Processing and Characterization of Oxide Dispersion Strengthened 14YWT Ferritic Alloys

102 Pages·2016·13.25 MB·English

by  Michael Keith West

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Doctoral Dissertations Graduate School
8-2006
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Michael Keith West
University of Tennessee - Knoxville
Follow this and additional works at: https://trace.tennessee.edu/utk_graddiss
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West, Michael Keith, "Processing and Characterization of Oxide Dispersion Strengthened 14YWT Ferritic
Alloys. " PhD diss., University of Tennessee, 2006.
https://trace.tennessee.edu/utk_graddiss/1890
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To the Graduate Council:
I am submitting herewith a dissertation written by Michael Keith West entitled "Processing and
Characterization of Oxide Dispersion Strengthened 14YWT Ferritic Alloys." I have examined the
final electronic copy of this dissertation for form and content and recommend that it be
accepted in partial fulfillment of the requirements for the degree of Doctor of Philosophy, with a
major in Materials Science and Engineering.
George M. Pharr, Major Professor
We have read this dissertation and recommend its acceptance:
Easo P. George, John D. Landes, Carl J. McHargue
Accepted for the Council:
Carolyn R. Hodges
Vice Provost and Dean of the Graduate School
(Original signatures are on file with official student records.)
To the Graduate Council:

I am submitting herewith a dissertation written by Michael Keith West entitled
"Processing and Characterization of Oxide Dispersion Strengthened 14YWT Ferritic
Alloys." I have examined the final electronic copy of this dissertation for form and
content and recommend that it be accepted in partial fulfillment of the requirements
for the degree of Doctor of Philosophy, with a major in Materials Science and
Engineering.

George M. Pharr
_______________________
Major Professor

We have read this dissertation
and recommend its acceptance:

Easo P. George
____________________________

John D. Landes
____________________________

Carl J. McHargue
____________________________

Accepted for the Council:

Anne Mayhew
_______________________
Vice Chancellor and
Dean of Graduate Studies

(Original signatures are on file with official student records.)
PROCESSING AND CHARACTERIZATION OF OXIDE
DISPERSION STRENGTHENED 14YWT FERRITIC ALLOYS

A Dissertation
Presented for the
Doctor of Philosophy
Degree
The University of Tennessee, Knoxville

Michael Keith West
August 2006
ACKNOWLEDGMENT
I would like to thank my advisor Prof. George M. Pharr for his patience and
guidance over the years. I would like to thank my committee members Prof. Charlie
Brooks, Prof. Easo George, Prof. Carl McHargue, and Prof. John Landes. I am
greatly indebted to the many scientists and staff members at Oak Ridge National
Laboratory who provided me with countless hours of help and advice. Foremost of
these, is David Hoelzer, without whose help this work could not have been finished. I
would also like to thank Claudia Rawn, Scott Speakman, Andrew Payzant, Wally
Porter, Ed Kenik, Mike Miller, Jim Bentley, Jackie Mayotte, Cecil Carmichael, and
Shirley Waters. I am grateful to the faculty and staff in the Materials Science and
Engineering Department at the University of Tennessee and my lab-mates James
Lamanna, Jeremy Strader, Songqing Wen, Sangjoon Sohn, and Dongyun Lee for their
help and friendship. Finally, I would like to thank my family and friends for their
moral support over the years.
This research was sponsored by the Office of Fusion Energy Sciences, U. S.
Department of Energy and the SHaRE Collaborative Research Center at Oak Ridge
National Laboratory under contract DE-AC05-00OR22725 with UT-Battelle, LLC.

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ABSTRACT
Oxide dispersion strengthened (ODS) ferritic steels are currently being
investigated as candidate materials for nuclear applications due to their increased high
temperature strength and low activation characteristics. Recent studies have shown
that ODS ferritic steels containing Ti exhibit enhanced high temperature properties
due to the formation of a very fine dispersion of nanometer-sized oxide clusters based
on Ti, Y, and O. Studies are currently underway to examine so called 14YWT alloys
with nominal compositions of Fe-14Cr-3W-0.4Ti (wt. %) mechanically alloyed with
0.25 (wt.%) Y O . The focus of this study was to investigate how the early stages of
2 3
processing of 14YWT alloys during mechanical milling, heat treatment, and
consolidation affect the structure and properties of the alloys.
The 14YWT alloys were milled at different times up to 80 hours, along with
alloy powder compositions of Fe-14Cr + 0.25 wt.% Y O (14Y) and Fe-14Cr without
2 3
Y O (Fe-14Cr). The evolution of the microstructure and mechanical properties
2 3
during milling was examined with a combination of optical metallography, x-ray
diffraction, electron microscopy, atom probe tomography, and nanoindentation.
Alloy powders were also heat treated and studied using high temperature x-ray
diffraction and differential scanning calorimetry methods. Special attention was paid
to milling parameters and temperature ranges which lead to the formation of nano-
sized oxide clusters in the alloys. Finally, the microstructure of consolidated alloys
was examined and related to milling and heat treatment methods.
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Mechanical properties and microstructure during milling were similar in the
three alloy powders examined regardless of dispersoid or alloy addition. Mechanical
mixing of the alloy powders was inefficient after 40 hours of milling. Milling did not
produce bulk amorphous phases but quickly reduced the crystallite size to ~10-20 nm.
Milling also resulted in relatively uniform dissolution of Y O . Thermal studies in
2 3
the range of 600-850 °C showed precipitation of oxide clusters occurred before
consolidation which blocked recrystallization and grain growth. Results also showed
retention of sub-micron sized grains at high temperature in both the annealed powders
and extruded material was seen to be direct evidence of nano-cluster formation.
Alloy extrusions conducted at 1175 °C showed coarse oxide precipitation while
extrusions at 850 °C showed nanometer-sized clusters. The results indicated that the
heat treatment temperature before consolidation was very important for producing the
most favorable microstructure.

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TABLE OF CONTENTS
1 INTRODUCTION.............................................................................................1
1.1 Mechanical Alloying..................................................................................1
1.2 ODS Ferritic Alloys...................................................................................7
1.3 Research Objectives.................................................................................12
2 EXPERIMENTAL PROCEDURES.................................................................15
2.1 Mechanical Milling of Alloy Powders......................................................15
2.2 Alloy Extrusion........................................................................................15
2.3 Metallography..........................................................................................18
2.4 Powder Morphology.................................................................................18
2.5 Nanoindentation.......................................................................................19
2.6 X-ray Diffraction......................................................................................19
2.7 Heat Treating of Powders.........................................................................22
2.8 Differential Scanning Calorimetry............................................................22
2.9 Electron Microscopy and Atom Probe Characterization............................22
3 RESULTS AND DISCUSSION.......................................................................24
3.1 Examination of Alloy Powders Before Milling.........................................24
3.2 Milled Powder Morphology.....................................................................24
3.3 Chemical Composition and Contamination of Milled Powders.................29
3.4 Structure of Milled Alloy Powders...........................................................33
3.5 Mechanical Properties of Milled Powders................................................46
3.6 Thermal Effects on Powders.....................................................................49
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3.7 Alloy Extrusions......................................................................................70
3.8 Discussion................................................................................................78
4 CONCLUSION...............................................................................................81
REFERENCES........................................................................................................83
VITA.......................................................................................................................88

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LIST OF TABLES
Table 1 Compositions of some commercially available ODS ferritic steels................8
Table 2 Alloy powder compositions before milling..................................................16
Table 3 Amount of important trace elements after milling for 80 hours....................34
Table 4 Crystallite size and strain measurements for FeCr, 14Y, and 14YWT alloys
after 80 hours milling.......................................................................................47

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