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Microstructural CharacterizationUsing EBSD

Steven R. Claves

Electron Backscatter Diffraction Sometimes referred to as

Backscattered Kikuchi Diffraction (BKD)

A diffraction technique for obtaining microtexturalinformation from small areas of bulk samples in thescanning electron microscope (SEM)

Advantages Imaging & Crystallography Orientation of individual grains Simple sample preparation

Disadvantages Crystalline samples Free of excessive

plastic strain

Components of an EBSD system

SEM source High current High brightness

Sample ~70 tilt

BSE yield Forward scattered Detector position

resolution Tilt correction Dynamic focus

OXFORD

Components of an EBSD system

Phosphor Screen Size Position

# polesDetail

Camera TV rate (speed/cost) CCD (quality)

OIM* Computer scan control Data analysis

*Sometimes referred to as ACOM(automated crystallographicorientation measurement)

OXFORD

OIM* - (orientation imaging microscopy)

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How the Pattern is Formed

high energy electrons are elastically scattered byatomic planes in a crystallographic sample

OXFORD

EBSP - Electron Backscatter Diffraction Patterns

Si single crystal

Map of the angular relationships between the atomic planes Orientation determined by indexing the EBSPs

Three Euler Angles

EBSP - Electron Backscatter Diffraction Patterns

Si single crystal

As the sample is rotated, the pattern changes toreflect the new orientaion

0 5

10 15 20 25 30

35 40 45 50 55

EBSD Samples BSE yield depends upon elements

Lateral resolution & depth as well

EBSD is a surface sensitive technique Flat surface (irregularities ok for non-mapping applications) High dislocation densities mar pattern quality

Mechanical polishing, 0.05 m colloidal silica Light etch to remove surface deformation Alternative final steps

Lapping Electropolish / electroetch (non-anodizing) Chemical polish

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Step 1: Loading the Sample Orientation data (i.e. angles) with respect to what?

Sample must be aligned properly in microscope

detector

Crystallographic orientationrelated to sample orientation

System geometry

TSL

TSL

Step 2: Acquire a Background Use fast scan at low mag. to acquire an average

pattern from many differently oriented grains Flat, even intensity

Step 3: Single Grain EBSP Fix beam position (spot mode) within a grain Raw pattern will have weak contrast

Step 4: Subtract Background Remove constant bkg to increase contrast (bkgs may also be divided or other manipulations)

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History EBSP first observed in 1954 (Alam et.al.)

Venables & Harland 1973 SEM with video rate camera

Background correction / flat fielding Eventually high gain CCD cameras

Burns algorithm for edge detection (Wrights,Adams92)

Hough Transform(Krieger Lassen, Jensen 92)

Orientation Imaging: The Emergence of a New Microscopy B.L. Adams, S.I. Wright, K. Kunze Met Trans 24A 1993

Hough Transform Sum up pixel intensities along line

Move the position and angle of the line Convert Kikuchi bands (2D) to a point (1D)

Hough Transform

angle

d i s t a n c e

start with vertical linemove counterclockwise

Step 5: Load Phase Information

Phase page fromTSL software

Calculated positionof poles from:

Point group

lattice parameters

Calibration

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EBSPs Poles are identified by their inter-relationships

based upon known interplanar angles Can identify poles by eye using crystal symmetry

3D cube showing the symmetryof the m( -3)m space group.

4/m

2/m

Interplanar Angles Lattice parameter and space group will determine the

interplanar angles (measured by distance on the pattern)

45

FCC - Aluminum

F m3m a = 4

Step 6: Calibration & Indexing Distances between poles are fixed for system setup

Calibration based upon screen position & WD Location of the poles on the pattern determines the

orientation

EBSD Mapping

Incidente- beam

crystallinesample

diffractedelectrons

phosphor screen

Diffracted patternsappear on screen

Fiber-optic cableto video camera

e- beam rasters across sample generating EBSPs Computer instantaneously indexes each point Records orientation data for each beam position

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Microtexture A population of orientations measured on a

grain-by-grain basis

6xxx Al Alloy

x,y 1, , 2 CI IQ phase

Grain Mapping Computer determines point-to-point misorientations

When angle exceeds a certain threshold a newgrain is declared

Large Areas Capability to perform

large area scans as well Stage scan Stage/beam scan combo.

Limited by size of chamber and system geometry

Stage / beam scanStage / beam scan

Small Area Maps Unique Grain Map Each grain given specific color

2.5 m step size

EBSD limited by spatialresolution of SEM

! W ~ 0.5 m step size! Schottky ~ 5 nm

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Orientation Maps

Inverse pole figure grain map

Grains of similar orientations are given like colors

Note poor pattern quality atgrain boundaries

Applications of EBSD

[Micro]Texture Analysis Misorientation Angles & Special Grain

Boundaries Grain Size (Pseudo) Strain Mapping via IQ index Phase Identification

(phase determination)

PF, IPF, & ODF

Discrete -Pole Figure

Contour - InversePole Figure

Intensity - OrientationDistribution Function

Pole Figures Display orientations of

highest probability

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Inverse Pole Figure Maps Orientation given with respect to a

specific direction (ND, TD, RD)

Extruded Al -fiber texture along RD

EBSD Comparison

Individual grains are given unique colors Good Agreement between the simulated

grain map and the backscattered electronimage

15

3

Grain Size Maps

(Can also use linear intercept method)

Misorientation Angle

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Special Grain Boundaries Coincident Site Lattices (CSL)

Grain boundary engineered Cu = 3,9,27

Special Grain Boundaries Coincident Site Lattices (CSL)

Grain boundary engineered Cu = 3,9,27 65% 3 twins

Strain Measurements Wilkinson & Dingley(91)

Plastic Strain Sharpness of band edges

Wilkinson(96/97) Increase distance to camera Small (mrad) shifts in location of zone axes

70

Phase IdentificationPhase determination

Known phasesdifferent crystallographic properties

Phase Identification Unknown phases In combination with EDS Searchable databases

Phase Identification in a Scanning Electron Microscope Using

Backscattered Electron Kikuchi Patterns R.P. Goehner & J.R. Michael J. Res. Natl. Inst. Stand. Technol., 101 , 301 (1996)

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-Al8Fe2Si -Al9Fe2Si2

Varying SEM Conditions Spot size / beam current, (nm / nA) Accelerating voltage, kV Working Distance, WD (mm) Tilt Angle, degrees ()

Effect of Beam Size Greater beam sizes = more current

Spot size / C1 current Gun bias

Lower acquisition times Better quality patterns (increased accuracy)

Largest possible beam size depends upon Grain size Desired step size

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[ Cast 6xxx Al Sample ]2x2 binning (650 x 525)

Spot size = 3 (35 nm) Current = ~ 0.1 nA

Acquire time = 8s

Spot size = 7 (0.58 m) Current = ~ 14 nA

Acquire time = 0.2 s

Effect of kV Changes width of bands

Poles stay in same positions

5 kV

10 kV

15 kV

20 kV

25 kV

30 kV

Effect of Working Distance

Moves pattern center (Changes the area of

maximum BSE intensity) Calibration is very

important

e-

y*x*

z*

TSL

16 mm 20 mm 24 mm 28 mm

Effect of Tilt Angle Modifies BSE yield Changes the distance angle

conversion Calibration is very important

e-

y*x*

z*

TSL

58

62

66

70

74

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EBSD 2 Metal Forming Examples

Microtexture Variation

X [100] ND{hkl}

Z [001] RD

S ur f a c e

s ur f a c e

Pole Figures

Small, Equiaxed Cube texture

{001}

Large, Elongated FCC shear texture

{111}

Macro Photographs I Dead Metal Zone(DMZ)

II Shear Intensive Zone(SIZ)

III Transition Zone(SIZ 2)

IVa Main DeformationZone (MDZ)

IVb - Negative Flow Angle(MDZ 2)

x

z

y

I

II

IIIIVa

IVb

Dieface

10 mm

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Micrographs

Etched in BarkersReagent Grain contrast under

polarized light Boundaries appear

dark

Record angle withrespect to extrusiondirection

extrusion direction

BILLETcenterline surface

49 III

Micrographs

Record metal flow direction (angle)

c e n t e r

c e n t e r

s ur f a c

e

PoleFigures Microstructural Detail

Small step size