Doctoral Dissertations

Keywords and Phrases

Atom probe tomography; Orientation image microscopy

Abstract

"Alloy modifications by addition of niobium (Nb ), vanadium (V), nitrogen (N) and cobalt (Co) to cast 17-4 PH steel were investigated to determine the effect on mechanical properties. Additions of Nb, V, and N increased the yield strength from 1120 MPa to 1310 MPa while decreased the room temperature charpy V notch (CVN) toughness from 20 J to four Joules. The addition of Co to cast 17-4 PH steel enhanced the yield strength and CVN toughness from 1140 MPa to 1290 MPa and from 3.7 J to 5.5 J, respectively. In the base 17-4 PH steel, an increase in block width from 2.27 ± 0.10 µm in the solution treated condition to 3.06 ± 0.17 µm upon aging at 755 K was measured using orientation image microscopy. Cobalt inhibited recrystallization and block boundary migration during aging resulting in a finer martensitic block structure. The influence of Co on copper (Cu) precipitation in steels was studied using atom probe tomography. A narrower precipitate size distribution was observed in the steels with Co addition. The concentration profile across the matrix / precipitate interface indicated rejection of Co atoms from the copper precipitates. This behavior was observed to be energetically favorable using first principle calculations. The activation energies for Cu precipitation increased from 205 kJ/ mol in the non-cobalt containing alloy, to 243 kJ/ mol, and 272 kJ/ mol in alloys with 3 wt. %Co, and 7 wt. %Co, respectively. The role of Co on Cu precipitation in cast 17-4 PH steel is proposed as follows: (i) Co is rejected out of the Cu precipitate and sets up a barrier to the growth of the Cu precipitate; (ii) results in Cu precipitates of smaller size and narrower distribution; (iii) the coarsening of Cu precipitates is inhibited; and (iv) the activation energy for Cu precipitation increases"--Abstract, Page iv.

Advisor(s)

Richards, Von

Committee Member(s)

Van Aken, David C.
Lekakh, S. N. (Semen Naumovich)
Medvedeva, Julia E.
OKeefe, Matt

Department(s)

Materials Science and Engineering

Degree Name

Ph. D. in Materials Science and Engineering

Sponsor(s)

Casting for Defense Readiness (CIDR)
Steel Founders' Society of America
Benét Laboratories

Publisher

Missouri University of Science and Technology

Publication Date

Fall 2012

Pagination

xiv, 135 pages

Note about bibliography

Includes bibliographical references.

Rights

© 2012 Arpana Sudershan Murthy, All rights reserved.

Document Type

Dissertation - Open Access

File Type

text

Language

English

Library of Congress Subject Headings

Austenite
Stainless steel
Strength of materials

Thesis Number

T 10036

Print OCLC #

847527065

Electronic OCLC #

847527908

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