Doctoral Dissertations

Keywords and Phrases

EBSD; Martensite; Phase Transformation; TRIP Steels

Abstract

"A regular solution model was developed to calculate the chemical driving force for α-martensite formation, ΔGλ→ αChem. A model for the strain energy, ΔGλ→αstr, was formulated utilizing the Young’s modulus (E), lattice misfit squared (δ²), and molar volume (Ω) which opposed the chemical driving force for α-martensite formation. The MαS was determined at a temperature at which ΔGλ→αChem + ΔGλ→αstr = 0. In conjunction with a previously developed ε-martensite model, a means of predicting the volume fraction of λ-austenite was determined; and it was shown that for values of ΔMs < 0, defined as Msε - Msα produced the greatest amounts of retained γ-austenite in the as quenched microstructure. These models were tested, and confirmed, with a new alloy formulated to produce a steel with chromium replacing the traditional aluminum to obtain a ΔMs = -100 C° that exhibited the two-stage TRIP behavior. From this substitution the dynamic strain aging response could be mitigated through M₂₃(C,N)₆ precipitation trapping carbon and nitrogen. The work hardening behavior of these steels was found to be due to the Stage II (ε→α) martensitic reaction and not the dynamic strain aging of the steels. Eight medium-Mn steels were processed and it was found that when the intrinsic stacking fault energy was less than 10.5 mJ/m² the two-stage TRIP response was activated. Empirical relationships for the strength and ductility were determined for the two-stage TRIP steels. The developed models have been used to optimize alloy composition and a designed steel with composition Fe-13.8Mn-1.0Si-3.0Cr-0.15C-0.003N (wt. pct.) is recommended for future investigation"--Abstract, page iv.

Advisor(s)

Van Aken, David C.

Committee Member(s)

O'Keefe, Matthew
O'Malley, Ronald J.
Fahrenholtz, William
Medvedeva, Julia E.

Department(s)

Materials Science and Engineering

Degree Name

Ph. D. in Metallurgical Engineering

Sponsor(s)

Kent D. Peaslee Steel Manufacturing Research Center

Comments

This work has been supported by the Kent D. Peaslee Steel Manufacturing Research Center (PSMRC) as part of the project development of New 3rd Generation Advanced High Strength Steels.

Publisher

Missouri University of Science and Technology

Publication Date

Spring 2018

Journal article titles appearing in thesis/dissertation

  • On the prediction of α-martensite temperatures in medium manganese steels
  • Dynamic strain aging phenomena and tensile response of medium-Mn two-stage TRIP steel
  • Chemistry and properties of medium-Mn two-stage TRIP steels

Pagination

xvi, 157 pages

Note about bibliography

Includes bibliographic references.

Rights

© 2018 Daniel Morye Field, All rights reserved.

Document Type

Dissertation - Open Access

File Type

text

Language

English

Thesis Number

T 11279

Electronic OCLC #

1041858463

Included in

Metallurgy Commons

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