Determining the fcc / bcc orientation relationship in plessite regions of iron meteorites

Transcription

1 8 Determining the fcc / bcc orientation relationship in plessite regions of iron meteorites Question: Which model best characterises the fcc-bcc orientation relationship in plessite regions from meteorites? Introduction The phase transformation between face-centred cubic (fcc or γ) and body-centred cubic (bcc or α) structures has been widely studied due to its importance in the processing of steel. However, understanding the transformation is of interest in other areas, such as determining the history of meteorites. One of the keys to understanding the transformation is accurate characterisation of the orientation relationship (OR) between the two phases. This can be achieved through orientation mapping on sections of partiallytransformed material and comparing the results with models. Various models that have been developed to describe the OR between the two phases are shown in Table 1. While the majority of papers today distinguish only between Kurdjumov-Sachs [1] and Nishiyama-Wassermann [2, 3], experimental data often show a deviation from the exact orientation relationship that is bigger than the accuracy of the applied technique [4]. This application note presents a characterisation of the OR between bcc and fcc phases in the plessite region of an iron meteorite and the assessment of its fit with the commonly accepted models in Table 1. The assessment is done by comparing contoured pole figures of orientation data with similar pole figures generated using a simulation process described in [7]. The simulated pole figures for the Nishiyama-Wassermann, Greninger- Troiano and Kurdjumov-Sachs relationships are shown in Figure 1. Table 1: Commonly used γ/α orientation relationship models using crystallographic planes and directions: Bain, K S (Kurdjumov- Sachs), N W (Nishiyama-Wassermann), G T (Greninger- Troiano [5]), Pitsch [6]. The last column shows what is closelypacked in the given model: the planes used (p), the given directions (d), both, or nothing. 40

2 N-W G-T K-S Figure 1: Simulation of pole figures for selected models: N W, K S, and G T as the intermediate model. The intensity of the N W-pole figures has been divided by 2 in order to see the multiple occupations (dark and light spots), i.e. the actual intensity of the N W is twice as high as shown. Deviations from the ideal relationships are best quantified by the use of Euler angles. The angles for the models from Table 1 are shown in Table 2. Table 2: Characterization of the orientation relationship models using Euler angles (Bunge convention): φ1,ψ,φ2. The last column gives the number of symmetry-independent variants. 41

3 EBSD Analysis EBSD was used to collect orientation maps on plessite regions of seven different meteorites. Results from the Gibeon iron meteorite are presented here. Table 1 contains details of the experimental conditions. Table 1: Details of EBSD analysis Sample Preparation: Mechanically polished with final polish using colloidal silica SEM Type: FEG-SEM EBSD System: HKL CHANNEL5 with with Nordif CI detector Accelerating voltage: 20 kv Probe Current: Unknown Results Plessite from the Gibeon iron meteorite Figure 2 shows the orientation map for a µm plessite region from the Gibeon meteorite. The phases present are taenite (fcc) and kamacite (bcc). Both phases are coloured using the inverse pole figure scheme shown in the legend. Figure 2: Orientation map of a plessite region in the Gibeon meteorite using inverse pole figure colouring in the Y0 direction for both phases, taenite (fcc) and kamacite (bcc). 42

4 The left-hand column of Figure 3 shows the raw orientation data (more than points) from the orientation map in Figure 2. The characteristic distribution of poles described by the OR-models becomes visible. However, the scattering around the ideal positions is so unfavorable that the most closely fitting model can not be discerned using the raw data. Closed broad rings but no single spots appear in the {100} pole figure. The accuracy of the orientation measurements is clearly better than the observed scattering. The scattering in the data may imply that a range of different ORs occur which are not described exactly by the models and that the whole range of orientations between the models can be detected in the same meteorite. Alternatively, the scattering may result from deformation in the parent phase or in the transformed phase due to collision events during the history of the meteorite. Since the origin of the scattering is unknown it seems to be not useful to interpret single scattered data, particularly because the reference orientation (the local fcccrystal orientation) is not known exactly. raw data convoluted data simulation Figure 3: Comparison of standard pole figures containing the raw orientation data of fig. 3, the convoluted data considering the experimentally detected frequency distribution, and the simulation of ideal pole figures reflecting all variants with the ideal frequency distribution. For the simulation, the Euler angles { φ1,ψ,φ2}={4.8, 46.1, 4.8 } have been applied. 43

5 The main OR present in the material can be determined by considering the frequency distribution of the orientation data. To this end Channel 5 offers a convolution procedure (contouring plot). By applying the smallest half width (2 ) the raw data distribution can be transformed into an intensity distribution showing the main orientations of the bcc phase (middle column of Figure 3). Well defined spots appear and prove that the frequency of far distant raw data is low and therefore not important for the main OR. In Figure 3, the spots in the convoluted data within the {100} pole figure show a different intensity and therefore one can conclude that each variant was not detected with the same statistical significance. Nevertheless, all variants are visible and for the determination of the OR the intensity distribution is insignificant. Comparing the spot distribution in the {100} pole figure with that of the models given in Figure 1, it is immediately clear that neither G T nor N W describe the detected OR. However, also the K S model does not fit the experimental OR although the Bain circles in the {100} pole figure show a principle agreement. All other poles in the {100} pole figure show a good visible deviation from K S. The most important characteristics for determining the Euler angles to describe the OR are visible in the {111} pole figure. Here, the outer pole positions of each of the 6 specific pole arrangements come close together whereas for K S as well as for G T they drift apart. With detailed analysis, the OR was found to be best represented by the Euler angles {φ1,ψ,φ2}={4.8, 46.1, 4.8 }, used for the simulation shown in the right-hand column of Figure 3. The OR can be described as K-S with a slight tendency to Bain. The relationship of the observed OR to the ideal ORs for the models in Table 1 is illustrated in Figure 4. 44

6 a b Figure 4: Bain circle (a) as a small part of a pole figure and Euler subspace (b). The region of the chosen Euler subspace in the Bain circle is shown as parallelogram. Applying the point-group symmetry of the crystal all variants can be generated using the subspace in b). The main orientation of the plessite from the Gibeon iron meteorite is given as a dark-blue circle and shows distinct deviations from K S. In fig. a) it would hit nearly the exact center of the parallelogram. The fcc-bcc ORs in plessite regions of six other meteorites (Agpalilik, Chinga, North Chile, Taza, Toluca, Watson) were investigated in a similar manner. None of the main ORs fitted a single model accurately. There were always small deviations. Even in a single meteorite (Agpalilik) different main ORs were detected on different plessitic regions. Conclusion The main orientation relationship in plessite regions from meteorites always shows a small deviation from the commonly accepted models. The most closely-fitting model and the nature of the deviation varies from meteorite to meteorite and even between different plessite regions in a given meteorite. 45 References 1. Kurdjumov, G. and Sachs, G, Zeitschrift für Physik, 1930, 64, Nishiyama, Z, Sci. Repts. Tohoku Imp. Univ. Tokio, 1934, 23, Wassermann, G, Mitt. K.-Wilh.-Inst. Eisenforsch, 1935, 17, Shek, C. H., Dong, C., Lai, J. K. L. and Wong, K. W., Metall. Mat. Trans., 2000, 31A(1), Greninger, A. B. and Troiano, A. R., Metals Transactions, 1949, 185, Pitsch, W., Archiv für das Eisenhüttenwesen, 1967, 38, Nolze, G., Z. Metallkd., 2004, 95, 744. Acknowledgement This application note was written in collaboration with Dr. Gert Nolze (Federal Institute for Materials Research and Testing, Berlin, Germany) and is based on work published in Nolze, Proceedings of the CHANNEL Users Meeting 2004, Ribe, Denmark (2004), pp The meteorite samples were provided by V. Geist (Leipzig University). The help of R. Saliwan Neumann and M. Buchheim during sample preparation and the EBSD measurements is gratefully acknowledged. Thanks to A. Epishin W. Swiatnicki, A. Głowacka and C.T. Chou for their helpful contributions.