Synthesis and Properties of Nanocrystalline Tungsten Heavy Alloy Powders Consolidated by

130 Pages·2014·9.14 MB·English

by  Steven James Livers

SYNTHESIS AND PROPERTIES OF NANOCRYSTALLINE TUNGSTEN HEAVY
ALLOY POWDERS CONSOLIDATED BY PULSED ELECTRIC CURRENT
SINTERING

by
Steven James Livers

A thesis
submitted in partial fulfillment
of the requirements for the degree of
Master of Science in Materials Science and Engineering
Boise State University

May 2014
© 2014
Steven James Livers
ALL RIGHTS RESERVED
BOISE STATE UNIVERSITY GRADUATE COLLEGE

DEFENSE COMMITTEE AND FINAL READING APPROVALS

of the thesis submitted by

Steven James Livers

Thesis Title: Synthesis and Properties of Nanocrystalline Tungsten Heavy Alloy
Powders Consolidated by Pulsed Electric Current Sintering

Date of Final Oral Examination: 10 December 2013

The following individuals read and discussed the thesis submitted by student Steven
James Livers, and they evaluated his presentation and response to questions during the
final oral examination. They found that the student passed the final oral examination.

Megan Frary, Ph.D. Chair, Supervisory Committee

Rick Ubic, Ph.D. Member, Supervisory Committee

Dmitri Tenne, Ph.D. Member, Supervisory Committee

The final reading approval of the thesis was granted by Megan Frary, Ph.D., Chair of the
Supervisory Committee. The thesis was approved for the Graduate College by John R.
Pelton, Ph.D., Dean of the Graduate College.
ACKNOWLEDGEMENTS
I would like to thank my faculty advisors, Dr. Megan Frary and Dr. Michael
Hurley, for their continued support and advice throughout my graduate program. I would
also like to thank my thesis committee members, Dr. Frary, Dr. Ubic, and Dr. Tenne for
their advice and contributions to completing this work.
A special thanks to the collaborators at MIT for their contributions in the powder
processing required for this project. I would also like to thank the Defense Threat
Reduction Agency for funding this project for basic research to improve our capabilities
to counter threats. Additional thanks to the Center for Advanced Energy Studies, which
housed the pulsed electric current sintering machine and Dr. Allahar and Brian Forsmann
for their expertise with its operation.
This work wouldn’t have been completed in a timely manner without
undergraduate support and the Frary Research group. Megan Beck, Kosette Leperi, and
Chris Stifter contributed hard work and dedication to this project, which was greatly
appreciated.
Family support also contributed to this work. Thank you, Peter Livers and
Barbara Livers, for your continued support and words of encouragement. Special thanks
to James Livers, my grandfather, who has helped all of his grandchildren complete
graduate school.
iv
ABSTRACT
Amorphous and nanocrystalline W-based materials are candidate alloys where
high toughness and abrasion resistance are required, such as for kinetic energy
penetrators (KEPs). Using a bottom-up approach to produce a tungsten heavy alloy
(WHA), W-based alloys, including W-Mo-Fe and W-Cr-Fe, have been produced using
high-energy ball milling and consolidated with pulsed electric current sintering (PECS).
Powder consolidation using PECS allows for fast consolidation and for high relative
densities to be achieved after sintering at a fraction of the melting temperature. Rapid
consolidation of the alloyed powders also reduces grain growth, resulting in improved
mechanical properties. During consolidation of mechanically alloyed powders, the
structure evolves with sintering conditions due to the residual stress introduced during
ball milling and the addition of other component elements added to promote
amorphization. Multiple nanoscale crystalline microstructures were seen in the W based
alloys depending on processing conditions. Negligible porosity was achieved at nearly
one third of homologous temperature, 1050 °C for W-Mo-Fe and 1200 °C for W-Cr-Fe,
using nanocrystalline powders and consolidating with PECS. Specimens containing
nanocrystalline microstructures showed extremely high hardness and improved
mechanical properties over several baseline microcrystalline tungsten alloys.
Consolidated alloys had improved hardnesses over baseline materials, up to 300%.
Intermetallic phase formation aided sintering by filling in pores and acting as a binder
phase between nanocrystalline powder particles. At temperatures above 1500 °C, liquid-
v
phase sintering characteristics were observed, including the formation of spheroidal
grains and rapid grain growth. PECS offers a novel processing route for tailoring the
structure and properties of these advanced alloys by altering the grain size. The ballistic
performance of these alloys under selected process parameters will be evaluated during
subsequent studies and will determine whether or not these alloys can be used as effective
KEPs.
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TABLE OF CONTENTS
ACKNOWLEDGEMENTS ..................................................................................................... iv 
ABSTRACT .............................................................................................................................. v 
LIST OF TABLES .................................................................................................................... x 
LIST OF FIGURES ................................................................................................................ xii 
LIST OF ABBREVIATIONS .............................................................................................. xviii 
CHAPTER ONE: INTRODUCTION ....................................................................................... 1 
CHAPTER TWO: BACKGROUND ........................................................................................ 3 
2.1 Kinetic Energy Penetrators ..................................................................................... 3 
2.1.1 Depleted Uranium .................................................................................... 3 
2.1.2 Tungsten Heavy Alloys............................................................................ 4 
2.2 Amorphous Metals .................................................................................................. 6 
2.2.1 Amorphous Metal Glass-Forming Principles .......................................... 6 
2.2.2 Amorphous Metal Processing .................................................................. 6 
2.2.3 Amorphous Tungsten ............................................................................... 7 
2.2.4 Amorphous Metal Deformation ............................................................... 8 
2.3 Nanocrystalline Metals ........................................................................................... 9 
2.3.1 Nanocrystalline Metals Processing .......................................................... 9 
2.3.2 Nanocrystalline Tungsten ...................................................................... 10 
2.3.3 Hall-Petch Relationship and Breakdown ............................................... 11 
vii
2.4 Powder Metallurgy................................................................................................ 11 
2.4.1 Mechanical Alloying .............................................................................. 12 
2.4.2 Solid-State Amorphization ..................................................................... 13 
2.4.3 Sintering ................................................................................................. 14 
2.4.4 Pulsed Electric Current Sintering ........................................................... 15 
2.4.5 Pulsed Electric Current Sintering of Amorphous and Nanocrystalline
Powders ........................................................................................................... 16 
CHAPTER THREE: EXPERIMENTAL ................................................................................ 18 
3.1 Mechanical Alloying ............................................................................................. 18 
3.2 Die Assembly ........................................................................................................ 19 
3.3 Pulsed Electric Current Sintering.......................................................................... 20 
3.4 Specimen Preparation ........................................................................................... 23 
3.5 Analysis Techniques ............................................................................................. 25 
3.5.1 Vickers Microhardness .......................................................................... 25 
3.5.2 Archimedes Principle ............................................................................. 25 
3.5.3 Scanning Electron Microscopy .............................................................. 26 
3.5.4 Grain Size Analysis ................................................................................ 27 
3.5.5 Transmission Electron Microscopy ....................................................... 27 
3.5.6 X-ray Diffraction ................................................................................... 28 
3.5.7 Williamson-Hall Technique ................................................................... 29 
3.5.8 Lattice-Parameter Determination ........................................................... 29 
3.6 Baseline Kinetic Energy Penetrator Materials ...................................................... 30 
CHAPTER FOUR: RESULTS AND DISCUSSION ............................................................. 32 
4.1 Powder Analysis ................................................................................................... 32 
viii
4.2 Density of Consolidated Alloys ............................................................................ 39 
4.3 Porosity ................................................................................................................. 41 
4.4 Vickers Microhardness ......................................................................................... 44 
4.5 Consolidated Specimen Morphology .................................................................... 46 
4.5.1 W+ Alloy Morphology .......................................................................... 46 
4.5.2 WCr Alloy Morphology ......................................................................... 51 
4.5.3 WMo Alloy Morphology ....................................................................... 61 
4.5.4 Intermetallic Phase Evolution ................................................................ 66 
4.5.5 Liquid-Phase Sintering ........................................................................... 72 
4.6 Crystal Structure of Milled Powders and Consolidated Specimens ..................... 76 
4.6.1 Lattice-Parameter Determination ........................................................... 81 
4.7 Grain Size Analysis............................................................................................... 82 
4.7.1 Crystallite Size Analysis ........................................................................ 83 
4.7.2 Grain Size vs. Hardness ......................................................................... 87 
4.7.3 Hall-Petch Relationship ......................................................................... 88 
CHAPTER FIVE: CONCLUSIONS ...................................................................................... 91 
CHAPTER SIX: FUTURE WORK ........................................................................................ 93 
REFERENCES ....................................................................................................................... 95 
APPENDIX A ....................................................................................................................... 107 
Other Common Names for Pulsed Electric Current Sintering87 ............................... 107 
APPENDIX B ....................................................................................................................... 110 
Results for PECS W+, W9Cr1, W8Cr2, W9Mo1, and W8Mo2 Specimens ............ 110 
ix
LIST OF TABLES
Table 1  HP specimen matrix for 1 min and 20 min dwell times. .......................... 22 
Table 2  LP specimen matrix for 1 min and 20 min dwell times. ........................... 23 
Table 3  Powder composition measured by EDS in at.% for candidate alloys milled
for 10 and 20 hours, estimated error +/- 0.5 (at.%)................................... 32 
Table 4  Crystallite size and mean particle diameter for initial powders. ............... 38 
Table 5  Highest hardnesses of HP W+, W9Cr1, W8Cr2, W9Mo1, and W8Mo2
alloys with consolidation temperature, dwell time and homologous
temperature. .............................................................................................. 46 
Table 6  EDS results in at.% for W+-1100C-1min-HP, corresponding with Figure
19a. ............................................................................................................ 48 
Table 7  EDS results in at.% for W9Cr1-1100C-1min-HP, corresponding to Figure
23............................................................................................................... 53 
Table 8  EDS results in at.% for W8Cr2-1100C-20min-HP, corresponding to
Figure 25. .................................................................................................. 57 
Table 9  EDS results in at.% for the intermetallic phase seen in W8Mo2-1100C-
20min-HP, corresponding to spectrum locations in Figure 31. ................ 65 
Table 10  EDS results in at.% for the intermetallic phase seen in W8Mo2-1100C-
20min-HP, corresponding to spectrum locations in Figure 32. ................ 66 
Table 11  Selected point EDS analysis in at.% of the intermetallic phase in W9Cr1-
1400C-20min-LP, corresponding to spectrum point in Figure 33. ........... 68 
Table 12  Selected point EDS analysis in at.% of the W solid solution in W9Cr1-
1400C-20min-LP, corresponding to spectrum point in Figure 34. ........... 68 
Table 13  EDS results in at.% of a drip from W9Cr1-1600C-1min-LP, corresponding
to Figure 41. .............................................................................................. 75 
x