1. Distinguishing Microstructures in TRIP Steel using EBSD And Optical Imaging
Julie Smith
Washington State University
Materials Science and Engineering Department

2. TRIP Steels Transformed Induced Plasticity
Ret. Austenite to Martensite
High in Strength and Elongation
Automotive Industry
Cooling Problems

3. The Problem FCC- Austenite BCC- Ferrite, Martensite and Bainite
Advanced Methods Can’t be Used
Limitations in Analysis and Advancements

4. Optical Microscopy-Color Etchants
Distinguishable Microstructures
Problems
2-5%Nital followed with 10% aq. sodium metabisulfite
Ferrite - Off White
Bainite, Pearlite and Martensite- Dark
Austenite- White
Bainite, Pearlite, and martensite are indistinguishable
2% Nital followed with heat treating
Ferrite and Bainite - Baige
Martensite- Dark Blue
Austenite- Purple
Have to heat treat sample.
Ferrite and Bainite are the same.
1% Aq. Metabisulfite followed with 4% Picral
Ferrite - Yellowish Blue
Bainite - Brown
Austenite and Martensite- White
Austenite and martensite indistinguishable.
4% Picral followed by 2% aqueous metabisulfite
Ferrite - Brown
Martensite and Austenite- White
4% Picral + HCl followed by 10% aqueous metabisulfite
Ferrite - Off White
Bainite- Bluish Black
Martensite- Straw Colored
Ferrite and Austenite close in color.

5. AFM Topography Measurements
Martensite and Retained Austenite= Same Height
Heat Treatment
Small Scans

6. SEM - EBSD Image Quality Image Quality and Confidence Index
Distinguish Between Bainite and Ferrite
Operator Variation
No Strain or Stress
Image Quality and Confidence Index
Ret. Austenite, Bainite and Ferrite
High Magnification and Multiple Scans

7. STILL IN NEED OF A SOLUTION!

8. Techniques EBSD/OIM and Optical Other Property Analysis
Ability to Reprogram OIM Software
Medium Size Scans
Easy Sample Prep

9. Proposed EBSD Compare OIM phase with Etched Sample
Austenite and Ferrite- same color with etchant
Austenite and Ferrite- Different Crystal Structure
Misorientations between Grains (future)
Relationship Between Grains (future)

10. Procedure Trial 1 Diamond Polish then Vibro Polish Make Markings OIM
Plasma Etch
2-step etch (4% Picral + HCl followed by 10% aqueous metabisulfite)
Optical Imaging
Compare - Phase and Optical to get all 4 microstructures

11. Optical Imaging Results
Bainite: Blue
Martensite: Gold
Ferrite and Austenite: Off- White

12. EBSD/ OIM Results

13. Procedure Cont. Problems with Trial 1
Plasma Etch appears to remove material in addition to hydrocarbons.
Etchant removes too much material.

14. Procedure Cont. Trial 2 Diamond Polish then Vibro Polish Make Markings
2- step Etch (4% Picral + HCl followed by 10% aqueous metabisulfite)
8 sec. Picral and sec aq. metabisulfite
Optical Imaging
Vibro Polish (.02μm Colloidal Silica)
OIM
Compare - Phase and Optical to get all 4 microstructures

15. Etching Etchant Time 1st part- Etches 2nd Part - Colors
2- step Etch (4% Picral + HCl followed by 10% aqueous metabisulfite)
1st part- Etches
Too long- over-etch. Too Short- no grains
8 sec. Optimum
2nd Part - Colors
Too Short- not enough color, Too long- darkens
15-18 sec Optimum

16. 0 Hours 1.5 Hours 2.5 Hours 4 Hours
Vibro Polishing
Vibro Polishing Time (.02μm Colloidal Silica)
0 Hours Hours Hours Hours
No Kikuchi Patterns
No Kikuchi Patterns

17. Vibro Polishing- 4 Hours
Vibro Polishing Time
4 Hours.... Too Long

18. Vibro Polishing- 4 Hours
Vibro Polishing Time
4 Hours
After- No Fish
Before- Fish Grain

19. Vibro Polishing- 2.5 Hours
Vibro Polishing Time
2. 5 Hours

20. Vibro Polishing- 2.5 Hours

21. Conclusions
Optical etch shows all phases except it cant distinguish between austenite and ferrite.
EBSD distinguishes between austenite and ferrite, martensite, bainite.
Method unsuccessful due to high material removal rate in comparison to the grain size.
vibro polishing (.02 Colloidal Silica) in combination with 2-part etchant (4% Picral + HCl followed by 10% aqueous metabisulfite)

22. STILL IN NEED OF A SOLUTION!

23. Future Work Etchant/ Optical Try a new etchant with same procedure.
Program OIM to differentiate Relationships Between Grains (distinguish martensite)
AFM- MFM Technique

24. EBSD Background Transformation to Martensite- Orientation
Kurdjumov-Sachs Relationship
(110)bcc//(111)fcc, [1-11]bcc//[0-11]fcc
Study: Misorientation Relationships- Martensite

25. Distinguishing Martensite

26. Thank You!! Questions??????

27. AFM and MFM All Phases Shown to have some Magnetism AFM MFM
Cantilever Tip
Contact- Repulsion
Non-contact- Attraction
MFM
Special Tip
Resolution 25 to 50 nm
Promising Solution