibit a dense scratch pattern (from residual preparation-induced damage) after tint etching.

Many of the tint etchants that metallographers use were developed by the late Israeli metallographer/chemist Emanual Beraha.  Several of his tint etchants deposit a thin sulfide film on a wide range of metals and alloys.  These films are produced in two ways.  For solutions that contain either potassium or sodium metabisulfite, the iron, nickel or cobalt cation in the sulfide film comes from the specimen while the sulfide anion comes from the reagent after it decomposes.  The other sulfide film type is produced by a metal-thiosulfate complex in the reagent, an aqueous solution of sodium thiosulfate, citric acid, and cadmium acetate or cadmium chloride.  The specimen acts as a catalyst and the film that forms is cadmium sulfide.

The tint etchants are used by immersion, that is, the polished specimen is placed face up in a container holding about 100 mL of the solution.  Swabbing of the surface with a cotton tuft soaked with the solution is never performed as the film will not form under such conditions.  Etch time varies between about 45 and 300 seconds, depending on the solution used.

Color Metallographic Examples

One of the simplest color etchants is an aqueous solution of 10% sodium metabisulfite (Na₂S₂O₅).  This solution can be used to tint etch most carbon and alloy steels, including the ultra-high strength, ultra-high toughness, Ni-Co steels.  To get good color effects, it is necessary to view the specimen with nearly crossed polarized light and a sensitive tint plate.  Figure 2 shows the microstructure of Odessa etched with this solution and viewed in this manner.  In the lower right corner there are patches of plessite which, because of the lamellar pattern of the kamacite and taenite, respond brightly.  In the upper and lower left side we see pink phosphides (schreibersite) containing cracks.  They are pink because of the sensitive tint plate used with nearly crossed polarizers; in bright field, they would be white.  The matrix consists of several kamacite grains colored light violet, purple and blue.  Note that there are numerous straight or slightly curved red lines in the kamacite.  These are Neumann bands from extraterrestrial shock events.  In the blue kamacite region, some very faint Neumann bands can be seen.  These are probably from much older shock events.  Also in the blue kamacite, we observe a series of fine blue lines that are interconnected.  These are subgrain boundaries.  A few very small phosphides are observed on these lines.  Note also that there is a variation in the blue region of kamacite on the left half of the micrograph.  This is from either chemistry variations or from residual stress in this area.

Figure 1:  Martensite in a patch of taenite (colored magenta) in Odessa observed using nearly crossed polarized light and a sensitive tint plate (etched with 2% nital, 1000X).	Figure 2:  Color micrograph of Odessa, a coarse octahedrite, tint etched with sodium metabisulfite and viewed with nearly crossed polarized light and sensitive tint revealing (50X) plessite (lower right corner), Neumann bands, schreibersite (left side), a kamacite matrix and subgrain boundaries, most visible in the light bluish kamacite grain covering most of the left side of the image	Figure 3:  Color micrograph of Coahuila, a hexahedrite that fell in Mexico, after tint etching with sodium thiosulfate and potassium metabisulfite revealing (200X) a kamacite matrix, Neumann bands (fine parallel lines), and prismatic- and plate-shaped rhabdites (white particles).

Figure 3 shows the microstructure of Coahuila, a hexahedrite that fell in Mexico.  It was etched with an aqueous solution containing 10% sodium thiosulfate (Na₂S₂O₃) and 3% potassium metabisulfite (K₂S₂O₅) and is viewed with bright field.  As for nearly all hexahedrites, the Coahuila specimen is a single crystal, that is, the kamacite matrix has no internal grain boundaries.  Note that all of the kamacite is colored blue.  There are a number of parallel light blue lines running diagonally left to right.  These are Neumann bands.  The white particles are phosphides, both prismatic and plate shaped (in three dimensions).  Note that one prismatically-shaped rhabdite in the lower right corner has cracked due to two intersecting Neumann bands.  Note also the variation in color around the large prismatically-shaped rhabdite in the center of the image.  This is due to compositional variations around the phosphide.  The kamacite background has a rough appearance due to the presence of many very small (<1 μm diameter) phosphides.

Figure 4 shows Gibeon, a fine octahedrite that fell in Southwest Africa ~50,000 years ago, after color etching with this same solution.  Being a fine octahedrite, the kamacite bandwidth is fine enough that a low magnification photograph can show how coloring of a matrix phase, much as kamacite, varies with crystal orientation.  Note that the structure exhibits elongated kamacite grains colored various shades of brown and blue.  Fine, parallel Neumann bands are observed within each kamacite grain.  The white films between the kamacite grains are taenite.  Note also a number of different types of plessite patches between kamacite grains.

Figure 4:  Color micrograph of Gibeon, a fine octahedrite, after tint etching with sodium thiosulfate and potassium metabisulfite revealing (20X): elongated kamacite grains, colored according to their crystallographic orientation, containing fine, parallel Neumann bands (mechanical twins), fine white taenite films at the edges of the much wider kamacite grains and patches of two-phase plessite of several types.	Figure 5:  Color micrograph of another region in Gibeon, after tint etching with sodium thiosulfate and potassium metabisulfite revealing (50X) two horizontal bands of two-phase plessite of several types, taenite films along the edges of the plessite, Neumann bands in the kamacite and a kamacite matrix which is all blue (same crystallographic orientation) except at the extreme top of the field where it is brown.	Figure 6:  Color micrograph (20X) of Henbury, a medium octahedrite that fell in Australia, etched with sodium thiosulfate and potassium metabisulfite revealing the effects of a violent deformation event in outer space.  Note the strong deformation pattern and the extensive fine Neumann bands.

Figure 5 shows another view of Gibeon, using the same etch, showing two parallel bands of plessite of several types running horizontally across the field of view.  Note the white films of taenite at plessite-kamacite or kamacite-kamacite boundaries.  The fine lines in the kamacite matrix are Neumann bands.

Figure 6 shows the microstructure of Henbury, a medium octahedrite that fell in Australia after etching with sodium thiosulfate and potassium metabisulfite.  This meteorite was subjected to a violent event that ripped it apart, probably in outer space.  The structure is mostly highly deformed kamacite containing many fine mechanical twins (Neumann bands).  In the upper right corner we see a large phosphide.

Beraha’s complex thiosulfate tint etch that forms a cadmium sulfide interference film works very well with meteorites.  Figure 7 shows the microstructure of Arispe, a coarse octahedrite that fell in Sonora, Mexico, tint etched with Beraha’s CdS reagent.  This shows a high magnification view of plessite in Arispe where both kamacite and taenite are colored.  Surrounding the plessite, we observe kamacite colored brown.  In the upper right corner, some non-colored cohenite is present.  At the edge of the plessite there is a tan rim of taenite.  The kamacite within the plessite is light blue while the taenite particles in the plessite are tan.

Figure 7: Color micrograph of Arispe, a coarse octahedrite that fell in Mexico, tint etched with Beraha’s complex CdS tint etch revealing (400X) dark brown kamacite, plessite where both the kamacite and taenite (tan) are colored, and cohenite at the top edge on the right, which is not colored.	Figure 8: Color micrograph (400X) of Odessa, a coarse octahedrite, etched with Beraha’s complex CdS tint etch revealing a taenite wedge containing a clear taenite (light blue) CT1 zone as the outer rim, then the colored cloudy zone, CZ, with cracks, then an inner clear taenite (white) zone, CT2, and bluish martensite “needles,” surrounded by kamacite at the top and bottom of the field-of-view, containing a few Neumann bands at the top.	Figure 9: Color micrograph (50X) of Canyon Diablo, a coarse octahedrite, etched with Beraha’s selenic acid reagent revealing two patches of cohenite, one of which has adhering greenish schreibersite, cream and tan taenite, and a deformed kamacite matrix.

Figure 8 shows a taenite wedge in Odessa (similar to that shown in Part II) after tint etching with Beraha’s CdS reagent.  The surrounding kamacite is yellow at the top and contains reddish Neumann bands.  At the edge of the taenite wedge there is a very thin light blue film, the outer clear taenite (CT1) zone.  Next to this is the cloudy zone where the color varies across the zone as the nickel content decreases.  Next to this is a white zone of clear taenite (CT2) and in the center we see bluish plates of martensite (Ni <25 wt. %) surrounded by yellow-tan colored kamacite .

As a final example, Figure 9 shows the microstructure of Canyon Diablo etched with Beraha’s selenic acid tint etch.  There is no fixed composition to this reagent.  It consists of hydrochloric acid (up to 20 mL), selenic acid (up to 3 mL) and ethanol (amount added so that the total is 100 mL).  One must experiment to find the best composition.  Pre-etching with nital is required for best results.  Figure 9 shows two patches of cohenite colored orange brown.  Note that in the center of the field, there are greenish patches of schreibersite attached to the cohenite.  The cream and tan patches are taenite. The matrix is highly deformed kamacite containing some faint subgrain boundaries.

Conclusions

Color metallography adds an extra dimension to the study of the microstructure of meteorites.  Tint etchants are very sensitive to phase composition and to other effects, such as crystal orientation.  However, unlike nital where the strength of the attack varies with crystal orientation so that not all of the structure is properly revealed, tint etchants show crystal orientation differences by variations in color.  Phase identification is usually much easier using tint etchants.  There are a number of tint etchants that will selectively etch only certain second phase constituents, thus aiding in their identification without need to use expensive electron beam analytical methods.  Tint etchants are also sensitive to chemical analysis variations or the effects of residual deformation while the standard “black and white” etchants are not.  

Reference

[1] G.F. Vander Voort, “Color Metallography,” in Vol. 9, Metallography and Microstructures, ASM Handbook,  G. F. Vander Voort (ed.), ASM International, Materials Park, Ohio, 2004, pp. 493-512.

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