Electron channelling contrast imaging (ECCI) an amazing tool for observations of crystal lattice defects in bulk samples

Transcription

1 Electron channelling contrast imaging (ECCI) an amazing tool for observations of crystal lattice defects in bulk samples Stefan Zaefferer with contributions of N. Elhami, (general & steels) Z. Li F. Ram, M. Hafez Haghighat (creep in superalloys) Z. Tarzimoghadam (hydrogen embrittlement) D. An (in situ observations) A TEM ECCI comparison as motivation TEM in-situ observation of nano indentation in SrTiO 3 (Kondo et al., Science Advances, 2 (2016), e ) 200 nm SEM- ECCI observation of nano indentation in a bulk TWIP steel sample S. Zaefferer: ECCI, Advances & Applications 1 Düsseldorf 1

2 A TEM ECCI comparison as motivation S. Zaefferer: ECCI, Advances & Applications 2 Outline Introduction to ECCI Basics, Defect observation Origin of creep dislocations in Ni based superalloys observations, dislocation dynamics modelling, 3D ECCI Hydrogen embrittlement in superalloys Dislocation characterization, Dislocation image modelling S. Zaefferer: ECCI, Advances & Applications 3 Düsseldorf 2

3 Outline Introduction to ECCI Basics, Defect observation Origin of creep dislocations in Ni based superalloys observations, Based dislocation on: dynamics modelling, 3D ECCI S. Zaefferer, N. N. Elhami, Hydrogen embrittlement Theory and application in superalloys of electron Dislocation channelling characterization, contrast Dislocation imaging image under modelling controlled diffraction conditions, Acta Materialia 75 (2014), S. Zaefferer: ECCI, Advances & Applications 4 Diffraction techniques in SEM incident beam Electron channelling ~ 10 pattern (ECP) BSE Detector ~ 7 ~ 90 on 6-axis stage (tilted to 2-beam conditions) ~ 15 Electron channelling contrast image (ECCI) 70 on pre-tilted stage Specimen chamber ~ 90 FSE Detector ~ 30 ~ 35 EBSD Detector Electron backscatter diffraction pattern (EBSD) Orientation microscopy image (OMI) S. Zaefferer: ECCI: Advances & Applications 5 Düsseldorf 3

4 ECCI: the physical picture e - S. Zaefferer: ECCI, Advances & Applications 6 The total electron wave density 30 w = w = w = w = w = 3.0 w < 0: high backscattering w > 0: little backscattering Scattering cross section Number of atoms 20 ~ dz z Electron density P>P _ x P<P _ x x x x S. Zaefferer: ECCI, Advances & Applications 7 Düsseldorf 4

5 The backscattering profile for the two beam case 2 x w B w C -1 Intensity (a.u.) -2-3 Kikuchi band Kikuchi line Bragg position deviation parameter, w S. Zaefferer: ECCI, Advances & Applications 8 Calculation of defect contrast (stacking fault) channelling S. Zaefferer, N. Elhami, Acta Materialia 75 (2014) backscattering y z z x y x Integration of channelling and backscattering conditions Δ Δ Δ 0 Straight bright line at intersection with surface Fading contrast on opposite side Intensity oscillations with depth S. Zaefferer: ECCI, Advances & Applications 9 Düsseldorf 5

6 (a) (2 2 0) (2) ( 1 1 1) (1) (2) 160 nm S. Zaefferer: ECCI, Advances & Applications 10 ECCI under controlled diffraction conditions (cecci) 70 nm y:4º/ z: 83º matrix twin The following steps are required: 1. Measure crystal orientations using EBSD 2. Simulate electron channeling pattern (ECP) 4. Use simulation y:16º/ z: 83º to determine tilt angles for 2-beam conditions 3. Tilt sample to the determined 2-beam conditions I. Gutierrez-Urrutia, S. Zaefferer, D. Raabe (2009), Scripta Materialia 61, S. Zaefferer: ECCI: Advances & Applications 11 Düsseldorf 6

7 Observation of dislocations (Fe Si alloy, bcc) S. Zaefferer: ECCI, Advances & Applications 12 g = (1-10) (01-1) (11-1) (-110) (110) (10-1) (011) (-111) (111) (101) (1-11) To obtain a good image you need: small beam convergence small spot size high beam current sensitive and quick BSE detector 12 nm S. Zaefferer: ECCI, Advances & Applications 13 Düsseldorf 7

8 Outline Introduction to ECCI Basics, Defect observation Some examples in overview Origin of creep dislocations in Ni based superalloys observations, dislocation dynamics modelling, 3D ECCI Hydrogen embrittlement in superalloys Dislocation characterization, Dislocation image modelling S. Zaefferer: ECCI, Advances & Applications 14 Planar slip in a high entropy alloy High entropy alloy: Courtesy Nelli Wanderka, Berlin S. Zaefferer: ECCI, Advances & Applications 15 Düsseldorf 8

9 Slip band development during low cycle fatigue g = (1-1 -1) 1 µm S. Zaefferer: ECCI, Advances & Applications 16 Local plasticity in a ferrite martensite sample Dislocation emission at martensite interfaces dislocations and elastic strain field S. Zaefferer: ECCI, Advances & Applications 17 Düsseldorf 9

10 Presentation on CMCS, May Large area defect observatio on bulk samples (iv ) (iii) (ii) (i) (i) (iii) (i) (iv ) (iii) (ii) (ii) S. Zaefferer: ECCI, Advances & Applications 18 In situ experiments: fatigue of a TWIP steel Cycles Cycles Cycles 500 nm S. Zaefferer: ECCI, Advances & Applications Düsseldorf 19 10

11 Nano Indentation S. Zaefferer: ECCI, Advances & Applications 20 ECCI on Al alloys: overlay of ECC and Z contrast S. Zaefferer: ECCI, Advances & Applications 21 Düsseldorf 11

12 Further examples (non metals) GaN: Picard. Scripta Mater. 2009;61: see also: K. Hite: Advances in Scientific Research and Education A. Vilas Mendez (ed.). GaN: Jones et al., ARL TR 5903 January 2012 Ferropericlase (Fe,Mg)O with NaCl Structure: Courtesy of. N. Miyajima (U Bayreuth) 2 µm S. Zaefferer: ECCI, Advances & Applications 22 Materials that have been observed with ECCI Metals: superalloys (very good), steels (bcc & fcc), Al alloys, Mg alloys (difficult), Ti alloys Semiconductors: GaN (very good), GaAs, CdTe (very good), Si (difficult) Ceramics: SrTi0 3 (images of dislocation intersections) Minerals: Ferropericlase (Mg,Fe)O (relatively dense structure) S. Zaefferer: ECCI, Advances & Applications 23 Düsseldorf 12

13 Summary: Dislocations & Stacking Faults ECCI offers TEM like characterization of individual dislocations and other defects on bulk sample Potentially this enables observation of defects in a more natural arrangement (i.e. no stress release) in situ observations of deformation easier than in TEM ECCI TEM Lateral resolution nm 1 nm Depth of observation nm nm Observable area 10 8 µm² 10 4 µm² Sample bulk thin foil S. Zaefferer: ECCI, Advances & Applications 24 Outline Introduction to ECCI Basics, Defect observation Origin Based of creep on: dislocations in Ni based superalloys F. Ram, Z. Li, M. Hafez et al. observations, dislocation dynamics modelling, 3D ECCI On the origin of creep dislocations in a Ni-base, Hydrogen single-crystal embrittlement superalloy: in superalloys an ECCI, EBSD, and Dislocation characterization, Dynamics-based Dislocation study image, modelling Acta Materialia 109 (2016), S. Zaefferer: ECCI, Advances & Applications 25 Düsseldorf 13

14 On the origin of creep dislocation in superalloys collaboration with R. Reed, Oxford University σ horizontal channels [100] On horizontal channels (001) glide dislocations b 2 b 1 [001] b 3 b 5 b 4 b 1 b 4 : 60 mixed b 5 :screw [010] σ [100] sessile dislocations b 7 [001] b 6 b 6 b 8 : edge b 8 vertical channels γ γ On vertical channels (100) and (010) [001] σ b 9 : screw b 12 b [010] 10. b 11 : 60 b 9 mixed b 12 : pure edge b 10 [010] [100] b 11 S. Zaefferer: ECCI, Advances & Applications 26 Creep specimens Ni 8Cr 10Co 3Re 8.5W 5.9Al 8.5Ta (wt.%)* Creep test: 900 C, 450MPa deformed up to fracture at about 20 % Dendrites in undeformed material Sample Analysed surfaces [001] Sample % of strain,surface orientation (001) Sample 2 0.3% strain, surface orientation near (101) * R. C. Reed et al. / Acta Materialia 57 (2009) 5898 S. Zaefferer: ECCI, Advances & Applications 27 Düsseldorf 14

15 Interpretation of misorientations as dislocation structures S. Zaefferer: ECCI, Advances & Applications 28 Dislocations inside dendrites (horizontal channels) g=(020) S3 σ S. Zaefferer: ECCI, Advances & Applications 29 Düsseldorf 15

16 Dislocations at dendrite interfaces (vertical channels) g=(-20-2) g=(-20-2) g=(002) g=(002) σ S. Zaefferer: ECCI, Advances & Applications 30 Modelling of dislocation evolution by DDD individual sources DDD: discrete disloction dynamics Results: dislocation fields from individual sources develop at least as quick as those from subgrain boundaries. subgrain boundaries S. Zaefferer: ECCI, Advances & Applications 31 Düsseldorf 16

17 Conclusions on superalloy observations Combination of EBSD and ECCI allows much improved materials characterization and understanding Here: creep dislocation origin more effectively from individual sources inside of dendrites than from dendrite boundaries Combination with discrete dislocation dynamics modelling is ideal 3D ECCI appears to be a realizable and valuable extension S. Zaefferer: ECCI, Advances & Applications 32 Outline Introduction to ECCI Basics, Defect observation Origin of creep dislocations in Ni based superalloys observations, dislocation dynamics modelling, 3D ECCI Based on: Hydrogen embrittlement Z. Tarzimoghadam, in superalloys D. Ponge Dislocation characterization, Dislocation image modelling S. Zaefferer: ECCI, Advances & Applications 33 Düsseldorf 17

18 Hydrogen induced embrittlement in superalloys Alloy In718, Chemical composition (approx.): Ni (bal) Cr Fe Mn Si Mo Ti Nb C R Tensile testing under H-charging conditions: charging solution: 5% H 2 SO % NH 4 SCN current density: 100 ma/cm µm Fracture surface without H 100 µm Fracture surface with H S. Zaefferer: ECCI, Advances & Applications 34 Hydrogen charging and deformation experiment extension compression 1. Electrochemical hydrogen charging 2. Bending of sample in micro-bending device keep sample loaded 3. Observation of initial microstructure (ECCI) 4. Out-diffusion of hydrogen in the SEM 5. Continuous observation during discharging (ECCI) 6. EBSD measurements on observed area 7. Crystallographic interpretation of results S. Zaefferer: ECCI, Advances & Applications 35 Düsseldorf 18

19 Dislocation observation during H discharging 1 h after H-charging Pileup 2 Pileup nm S. Zaefferer: ECCI, Advances & Applications 36 Dislocation observation during H discharging 1.5 h after H-charging S. Zaefferer: ECCI, Advances & Applications 37 Düsseldorf 19

20 Dislocation observation during H discharging 2.5 h after H-charging S. Zaefferer: ECCI, Advances & Applications 38 Dislocation observation during H discharging 3.5 h after H-charging S. Zaefferer: ECCI, Advances & Applications 39 Düsseldorf 20

21 Dislocation characterization I (slip plane) slip plane determination from trace analysis (based on EBSD measurements) HAGB Ʃ3 Ʃ9 111 plane trace Slip trace is (-1-1 1) S. Zaefferer: ECCI, Advances & Applications 40 Dislocation characterisation II (Burgers vector) g=(1-1 1) n s = (-1-11) b 2 = [-110], mixed b 1 = [101], almost edge Set 1: (immobile during H-release): visibility for g=(1-11) invisibility for g=(020) b 1 =[101] because it is in the (-1-11) plane Set 2: (mobile during H-release): visibility for g=(1-11) and g=(020) b has to be on the (-1-11) plane b 2 =[-110] g=(020) (1-11) (0-11) (101) (-1-11) (-1-10) (111) (-101) (011) (-111) (-110) S. Zaefferer: ECCI, Advances & Applications 41 Düsseldorf 21

22 Interpretation of dislocation movement 100 nm 100 nm b 2 = [-110] mixed Burgers vector at large angle to surface Low pinning force Easy movement under H-release b 1 = [101] almost edge Burgers vector almost parallel to surface High pinning force No movement under H-release Proposed mechanism of hydrogen embrittlement: Hydrogen-Enhanced Localized Plasticity (HELP) Hydrostatic stress field at crack tip attracts hydrogen Hydrogen leads to shielding of stress fields of dislocations Dislocations move easier, yield stress is locally reduced Earlier formation of micro pores in front of crack tip S. Zaefferer: ECCI, Advances & Applications 42 Detailed observation of dislocation structures Two types of dislocations visible? S. Zaefferer: ECCI, Advances & Applications 43 Düsseldorf 22

23 Modelling of ECC dislocation images Picard et al: Ultramicroscopy146(2014)71 78 Modelling a dislocation in SrTiO3 using scattering-matrix approach A screw and an edge dislocation are treated Dislocations with and without surface relaxation EMSoft at: S. Zaefferer: ECCI, Advances & Applications 44 Conclusions on hydrogen embrittlement ECCI allows direct observation of dislocation movement due to hydrogen release Direct indication for the HELP mechanism! cecci allows for a comprehensive dislocation characterization by combination of trace analysis and tilt experiments Defect contrast simulations are very helpful for effective defect characterization S. Zaefferer: ECCI, Advances & Applications 45 Düsseldorf 23

24 General conclusions ECCI is a powerful method, complementary to EBSD Observation of indivdual lattice defects like in TEM but with reduced contrast and resolution The big advantage: it works on bulk samples! possibility to perform in situ experiments possibility to perform 3D observations Disadvantage (no longer?): diffraction blind technique smallangle channelling patterns is a good idea calibration with EBSD may work as well S. Zaefferer: ECCI, Advances & Applications 46 Düsseldorf 24