MICROSTRUCTURAL CHARACTERIZATION AND ... - Course Notes

MICROSTRUCTURAL CHARACTERIZATION AND MECHANICAL BEHAVIOUR OF HIGH MANGANESE STEELS

MSE 701 Graduate Seminar

Master Graduate Student: Xin Liang Profs. Hatem Zurob, Joseph McDermid and David Embury

Agenda 2

 

Introduction and Research Objectives Experimental, Results and Discussions 

Part I: Microstructural Characterization of High Mn Steels  



  

Microstructural characterization of Fe-24Mn & Fe-30Mn Newly developed characterization method

Part II: Preliminary study of mechanical behaviour of Fe-24Mn & Fe-30Mn Steels

Summaries and Conclusions Future Work Acknowledgements

3

Introduction and Research Objectives

Introduction 4



Why high Mn steels?

[1] Bleck, W. & Phiu-On, K. Materials Science Forum. Vol. 500-501 97-112

Introduction – Literature Review Plasticity + Phase Transitions → strength & ductility Phase transitions: mechanical twinning and g → e (→ a’) martensitic phase transformation

If phase transitions occur

upward

 flow    flow    f (V f (e ,a ',twin ) ) two twin sys. cell structures 2 sets of e Tensile behaviour of Fe-22Mn-0.6C at different temperatures1

TEM micrograph, Fe-22Mn0.6C,40% 34%strain, strain,673K 77K11 52% 293K 0.6C,

[1] Allain, S. et al. Materials Science & Engineering A 387-389, 158-162 (2004).

5

Literature Review and Research Motivation 

Phase transitions – thermodynamic driving force

Fe-24Mn

e+

The calculated iso-SFE lines in the carbon/manganese (wt.%) map at 300K1

Fe-30Mn

e + twin +

Is this sufficient?

[1] Allain, S. et al. Materials Science & Engineering A 387-389, 158-162 (2004).

6

Research Objectives 7



Research objective: interrelationship between microstructures and mechanical behaviour Composition / microstructure → ? mechanical behaviour Mechanical path → ? microstructure



We need to Characterize microstructures Investigate the mechanical behaviour

8

Experimental, Results and Discussions 

Part I: Microstructural Characterization of High Mn Steels 



Microstructural characterization of Fe24Mn and Fe-30Mn steels Newly developed characterization method

Microstructural Characterization of Fe24Mn and Fe-30Mn Steels 9



Materials under study and experimental Elemental analysis

Fe Mn C

Fe-24Mn 74.35~75.57% 23.74~24.30% 0.016%

Fe-30Mn 66.91~69.08% 29.62~30.79% 0.0122%

Heat treatment

Sample preparation • Optical metallography and EBSD: “EBSD Prep” mode on Struers® Automatic Polisher, up to 0.05 micron final polishing, load ~ 5N • TEM: Preliminary thinning, final thinning using twin-jet electropolishing (10% perchloric acid dissolved in HPLC methanol, temperature around - 40 ºC, 38 V, 30 sec)

Microstructural Characterization of Fe24Mn and Fe-30Mn Steels 10



Optical metallography: Fe-24Mn steel

Tint etching: 5% nital + Klemm’s I etchant (50 ml saturated aqueous sodium thiosulfate + 1 gram potassium metabisulfite); Observation was done in Axioplane2 Imaging System at Zeiss ® optical microscope, polarizing light.

Microstructural Characterization of Fe24Mn and Fe-30Mn Steels 11



Optical metallography: Fe-30Mn steel

A combination of characterization techniques are required to cover a range of length scales. Same etchant and observation mode as Fe-24Mn steel.

Microstructural Characterization of Fe24Mn and Fe-30Mn Steels 12



Phase analysis - phase diagram calculations

g

Fe-24Mn Fe-30Mn

R.T.

e

ge

• a phase was suspended; no carbon involved in this alloy system

Microstructural Characterization of Fe24Mn and Fe-30Mn Steels 13

 a

b

Phase analysis – XRD measurements c

1

d

a

b

c

d

2

• Bruker5® Smart Apex II Mo X-ray diffractometer, beam size 0.5 mm • GADDS, Merge and TOPAS software package

Microstructural Characterization of Fe24Mn and Fe-30Mn Steel 14



EBSD analysis: Fe-24Mn steel

Phase map (yellow-g, red-e, black line-GB>15⁰)

Crystal orientation map

• JEOL JSM-7000F FEG-SEM, 20 kV, 70° tilted, WD=20 mm, step size=0.3 mm • CCD detector and HKL® Channel 5 package (Falmenco and Tango)

Microstructural Characterization of Fe24Mn and Fe-30Mn Steel 15



EBSD analysis: Fe-30Mn steel

Phase map (yellow-g, black lineGB>15⁰,ligth blue-annealing twins)

Crystal orientation map

• JEOL JSM-7000F FEG-SEM, 20 kV, 70° tilted, WD=20 mm, step size=0.5 mm

Microstructural Characterization of Fe24Mn and Fe-30Mn Steel 16



TEM investigations: Fe-24Mn steel

SAD pattern, HCP [5 -1 -4 3]

SAD pattern, FCC [111]

BF image of g matrix and e plates

• Philip® CM 12 Transmission Electron Microscope, 120 kV

Epsilon plates in austenite matrix

Microstructural Characterization of Fe24Mn and Fe-30Mn Steel 17



TEM investigations: Fe-30Mn steel1

Equi-axed austenite grains

Annealing twins

[1] TEM investigation of Fe-30Mn steel was done by Dr. Xiang Wang.

18

Experimental, Results and Discussions 

Part I: Microstructural Characterization of High Mn Steels 



Microstructural characterization of Fe24Mn and Fe-30Mn steels Newly developed characterization method

Newly Developed Characterization Method: Issue of Sample Preparation 19

 

Problem associated with mechanical preparation Problem associated with electropolishing and TEM investigation

20 mm Low magnification view of thin areas in TEM specimen, Fe-24Mn

2 mm SEM images of one thin area in TEM specimen, Fe-24Mn

Newly Developed Characterization Method: What is it and why is it better? 20

Electropolish the bulk specimen No deformation damage

Plenty of material, efficient

Scanning electron microscopy (SEM) Examine large areas

Reveal → see before“run”

Electron backscattered diffraction (EBSD) Identify phases and features

Correlating information

Newly Developed Characterization Method: Resolving Microstructural Features 21



Characterizing e martensite

100 mm

10 mm

SEM images of as-electropolished Fe-24Mn steel (annealed), 250X and 1000X.

• Fe-24Mn, annealed at 900°C for 2 hrs, Ar protected, oil quenched, • Electropolishing: 10% perchloric acid in HPLC methanol, 45-50 v, 1 min

Newly Developed Characterization Method: Resolving Microstructural Features 22



Characterizing e martensite

SEM image, Fe-24Mn

EBSD phase map, Fe-24Mn (yellow – g; red – e; black line – GB>15°)

• JEOL JSM-7000F FEG-SEM, 20kV, 70° tilted, WD=20 mm, step size=0.1 mm • CCD detector and HKL® Channel 5 package (Falmenco and Tango)

23

Experimental, Results and Discussions 

Part II: Preliminary Study of Mechanical Behaviour of Fe-24Mn & Fe-30Mn Steels

Preliminary Study of Mechanical Behaviour of Fe-24Mn & Fe-30Mn Steels 24



Monotonic tensile behaviour Fe-24Mn Fe-30Mn

Fe-24Mn Fe-30Mn

1200 1000

True Stress, MPa

Engineering Stress, MPa

800

600

400

800 600 400

200 200 0 0.0

0.1

0.2

0.3

0.4

0.5

Engingeering Strain

0.6

0.7

0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50

True Strain

Fe-24Mn vs. Fe-30Mn: euniform and flow • Fe-24Mn and Fe-30Mn, 900°C, 2 hrs, Ar protected, oil quenched • Tensile testing machine: Instron 5566; strain rate: 1 mm/min

Preliminary Study of Mechanical Behaviour of Fe-24Mn & Fe-30Mn Steels 25



Fracture strain and stress

1800 1600

Fe-24Mn

True stress, MPa

1400 1200 1000 800 600 Fe-24Mn Fracture stress Fe-30Mn Fracture stress Fe-24Mn True stress Fe-30Mn True stress

400 200 0 0.0

0.5

1.0

1.5

Fe-30Mn

True strain

True stress-strain curves, with fracture points • Imaging analysis (Zeiss® stereoscope and North Eclipse® v6.0 imaging software)

Preliminary Study of Mechanical behaviour of Fe-24Mn & Fe-30Mn Steels 26



Fractography Fe-25Mn steel

2 mm

Fe-30Mn steel

2 mm

• JEOL JSM-7000F FEG-SEM, 10 kV, WD=10 mm

Preliminary Study of Mechanical Behaviour of Fe-24Mn & Fe-30Mn Steels 27

Work hardening behaviour Fe-24Mn WHR Fe-30Mn WHR T = T

20000

15000

10000

5000

0 100 200 300 400 500 600 700 800 900 1000 1100 1200

Work hardening rate / true stress, MPa

Work hardening rate / true Stress, MPa



4000 Fe-24Mn WHR Fe-30Mn WHR Fe-24Mn True stress Fe-30Mn True stress

3500 3000 2500 2000 1500 1000 500 0 0.00

0.05

0.10

0.15

Necking

0.25

True strain

True stress, MPa

criterion1

0.20

d  de

[1] Kocks, U.F. & Mecking, H. Progress in Materials Science 48, 102 (2003).

0.30

0.35

0.40

Preliminary Study of Mechanical Behaviour of Fe-24Mn & Fe-30Mn Steels d 

Work hardening behaviour

Relationship: mechanical behaviour & microstructure g

Fe-24Mn

1  M.F.P.

Dislocation strain hardening2

1

Fe-30Mn

 M.F.P. ge

de



28



1 dgrain

k 

Interface strengthening3

1  M.F.P.



1 dgrain

k  k'

[1] Kocks, U.F. & Mecking, H. Progress in Materials Science 48, 102 (2003). [2] Allain, S., et al. Materials Science and Engineering A 387-389, 143-147 (2004). [3] David Embury, Joint UBC-McMaster Workshop, Vancouver, September 2007.

1 linterface

1

Preliminary Study of Mechanical Behaviour of Fe-24Mn & Fe-30Mn Steels

29

Distinguishing between contributions1,2 – Bauschinger effect

 flow   0   iso   kin Isotropic contribution: dislocation accumulation, non-directional

Kinematic contributions (back stress): contributions other than dislocation accumulation, directional

F

 F   0   iso   kin  F   0   iso   kin  kin 

 F  R 2

[1] Bate, P.S. & Wilson, D.V. Acta Metallurgica 34, 1097-1105 (1986). [2] Spencer, K. Ph.D. thesis, McMaster University (2004).

R

Preliminary Study of Mechanical Behaviour of Fe-24Mn & Fe-30Mn Steels 30

800

Fe-24Mn Fe-30Mn

700

Backstress, MPa

 flow   0   iso   kin

Bauschinger effect - results

600 500 400 300

0.01% offset

200 100

0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 1/2

Pre-strain

True stress / backstress, MPa



1200 1000

1

Fe-24Mn True stress Fe-30mn True stress Fe-24Mn Backstress Fe-30Mn Backstress

800 600 400

0.01% offset

200

0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50

True Strain

• Bauschinger effect - microstructure

• Kinematic hardening contribution to the flow stress [1] Bate, P.S. & Wilson, D.V. Acta Metallurgica 34, 1097-1105 (1986).

31

Summaries and Conclusions

Summaries and Conclusions 32



Microstructural characterization of annealed Fe24Mn and Fe-30Mn steels Phase compositions are identified Microstructures are well characterized Phases, morphology and crystallographic information are effectively related



Newly developed characterization method Problems and solutions Resolving e martensite

Summaries and Conclusions 33



Preliminary study of mechanical behaviour of Fe-24Mn and Fe-30Mn steels Fe-24Mn has superior mechanical properties over Fe30Mn due to its high work hardening rate Relationship between the microstructure and mechanical behaviour was investigated Distinguishing between contributions to work hardening behaviour by looking at Baushinger effect 



Large contribution of kinematic hardening to the overall hardening behaviour in Fe-24Mn steel Saturation of kinematic hardening in Fe-30Mn steel indicates the dominant role of isotropic hardening in the later stage of plastic deformation

Future Work 34



Investigation of deformation mechanisms Interrupted tensile tests with metallography (OM, EBSD, TEM) and XRD analysis → prove the model, development of microstructures with strain CRSS for deformation twinning and e phase transformation Effects of temperature and strain rate



Revist Allain’s SFE model1 SFE and deformation mechanisms? (e.g. SFE of iridium ~ 480 mJm-2 and that of silver ~ 21 mJm-2, but both twin fairly readily2)

[1] Allain, S. et al. Materials Science & Engineering A 387-389, 158-162 (2004). [2] Warner, D.H., Curtin, W.A. & Qu, S. Nature Materials 6, 876-881 (2007).

Acknowledgements 35



Supervisors Prof. Hatem Zurob Prof. Joseph McDermid Prof. David Embury



NSERC and Arcelor-Mittal



MSE Dept. Support Mr. Doug Culley, Mr. John Rodda and Mr. Rob Lemmon



BIMR and CCEM Support Mr. Chris Butcher, Dr. James Britten, Dr. Steve Koprich, Dr. Glynis de Silveira, Mr. Andy Duft, Mr. Jim Garret, and Mr. Fred Pearson



Researchers and Grad. Students at McMaster Dr. Florent Lefevre-Schlick, Dr. Xiang Wang, Dr. Yan Li, Dr. Gordana Avramovich-Cingara, Yankui Bian and Erika Bellhouse

36

Thank you for attention! Questions and comments are welcome!