RMS Summer School 2010, Leeds SEM ELECTRON BACK-SCATTERED DIFFRACTION

Presentation on theme: "RMS Summer School 2010, Leeds SEM ELECTRON BACK-SCATTERED DIFFRACTION"— Presentation transcript:

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)

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

Basic Principals

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

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

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

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

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

EBSD Pattern Recognition

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. dhkl) 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

diffraction from specific lattice plane
EBSD Patterns pyrite FeS2 diffraction from specific lattice plane width = 1/d-spacing 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: major crystal ‘pole’ HOLZ ring 2nd order diffraction 1st order diffraction

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

EBSD pattern ‘range’ Determined by crystal symmetry – defines the crystallographic unit triangle that repeats the range of patterns over a sphere Example: fcc (Cu) Example: trigonal (quartz) unit triangle 0001 11-20 -12-10 -2110 unit triangle 001 101 111 011 symmetrically equivalent unit triangle rotate

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

Creating a crystal database
1. Select a crystal e.g. aluminium 2. Input lattice parameters e.g. a = b = c = 0.405nm a = b = c = 90 3. Input crystal symmetry e.g. cubic Laue group = m3m Space group = 225 or Fm-3m 4. Input crystal unit cell symmetry indicates 4 atomic positions e.g. Atom x y z Occ Al Al Al Al

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 number of atoms lattice planes structure factor for (hkl) plane atomic scattering factor atomic position of atom g

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

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

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): quartz trigonal but can appear pseudo-hexagonal olivine orthorhombic but can appear pseudo-hexagonal plagioclase triclinic but can appear pseudo-monoclinic &/or pseudo- hexagonal

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

(e.g. http://www.gel.usherbrooke.ca/casino/index.html)
Spatial resolution Example: R P Depends on the penetration & deviation of electrons into a sample (plus beam diameter) Typically ranges from ‘few’ mm 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) note ‘down slope’ effect of tilting – scan ‘uphill’ Several Monte Carlo based simulation packages are available via the Web (e.g.

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)

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

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

Orientation Contrast Imaging

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

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 Forescattered electrons (FSE) with intensities determined by penetration (i.e. crystal orientation) are emitted towards the EBSD detector quartzite FSE signal detected by silicon devices attached to EBSD detector FSE Orientation Contrast image of variation in crystal orientation - contrast variations only qualitative (next slide)

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

OC images not quantitative !
(after D. J. Prior) 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

Automated EBSD Analysis

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!

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

Specimen Requirements

Specimen Preparation Polished blocks, thin-sections, natural fractured or grown surfaces Surface damage (mm-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

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

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

Interim Summary Orientation contrast images: EBSD patterns:
variations in crystallographic orientation & sample microstructure EBSD patterns: full crystallographic orientation of any point in OC image Spatial resolution: ~100nm (FEG, metals) to ~1mm (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)

EBSD Applications

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 (Cij) from grain crystal texture – used to predict material properties (e.g. thermal, electric, acoustic, magnetic, etc.)

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.

Example applications

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

CPO/texture analysis SEM/EBSD: BUT! only surficial analysis:
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)

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

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

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)

projected 2010 (~3500) April 2010

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, 3-19. 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,

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:

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