ebsd(electron backscattered diffraction)

8/4/2019 EBSD(ELECTRON BACKSCATTERED DIFFRACTION)

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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

ebsd(electron backscattered diffraction)

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