Introduction to Electron Backscattered Diffraction. TEQIP Workshop HREXRD Feb 1 st to Feb 5 th 2016

Size: px

Start display at page:

Download "Introduction to Electron Backscattered Diffraction. TEQIP Workshop HREXRD Feb 1 st to Feb 5 th 2016"

Donna Wood
6 years ago
Views:

$Diffraction 1 TEQIP Workshop HREXRD Feb 1 st to$

1 Introduction to Electron Backscattered Diffraction 1 TEQIP Workshop HREXRD Feb 1 st to Feb 5 th 2016

2 SE vs BSE 2

3 Ranges and interaction volumes 3 (1-2 m)

4 Backscattered Electrons 4

5 Topographic Contrast 5 Image from Characterization Facility Manual, University of Minnesota

6 Secondary and backscattered Electrons 6 Backscattered electrons can also produce secondary electrons. Secondary electrons are generated throughout the interaction volume, but only secondary electrons produced near the surface are able to escape (~5 nm in metals). For this reason, secondary electron imaging (SEI) yields high resolution images of surface features. By definition, secondary electrons have energy <50 ev, with most <10 ev.

7 EBSD: Theory to Technique 7 Some slides borrowed from Prof. Sudhanshu Shekhar Singh and TSL OIM Training Program

8 Electron backscattered Diffraction (EBSD) 8

$Diffracting$ plane Cone of

$Diffraction$ Cones Electron

9 EBSD Setup 9 SEM vacuum chamber Diffracting plane Cone of intense electrons Diffraction Cones Electron beam EBSD detector Cone of deficient electrons Sample at 70 tilt Kikuchi pattern Kikuchi lines

10 Interaction of electrons with materials Kikuchi pattern (map) 10

11 Setup for EBSD in SEM 11 Principal system components Sample tilted at 70 from the horizontal phosphor screen (interaction of electrons) Sensitive CCD video camera (capture the image on phosphor screen) T. Maitland et. al., 2007 V. Randle et. al, 2000

12 Bragg s Law 12 d n = 2d sin B

13 Formation of Kikuchi lines 13

$diffracted electrons form hyperbolae on the phosphor$

14 Conic Sections to Kikuchi Bands 14 The cones of diffracted electrons form hyperbolae on the phosphor screen

15 Properties of Kikuchi pattern 15 Each band : diffraction of a family of planes Intersections of bands : intersections of planes = zone axes Angles between bands : angles between planes Band widths : proportional to d(hkl) related to lattice parameters Middle line of a kikuchi band represents plane Excess line Zone axis Deficient line Kikuchi lines Kikuchi/EBSP pattern at a point

16 Indexing: Identifying various planes 16 Look Up Table (LUT) The angles between these bands formed by planes are measured from the Kikuchi pattern These values are compared against theoretical values of all angles formed by various planes for a given crystal system When the h-k-l values of a pair of lines are identified, it gives information about the pair of planes, but this does not distinguish between the two planes and hence this alone cannot be used to identify the orientation of the sample At least 3 sets of lines are required to completely identify the individual planes and hence find the orientation of the sample, as shown in Figure Angle (hkl) 1 (hkl)

17 Band Identification: Image processing 17

18 Hough Transform 18

19 Hough Transform 19

20 Hough Transform 20

21 Hough Transform 21

22 EBSD Analysis 22

23 In order to specify an orientation, it is necessary to set up terms of reference, each of which is known as a coordinate system There are two coordinate systems: Sample (specimen) coordinate system Crystal coordinate system Coordinate systems 23 Specimen coordinate system: Coordinate system chosen as the geometry of the sample Crystal coordinate system: Coordinate system based on crystal orientation. In general [100], [010], [001] are adopted V. Randle et. al., 2000

24 24 orientation is then defined as 'the position of the crystal coordinate system with respect to the specimen coordinate system', i.e. where Cc and CS are the crystal and specimen coordinate systems respectively and g is the orientation matrix The fundamental means for expressing g is the rotation or orientation matrix The first row of the matrix is given by the cosines of the angles between the first crystal axis, [l00], and each of the three specimen axes, X, Y, Z, in turn In general sample coordinate system is the reference system

25 Orientation Maps 25 =100 µm; IPF; Step=1 µm; Grid300x200 =100 µm; BC; Step=1 µm; Grid300x200 Inverse Pole Figure Image Quality Map

26 Phase Maps 26 Titanium Aluminate Alumina Erbium Oxide Zirconium Oxide

27 Various kinds of boundaries 27

28 Charts: Misorientation Angle Distribution 28

29 Charts: Misorientation Profile 29

30 Charts: Grain Size 30 The area (A) of a grain is the number (N) of points in the grain multiplied by a factor of the step size (s). For square grids: A = Ns 2 For hexagonal grids: A = N 3/2s 2 The diameter (D) is calculated from the area (A) assuming the grain is a circle: D = (4A/p) 1/2.

31 Pole Figures Consider a cubic crystal in a rolled sheet sample with "laboratory" or "sample" axes as shown below. 31 The Pole Figure plots the orientation of a given plane normal (pole) with respect to the sample reference frame. The example below is a (001) pole figure. Note the three points shown in the pole figure are for three symmetrically equivalent planes in the crystal.

32 Pole Figure: Texture Analysis 32

33 Orientation Distribution Function (ODF) 33 Although an orientation can be uniquely defined by a single point in Euler space, 3D graphs are hard to interpret Therefore ODF is a 2D representation of Euler Space Euler Space is divided into slices with interval of 5 o aluminum.matter.org.uk Slices arranged in gird called ODF

34 t-ebsd 34

9 a=0 tool indexable Large areas where the orientation cannot be determined (by indexing of Kikuchi patterns) 1.

35 SEM EBSD analysis of the microstructure in 316L chips formed with both the 0 and 20o raking angle 20 o tool angle: g = 1.5 not indexable a=+20 0 o tool angle: g = 1.9 a=0 tool indexable Large areas where the orientation cannot be determined (by indexing of Kikuchi patterns) 1. Due to refinement of the microstructure beyond the resolution limit of the SEM 2. Introduction of large amounts of colddeformation strain => decreasing the quality of the Kikuchi pattern Nothing could be indexed G. Facco; S. Shashank; M.R. Shankar; A.K. Kulovits; J.M.K. Wiezorek, MRS2010 Boston

G. Facco; S. Shashank; M.R. Shankar; A.K.

BF images show the formation of dislocation walls

plastic deformation facilitated by conventional

OIM imaging shows large grains that contain low

36 G. Facco; S. Shashank; M.R. Shankar; A.K. Kulovits; J.M.K. Wiezorek, MRS2010 Boston TEM based OIM Analysis (+20 rake) 0.4 m 0.4 m 0.4 m 0.4 m Orientation spread 0.2 m 1. BF images show the formation of dislocation walls sub cell structure typical of large amounts of plastic deformation facilitated by conventional plastic deformation 2. OIM imaging shows large grains that contain low angle mis-orientations 3. OIM observations are consistent with BF image contrast of the dislocation wall sub cell structure

Analysis (0 rake) 0.4 m 0.4 m 0.4 m 0.4 m 1.

37 G. Facco; S. Shashank; M.R. Shankar; A.K. Kulovits; J.M.K. Wiezorek, MRS2010 Boston TEM based OIM Analysis (0 rake) 0.4 m 0.4 m 0.4 m 0.4 m 1. OIM imaging shows much smaller grains separated by High Angle Grain Boundaries HAGB s => grain refinement took place 2. 0 raking constitutes a severe plastic deformation process

38 Cross-correlation technique to determine elastic strain 38

39 In-situ Recrystallization 39 (a) 26R (b) 500 C (c) 15min (d) 30min (e) 90min (f) 120min N. Sharma, S. Shashank; submitted to J. Microscopy

40 Band Contrast Intensity as userindependent parameter 40 N. Sharma, S. Shashank; submitted to J. Microscopy

41 Recovery Parameter 41 (a) 26R, (b) 200 C and (c) 450 C. N. Sharma, S. Shashank; submitted to J. Microscopy

42 MAD as user-independent parameter 42 N. Sharma, S. Shashank; submitted to J. Microscopy

43 Summary EBSD is a very powerful technique for quantitative microscopy It is based on diffraction and hence can be used for any crystalline materials This method provides trove of data related to orientation, misorientation and can be extrapolated to represent strains, extent of recovery, recrystallization and may more things 43

EBSD Basics EBSD. Marco Cantoni 021/ Centre Interdisciplinaire de Microscopie Electronique CIME. Phosphor Screen. Pole piece.

EBSD Basics EBSD. Marco Cantoni 021/ Centre Interdisciplinaire de Microscopie Electronique CIME. Phosphor Screen. Pole piece. EBSD Marco Cantoni 021/693.48.16 Centre Interdisciplinaire de Microscopie Electronique CIME EBSD Basics Quantitative, general microstructural characterization in the SEM Orientation measurements, phase