Process Kinetics of Transient Liquid Phase Sintering in a Binary-Isomorphous Alloy System

319 Pages·2007·11.67 MB·English

Process Kinetics of Transient Liquid Phase
Sintering in a Binary-Isomorphous Alloy
System

by

Dennis Michael Ryan Turriff

A thesis
presented to the University of Waterloo
in fulfillment of the
thesis requirements for the degree of
Doctor of Philosophy
In
Mechanical Engineering

Waterloo, Ontario, Canada, 2007

© Dennis Michael Ryan Turriff 2007
Author’s Declaration

I hereby declare that I am the sole author of this thesis. This is a true copy of the thesis,
including any required final revisions, as accepted by my examiners.

I understand that my thesis may be made electronically available to the public.
ii
Abstract
The problem of inadequate measurement techniques for quantifying the isothermal
solidification process during transient liquid phase sintering (TLPS) in binary isomorphous
systems such as Ni-Cu, and the resulting uncertainty regarding the solidification mechanism
and its sensitivity to important process parameters, has been investigated. A unique
combination of differential scanning calorimetry (DSC), neutron diffraction (ND), and
metallographic techniques has enabled the quantitative characterization of important TLPS
stages (i.e., solid-state sintering, melting and dissolution, isothermal solidification, and
homogenization) as well as verifying the re-melt behaviour of post-sintered specimens and
measuring variable melting point (VMP) properties. This has resulted in the advancement of
the fundamental understanding of liquid formation and its removal mechanism during
isothermal, or diffusional, solidification. The Ni-Cu system was chosen for experimentation
due to its commercial relevance as a braze filler material and also because it is an ideal model
system (due to its isomorphous character) that is not well understood on a quantitative or
phenomenological basis. Samples consisted of elemental Ni and Cu powder mixtures of
varying particle size and composition.
In DSC experiments, the progress of isothermal solidification was determined by
measuring the enthalpy of melting and solidification after isothermal hold periods of varying
length and comparing these to the measured enthalpy of pure Cu. The low melting enthalpies
measured for all Ni/Cu mixtures heated just past the Cu melting point (1090°C) indicate that
solid-state sintering and interdiffusion during heat-up significantly suppress initial liquid
formation and densification from the wetting liquid. For samples heated well past the Cu
melting point (1140°C), Ni dissolution causes increased initial liquid fractions and
densification. It was found that significantly more time was required for complete liquid
removal at 1140°C vs. 1090°C. This is attributed to the observed increase in initial liquid
fractions formed at higher processing temperatures due to the dissolution of Ni. This
effectively counteracts the increased diffusivities at these temperatures, and thus more time is
required to completely remove the increased liquid content. TLP mixtures sintered at 1140°C
using three different particle sizes revealed that fine base metal Ni particles cause high degrees
of solid-state interdiffusion during heat-up, small initial liquid fractions, and accelerated liquid
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removal rates due to high surface area/volume ratios. A diffusion-based analytical model was
developed to account for these effects (i.e., particle size, temperature, solid-state sintering, and
dissolution). Comparison with experimental DSC results reveals that this model can accurately
predict liquid removal given accurate diffusivities. Metallographic analysis of post-sintered
DSC specimens via SEM and EDS indicates that isothermal liquid solidification leaves behind
Ni-rich cores surrounded by Cu-rich matrix regions having compositions given by the Ni-Cu
phase diagram solidus (C ) at a selected isothermal processing temperature (T ).
S P
ND experiments were used to investigate the melting event and interdiffusion during the
isothermal hold segment by analyzing the evolution of the {200} FCC peaks of Ni and Cu.
ND patterns were collected in situ at 1 minute intervals during prolonged sintering cycles for
larger powder specimens. The Cu melting event was characterized by an abrupt decrease in Cu
peak intensity at 1085°C as well as a shift towards higher 2θ angles corresponding to lower Cu
contents. This shifted residual peak (hereafter referred to as the C peak) originates from
S
regions of the specimen having compositions near solidus at T . Immediately following the
P
melting event, the evolution of ND patterns shows that these C peaks grow rapidly, indicating
S
the isothermal growth of a Cu-rich phase. These in situ findings confirmed the metallographic
and DSC data and indicated that isothermal solidification of the liquid phase proceeds via the
growth of a solute-rich solid solution layer surrounding the Ni particles. This occurs by the
transient progression of the solid/liquid interface at compositions given by the liquidus and
solidus (C /C ). During sintering, diffraction intensities gradually increased at intermediate 2θ
S L
angles between previous Ni and Cu peaks. ND patterns gradually evolved from initially
having a broad double-peak profile to a sharper single-peak profile due to increased Ni-Cu
interdiffusion. The 2θ position and width of the post-sintered peaks indicated very
homogeneous sintered alloys. Metallographic analysis of post-sintered specimens having
undergone prolonged sintering and homogenization revealed extensive Kirkendall pore
formation from unequal diffusivities (D > D ).
Cu Ni
In this study, the unique combination of diffusion-based modelling as well as DSC, ND,
and supporting metallographic analysis has enabled the identification of characteristic sintering
behaviour, important process parameters, and processing windows for TLPS in Ni-Cu systems.
Quantitative and in situ information of this nature is absent in the previous TLPS literature.
iv
Acknowledgements
I thank my supervisor, Dr. Stephen Corbin, for my introduction to research and the
opportunity to work together on this project. I appreciate the support and freedom he has given
me to pursue various academic and research interests as well as the timely guidance and insight
provided when it was needed. Without his influence and mentorship, I would likely not have
pursued this path to the extent that I have. Thank you for sparking the curious researcher in
me.

I thank the numerous co-workers and friends who have supported me throughout my
time here. I would like to particularly acknowledge certain people for their support and
friendship over the years, namely: Aarati Vigneswaran, Alex Bardelcik, Caroline Amyot,
Gregory Bourne, Roydyn Clayton, Ryan Clemmer, Mike Kuntz, Jeff McIsaac, Valentina Ngai,
Saleh Tabandeh, Mark Whitney. I have shared memorable experiences, and learned so much
from all of you. You have all contributed to make this a fruitful and enjoyable journey. Alex
and Valentina, I am so glad to have met you during my studies. Thank you for your friendship
and everything else that is much too long for these pages to rightly describe.

Many thanks must also go to Lachlan Cranswick, Mike Watson, and the Canadian
Neutron Beam Centre (CNBC, Chalk River, Ontario, Canada) for their support and important
technical contributions, which enabled novel neutron diffraction studies. This work has also
been gratefully supported by funding from Materials and Manufacturing Ontario (MMO), an
Ontario Centre of Excellence (OCE), and the Natural Sciences and Engineering Research
Council of Canada (NSERC). Additional financial support from the University of Waterloo is
gratefully appreciated.
v
For my parents, who have raised me to be who I am today and have made this possible. For
my sister who will always be so close to me.

And for Valentina, who has been with me almost every step of the way.

"Fear paralyzes; curiosity empowers. Be more interested than afraid."
- Patricia Alexander

vi
Table of Contents
Abstract.......................................................................................................................................iii
Acknowledgements....................................................................................................................vii
List of Tables...............................................................................................................................x
List of Figures.............................................................................................................................xi
1. Introduction........................................................................................................................1
1.1. TLP Sintering Background.........................................................................................1
1.2. Problem.......................................................................................................................4
1.3. Objectives....................................................................................................................5
2. Literature Review and Research Justification...................................................................7
2.1. Sintering Background and Basic Theory....................................................................7
2.2. Process Variations.....................................................................................................13
2.3. Solid-State Sintering.................................................................................................14
2.4. Liquid Phase Sintering..............................................................................................17
2.5. Transient Liquid Phase (TLP) Sintering...................................................................21
2.5.1. Process description and nomenclature...................................................................21
2.6. Non-DSC Studies and Important Process Parameters..............................................24
2.7. Ni-Cu Sintering Studies............................................................................................29
2.8. Modelling of TLPS...................................................................................................34
2.8.1. Mathematics of Binary Diffusion..........................................................................34
2.8.2. Analytical Solution for Spherical Particles............................................................36
2.8.3. Simplified TLPS Isothermal Solidification Model................................................39
2.8.4. Numerical Modelling.............................................................................................44
2.9. Mass Transport Considerations.................................................................................48
2.9.1. Grain Boundary Effects.........................................................................................50
2.9.2. Concentration Effects............................................................................................55
2.10. Differential Scanning Calorimetry............................................................................58
2.10.1. Using DSC to Measure TLPS Kinetics..............................................................63
2.11. X-ray and Neutron Diffraction Techniques..............................................................65
2.11.1. Powder Diffraction Theory.................................................................................66
2.11.2. Lattice Parameter Measurement.........................................................................73
2.11.3. X-ray Diffraction (XRD)....................................................................................74
2.11.4. Neutron Diffraction (ND)...................................................................................77
2.11.5. Temperature Effects3.........................................................................................82
2.11.6. Interdiffusion and Alloying Effects/Studies.......................................................85
2.11.7. Quantitative Analysis and Sources of Error.......................................................91
2.12. Justification of Current Work...................................................................................97
2.13. Scope, Criteria and Constraints...............................................................................100
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3. Experimental Methods...................................................................................................101
3.1. Materials..................................................................................................................101
3.2. DSC Experiments....................................................................................................104
3.2.1. Equipment............................................................................................................104
3.2.2. Sample Preparation Techniques...........................................................................107
3.2.3. DSC Operation and Temperature Programs........................................................108
3.2.4. Analysis of DSC results.......................................................................................112
3.3. Neutron Diffraction Experiments............................................................................115
3.3.1. Material Characterization....................................................................................116
3.3.2. C2 Neutron Powder Diffractometer.....................................................................121
3.3.3. C2 Sample Environment and Temperature..........................................................123
3.3.4. Temperature Programs.........................................................................................128
3.3.5. Data Acquisition..................................................................................................129
3.3.6. Analysis of ND Results........................................................................................132
3.4. Metallographic Analysis.........................................................................................134
3.4.1. Sample Preparation..............................................................................................134
3.4.2. Optical Microscopy..............................................................................................134
3.4.3. Scanning Electron Microscopy............................................................................135
4. DSC experiments...........................................................................................................136
4.1. Results and Analysis...............................................................................................136
4.1.1. Characterization of Pure Cu Powder Melting......................................................136
4.1.2. Initial Characterization of DSC traces for Cu-Ni TLPS Mixtures.......................139
4.1.3. Solid-State Sintering and Interdiffusion..............................................................142
4.1.4. Melting & Dissolution.........................................................................................143
4.1.5. Isothermal Solidification......................................................................................150
4.2. Homogenization and Reheating..............................................................................161
4.3. Discussion...............................................................................................................163
4.3.1. Temperature Effects.............................................................................................163
4.3.2. Isothermal Solidification Rates............................................................................166
4.3.3. Sample Type and Liquid Distribution Effects.....................................................167
4.3.4. Ni Particle Size Effects........................................................................................168
4.4. Summary.................................................................................................................170
5. Neutron Diffraction Experiments..................................................................................171
5.1. Results and Analysis of Preliminary Experiments..................................................172
5.1.1. ND Pattern Assessment and Powder Characterization (Sample 1).....................172
5.1.2. Cu Melting and Thermal Expansion (sample 2)..................................................177
5.1.3. Non-interacting Cu + Ni Experiment (Sample 3)................................................191
5.2. Cu-Ni TLPS Experiments.......................................................................................198
5.2.1. Results..................................................................................................................198
5.3. Discussion...............................................................................................................206
5.3.1. Solid-state Sintering Stage...................................................................................206
5.3.2. Isothermal Solidification Stage............................................................................214
5.3.3. Temperature Effects.............................................................................................223
5.4. Metallographic Analysis.........................................................................................236
5.5. Summary.................................................................................................................242
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6. TLPS Modelling in Isomorphous Systems....................................................................245
6.1. Limitations and Implications of the Simplified Model...........................................246
6.2. Model Development................................................................................................246
6.3. Solid-state interdiffusion.........................................................................................251
6.4. Isothermal Solidification Predictions at T .............................................................257
A
6.5. Dissolution and Melt-Back Considerations for T > T .........................................261
P A
6.6. Comparison of Model to Experimental Results......................................................265
6.6.1. Diffusivity Effects................................................................................................265
6.6.2. Mixture Inhomogeneity Effects...........................................................................273
6.6.3. Particle Size Effects.............................................................................................275
6.7. Summary.................................................................................................................277
7. Conclusions....................................................................................................................279
7.1. Differential Scanning Calorimetry Results.............................................................279
7.2. Neutron Diffraction Results....................................................................................282
7.3. Modelling Results...................................................................................................284
7.4. Recommendations and Future Work.......................................................................286
8. References......................................................................................................................296

ix
List of Tables
Table 2-1: Lattice and boundary diffusivity data for Cu in pure Ni [1,51]...............................50
Table 2-2: Penetration depths (I = 0.1I , in mm) of thermal neutrons and x-rays in various
O
materials [74]..........................................................................................................80
Table 3-1: Powder data............................................................................................................103
Table 3-2: X-ray diffractometer specifications........................................................................117
Table 3-3: GSAS LeBail fit results for INEL and Bruker XRD data......................................121
Table 3-4: C2 Neutron powder diffractometer specifications.................................................121
Table 3-5: C2 Furnace conditions............................................................................................124
Table 4-1: Measured enthalpies (∆H ) for pure Cu powder heated to T = 1090°C...............138
m P
Table 4-2: Measured enthalpies (∆H ) for pure Cu powder heated to T = 1140°C...............138
m P
Table 4-3: Measured enthalpies (∆H ) for 65 wt% Cu Type B mixtures heated to T = 1140°C.
m P
..............................................................................................................................142
Table 4-4: DSC and density data for Type A and B mixtures heated to T and immediately
P
cooled....................................................................................................................145
Table 4-5: Exotherm enthalpy, ∆H (J/g) and liquid fraction (W ) data from DSC at increasing
m A
hold times for Type A and B mixtures TLP sintered at 1090°C and 1140°C......152
Table 4-6: Melting point shift data for layered/pressed mixtures (Type A)............................162
Table 5-1: List of ND experiments..........................................................................................171
Table 5-2: Comparison of Al O lattice parameters calculated by GSAS for XRD and ND data.
2 3
..............................................................................................................................174
Table 5-3: Ni and Cu lattice parameters calculated by GSAS for ND data.............................174
Table 5-4: Cu and Ni neutron diffraction peak locations at room temperature (λ = 1.33069Å)
..............................................................................................................................175
Table 5-5: Measured Cu peak locations and calculated thermal expansion values (sample 2)183
Table 5-6: Measured Al O peak locations and calculated thermal expansion values (sample 2)
2 3
..............................................................................................................................184
Table 5-7: Comparison of measured thermocouple temperatures and calculated temperatures
for sample 2..........................................................................................................187
Table 5-8: Expected expansion of Al O internal standard in ND experiments (λ = 1.33069Å).
2 3
..............................................................................................................................189
Table 5-9: Summary of expected temperature effects on TLPS (↑ T )...................................224
P
Table 6-1: List of parameters and nomenclature.....................................................................248
Table 6-2: Values of model parameters following solid-state sintering and dissolution.........251
x