Masters Theses

Keywords and Phrases

3rd Generation AHSS; Alloy Development; EBSD; Epsilon Martensite; Phase Transformation; Thermodynamic Modeling

Abstract

"FeMnAlSiC steels which exhibit two-stage transformation induced plasticity (TRIP) behavior characterized by the γ→ε→α' dual stage martensitic transformation promise to take a leading role in the development of 3rd generation advanced high strength steels. The crystallographic orientation relationship of the γ→α' and γ→ε athermal martensitic transformations in these steels has been determined as the Kurdjumov-Sachs and the Shoji-Nishiyama, respectively. Six crystallographic variants of α-martensite consisting of three twin-related variant pairs were observed in ε- bands. A planar parallelism of {0001}ε || {110}α' and a directional relation of α' lying within 1⁰ of ε existed for these variants. Two regular solution models have been developed to describe the thermodynamics for the γ→ε, γ→α', and subsequent ε→α' martensitic transformations which best described the behavior and microstructure of various FeMnAlSiC TRIP compositions when compared against other thermodynamic models from literature. The role of available nucleating defects of critical size, n*, has been linked to the intrinsic stacking fault energy (SFE) necessary to observe the athermal γ→ε transformation and it is thus proposed that the amount of ε-martensite in the quenched microstructure is a function of material processing history as well as thermodynamic driving force. The developed thermodynamic model has been used to optimize alloy compositions that produce ideal two-stage TRIP behavior. Compositions with Al contents near 1.5 wt% adequately balance ε- and α-martensite start temperatures such that retained austenite is expected upon quenching to room temperature while also maintaining adequate transformation driving forces to ensure full two-stage TRIP behavior"--Abstract, page iv.

Advisor(s)

Van Aken, David C

Committee Member(s)

OKeefe, Matt
Asle Zaeem, Mohsen
Link, Todd M.

Department(s)

Materials Science and Engineering

Degree Name

M.S. in Metallurgical Engineering

Sponsor(s)

Kent D. Peaslee Steel Manufacturing Research Center
National Science Foundation (U.S.)

Comments

This work has been fully supported by the Kent D. Peaslee Steel Manufacturing Research Center as part of the project Development of New 3rd Generation Advanced High Strength Steel. Scanning electron microscopy was performed using an FEI Helios NanoLab dual beam FIB obtained with a Major Research Instrumentation grant from the National Science Foundation under contract DMR-0723128.

Research Center/Lab(s)

Peaslee Steel Manufacturing Research Center

Publisher

Missouri University of Science and Technology

Publication Date

2014

Journal article titles appearing in thesis/dissertation

  • Thermodynamic driving force of the γ→ε→α' transformation in FeMnAlSiC steels
  • Crystallographic orientation of the ε→α' martensitic (athermal) transformation in a FeMnAlSiC steel

Pagination

x, 84 pages

Note about bibliography

Includes bibliographical references.

Rights

© 2014 Scott Thomas Pisarik, All rights reserved.

Document Type

Thesis - Open Access

File Type

text

Language

English

Subject Headings

Steel alloys -- Analysis
Steel, High strength -- Mechanical properties
Steel -- Hardenability
Phase transformations (Statistical physics
Crystallography

Thesis Number

T 10849

Electronic OCLC #

953991678

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Included in

Metallurgy Commons

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