ebsd(electron backscattered diffraction)
TRANSCRIPT
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DIFFRACTION PHENOMENA IN SEM
byMuhammad faheem khan
Roll# MM-11
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TheEBSD technique has been known for 50 years, but onlywidely applied in the last 10 years. EBSD is now a fast,automated technique applicable to most crystalline materialsthat can provide microstructuralnformation in the form of grain
size and shape, phase identification and distribution,crystallographicsample texture, phase and grain boundarycharacteristics.
Electron backscatter diffraction (EBSD) is a powerful
technique which allows crystallographic Information to beobtained from samples in the scanning electron microscope(SEM).
INTRODUCTION
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What is EBSD
Accelerated electrons in the primary beam of a scanningelectron microscope (SEM) can be diffracted by atomic layers in
crystalline materials. These diffracted electrons can be detectedwhen they impinge on a phosphor screen and generate visiblelines, called Kikuchi bands, or "EBSP's" (electron backscatterpatterns).
http://serc.carleton.edu/research_education/geochemsheets/techniques/SEM.htmlhttp://serc.carleton.edu/research_education/geochemsheets/techniques/SEM.html -
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Typical SEM EBSD set-up
incident electron beam:8-40kV, 0.01-50nA
Specimen:Surface normal
typicallyinclined 60-80
to beam
EBSD detector - positionusually constrained by
chamber geometry
EBSD detector distanceset to give ~90
angular range in EBSP
emittedelectrons
low-light sensitive (now
digital; originally analogue)
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Diffraction Pattern-Observation Events
OIM computer asks Microscope Control Computer to place afixed Diffraction Pattern-Observation Events electron beam on aspot on the sampleA cone of diffracted electrons is intercepted by a specificallyplaced phosphor screenIncident electrons excite the phosphor, producing photonsA Charge Coupled Device (CCD) Camera detects and amplifies
the photons and sends the signal to the OIM computer forindexing
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Diffraction Patterns-Source
Electron Backscatter Diffraction Patterns(EBSPs) are observed when a fixed, focusedelectron beam is positioned on a tiltedspecimenTilting is used to reduce the path length ofthe backscattered electronsTo obtain sufficient backscattered electrons,the specimen is tilted between 55-75o,where 70o is considered idealThe backscattered electrons escape from30-40 nm underneath the surface, hence
there is a diffracting volumeNote that and
y 2.5 to 3 times spot siz
y 2.5 to 3 times spot siz
e- beam
z
y
x
20-35o
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Diffraction Patterns
There are two distinct artifacts
BandsPoles
Bands are intersections of diffractioncones that correspond to a family ofcrystallographic planes Band widthsare proportional to theinverse interplanar spacing Intersection of multiple bands(planes) correspond to a pole of thoseplanes (vector)
Note that while the bands are bright,they are surrounded by thin dark lineson either side
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Diffraction Patterns-Anatomy of a Pattern
There are two distinct artifacts
BandsPoles
Bands are intersections of diffractioncones that correspond to a family ofcrystallographic planes Band widthsare proportional to theinverse interplanar spacing Intersection of multiple bands(planes) correspond to a pole of thoseplanes (vector)
Note that while the bands are bright,they are surrounded by thin dark lineson either side
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Diffraction Patterns-SEM Settings
Higher Accelerating Voltage alsoproduces narrower diffraction bands (avs. b) and is necessary for adequatediffraction from coated samples (c vs.d) Larger spot sizes (beam current)may be used to increase diffractionpattern intensity
High resolution datasets and non-conductive materials require lowervoltage and spot size settings
Increasing the Accelerating Voltage increases the energy of theelectrons Increases the diffraction pattern intensity
a. b.
c. d.
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Pattern capture-Background
The background is the fixed variation in the captured frames due to the spatial
variation in intensity of the backscattered electrons Removal is done by averaging 8 frames (SEM in TV scan mode
Live signal Averaged signal
Note the variation of intensity in the images. The brightest point (marked with X)should be close to the center of the captured circle.
The location of this bright spot can be used to indicate how appropriate theWorking Distance is. A low bright spot = WD is too large and vice versa
X
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Detecting Patterns-Hough TransformLines in the captured pattern with points (xi,yi) are transformed into the
length of the orthogonal vector, rand an angle qA modified Hough Transform is used, and changes the reference frame ofthe pattern (transforms it)The average grayscale of the line (xi,yi) in Cartesian space is then assignedto the point (r,u) in Hough space
O x
y
r
q
I II
IVIIIr=0
r=n
r=-n
2n=Hough bin size
Transformed (Hough) space
I:0rn ; 0qp/2III: -n r
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Hough Transform
The Hough transform is also known as the Radontransform. The literature suggests that the actualtransformation used in OIM is a modification of the
original Radon transform. This modified transform isdesigned for use with digital images. The objective of the Hough transform is toconvert the parallel lines found in EBSD patterns
into points. These points can more easily beidentified and used in automatic computation.
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r=xcos+ysinwhere ris the perpendiculardistance from the origin and the angle withthe normal.
The coordinate transformation is such that pointsin the Cartesian planetransform to linesin the Hough plane. Or, more than one value of pand qcan satisfy the equation given above.
Thus, the numerical implementation of the transform is called anaccumulator: the intensity at each Cartesian point is added to the set ofcells in the Hough plane along the line that corresponds to that point. Thusthe intensity at points1,2 & 3 in the example above, contribute equally tothe points along lines1,2 & 3 in the Hough plane.
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Detecting Patterns-The Hough of one band
Since the patterns are composed of bands, and not lines, the
observed peaks in Hough space are a collection of points and not justone discrete point
Lines that intersect the band in Cartesian space are on averagehigher than those that do not intersect the band at all
Transformed (Hough) space
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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
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Indexing Patterns-Identifying Bands
Procedure:
Generate a lookup table from given lattice parameters andchosen reflectors (planes) that contains the inter-planar angles Generate a list of all triplets (sets of three bands) from thedetected bands in Hough space
Calculate the inter-planar angles for each triplet setSince there is often more than one possible solution for each triplet, amethod that uses all the bands needs to be implemented
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Indexing Patterns-Voting SchemeConsider an example where there exist:
Only 10 band triplets (i.e. 5 detected bands)Many possible solutions to consider, where eachpossible solution assigns an hklto each band. Only11 solutions are shown for illustration
Triplets are illustrated as 3 colored linesIf a solution yields inter-planar angles
within tolerance, a vote or an x ismarked in the solution columnThe solution chosen is that with mostnumber of votes
Confidence index (CI) is calculated as
Once the solution is chosen, it is comparedto the Hough and the angular deviation iscalculated as the fit
Solution #
# votes
Band
triplets
0.610
410
tripletsbandofnumber
S2ofvotes#-S1ofvotes#CI
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EBSD Pattern Recognition
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EBSD pattern recognition
EBSD patterns are uniquefor a specific crystal orientation
The pattern is controlled by the crystal structure: spacegroup symmetry, lattice parameters, precisecomposition
Within each pattern, specific bands (i.e. pairs of cones ofdiffraction) represent the spacing of specific lattice planes(i.e. dhkl)
EBSD pattern recognition compares the pattern of bandswith an atlas of all possible patterns in order to index thecrystal orientation depicted
This process WAS manual it is NOW automated!
Example - next slide
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pyrite
EBSD Patterns
Unique for crystal orientation &composition at the point ofbeam incidence
Can be >100 of total crystalprojection - easy to index assymmetry decreases
Spatial resolution (1m)
Some pattern details:
diffraction from
specific lattice plane
width = 1/d-spacing
1st order diffraction
2nd order diffraction
major crystalpole
HOLZ ring
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Example: pattern indexingOriginal pattern
manual/auto-indexed bands
Computer indexed pattern
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EBSD Problems
Spatial resolution
Angular resolution
Specimen preparation
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Spatial resolution
Depends on the penetration &deviation of electrons into a sample(plus beam diameter)
Typically ranges from few m forW-filament SEM to a few 100nm for
FEG SEM
Penetration depends on:
sample atomic number
accelerating voltagebeam current
(plus, coating depth &surface damage - seelater)
Several Monte Carlo basedsimulation packages are
available via the Web(e.g.
http://www.gel.usherbrooke.ca/casino/index.html)
Example:
RR
P
Pnote down slope effect of tilting
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Angular Resolution
Angular resolution of an individualEBSD pattern is typically~1
Also important when determining the misorientation betweentwo (adjacent) crystal lattices (e.g. grains)misorientationanalysis is becoming a popular application of EBSD as itprovides information on sample properties & behaviour
But, calculations of misorientation axes from 2 individualmeasurements with misorientation of
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Angular resolution 1:sample-detector considerations
Small detector distance Large detector distance
good for indexing butpoor angular resolution
poor for indexing but good angularresolution
important for constraining
misorientation axes.
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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
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Orientation Contrast
Imaging
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polycrystalline sample
Control of crystal orientationon emission signal
note variation in imagegrey-scale level - depends on penetration &emission, which depend on crystal orientation
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EBSD microstructural imagesElectron beam is scanned over an areaof a tilted sample, rather thanpositioning the beam on a point for EBSD patterns
quartzite
FSE Orientation Contrastimage of variation in crystal orientation -
contrast variations only qualitative (next slide)
FSEsignal detected
by silicon devicesattached to EBSDdetector
Forescatteredelectrons (FSE) withintensities determinedby penetration (i.e.
crystal orientation) areemitted towards theEBSD detector
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Automated EBSD Analysis
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Automated EBSD analysis
Computer controlled movementof the electron beam across asample
EBSD pattern captured at each
point
Indexing of EBSD patterns is viapattern recognition software
Software writes the crystal orientation
(3 Euler angles), & phase informationper pattern to a data-base for lateranalysis
BUT important to run a manual visualcheck of solutions beforethe
automated analysis!
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Automated EBSDanalyses
orientation contrastcrystal orientation variation pattern quality - strain
provides a variety ofinformation
crystal orientation pole figures
many other parameters:e.g. misorientation
(becoming very important inmicrostructural analysis)
P
T
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Specimen Requirements
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Specimen Preparation
Polished blocks, thin-sections, natural fractured or grownsurfaces
Surface damage (m-mm) created by mechanical polishingmust be removed:
chemical-mechanical (syton) polish
etching
electro-polishing
ion beam milling
Insulating samples may require very thincarbon coat, butuncoated samples mayperform OK - next
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Charging ProblemsReduce 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 3-5nmC coat
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Effect of coating on OC images
200m
uncoated ~4nm C ~8nm C
(after D.J. Prior)
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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 ~1m (W, rocks)
Angular resolution:~1 - 2 (misorientation >5 )
Materials:most metals & ceramics; many minerals - depends on composition
Automated analysis:100s of EBSD patterns/second (record ~800/sec via stage scanning)
but indexing accuracy may suffer (use of fast or sensitiveEBSD
detectors increasing depending on requirements)
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EBSD Applications
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What can EBSD be used for?
Measuring absolute (mis)orientation of known materials -most popular/obvious usage
Phase identification of known polymorphs - becomingpopular
Calculating lattice parameters of unknown materials -difficult, only possible for relatively simple structures?
Measuring elastic strain
Estimating plastic strain on the scale of the electronbeam activation volume
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Recommended applications
Tremendous significance for many types of materialsresearch, 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
- etc.
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Example applications
C t l i t ti d t f
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Crystal orientation data fromSEM/EBSD
Individual orientation measurements related tomicrostructure:
crystal lattice preferred orientations/texture analysis (i.e. inverse/polefigures, orientation distribution functions
misorientation data (similar types of plots)
Non destructive
data be collected from representativesamples
Automated
statistically large/viable data sets acquired
BUT! Samples mustbe oriented:
Materials - RD, ND, TD
Rocks X, Y, Z or NSEW
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Selected bibliography
Field, D.P. 1997. Recent advances in theapplication of orientation imaging.Ultramicroscopy67, 1-9.
Humphreys, F.J. 1999. Quantitativemetallography by electron backscattereddiffraction. Journal of Microscopy195, 170-
185.Electron backscattered Diffraction inMaterial science By Adam j.schwartz
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THANKS
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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