Dierk Raabe T W I P Steel 2009 M S T

Interface-related micromechanics in TWIP steels

I. Gutierrez-Urrutia, S. Zaefferer, D. Raabe

28. October 2009, MS&T, Pittsburgh

Acknowledgements: SFB 761 “Steel ab initio”;discussions with Marty Crimp and Tom Bieler (Michigan State University)

see also: talk at 2 pm Thursday room 328, EBSD session

Overview

Motivation

Characterization of interfaces in TWIP

Strain hardening behavior in TWIP

Conclusions

Dierk Raabe, MS&T, Pittsburgh, 28. Oct. 2009, MPIE

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200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 16000

10

20

30

40

50

60

70

80

tota

l elo

ngatio

n to f

ract

ure

[%

]

ultimate tensile strength [MPa]

TRIP and complex phaseTRIP and complex phase

martensiticmartensitic

maraging TRIPmaraging TRIP

dual phasedual phase

ferriticferritic

Ductility-strength profile for sheet steels

austenitic stainlessaustenitic stainless

advanced TWIP and TRIP


www.mpie.de

www.mpie.de

www.mpie.de

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200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 16000

10

20

30

40

50

60

70

80

tota

l elo

ngatio

n to f

ract

ure

[%

]

ultimate tensile strength [MPa]

TRIP and complex phaseTRIP and complex phase

martensiticmartensitic

maraging TRIPmaraging TRIP

dual phasedual phase

ferriticferritic

Ductility-strength profile for sheet steels

austenitic stainlessaustenitic stainless



Characterization

TEM, EBSD

High spatial resolution EBSD

3D EBSD (joint 2D EBSD plus serial sectioning)

Electron channeling contrast imaging (ECCI)

ECCI combined with EBSD: ECCI under controlled diffraction conditions


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Scaling issue in microscopy

?TEM EBSD

EBSD orientation contrastis not lattice defect contrast

Large scale mapping of lattice defects

TEM information in SEM

EBSD orientation contrastis not lattice defect contrast

Large scale mapping of lattice defects

TEM information in SEM

Electron Channeling Contrast Imaging

Example: Fe-22Mn-0.6C (wt%), TWIP

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EBSD

High spatial and angular resolution

ECCI

1 nm

10 nm

100 nm

1000 nm

0.001º 0.01º 0.1º 1º

Spatial resolution

Angular resolution

Limited spatial and

angular resolution

Good spatial and angular resolution

TEM

Overview characterization

Electron channeling contrast mechanism

Dislocation imaging

[A.J. Wilkinson, P.B. Hirsch, Micron 28 (1997) 279]

Requirement: Crystal at Bragg conditionImaging under controlled diffraction conditions

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Fe-22Mn-0.6C (wt%) TWIP steel

High resolution EBSD ECCItwins

Dislocation cells

Comparison EBSD-ECCI


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SEM

11

SEM

TEM

12

ECCI / SEM

(-220)

Dislocation cells are clearly seen by ECCI

TEM-Bright Field

Fe-22Mn-0.6C (wt%) TWIP steel


I. Gutierrez-Urrutia et al. Scripta Mater. 61 (2009) 737-740

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0

500

1000

1500

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3500

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0 0.1 0.2 0.3 0.4 0.5

True stressHardening (MPa)

Har

deni

ng/

Str

ess

(MP

a)

True strain

YS: 210 MPaUTS: 1200 MPaDuctility: 51%strain rate: 2.5x10-4 s-1

tensile test, room temperature10%

20%

40%

TA

twins

TA

step size: 50 nm

twins do not provide much strain but the 2nd hardening plateau

EBSD: Work hardening of TWIP steels, Fe-22Mn-0.6C (wt%)


0

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0 0.1 0.2 0.3 0.4 0.5


Har

den

ing/

Str

ess

(MP

a)

True strain


2% 10% 25% 35%

2nd hardening stage:

2%<e< 10%

-Planar slip structures + wavy structures-Few mechanical twins*

ECCI: Work hardening of TWIP steels, Fe-22Mn-0.6C (wt%)


2nd hardening stage:

2%<e< 10%


I. Gutierrez-Urrutia, et al. Mater. Sc. Engin. A 527 (2010) 3552

0

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0 0.1 0.2 0.3 0.4 0.5


Har

den

ing/

Str

ess

(MP

a)

True strain


2% 10% 25% 35%

3rd hardening stage:

10%<e< 25%

-Wavy structures: dislocation cells-Mechanical twins-3 Types of grains




10%<e< 25%



0

500

1000

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0 0.1 0.2 0.3 0.4 0.5


Har

den

ing/

Str

ess

(MP

a)

True strain


2% 10% 25% 35%

5th hardening stage:

35%<e< 50%

-Wavy structures: dislocation cells-Mechanical twins-3 Types of grains




35%<e< 50%



Type I


35%<e< 50%



Type II


35%<e< 50%



Type III


35%<e< 50%



0

20

40

60

80

100

0.05 0.1 0.2 0.4

DDW+CellsDDW+Cells+TwinsType IType IIType III

%

True strain

Type of grain

Interface statistics


Most of the grains follow the Schmid’s Law (3/4)

True strain: 40%

EBSD: Growth of deformation twins in TWIP steels


True strain: 40%Some of the grains

follow the Schmid’s Law



True strain: 40%

1-2activated twinning systems nucleated at

grain boundaries

No twins inside the grain (few degrees from [001])



Grains with “low” twinning activity:-Short twins (less than 5 microns length)

-Few twins



Grains with “high” twinning activity:-Long twins

-Thick bundles of twins-High population of twins

These results suggest a Sslip/Stwinning relationship

on twin growth



Type I


10%<e< 25%



Type II


10%<e< 25%



Type III


10%<e< 25%



32Fe22Mn0.6C TWIP steel

Mechanical twins

Dislocation cells





Set-up is proposed for ECCI under controlled diffraction conditions by means of EBSD in conventional FEG-SEM

Two positions set-up: low-tilt position for ECCI (high-tilt position (70º) for EBSD; optimized operation conditions: small working distance (<7 mm) and 10 kV

Dislocation cells and mechanical twins of 20 nm in thickness are imaged in a deformed TWIP steel

The ECCI technique under controlled diffraction conditions is a powerful tool for characterizing deformed bulk materials

Interface statistics in TWIP at different deformation regimes

Hall Petch parameter for twin-dislocation interaction below that for g.b.-dislocation interaction


Summary

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I. Gutierrez-Urrutia, S. Zaefferer, D. Raabe: Scripta Mater. 61 (2009) 737-740Electron channeling contrast imaging of twins and dislocations in twinning-induced plasticity steels under controlled diffraction conditions in a scanning electron microscope

I. Gutierrez-Urrutia, S. Zaefferer, D. Raabe: Mater. Sc. Engin. A 527 (2010) 3552-3560 The effect of grain size and grain orientation on deformation twinning in a Fe–22 wt.% Mn–0.6 wt.% C TWIP steel

References

Dierk Raabe T W I P Steel 2009 M S T

Business

workhardeningof twip

advanced twip

twip steelsi

ebsd sessionacknowledgements

scripta mater

high spatial resolution

d ebsd joint

types of grainsi