EBSD Electron BackScatter Diffraction Principle and Applications

Transcription

1 EBSD Electron BackScatter Diffraction Principle and Applications Dr. Emmanuelle Boehm-Courjault EPFL STI IMX Laboratoire de Simulation des Matériaux LSMX 1 Outline! Introduction! Overview of the EBSD technique! Application examples 2

2 Introduction 3 EBSD Basics EBSD = quantitative and general microstructural characterization technique in a SEM (Scanning Electronic Microscope). Near-surface technique Using diffraction patterns originating 20 nm 100 nm below the surface! Surface preparation is critical Aims : Orientation measurements & phase identification Materials analyzed Crystalline materials that survive under the beam! Metals, ceramics, minerals Conductors and insulators 4

3 =5000!m; tri-z; Step=8!m; Grid1890x882 Sample preparation! Requirements The backscattering volume (100 nm) below the surface sample must be crystalline and without excessive plastic deformation. Problems :! plastic deformation due to mechanical polishing (work-hardening)! foreign layers (oxide)! internal strain! Preparations methods " Mirror quality polishing (! 0.25 "m diamond grade), and :! chemical-mechanical polishing (silica or alumina suspension)! electro-polishing or chemical polishing/etching! ion-milling or plasma etching " Cleaved surface, growth surface " Insulating materials! carbon coating (< 100 Å) (degrades pattern quality)! low-vacuum SEM ( environmental SEM, pressure of a few Pa) 5 Uses of EBSD Maps Pole figures (orientation analysis) Strain analysis Phase identification Quantitative microstructural data 6

4 History of EBSD Kikuchi First observation of Kikuchi lines during the electron diffraction on a mica crystal, and interpretation Venable et Harland Micro-texture, micro-crystallography in a SEM (EBSP) Dingley, Wright, Adams, Schwarzer,... Automation of EBSD pattern analysis for determining crystal orientation Alam, Blackman et Pashley High-angle backscattered Kikuchi patterns (LiF, KI, NaCl, PbS2) 1990! TSL (EDAX), HKL, Oxford, Noran... Orientation imaging microscopy. - Kikuchi, Japanese Journal of Physics, V, 2 (1928) - M.N. Alam, M. Blackman, D.W. Pashley, High-angle Kikuchi Patterns, Proc. Roy. Soc. 222 (1954) J.A. Venables, C.J. Harland, Electron Back-Scattering Patterns - A New Technique for Obtaining Crystallographic Information in the Scanning Electron Microscope, Philosophical Magazine 27 (1972) Overview of the EBSD technique 8

5 EBSD setup Phosphor Screen Pole piece CCD Camera Sample Tilted at 70 EBSD pattern (= EBSP) Pseudo-kikuchi bands Phosphor screen + CCD camera in the FEI XLF-30 SEM at CIME 9 Formation of EBSPs! Interactions e-/matter! SE, BSE, X-Rays, Auger etc# BSE are used for : - images : chemical contrast (atomic number Z) - crystallographic analysis (EBSD).! Some of the BSE are diffracted by crystal planes diffraction if they are in Bragg condition! 2 diffraction cones Bragg : 2 d hkl sin " = n # F a i s c e a u d ' é l e c t r o n s P l a n d i f f r a c t a n t B a n d e d e K i k u c h i 20 kev $ # % nm $ " % 0.5 Angle of the cones close to 180! almost flat! Gnomonic projection on the 2D screen! 2 hyperbolas (Kikuchi bands). The middle of a band corresponds to the trace of the diffracting plane. C o n e s d e d i f f r a c t i o n E c r a n l u m i n e s c e n t 10

6 Indexation of EBSPs Each band Intersections of bands Angles between bands Position of bands: Band widths: = diffraction of a family of planes = intersections of planes = zone axes = angles between planes directly linked to the crystallographic orientation proportional to d hkl! related to lattice parameters Zone axis Si 40 kv 11 EBSP vs. accelerating voltage and tilt 5kV 10kV 20kV 30kV 40kV 70º 60º 50º (Also : current 1-15 na ; WD mm)

7 Spatial, angular resolutions & speed of acquisition Spatial resolution: 20 nm - 1 µm Limited by the overlapping of diffraction patterns near a boundary Depends on: " interaction volume (energy and Z) " size of the beam spot (probe current, focus, astigmatism). Angular resolution: relative % 2 absolute Limited by: " Accuracy on the localization of Kikuchi bands (Hough, #). " The weak signal/noise ratio of the images, and blur of Kikuchi bands. " Geometrical fluctuations of the conditions of diffraction, and calibration. " Absolute accuracy : sample position in the chamber. Speed of acquisition: & 25 ms/pt With Oxford-HKL Nordlys II detector (CIME) 13 Applications of EBSD EBSD patterns depend mainly on - crystal structure (symmetry) - crystal orientation! Micro-crystallography Determination of zone-axis symmetries : 1, 2, 3, 4, 6, m, 2mm, 3m, 4mm ou 6mm! identification of the crystal point group.! Possible indeterminations : (1 / -1, 3 / -3, 4 / -4, 6 / -6) Seldom used! Orientation measurements! Phase identification / discrimination 2 main applications 14

8 Orientation measurements 15 Indexing Cycle Position beam Collect EBSP Hough transform & band detection Phase and orientation Document Oxford Instruments Confirm match: Indexed EBSP Match to phase & orientation 16

9 Mapping grid Step size For each point :! Phase! Orientation (Euler angles) 17 Data analysis and orientation representations By EBSD : determination of Euler angles '1, (, '2 3 rotations needed to bring the sample referential {S} in coincidence with the crystallographic referential {C}. Caution : Euler angles are not unique (cubic crystal $ 24 equivalent triplets). Grey or coloured scale One map per Euler angle or one map including the three angles Combination of the 3 Euler angles = combination of 3 color scales 18

10 Data analysis and orientation representations Directional solidification of a Ni-base alloy Orientation image The scale is coloured considering the misorientation between each point and the <001> direction oriented along the normal direction (i.e. Z) Orientation matrix g : {C} = g {S} {S} sample ref, {C} crystallographic ref Angle (<001>, Z) & cos( ( 1) ' sin( ( 1) 0# & # & cos( ( 2) ' sin( ( 2) 0# g = $ sin( ( cos( ( 0! $ 0 cos( )) ' sin( ))! $ sin( ( cos( ( 0! $! $! $! % " % 0 sin( )) cos( ))" % 0 0 1" Misorientation between two points: )g = g('1,*, '2 ) t.g(''1,*', ''2) 0 55 Pole figure " Stereographic projection! Textures Sphere with center O, south pole S and point P on its surface. SP intersects the equatorial plane at p.! Any direction OP can be represented by a projection point p on the equatorial plane. 19 Phase identification 20

11 Phase identification Acquire Chemistry (EDX) Search Structural Databases At the same time (combined EDX-EBSD) or not (EDX or XRD done previously) Index Acquire EBSP Phase identified 21 Combination of EBSD and other techniques! EBSD and EDX! see slide 21! EBSD and FIB (focused ion beam)! possible at CIME but with 2 different machines! in-situ experiments " Heating or cooling stages " Tensile stage! EBSD in a TEM In a next future at EPFL-CIME 22

12 Application examples Directional solidification of a Ni-base alloy Cross section at 6 mm DS (gradient) Cross section at 2 mm Longitudinal section Angle (<001>, DS)

13 2. Galvanized steel sheets Natural convection Nitrogen flux of 25 l.min -1 Nitrogen flux of 50 l.min -1 Grain size distribution after refusion and cooling with different nitrogen fluxes 25% 20% Misorientation between the {1000} plane of zinc and the sheet plane 15% 10% 0 l/min 25 l/min 50 l/min 5% Number of grains # with flux (size $) and number of basal grains # (blue grains) 0% > Grain diameter [!m] A. Mariaux, M. Rappaz LSMX Carbide and nitride precipitates in a ferritic stainless steel FSE image EBSD map : phases BSE image Oxford Instruments document 26

14 4. Ultra-high resolution EBSD analysis of fine copper interconnect lines Black = grain boundaries (>10 ) Red = twins (78.6%) Average grain size = 0.71 "m High texture : <111> pole parallel to the normal direction. But it is not a true fibre texture as the in-plane texture is also strong. 27 Oxford Instruments document