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Explore improved methods to differentiate between microstructures in TRIP steel, focusing on the limitations of current techniques and proposing solutions involving EBSD and optical imaging.
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Distinguishing Microstructures in TRIP Steel using EBSD And Optical Imaging • Julie Smith • Washington State University • Materials Science and Engineering Department
TRIP Steels • Transformed Induced Plasticity • Ret. Austenite to Martensite • High in Strength and Elongation • Automotive Industry • Cooling Problems
The Problem • FCC- Austenite • BCC- Ferrite, Martensite and Bainite • Advanced Methods Can’t be Used • Limitations in Analysis and Advancements
AFM • Topography Measurements • Martensite and Retained Austenite= Same Height • Heat Treatment • Small Scans
SEM - EBSD • 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
Techniques • EBSD/OIM and Optical • Other Property Analysis • Ability to Reprogram OIM Software • Medium Size Scans • Easy Sample Prep
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)
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
Optical Imaging Results • Bainite: Blue • Martensite: Gold • Ferrite and Austenite: Off- White
Procedure Cont. • 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
Etching • 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
Vibro Polishing • Vibro Polishing Time (.02μm Colloidal Silica) 0 Hours 1.5 Hours 2.5 Hours 4 Hours No Kikuchi Patterns No Kikuchi Patterns
Vibro Polishing- 4 Hours • Vibro Polishing Time • 4 Hours.... Too Long
Vibro Polishing- 4 Hours • Vibro Polishing Time • 4 Hours After- No Fish Before- Fish Grain
Vibro Polishing- 2.5 Hours • Vibro Polishing Time • 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)
Future Work • Etchant/ Optical • Try a new etchant with same procedure. • Program OIM to differentiate Relationships Between Grains (distinguish martensite) • AFM- MFM Technique
EBSD Background • Transformation to Martensite- Orientation • Kurdjumov-Sachs Relationship • (110)bcc//(111)fcc, [1-11]bcc//[0-11]fcc • Study: Misorientation Relationships-Martensite
AFM and MFM • 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