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RMS Summer School 2010, Leeds SEM ELECTRON BACK- SCATTERED DIFFRACTION Geoffrey E. Lloyd School of Earth & Environment, Leeds University Acknowledgements:

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Presentation on theme: "RMS Summer School 2010, Leeds SEM ELECTRON BACK- SCATTERED DIFFRACTION Geoffrey E. Lloyd School of Earth & Environment, Leeds University Acknowledgements:"— Presentation transcript:

1 RMS Summer School 2010, Leeds SEM ELECTRON BACK- SCATTERED DIFFRACTION Geoffrey E. Lloyd School of Earth & Environment, Leeds University Acknowledgements: Niels-Henrik Schmidt, Austin Day, Pat Trimby (formerly HKL Software) Obducat CamScan (Dick Paden) Dave Prior (University of Liverpool)

2 Plan Basic principals EBSD pattern recognition EBSD problems Orientation contrast imaging Automated EBSD analysis Specimen requirements EBSD applications A few examples Concluding remarks

3 Basic Principals

4 Electron:Sample Interaction Single nucleus incident electrons ‘lost’ electrons emitted electrons ‘lost’ electrons emitted electrons Multiple scattering ‘focussed’ ‘unfocussed’

5 Effects of Scattering 1. creates a ‘point’ source of electrons, with all possible trajectories, within the material 2. crystalline materials: electrons are diffracted by the crystal lattice planes when the Bragg condition is satisfied note different lattice spacing

6 Scattering from single lattice planes (after A. Day) For SEM electron wave- lengths, opening cone angles are close to 180  Each lattice plane (hkl) gives rise to 2 diffraction ‘cones’

7 Scattering from 3D lattice planes A 20keV electron beam strikes a sample tilted at 65-75° The crystal structure at the point of incidence diffracts the electron beam according to Bragg’s law, n = 2dsin  Each lattice plane (hkl) gives rise to 2 diffraction ‘cones’ (after A. Day) (after J. Hjeiling) The image (EBSD pattern) is unique for the crystal orientation & composition at the point of incidence on the sample

8 Typical SEM EBSD set-up EBSD detector incident electron beam: 8-40kV, nA Specimen: Surface normal typically inclined 60°-80° to beam EBSD detector - position usually constrained by chamber geometry EBSD detector distance set to give ~90 ° angular range in EBSP emitted electrons Camera low-light sensitive (now digital; originally analogue) Pattern processing

9 EBSD Pattern Recognition

10 EBSD pattern recognition EBSD patterns are unique for a specific crystal orientation The pattern is controlled by the crystal structure: space group symmetry, lattice parameters, precise composition Within each pattern, specific ‘bands’ (i.e. pairs of ‘cones of diffraction’) represent the spacing of specific lattice planes (i.e. d hkl ) EBSD pattern recognition compares the pattern of bands with an ‘atlas’ of all possible patterns in order to index the crystal orientation depicted This process WAS manual – it is NOW automated ! Example - next slide

11 pyrite FeS 2 EBSD Patterns Unique for crystal orientation & composition at the point of beam incidence Can be >100° of total crystal projection - easy to index as symmetry decreases Spatial resolution (  1  m) Some pattern details: diffraction from specific lattice plane width = 1/d-spacing 1st order diffraction 2nd order diffraction major crystal ‘pole’ HOLZ ring

12 Example: pattern indexing Original pattern manual/auto-indexed bands Computer indexed pattern

13 EBSD pattern ‘range’ Determined by crystal symmetry – defines the crystallographic unit triangle that repeats the range of patterns over a sphere Example: fcc (Cu) unit triangle rotate Example: trigonal (quartz) 011 symmetrically equivalent unit triangle

14 Indexing requirements SEM geometry: beam energy, specimen & detector positions & orientations usually fixed per SEM Crystallography: sample lattice parameters & Laue/space group input per phase (i.e. composition) as required Diffraction characteristics: relative diffraction intensities from different (hkl) lattice planes calculated per phase as required

15 Creating a crystal database 1. Select a crystale.g.aluminium 2. Input lattice parameters e.g.a = b = c = 0.405nm  =  =  = 90  3. Input crystal symmetry e.g.cubic Laue group= m3m Spacegroup= 225 orFm-3m 4. Input crystal unit cell symmetry indicates 4 atomic positions e.g.AtomxyzOcc Al0001 Al Al Al

16 Create a diffraction database To index EBSD patterns, we must know the relative intensities of the (Kikuchi) bands (reflectors) in the patterns Most approaches use the kinematic electron diffraction model This model calculates the structure factor (intensity) for each (hkl) reflecting plane: intensity of (hkl) plane structure factor for (hkl) plane number of atoms atomic scattering factor lattice planes atomic position of atom g

17 Diffraction database Conventional software packages automatically calculate the diffraction (reflector) database of (relative) intensities e.g. aluminium:

18 EBSD Problems Pseudo-symmetry Spatial resolution Angular resolution Specimen preparation - see later

19 Pseudo-symmetry Occurs where 2 orientations cannot easily be distinguished due to an apparent n-fold rotation axis Especially common in lower symmetry crystal structures: e.g. the orthorhombic structure with a  b can appear to be tetragonal when viewed down the c-axis Specific examples (minerals): quartztrigonal but can appear pseudo-hexagonal olivineorthorhombic but can appear pseudo-hexagonal plagioclasetriclinic but can appear pseudo-monoclinic &/or pseudo- hexagonal

20 Example Quartz: 60° rotation about the c-axis Often results in similar EBSD patterns Recognition depends on the identification of ‘minor’ bands (black) - often not selected automatically negative rhomb positive rhomb distinguishes

21 Spatial resolution Depends on the penetration & deviation of electrons into a sample (plus beam diameter) Typically ranges from ‘few’  m for W-filament SEM to a few 100nm for FEG SEM Penetration depends on: sample atomic number accelerating voltage beam current (plus, coating depth & surface damage - see later) Several Monte Carlo based simulation packages are available via the Web (e.g. Example: R R P P note ‘down slope’ effect of tilting – scan ‘uphill’

22 Angular Resolution Angular resolution of an individual EBSD pattern is typically ~1  Also important when determining the misorientation between two (adjacent) crystal lattices (e.g. grains) – ‘misorientation analysis’ is becoming a popular application of EBSD as it provides information on sample properties & behaviour But, calculations of misorientation axes from 2 individual measurements with misorientation of <15° contain increasingly large errors Angular resolution depends typically on basic EBSD set-up configuration, EBSD pattern quality & hence indexing software (parameters, composition, pseudo-symmetry, etc.) Examples (next slides)

23 Angular resolution 1: sample-detector considerations Small detector distance Detector Large detector distance good for indexing but poor angular resolution poor for indexing but good angular resolution important for constraining misorientation axes.

24 Angular resolution 2: effect of angle imaged Changes in high resolution EBSD patterns can be used to define better rotation angles & more accurate misorientations large angular spread: low angular resolution low angular spread: good angular resolution

25 Orientation Contrast Imaging

26 polycrystalline sample Control of crystal orientation on emission signal note variation in image grey-scale level - depends on penetration & emission, which depend on crystal orientation

27 EBSD microstructural images Electron beam is scanned over an area of a tilted sample, rather than positioning the beam on a point for EBSD patterns quartzite FSE Orientation Contrast image of variation in crystal orientation - contrast variations only qualitative (next slide) FSE signal detected by silicon devices attached to EBSD detector Forescattered electrons (FSE) with intensities determined by penetration (i.e. crystal orientation) are emitted towards the EBSD detector

28 Warnings 2 grains with the same orientation can have different OC signals 2 grains with different orientations can have the same OC signal due to different ‘incident angles’ due to effectively the same ‘incident angles’ beam

29 OC images not quantitative ! Grey-levels cannot be inverted to give orientation Grains in the same orientation but different positions have different grey shades Grains in different orientations could have the same grey shade One image may show only 60% of boundaries – so, move the image slightly (after D. J. Prior)

30 Automated EBSD Analysis

31 Automated EBSD analysis Computer controlled movement of the electron beam across a sample EBSD pattern ‘captured’ at each point Indexing of EBSD patterns is via pattern recognition software Software writes the crystal orientation (3 Euler angles), & phase information per pattern to a data-base for later analysis BUT – important to run a manual visual check of solutions before the automated analysis!

32 Automated EBSD analyses orientation contrast crystal orientation variationpattern quality - ‘strain’ provides a variety of information crystal orientation ‘pole’ figures many other parameters: e.g. ‘misorientation’ (becoming very important in microstructural analysis) P T

33 Specimen Requirements

34 Specimen Preparation Polished blocks, thin-sections, natural fractured or grown surfaces Surface damage (  m-mm) created by mechanical polishing must be removed: chemical-mechanical (‘syton’) polish etching electro-polishing ion beam milling Insulating samples may require very thin carbon coat, but uncoated samples may perform OK - next

35 Charging Problems Reduce charging by coating but only at expense of image detail &/or resolution Note: specimen damage can occur in absence of charging K-feldspar. 20keV ~15nA (after D.J. Prior) Uncoated 3-5nm C coat

36 Effect of coating on OC images 200  m uncoated~4nm C~8nm C (after D.J. Prior)

37 Interim Summary Orientation contrast images: variations in crystallographic orientation & sample microstructure EBSD patterns: full crystallographic orientation of any point in OC image Spatial resolution: ~100nm (FEG, metals) to ~1  m (W, rocks) Angular resolution: ~1 - 2° (misorientation >5 °) Materials: most metals & ceramics; many minerals - depends on composition Automated analysis: 100’s of EBSD patterns/second (record ~800/sec via stage scanning) but indexing accuracy may suffer (use of fast or sensitive EBSD detectors increasing depending on requirements)

38 EBSD Applications

39 What can EBSD be used for? Measuring absolute (mis)orientation of known materials - most popular/obvious usage Phase identification of known polymorphs - becoming popular Calculating lattice parameters of unknown materials - difficult, only possible for relatively simple structures? Measuring elastic strain Estimating plastic strain on the scale of the electron beam activation volume Estimate aggregate elastic stiffness matrix (C ij ) from grain crystal texture – used to predict material properties (e.g. thermal, electric, acoustic, magnetic, etc.)

40 Recommended applications Tremendous significance for many types of materials research, including: - deformation & recrystallisation - understanding processing histories - effects of pre-heating & heat treatments - identifying phases in multi-component systems - microstructural characterisation & calibration (including boundary geometry, etc.) - modelling microstructural processes - constraining micro-chemical data - estimating physical properties (e.g. elastic, thermal, sonic, electric, magnetic) - etc.

41 Example applications

42 Crystal orientation data from SEM/EBSD Individual orientation measurements related to microstructure: crystal lattice preferred orientations/texture analysis (i.e. inverse/pole figures, orientation distribution functions misorientation data (similar types of plots) Non destructive data be collected from representative samples Automated statistically large/viable data sets acquired BUT! Samples must be oriented: Materials - RD, ND, TD Rocks – X, Y, Z or NSEW

43 CPO/texture analysis SEM/EBSD: faster than optical & gives full crystallographic information faster than X-ray goniometry & applicable to all crystals faster & simpler than neutron diffraction & applicable to all crystals does not suffer from problems associated with ODF calculations BUT! only surficial analysis: restricted to electron beam penetration depths not 3D (unless incorporate serial &/or three orthogonal sections)

44 Example: quartz colour-coded pole figures colour-coded automated pseudo-image colour-coded inverse pole figures misorientation angle profile subgrains Dauphine twins

45 Automated phase identification Phase 3: mica Crystal orientation (Euler) image 100  m Phase 1: quartz 100  m Phase 2: feldspar 100  m

46 In Situ Experiments “Crystal-probe” HT (1200  C) FEGSEM (D.J. Prior, University of Liverpool) Note: column tilted to 70° - allows horizontal sample movement & greater access to sample chamber (i.e. various other electron- specimen signal detectors)

47 projected 2010 (~3500) April 2010

48 Selected bibliography As of April 2010, searching for EBSD in Web of Knowledge results in ~3000 hits! So, here are some ‘early’ papers. Lloyd, G.E Atomic number and crystallographic contrast images with the SEM: a review of backscattered electron techniques. Mineralogical Magazine 51, Schmidt, N.-H. & Olesen, N.O Computer-aided determination of crystal lattice orientation from electron channelling patterns in the SEM. Canadian Mineralogist 27, Randle, V Microtexture Determination and its Application. The Institute of Materials, London 174pp. Randle, V The Measurement of Grain Boundary Geometry. Institute of Physics Publishing, Bristol, 169pp. Field, D.P Recent advances in the application of orientation imaging. Ultramicroscopy 67, 1-9. Wilkinson, A.J. & Hirsch, P.B Electron diffraction based techniques in scanning electron microscopy of bulk materials. Micron 28, Humphreys, F.J Quantitative metallography by electron backscattered diffraction. Journal of Microscopy 195, Prior, D.J Problems in determining the orientation of crystal misorientation axes for small angular misorientations, using electron backscatter diffraction in the SEM. Journal of Microscopy 195, Prior, D.J. et al The application of electron backscatter diffraction and orientation contrast imaging in the SEM to textural problems in rocks. American Mineralogist 84, Trimby, P.W. and Prior, D.J Microstructural imaging techniques: a comparison between light and scanning electron microscopy. Tectonophysics 303, Wilkinson, A.J Measurement of small misorientations using electron back scatter diffraction. Electron Microscopy and Analysis, Institute of Physics Conference Series, 161,

49 Crystallographic data sources Pearson’s Handbook, Desk Edition, Crystallographic data for intermetallic Phases, ASM international, ISBN American Mineralogist - International Tables for Crystallography Volume A: Space Group Symmetry. Edited by T. Hahn, revised edition, ISBN ; see also: Altwyk site:

50 Laboratory demonstration Aims to show: basic SEM/EBSD set-up orientation contrast imaging EBSD pattern capture & indexing (including crystal & diffraction database construction) automation questions & answers (?) plus general advice

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