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Characterization of the fcc/bcc orientation relationship by EBSD using pole figures and variants
(2004)
The orientation relationship (OR) between fcc and bcc lattices are described by crystallographic fundamentals using the example of Kurdjumov-Sachs (K-S) and Nishiyama-Wassermann (N-W). Complete pole figures containing all variants will be used to distinguish even between slightly different ORs. EBSD on iron meteorites and duplex steel has been used to analyse a large number of crystal orientations with regard to a high statistical significance and a high probability to capture all variants in a single measurement. It is shown that the use of fixed OR models like K-S, N-W, Bain, Pitsch, or Greninger-Troiano does not satisfacturally reflect the observed experimental pole distributions. It is not convenient to use high-indexed lattice planes and directions to describe the small deviations from the given models. The Euler subspace representation offers a readily comprehensible tool to get an idea about the characteristic of the experimentally detected OR.
Irrational orientation relationship derived from rational orientation relationships using EBSD data
(2008)
The determination of the orientation relationship (OR) between α-Fe matrix (bcc) and γ-Fe precipitates (fcc) is discussed using orientation data collected by electron backscatter diffraction (EBSD). The comparatively low accuracy of EBSD is compensated by the high number of measurements what allows a general statement regarding to the mean OR existing in a sample. The representation and discussion is realized on a part of the Bain zone in a {001} pole figure. A discussion of some selected rational OR which are commonly used for the phase boundary characterization between α and γ shows that the pole figures describing a transformation from γ → α are different to those for α → γ. A technique is proposed based on at least three misorientation angles between the experimental OR to the rational OR's as reference. For the misorientation angle distribution a refinement is applied to extract the mean values. They are used to detect the mean OR, what is also possible for only a few or even a single precipitate if the number of measurements describing the phase boundary is sufficiently high.
The orientation relationship (OR) between the bcc and fcc phase in the plessite microstructure of the iron meteorites Watson, Agpalilik and Gibeon has been analysed in a scanning electron microscope using electron back-scattered diffraction (EBSD). A very strong OR exists, independently on the analysed plessite type and the observed spreading of single orientation data. The agreement between the experimental orientation distribution and existing models varies for each meteorite. The black plessite in the Agpalilik corresponds to the Nishiyama-Wassermann model whereas the Duplex plessite of the Gibeon meteorite shows an OR close to the Kurdjumov-Sachs model. The Watson meteorite is strongly deformed so that a general OR is difficult to determine due to the blurred experimental orientation distribution.
Roaldite – Fe4N – has been identified in the São Julião de Moreira iron meteorite using electron backscatter diffraction (EBSD) and simultaneously acquired energy-dispersive x-ray spectroscopy (EDS). Mean-periodic-number images derived from raw EBSD patterns confirm this phase by an even higher spatial resolution compared to EDS.
Roaldite appears in the form of systematically and repetitively aligned plates. Despite the locally heavy plastic deformation, it is shown that the origin of the oriented precipitation of roaldite is linked to the orientation of the kamacite matrix. Roaldite can be considered to be precipitated from kamacite using an inverse Kurdjumov-Sachs (K-S) or Nishiyama-Wassermann (N-W) orientation relationship. A more accurate discrimination is impossible due to the accumulated shock deformation, which blurs the local reference orientation of kamacite. The habit plane of roaldite is found to be {112}R, which is most likely parallel to {120}K of kamacite. Some of the roaldite plates contain two orientation variants which repeatedly alternate. Their misorientation angle is about 12°.
The macro- and microstructure of iron meteorites provide valuable insights into both the inner structure of our planet and the history of our solar system. High speed collision events in the asteroid belt send the meteorites careening toward Earth. The collisions produce unique deformation microstructures. With cooling rates on the scale of a few degrees per million years, iron meteorites can consist of crystal sizes on the order of meters prior to the collision events. These extremely slow cooling rates result in phase transformations occurring at conditions near thermodynamic equilibrium. Preserving meteorite fragments is important for future studies of phase transformations, material behavior at high strain rates, and the origin of the universe.