Microstructures and Mechanical Behavior of NiAl-Strengthened Ferritic Alloys at Room and ...

188 Pages·2016·7.39 MB·English

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Doctoral Dissertations Graduate School
5-2015
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Zhiqian Sun
University of Tennessee - Knoxville, [email protected]
Follow this and additional works at: https://trace.tennessee.edu/utk_graddiss
Part of the Metallurgy Commons, and the Structural Materials Commons
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Sun, Zhiqian, "Microstructures and Mechanical Behavior of NiAl-Strengthened Ferritic Alloys at Room and
Elevated Temperatures. " PhD diss., University of Tennessee, 2015.
https://trace.tennessee.edu/utk_graddiss/3368
This Dissertation is brought to you for free and open access by the Graduate School at TRACE: Tennessee
Research and Creative Exchange. It has been accepted for inclusion in Doctoral Dissertations by an authorized
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[email protected].
To the Graduate Council:
I am submitting herewith a dissertation written by Zhiqian Sun entitled "Microstructures and
Mechanical Behavior of NiAl-Strengthened Ferritic Alloys at Room and Elevated Temperatures." 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.
Peter K. Liaw, Major Professor
We have read this dissertation and recommend its acceptance:
Hahn Choo, Yanfei Gao, John D. Landes
Accepted for the Council:
Carolyn R. Hodges
Vice Provost and Dean of the Graduate School
(Original signatures are on file with official student records.)
Microstructures and Mechanical Behavior of NiAl-
Strengthened Ferritic Alloys at Room and Elevated
Temperatures

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

Zhiqian Sun
May 2015
Copyright © 2015 by Zhiqian Sun.
All rights reserved.

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DEDICATION

The dissertation is dedicated to my beloved family (Weiping Sun, Lefang He, and Li
Sun) and girlfriend, Di Ai.

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ACKNOWLEDGEMENTS

I would like to thank all who have helped me in my pursuing of the Ph.D. degree at The
University of Tennessee. First of all, I am appreciated for the kind/great instructions and
help from my supervisor, Dr. Peter K. Liaw. I would like to thank all committee
members, including Dr. Hahn Choo, Dr. Yanfei Gao, and Dr. John D. Landes for their
suggestions. I also want to express my sincere appreciation to my colleagues and friends,
including Dr. Gongyao Wang, Dr. Shenyan Huang, Dr. Zhengke Teng, and Gian Song. I
enjoyed working with them and have learnt greatly from them. Finally, I want to thank all
collaborators in the project from Northwestern University and University of California,
Berkeley, including Dr. Morris E. Fine, Dr. Gautam Ghosh, Dr. David Dunand, Dr. Nhon
Q Vo, Michael Rawling, Dr. Mark D. Asta, Dr. Christian Liebscher, and Hong Ding.
The work was financially supported by the Department of Energy, Office of Fossil
Energy, under Grants # DE-09NT0008089 and DE-FE0005868 with Richard Dunst, Vito
Cedro, and Patricia Rawls as the program managers. The research benefitted from the use
of facilities at Advanced Photon Source (APS) of Argonne National Laboratory (ANL)
and Lujan Neutron Scattering Center at Los Alamos National Laboratory (LANL). The
ChemMatCARS Sector 15 located at APS is principally supported by National Science
Foundation under Grant # NSF/CHE-1346572. The use of APS was supported by the
U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under
Contract No. DE-AC02-06CH11357. LANL is operated by the Los Alamos National
Security LLC under Contract No. DE-AC52-06NA-25396. I would like to thank all
beamline scientists, including Dr. Jan Ilavsky (APS), Thomas A. Sisneros (LANL), Dr.
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Bjørn Clausen (LANL), and Dr. Donald W. Brown (LANL) for their kind help in the
experiments and data analysis.

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ABSTRACT

In order to improve the thermal efficiency and decrease the greenhouse gases emission,
it is required to increase the steam temperature and pressure in fossil-energy power
plants. In the United States, research has been performing in order to push steam
temperature to 760 ℃ and steam pressure to 35 MPa. However, the highest operational
temperature for current commercial heat-resistant ferritic steels is ~ 620 ℃. In this sense,
new advanced ferritic alloys with better creep resistance are needed, considering such
service conditions in next-generation ultra-supercritical fossil-energy power plants.
Coherent B2-ordered NiAl-type precipitates have been employed to reinforce the body-
centered-cubic iron for high-temperature applications in fossil-energy power plants. One
NiAl-strengthened ferritic alloy, designated as FBB8 (Fe-6.5Al-10Ni-10Cr-3.4Mo-
0.25Zr-0.005B, weight percent), has been investigated in this study. This study mainly
focuses on three critical issues in the development of advanced NiAl-strengthened ferritic
alloys: (1) the stability of NiAl-type precipitates at expected service temperatures (e.g.,
700, 800, and 950 ℃); (2) the poor ductility and fracture mechanisms at room
temperature; and (3) the creep resistance and underlying deformation mechanisms.
The present study achieves a systematical understanding of the microstructure-
mechanical properties relationship in NiAl-strengthened ferritic alloys. It furthers our
fundamental understanding of the phase stability and deformation mechanisms in
precipitate-strengthened alloys, and provides new insights into developing new advanced
materials.
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TABLE OF CONTENTS

CHAPTER I INTRODUCTION ........................................................................................ 1
1.1 Ultra-supercritical fossil-energy power plants .......................................................... 1
1.2 Materials for ultra-supercritical fossil-energy power plants ..................................... 2
1.3 The current state of ferritic heat-resistant steels for fossil-energy power plants ...... 3
CHAPTER II LITERATURE REVIEW ......................................................................... 12
2.1 Strengthening mechanisms in 9-12%Cr ferritic steels ............................................ 12
2.2 Precipitation strengthening ..................................................................................... 15
2.3 NiAl-strengthened ferritic alloys ............................................................................ 18
2.3.1 Physical properties of NiAl .............................................................................. 18
2.3.2 NiAl-strengthened ferritic alloys ..................................................................... 22
2.3.3 Critical issues in developing advanced NiAl-strengthened ferritic alloys ....... 24
CHAPTER III CORSENING BEHAVIOR OF NIAL-TYPE PRECIPITAETS ............ 36
3.1 Introduction ............................................................................................................. 36
3.1.1 Microstructural instability in 9-12%Cr ferritic steels ...................................... 36
3.1.2 Coarsening behavior of 𝛾′ in nickel-based superalloys ................................... 39
3.1.3 NiAl-type precipitates in ferritic alloys ........................................................... 39
3.2 Experimental methods ............................................................................................ 40
3.2.1 Sample preparation .......................................................................................... 40
3.2.2 Experimental setup and data analysis of USAXS ............................................ 40
3.2.3 Microstructural investigation by SEM and TEM............................................. 42
3.3 Results ..................................................................................................................... 42
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3.3.1 Characterization by TEM ................................................................................. 42
3.3.1 Characterization by SEM ................................................................................. 43
3.3.2 Characterization by USAXS ............................................................................ 44
3.4 Discussions ............................................................................................................. 46
3.4.1 Non-uniform distribution of NiAl-type precipitates ........................................ 46
3.4.2 Coarsening rate and interfacial energy between the matrix and NiAl-type
precipitates ................................................................................................................ 47
3.5 Conclusions ............................................................................................................. 50
CHAPTER IV DUPLEX PRECIPITATES AND THEIR EFFECTS ON THE RT
FRACTURE BEHAVIOR ................................................................................................ 75
4.1 Introduction ............................................................................................................. 75
4.2 Experimental methods ............................................................................................ 75
4.3 Results and discussions ........................................................................................... 76
4.3.1 Duplex precipitates in FBB8 ............................................................................ 76
4.3.2 Mechanical properties of FBB8 at RT ............................................................. 77
4.3.3 Strengthening effects from aged and cooling precipitates ............................... 78
4.3.4 Effects of temperatures .................................................................................... 80
4.4 Conclusions ............................................................................................................. 81
CHAPTER V LOAD PARTITIONING BETWEEN THE MATRIX AND NIAL-TYPE
PRECIPITATES ON MULTIPLE SCALES .................................................................... 93
5.1 Introduction ............................................................................................................. 93
5.2 Experiments and modeling ..................................................................................... 94
5.2.1 Materials preparation and characterization by TEM ....................................... 94
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