3. EDS support during phase assignment

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Clarence Sullivan
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1 Microstructure investigations of iron meteorites by EBSD and EDS analysis G. Nolze, R. Saliwan Neumann, M. Buchheim, E. Payton Federal Institute for Materials Research and Testing (BAM), Berlin, Germany 1. Introduction 2. For EBSD only 3. EDS support during phase assignment 4. When EBSD & EDS fails

$2 Electron backscatter diffraction (EBSD) Possibilities and limitations Benefits: Fast crystal orientation determination (Identification) of phases Enormous post processing capabilities:$

2 2 Electron backscatter diffraction (EBSD) Possibilities and limitations Benefits: Fast crystal orientation determination (Identification) of phases Enormous post processing capabilities: misorientations, boundary characterization, grain size and shape, and much more Challenges: High sample tilt (~70 deg), influences lateral resolution and requires flat surface Careful sample prep because of information depth of a few 10nm Electrically conductive samples required Quite insensitive to lattice parameters, e.g. cubic phases with the same structure like Cu, Ni, Ag, Al.cannot be differentiated

3 3 Energy dispersive spectroscopy (EDS) Possibilities and limitations Benefits: Fast local determination of the chemical composition or distribution of elements Nowadays easy to use i.e. artifacts like escape peaks or sum peaks are no longer a serious problem Challenges: Spatial resolution at high magnification is clearly worse than the image resolution Signal deconvolution for specific element combinations is difficult, especially for fast measurements Phase separation is based on chemistry only (cf. polytypes, light elements, similar chemistry) Collimating technique limits scan field size

4 4 EBSD & EDS coupling Major challenges: But: Comparison of the incomparable! EDS information volume is clearly bigger than for EBSD EDS information appears shifted in y-direction caused by sample tilt EDS shift is different for each phase because of absorption EDS acquisition is slower despite SDD technology A bird in the hand is worth two in the bush.

Size of the rings and spot distribution are characteristic of the orientation relationship, e.g. Kurdjumov-Sachs (KS) or Nishiyama-Wassermann (NW).

5 5 experiment simulation Widmannstaetten structure Gibeon meteorite (white plessite) The centers of the rings in the bcc pole figures define the {001} of the high-temperature fcc phase. Size of the rings and spot distribution are characteristic of the orientation relationship, e.g. Kurdjumov-Sachs (KS) or Nishiyama-Wassermann (NW). Nolze, Z.Metallk., (2004), 95, j 1 = 4.8 ; y =46.1 ; j 2 = 4.8

$However, if phases deliver similar diffraction patterns additional information is required.$ IPF-X map Dermbach is an iron meteorite consisting mainly of taenite, but also has numerous inclusions of troilite and rhabdite, and small kamacite

IPF-X map Dermbach is an iron meteorite consisting mainly of taenite, but also has numerous inclusions of troilite and rhabdite, and small kamacite

6 Phase recognition by EBSD Dermbach (ataxite, 42% Ni) Phase recognition using EBSD is for many phases quite successful. However, if phases deliver similar diffraction patterns additional information is required. IPF-X map Dermbach is an iron meteorite consisting mainly of taenite, but also has numerous inclusions of troilite and rhabdite, and small kamacite particles. Standard EBSD interpretations are based on the analysis of angles between crystal lattice planes. For cubic phases these angles are phaseindependent, i.e. mainly cubic phases complicate a correct phase recognition. IPF-X map 7 Dermbach_04

7 Phase verification by EDS Dermbach (ataxite, 42% Ni) P S O Cl Weathering within big phosphides is noticeable. Along the interface between metallic matrix and phosphide, EDS reflects an increase of Fe and O and a decrease of Ni. Additionally, a taenite small amount of Cl is detectable. schreibersite 8 troilite BSE image Dermbach_07

Phase recognition using EBSD & EDS Dermbach (ataxite, 42% Ni)

oxygen-bearing phase along the interface between taenite and

taenite, the EDS quantification of oxygen is used.

schreibersite ((Fe,Ni) 3 P, green) the EDS quantification is

8 Phase recognition using EBSD & EDS Dermbach (ataxite, 42% Ni) Pattern quality Phase distribution IPF-X IPF-Y + PQ The oxygen-bearing phase along the interface between taenite and phosphide has been identified as magnetite (Fe 3 O 4 ), and also appears within troilite. IPF-X map for magnetite For an absolute separation from taenite, the EDS quantification of oxygen is used. However, for all other phases like troilite (FeS, yellow) and schreibersite ((Fe,Ni) 3 P, green) the EDS quantification is actually not necessary. Unfortunately, EDS causes a few misinterpretations for taenite along the interface, cf. the arrow. 9 Dermbach_07

Advanced phase assignment by BSE Odessa (octahedrite)

Ni-spinel Phase map Phase map BSE The phase

The phase recognition can be improved by the use of

A histogram segmentation extracts 5 different phases.

9 Advanced phase assignment by BSE Odessa (octahedrite) EBSD & EDS kamacite rhabdite taenite magnetite Ni-spinel Phase map Phase map BSE The phase assignment by EBSD+EDS is erroneous, e.g. along interphases at higher resolution or low signal. The phase recognition can be improved by the use of BSE images. A histogram segmentation extracts 5 different phases. BSE upper: periodic number contrast (BSE), bright = high Z, dark = low Z lower: orientation contrast (FSE) Crystals of different orientations show different colors 08/01/

10 What causes corrosion of meteorites? Dronino (ataxite, 17% Ni) BSE image EDS map phase map EBSD+EDS 40µm Troilite nodules are surrounded by a Cl-bearing phase which appears dark in the BSE image (low Z). BUCHWALD & KOCH identified this phase by light microscopy in 1995 as hibbingite: b-fe 2 (OH) 3 Cl. Meteoritics 30 (1995)5, 493 kamacite troilite taenite magnetite hibbingite experiment simulation 08/01/

11 Take home messages EBSD is a technique for interpreting crystal orientations assuming the correct phase has been selected. If EBSD fails, a simultaneous EDS acquisition and quantification can help at lower magnifications when the signal difference is sufficient. Data can be re-analyzed in case of missing phases if spectra and diffraction information preferably the patterns have been saved. A phase identification needs some patterns which should be compared with dynamically simulated patterns in order to compare the intensity distribution. For higher magnifications, the information volume of EDS no longer fits to the EBSD signal and can cause misinterpretations. BSE images have a much better spatial resolution and can be used as an additional constraint for phase assignment. This can be applied even on amorphous phases. 08/01/

12 Abbildung im Rasterelektronen-Mikroskop: cohenite Orientierungskontrast Canyon Diablo kamacite

13 kamacite Campo del Cielo schreibersite

14 Cape York

15 Dronino (Hibbingite) 16

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

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?