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

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