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Julie Smith Washington State University

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Presentation on theme: "Julie Smith Washington State University"— Presentation transcript:

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


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