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Crystallographic Relations Between Face- and Body-Centred Cubic Crystals Formed Under Near-Equilibrium Conditions: Observations from the Gibeon Meteorite

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Crystallographic Relations Between Face- and Body-Centred Cubic Crystals Formed Under Near-Equilibrium Conditions: Observations from the Gibeon Meteorite Acta Materialia 54 (2006) 1323–1334 www.actamat-journals.com Crystallographic relations between face- and body-centred cubic crystals formed under near-equilibrium conditions: Observations from the Gibeon meteorite Youliang He a,*, Ste´phane Godet b, Pascal J. Jacques b, John J. Jonas a a Department of Materials Engineering, McGill University, 3610 University Street, Montreal, Que., Canada H3A 2B2 b De´partement des Sciences des Mate´riaux et des Proce´de´s, IMAP, Universite´ catholique de Louvain, Place Sainte Barbe 2, B-1348 Louvain-la-Neuve, Belgium Received 22 June 2005; received in revised form 21 October 2005; accepted 2 November 2005 Available online 6 January 2006 Abstract The orientations of the kamacite lamellae formed from a single prior-taenite grain were measured by analysing the electron backscat- ter diffraction patterns obtained using scanning electron microscopy. These are shown to be close to the Kurdjumov–Sachs and Nishiy- ama–Wassermann relations and their intermediate, i.e., the Greninger–Troiano relation. The orientations of the a grains in the plessite regions were also measured and these were found to be continuously distributed around the Bain circles formed by the variants of the common correspondence relationships, including the Pitsch one in this case. The local misorientations between individual face- and body-centred cubic crystals along their common interfaces were measured. These can be characterized by the orientation relationships mentioned above as long as a certain amount of tolerance is allowed. Orientation variations within individual kamacite lamellae were also analysed. The crystallographic data support the view that somewhat different mechanisms are involved in the formation of Widmansta¨tten structures and of the plessite in meteorites. Ó 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Phase transformation; Misorientation; Meteorites; EBSD 1. Introduction of the first iron–nickel phase diagram was based on infor- mation from iron meteorites [8–11]. The cooling rates Meteorites have long been of interest to mineralogists deduced from the analysis of Widmansta¨tten structures and geologists because of their importance in increasing have now become useful tools for determining the thermal our understanding of the origin and history of asteroidal history of asteroidal bodies [8,12–14]. bodies. Many investigations, e.g., Refs. [1–6], have been Although there is still some dispute regarding the mech- carried out to characterize the macro- and microstructures anism of formation of the Widmansta¨tten structures, e.g., of iron meteorites and to comprehend the formation of the Refs. [3,8,15,16], these are now generally believed to have Widmansta¨tten patterns found within them. Due to certain formed as the iron meteorites cooled from an initially similarities between metallic meteorites and engineering homogeneous c-iron phase into the a + c phase field at alloys, meteorites are also of interest to metallurgists. His- cooling rates of one to a few hundred degrees per million torically, even the invention of metallography was moti- years [12,13,16–19]. By contrast, for the formation of ples- vated by the study of meteorites [7] and the construction site, a martensitic transformation and decomposition pro- cess is generally assumed to have occurred [15,16,20,21]. In order to throw light on these mechanisms, the orienta- * Corresponding author. Tel.: +1 514 398 4755; fax: +1 514 389 4492. tion relationships between the a and c phases produced E-mail address: [email protected] (Y. He). during cooling can be examined. Thus, considerable work 1359-6454/$30.00 Ó 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2005.11.008 1324 Y. He et al. / Acta Materialia 54 (2006) 1323–1334 has been focused on the crystallographic features of the TSL and HKL Channel 5 software in field emission gun Widmansta¨tten structures and the plessite found in scanning electron microscopes were used as the main tools meteorites. to measure the orientations of the fcc and bcc phases. A An early study by Young [22] on the Carlton meteorite small specimen (25 mm · 15 mm) was sectioned from the carried out using X-ray diffraction resulted in the proposal available piece and a smooth surface was produced by of an orientation relationship that is actually identical to grinding and polishing. Nital (2%) was employed for the later well-known Kurdjumov–Sachs (K–S) relation. revealing the Widmansta¨tten pattern. For microstructural In their study of the Bethany meteorite using transmission characterization, an aqueous solution containing 10% electron microscopy, Hasan and Axon [23] showed that the sodium thiosulphate (Na2S2O3) and 3% potassium metab- orientation relationship of the primary kamacite with isulphite (K2S2O5) was also utilized to decorate various respect to the taenite is close to the Nishiyama–Wasser- phases in different colours. For the EBSD measurements, mann (N–W) relation, whereas for the plessite kamacite after the usual grinding and polishing procedure, the spec- it is close to the K–S relation. imen was finally polished using a 0.05 lm colloidal silica More recently, neutron diffraction methods have been suspension in a vibratory finishing machine or by hand employed to measure the pole figures of both taenite and on a conventional polishing cloth; the duration of such kamacite crystals in the Gibeon meteorite [24] and the ori- final polishing was approximately one hour. entations between the kamacite lamellae and the taenite The major phases of the Gibeon meteorite are kamacite were observed to be close to the N–W relationship. Bunge (ferritic iron with up to 7.5% nickel), taenite (fcc austenite et al. [25] measured the orientation distributions of the with more than 25% nickel) and plessite, a mixture of both Widmansta¨tten plates in a sample of the Gibeon meteorite phases. Depending on the nickel content, the lattice param- using high-energy synchrotron radiation. Their measure- eters of the taenite and kamacite vary slightly. The ments revealed a continuous range of orientations stretch- measured lattice parameters for taenite and pure fcc ing out from both sides of the N–W orientation to the two iron–nickel alloys are in the range of about 0.351– adjacent K–S orientations. Electron backscatter diffraction 0.360 nm and those for kamacite and bcc iron–nickel fall (EBSD) measurements on the Gibeon and other meteorites between 0.286 and 0.289 nm [22,29,30]. [26–28] have also produced similar results. EBSD maps were collected using typical settings for However, none of these studies has provided informa- steels, e.g., 20 kV, 70° tilt. The measured orientations were tion about the orientation distribution of individual kama- represented in the format of Euler angles with respect to cite lamellae within specimens and about how these the sample reference frame. For metal processing, this is orientations vary within a particular a grain. These aspects usually directly related to the reference directions of mate- are the subjects of the present study. Moreover, local orien- rials processing. However, in the case of meteorites, there tation relations along the c/a boundaries were also mea- are no meaningful reference directions. The orientations sured and compared with the common orientation of both the fcc and bcc phases were measured simulta- relationships. These are intended to assist in interpreting neously in each EBSD scan. Other phases (e.g., martens- the mechanisms proposed to explain the formation of Wid- ite), inclusions, small particles and grain boundaries mansta¨tten patterns and plessite structures in meteorites. could not be identified and remained unresolved on the Here, the orientations of both the body-centred cubic EBSD maps. The angular accuracy of the EBSD system (bcc) and face-centred cubic (fcc) phases were measured is about 1° using the operational parameters for meteorites simultaneously by means of EBSD techniques and the ori- and steels. The orientations measured within a ferrite grain entation relationships observed are represented in pole fig- transformed from deformed austenite in a transformation- ure form as well as in Rodrigues–Frank (R–F) space. induced plasticity steel indicate a spread of less than 1° [31]. 2. Method 3. Results The material investigated was a piece of an iron meteor- 3.1. The Widmansta¨tten pattern and microstructures ite collected from the Gibeon shower. It consists principally of iron and nickel, together with some trace elements. The An example of the Widmansta¨tten pattern revealed by main chemical composition is (wt.%) Ni 7.93, Co 0.41, P optical microscopy is illustrated in Fig. 1(a). It is evident 0.04 and balance Fe [3]. The Gibeon is classified as falling that the kamacite lamellae cross each other at various into the fine octahedrite category (Group IVA). The piece angles and enclose areas of grey or dark plessite of different examined here was a slice weighing about 103 g and mea- sizes between them. The detailed microstructures can be suring approximately 60 mm · 60 mm · 5 mm; it was cut seen more clearly in Fig. 1(b) and (c); here tint etching from a larger sample. was carried out using the solution mentioned above. The Ordinary metallographic methods, i.e., optical micros- three major phases, namely kamacite, taenite and plessite, copy, were employed to reveal first the Widmansta¨tten pat- can be readily distinguished. The kamacite lamellae, which tern and then the major phases and microconstituents of are about 0.3 ± 0.05 mm in thickness, are in colour and are the meteorite. Automated EBSD systems equipped with sheathed by thin white films of taenite. Within the kamacite Y. He et al. / Acta Materialia 54 (2006) 1323–1334 1325 lamellae, numerous Neumann bands (mechanical twins) can be seen. These are believed to have formed as a result of extraterrestrial collisions, not by impact with the earth [3,6].
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