Distinguishing Microstructures in
TRIP Steel using EBSD And Optical Imaging
Distinguishing Microstructures in
TRIP Steel using EBSD And Optical Imaging
Julie SmithWashington State University
Materials Science and Engineering Department
Julie SmithWashington State University
Materials Science and Engineering Department
• Transformed Induced Plasticity
• Ret. Austenite to Martensite
• High in Strength and Elongation
• Automotive Industry
• Cooling Problems
TRIP Steels
• FCC- Austenite
• BCC- Ferrite, Martensite and Bainite
• Advanced Methods Can’t be Used
• Limitations in Analysis and Advancements
The Problem
Optical Microscopy-Color Etchants
Etchant Distinguishable Microstructures
Problems
2-5%Nital followed with 10% aq. sodium metabisulfite
Ferrite - Off WhiteBainite, Pearlite and Martensite- DarkAustenite- White
Bainite, Pearlite, and martensite are indistinguishable
2% Nital followed with heat treating
Ferrite and Bainite - BaigeMartensite- Dark BlueAustenite- Purple
Have to heat treat sample.Ferrite and Bainite are the same.
1% Aq. Metabisulfite followed with 4% Picral
Ferrite - Yellowish BlueBainite - BrownAustenite and Martensite- White
Austenite and martensite indistinguishable.
4% Picral followed by 2% aqueous metabisulfite
Ferrite - BrownMartensite and Austenite- White
Austenite and martensite indistinguishable.
4% Picral + HCl followed by 10% aqueous metabisulfite
Ferrite - Off WhiteBainite- Bluish BlackAustenite- WhiteMartensite- Straw Colored
Ferrite and Austenite close in color.
• Topography Measurements
• Martensite and Retained Austenite= Same Height
• Heat Treatment
• Small Scans
AFM
• Image Quality• 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
SEM - EBSD
STILL IN NEED OF A SOLUTION!STILL IN NEED
OF A SOLUTION!
• EBSD/OIM and Optical • Other Property Analysis
• Ability to Reprogram OIM Software
• Medium Size Scans
• Easy Sample Prep
Techniques
• 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)
Proposed EBSD
• 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
Procedure
Optical Imaging Results
• Bainite: Blue
• Martensite: Gold
• Ferrite and Austenite: Off- White
EBSD/ OIM Results
• Problems with Trial 1• Plasma Etch appears to remove material in addition to
hydrocarbons.
• Etchant removes too much material.
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 15-18 sec aq. metabisulfite
• Optical Imaging
• Vibro Polish (.02μm Colloidal Silica)
• OIM
• Compare - Phase and Optical to get all 4 microstructures
Procedure Cont.
• Etchant Time• 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
Etching
0 Hours 1.5 Hours 2.5 Hours 4 Hours
Vibro Polishing• Vibro Polishing Time (.02μm Colloidal
Silica)
No Kikuchi Patterns
No Kikuchi Patterns
Vibro Polishing- 4 Hours
• Vibro Polishing Time
• 4 Hours.... Too Long
• Vibro Polishing Time
• 4 Hours
Before- Fish Grain
After- No Fish
Vibro Polishing- 4 Hours
• Vibro Polishing Time
• 2. 5 Hours
Vibro Polishing- 2.5 Hours
Vibro Polishing- 2.5 Hours
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)
STILL IN NEED OF A SOLUTION!STILL IN NEED
OF A SOLUTION!
• Etchant/ Optical
• Try a new etchant with same procedure.
• Program OIM to differentiate Relationships Between Grains (distinguish martensite)
• AFM- MFM Technique
Future Work
• Transformation to Martensite- Orientation
• Kurdjumov-Sachs Relationship• (110)bcc//(111)fcc, [1-11]bcc//[0-11]fcc
• Study: Misorientation Relationships-Martensite
EBSD Background
Distinguishing Martensite
Thank You!!Questions??????
Thank You!!Questions??????
• All Phases Shown to have some Magnetism
• AFM• Cantilever Tip
• Contact- Repulsion
• Non-contact- Attraction
• MFM• Special Tip
• Resolution 25 to 50 nm
• Promising Solution
AFM and MFM