A Note on Metallographic I Meteorites George F. Vander Metal Physics Research, Carpenter Technology Corporation, Reading, PA 19612 Meteorites are particularly fascinating subjects for metallographic examination, not merely because of their extraterrestial origin, but also because of the rich variety of microstructural constituents encountered. While their structure can be assessed use of the familiar nital and picral etchants used extensively for steels, application of selective etchants, including those that produce selective coloration, greatly increases the amount of infor­ mation obtained by light microscopy. The article describes efforts to preferentially darken or color microstructural constituents in iron meteorites: kamacite (ferrite), Neumann bands (mechanical twins), taenite (austenite), cohenite (Fe-Ni carbide similar to ce­ mentite), martensite and schreibersite/rhabdite (Fe-Ni phosphides). The influence of con­ centration gradients, deformation, and reheating on the structure can be clearly observed by the use of tint etchants. INTRODUCTION iron portion of meteorites, as in the iron meteorites, is opaque to and Meteorites have fascinated engineers and requires reflected-light studies. scientists for many years. Research on me­ The classification of iron meteorites is a teorites began with mineralogists and ge­ complex Classification is based on ologists and the major phases and constit­ macrostructural and microstructural char­ uents in iron meteorites were identified acteristics or chemical ,-,,..,.,.,,.. -e-,,..,.,,i.-ir'n and named prior to Henry Clifton Sorby's Ga, Ge, Ir contents). The two "UQYIPn,,, first observation in 1863 of the microstruc­ tunately. are consistent. In terms, ture of polished and etched manmade met­ there are three categories of iron meteo- als. Iron meteorites are but one of several rites-hexahedrites, and general categories of meteorites. ataxites. Those that do not these Fe-Ni alloys with nickel contents of three groups are cate- 4.3-34%, although few have levels >20%; gory-anomalous. and most are in the range of 5-10% Ni. The hexahedrites Small amounts of cobalt 0.4- crystals of kamacite 1.0%), sulfur (up to about phos- boundaries are phorus (up to about 0.3%), and carbon subgrain boundaries are common. Their to about 0.2%) are usually present, while structure may be altered trace levels of many other elements are de­ cosmic which tectable. There are also numerous types of the kamacite stony and stony-iron meteorites. The stony grains. Most contain Ni in the matter can be studied using reflected range of about 5.2-5.8%. or transmitted light on thin sections. The Octahedrites received name based 223 ©Elsevier Science Publishing Co., Inc., 1992 MATERIALS CHARACTERIZATION 29:223-241 (1992) 655 Avenue of the Americas, New York, NY 10010 1044-5803/92/$5.00 224 G. F. Vander Voort on their macrostructural charac­ crostructure may also be altered cosmic teristic, visible to the unaided eye after shock and reheating events, as well as etching, caused the of kamacite terrestial corrosion. on the octahedral of the n1"'ln1"_.t-",~'n_ Ataxites contain higher Ni contents than ite (i.e., The octahedrites typically about 15-18%, but contain of Ni than the hex- do not exhibit gross macrostructural ahedrites, in the range of n-""r'TH~·h patterns. Instead, their macro­ although a have Ni contents. structure is basically featureless. Relatively The kamacite in octahedrites con- few ataxites have been found. The kama­ tains more Ni in hexahedrites, 5.5- cite are equiaxed and small in size, 7.5% versus 5.0-5.5%. There are six basic typically <30/-Lm in diameter. types of octahedrites; five are based on the width of the kamacite grains. As the Ni content increases, the kamacite width de­ MICROSTRUCTURAL FEATURES creases. Thus, octahedrites are classed in five steps from coarsest (lowest Ni) to fin­ In 1861, Reichenbach named the two major est (highest Ni) and then plessitic, where phases in iron meteorites, kamacite and the kamacite width is <0.2 rnm, and they taenite, and their mixture, plessite. Ka­ exist in discontinuous short fibers or spikes macite is the body-centered cubic ex phase with an octahedral orientation. The mi- known to metallurgists as ferrite, while (a) (b) FIG. 1. Examples of plessite morphologies: (a) comb plessite in Odessa (4% picral); (b) pearlitic plessite in Odessa (unetched, DIC); (c) comb and spheroidized plessite in Odessa (unetched, DIC); (d) comb, pearlitic and spheroidized plessite in Odessa (2% nital): (e) pearlitic plessite in Arispe (4% picral): (f) net (right) and cellular plessite in Gibeon (2% nital): (g) black plessite in Gibeon (4% picral): (h) acicular plessite in Canyon Diablo (4% picral); (i) altered plessite in Box (c) Hole (4% picral). Metallography of Fe Meteorites 225 taenite is the face-centered cubic phase alloys. Plessite 1) occurs in numerous known to metallurgists as austenite. Ples­ configurations two that resemble site is a mixture of kamacite and taenite ferrite-cementite mixtures in steels, that (ferrite and austenite) that occurs in me­ is, pearlitic and spheroidized ples­ teorites but not in manmade, iron-based site. Note that meteorites are named after (d) (e) (g) (h) FIG. 1. (Continued) 226 G. F. Vander Voort the place where they fell. Table 1 lists de­ grains as films or as triangular patches tails of the meteorites whose microstruc­ ("wedges") at kamacite junctions. The Ni tures are shown in this article. content within taenite is nonuniform [2, Neumann bands, that is, mechanical despite the extraordinarily slow twins, are observed in kamacite, be it in rate of meteorites (typically from 1 to 250°C hexahedrites, octahedrites, or ataxites (as per 106 years between 700 and 400°C long as the kamacite is more than about Determination of the prior-taenite 20/-Lm wide [1]). Because hexahedrites are size has rarely been done, mainly single crystals, Neumann bands can be one must have an exceptionally large quite long; up to several centimeters is not imen. Buchwald [1], in his study the unusual. The vast majority are formed by 20,140kg Agpalilik Cape York extraterrestrial collisions, not by their im­ found a prior-taenite grain size pact with the earth. The band width is usu­ x 1.5 x 1.5m (yes, meters!). ally narrow, typically 1-10/-Lm (Fig. The structure of the taenite generally quite complex and very Plessite is observed in octahedrites ing (Fig. Starting at the kamacite-facn­ where it becomes more common as the Ni ite interface and moving inward, ob­ content increases. The morphology of the serve that etching with nitaI reveals a series plessite mixture also changes with Ni con­ of zones. First, we see a very narrow zone, tent, and a variety of forms has been ob­ 1-2/-Lm wide, called clear taenite 1 served and classified (creating jargon to which contains 51.4-45.6 ± 1.3% Ni and rival anything developed by metallogra­ is ordered Fe-Ni [5]. Next is a phers!). Also in octahedrites, taenite can be brownish appearing zone, up to about observed between adjacent kamacite 25/-Lm wide, called the "cloudy zone" Table 1 Meteorites Examined in This Work Composition (Wt. %)[11 Meteorite" Fall location Type Ni Co p s c Arispe Sonora, Mexico Coarse 6.70 0.47 0.3 Octahedrite Box Hole Northern Territory, Medium 7.67 0.49 0.11 0.05 Australia Octahedrite Canyon Diablo Arizona, USA Coarse 7.10 0.46 0.26 ~1.0 ~1.0 Octahedrite Coahuila Mexico Hexahedrite 5.59 0.45 0.28 Gibeon Southwest Africa Fine 7.93 0.41 0.04 Octahedrite Henbury Northern Territory, Medium 7.51 0.45 0.09 Australia Octahedrite Mundrabilla Western Australia Medium 7.8 ~0.7 0.26 ~8.0 ~1.0 Octahedrite North Chile Tocopilla, Chile Hexahedrite 5.59 0.48 0.30 Odessa Texas, USA Coarse 7.35 0.48 ~0.25 0.5 ~O.2 Octahedrite Toluca Mexico Coarse 8.14 0.49 0.16 0.7 Octahedrite Washington Colorado, USA Ataxite 9.9 0.6 0.39 a Meteorites are named after the place where the fall occurred. Metallography of Fe Meteorites K ,N tQOfL m (a) - (b) (b) FIG. 3. Examples of taenite wedges and martensite (c) meteorites: (a) taenite wedge in Odessa showing thin CT-l zone at the a--y interface, the dark CZ, 2. Examples of Neumann bands (mechanical terior retained taenite (CT-2) and martensite (very twins) in meteorites: (a) Neumann bands (N), light because no tempering has occurred) (2% nital); subgrain boundaries (sb), piessite (P), taenite (T) and (b) taenite wedge in Canyon Diablo showing the thin kamacite (K) in Gibeon (2% nital); (b) Neumann bands CT-l, the dark CZ, retained taenite (CT-2) and dark Odessa (2% nital); (c) Neumann bands intersecting etching martensite in the interior (2% nital); and (c) prismatic and platerhabdites in North Chile (2% nital, acicular piessite in Arispe containing martensite (4% DIC). picral). 228 G. F. Vander Voort where spinodal uecomposmon rites. The most common is called coh­ very fine, enite which is orthorhombic and ordered Fe-Ni identical to cementite a surrounding cohenite contains Ni Ni) of martensite and cobalt accord- serve a zone of about to Scott (quoted by Buchwald [1]), and the same width called the dear taenite 2 has the formula (Fe,Ni,CohC. It appears (CT-2) zone 25.8-28.1% white when being quite which may be ordered [5]. The struc- hard 1100 it will stand in relief. tural details in these three zones can only an alloy carbide with a cubic be observed by transmission electron mi- structure and the formula croscopy. In the interior where the (Fe,Ni,Coh3C6 , has been observed in only nickel content is martensite plate- a few meteorites. It is smaller in size than lets are observed within a retained taenite slightly less hard, and also white matrix.