I'd like to thank my beloved Anna Cieslinska for her love and support during the crucial ...... G. Heiberg, K. Nogita, A. K. Dahle, L. Arnberg, Acta Mat., 50(10) , pp.
1 ABSTRACT
Studying solid-liquid interfaces in multi-component alloys is important for understanding the thermodynamic and kinetic behaviors of phases during solidification and melting. Previous investigations pertaining to characterization of solid-liquid interfaces have been limited due to restricted experimental accessibility. In this study, solid-liquid interfaces in atomized powders of hypereutectic Al-Si based alloy were investigated using in-situ transmission electron microscopy (TEM). A thermal shield developed during the study allowed chemical characterization of solid-liquid interfaces and phases to be performed as a function of temperature, thereby directly determining solute partitioning and the metastable phase boundary of the undercooled liquid. In addition, kinetic analyses involving the nucleation, growth and dissolution behaviors of primary Si and the Al solid-solution were also performed. The morphological evolutionary paths of primary Si and the Al solid-solution were found to be fundamentally different due to the underlying interfacial energetics. Moreover, electron energy-loss spectroscopy (EELS) was used to study the variation in plasmon energy as a function of temperature in liquid and solid phases and across the solid-liquid interface. The plasmon energy-temperature trend in the liquid alloy was found to be markedly different from the case of pure liquid Al, revealing the electronic effects of alloying additions to the liquid phase. The energy-loss near edge structures (ELNES) of the liquid alloy also showed remarkably different electronic structure of the unoccupied density of states (DOS) in comparison to pure liquid Al, indicating a fundamental electronic structure variation in the liquid due to solute additions.
2 ACKNOWLEDGEMENTS
First and foremost, I’d like to sincerely thank Prof. James M. Howe for his valuable feedback, scholarly guidance and for creating a very friendly atmosphere to carry out this research. His patience and timely availability for discussions greatly contributed to successful completion of this work. I consider myself extremely lucky to be under his tutelage for my professional and personal development. His dedication, sincerity and creative methods for scientific research will continue to inspire me.
My committee members, Profs. Gary J. Shiflet, S. Joseph Poon, William A. Soffa and Leonid V. Zhigilei were of great help and guidance during this work. I’m deeply indebted for their feedback and deep insights.
Thanks are also due to Dr. G. Muralidharan, Scientific Staff, Oak Ridge National Laboratory, TN, for his contribution related to the calculated phase diagrams using JMatPro. I’d also like to thank Mr. Peter Schare for his help with developing the thermal shield, Mr. Eric Lass for his help with the DTA experiments, Dr. Mitsu Murayama for training me on the analytical TEM and Mr. Richard White for maintaining the microscopes.
Financial support for this research by the National Science Foundation under Grant DMR-0554792 is acknowledged with thanks.
I’d like to thank my beloved Anna Cieslinska for her love and support during the crucial stages of this work. Thanks also go to my friends for making my stay in Charlottesville a very pleasant and memorable one. Notably, Abhay (my great room-mate for 4 years), Avinash, Nitin, Vipul, Sakya, Jasons (Hadorn & Wang) and others.
Last, but not the least, I thank my parents and brothers for their love and support without which this work would have been impossible.
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DEDICATED TO MY PARENTS AND TEACHERS
4 TABLE OF CONTENTS 1
INTRODUCTION ...............................................................................................................................8 1.1 1.2 1.2.1 1.2.2 1.3 1.4
2
EXPERIMENTAL PROCEDURES ................................................................................................16 2.1 2.2
3
INTRODUCTION......................................................................................................................22 EXPERIMENTAL PROCEDURE.............................................................................................22 RESULTS ..................................................................................................................................25 DISCUSSION ............................................................................................................................26 SUMMARY ...............................................................................................................................28
CHEMICAL CHARACTERIZATION OF SOLID-LIQUID SYSTEMS....................................30 5.1 5.2 5.2.1 5.2.2 5.3 5.4 5.5 5.5.1 5.5.2 5.5.3
6
X-RAY DIFFRACTION STUDY.......................................................................................................19 DTA AND DSC ANALYSES .........................................................................................................20
DEVELOPMENT OF THERMAL SHIELD ..................................................................................22 4.1 4.2 4.3 4.4 4.5
5
SAMPLE PREPARATION METHOD ................................................................................................16 TEM AND ASSOCIATED ANALYTICAL TOOLS .............................................................................17
PRELIMINARY CHARACTERIZATION OF THE ALLOY......................................................19 3.1 3.2
4
THE AL-SI SYSTEM .......................................................................................................................9 RELEVANT PREVIOUS STUDIES ON THE AL-SI SYSTEM ...............................................................10 Kinetic Studies in Al-Si Alloys ...............................................................................................10 Electronic Structure Studies ..................................................................................................11 MOTIVATION FOR CURRENT WORK ............................................................................................12 OVERALL LAYOUT OF THESIS .....................................................................................................14
INTRODUCTION......................................................................................................................30 RESULTS ..................................................................................................................................31 Concentration Profile across Solid-Liquid Interface ............................................................31 Determination of Metastable Phase Boundary......................................................................35 DISCUSSION ............................................................................................................................38 SUMMARY ...............................................................................................................................40 APPENDIX ................................................................................................................................43 Quantification of X-ray Spectra ............................................................................................43 Additional Considerations Related to Interpretation of the Results ......................................44 Complementary Experimental Methods ................................................................................46
ANALYSIS OF INTERFACIAL ENERGETICS...........................................................................47 6.1
SUMMARY ...............................................................................................................................49
7 KINETIC ANALYSES OF THE GROWTH AND DISSOLUTION PHENOMENA OF PRIMARY SI AND α-AL...........................................................................................................................50 7.1 7.2 7.3 7.3.1 7.3.2 7.3.3 7.3.4 7.3.5 7.4 8
INTRODUCTION......................................................................................................................50 EXPERIMENTAL PROCEDURE.............................................................................................52 RESULTS AND DISCUSSION.................................................................................................55 Growth and Dissolution of Primary Si ..................................................................................55 Nucleation of Si Crystal and Growth Twins ..........................................................................66 Morphology of Si Crystals.....................................................................................................68 Comparison with Bulk Alloys ................................................................................................70 Nucleation, Growth and Dissolution of α-Al from Liquid.....................................................71 CONCLUSIONS........................................................................................................................73
ELECTRON ENERGY-LOSS SPECTROSCOPY OF SOLID-LIQUID AL-SI ALLOY..........75 8.1
INTRODUCTION ...........................................................................................................................75
5 8.2 EXPERIMENTAL PROCEDURE.............................................................................................79 8.3 RESULTS AND DISCUSSION.................................................................................................81 8.3.1 Temperature Dependence of Plasmon Energies....................................................................81 8.3.2 Interfacial Plasmon ...............................................................................................................91 8.3.3 Energy-Loss Near-Edge Structures .......................................................................................95 8.4 CONCLUSIONS......................................................................................................................100 9
OVERALL CONCLUSIONS .........................................................................................................101
10
SUGGESTED FUTURE WORK....................................................................................................104
11
REFERENCES ................................................................................................................................106
6 LIST OF FIGURES Figure 1.1 The bulk Al-Si binary phase diagram at 1 atm [12]. 9 Figure 3.1: Room temperature X-ray diffraction pattern obtained from several milligrams of as-received atomized powders of the Al-Si based alloy 19 Figure 3.2: DTA results from the Al-Si based alloy. 20 Figure 3.3: DSC results from the Al-Si based alloy. 21 Figure 4.1: (A) Picture of the shield, (B) positioned on the heating holder viewed from the top, which faces the X-ray detector, and (C) viewed from the bottom, where the specimen cup/furnace can be seen. 23 Figure 4.2: Energy-dispersive X-ray spectra from an Al-Si-Cu-Mg alloy particle obtained without a shield at (A) 773 K and (B) 823 K, and with the shield in place at (C) 823 K and (D) 873 K. Note the peak shifts in B. The peak labels are located where the peaks would be without the energy shift. 24 Figure 5.1: (A) Bright-field TEM image of a partially molten Al-Si-Cu-Mg powder particle taken at a temperature of 828 K. The faint, dashed line parallel to the interface in (A) indicates the approximate appearance of the column of material sampled by the electron beam normal to the plane of the figure. (B) Bright-field TEM image of the same powder particle after further heating to a temperature of 878 K. (C) Concentration profiles of Al and Si obtained from x-ray spectra taken at 828 K (solid lines) and at 878 K (dashed lines). (D) Concentration profiles of Cu and Mg obtained from x-ray spectra taken at 828 K (solid lines) and at 878 K (dashed lines). 41 Figure 5.2: (A) Bright-field TEM image of a second particle at 833 K. An arrow points to the position of the electron probe in the liquid in the analysis and the bright spot in the lower-left corner is a image of the actual probe used. (B) Image at 753 K with an inset electron diffraction pattern showing the diffuse ring pattern corresponding to the liquid phase, as well as Si reflections. (C) Image at 743 K with an inset electron diffraction pattern showing additional spots due to the formation of crystalline Alrich phase. Faint rings in the diffraction pattern correspond to a small fraction of remaining liquid. (D) Image at 743 taken with a lower electron-beam brightness to eliminate beam heating. Arrow points to Al2Cu phase identified by EDXS. Note that the electron beam is condensed to just illuminate the particle in all of the images, so that the remaining background appears dark. The contrast visible in the Si crystal is due to twins, which change their orientation and contrast slightly with temperature. 42 Figure 5.3: (A) Si and (B) Cu concentrations in the undercooled liquid phase plotted as a function of temperature in pseudobinary phase diagrams along with the calculated equilibrium liquidus, solidus and solvus lines for the elements. 43 Figure 6.1: Same as Figure 5.1A reproduced here with the interfacial energies of the phases in this system (solid Al, solid Si, liquid Al-Si, solid aluminum-oxide) indicated. 48 Figure 7.1: A secondary SEM micrograph of the microstructure of as-cast A390 alloy showing the primary Si particles. The other intermetallics present are Al15(Fe,Mn)3Si2 with a script morphology, CuAl2, and small Al5Cu2Mg8Si6 51 particles. [19].
7 Figure 7.2: A picture of the video camera relative to the TEM column. 54 Figure 7.3: Frames captured from a video sequence showing the morphology of primary Si during growth and dissolution cycles. 59 Figure 7.4: Traces of solid-liquid interfaces during (A) growth, (B) dissolution and (C) growth after the twin formation indicated by arrow-head. 59 Figure 7.5: Schematic of the growth traces of primary Si obtained by ex-situ methods [22]. 60 Figure 7.6: Traces of the solid Si-liquid interface during (A) growth and (B) dissolution. 61 Figure 7.7: A schematic illustration of the fast and slow growth/dissolution directions. 62 Figure 7.8: The positions and corresponding velocities of the solid Si-liquid interface for a temperature drop of 10 K. 65 Figure 7.9: A frame captured from the video showing the presence of a ledge during the growth of primary Si. The ledge height is approximately 9 nm. 66 Figure 7.10: An optical micrograph of a hypereutectic Al- 15wt% Si alloy, chemically similar to the one used in this study, illustrating that the alumina fiberreinforcements acts as preferred nucleation sites for primary Si [75]. 68 Figure 7.11: A schematic illustration of some common Si crystal morphologies coexisting with the liquid. 70 Figure 7.12: Frames A and B captured from a video recording of (Al) nucleation from the liquid phase. Temporally, frames A and B are separated by 1/30th of a second, implying an average growth front velocity greater than 11µm/s. 72 Figure 8.1: EEL spectra of pure Al at various temperatures. 83 Figure 8.2: Variation in Ep as a function of temperature in pure Al 84 Figure 8.3: EFTEM image obtained using a 2 eV energy window centered at 6 eV energy-loss. 85 Figure 8.4: EEL spectra of Al-Si based alloy at various temperatures 89 Figure 8.5: Variation of Ep as a function of temperature in Al-Si based alloy 90 Figure 8.6: Image of the solid Si - liquid Al-alloy interface with the approximate size and position of the entrance aperture indicated by dashed circles. The interface position is indicated by the arrow. 92 Figure 8.7: Low-loss spectra obtained from A: liquid Al alloy, B: solid Si and C: solidliquid interface. 93 Figure 8.8: L2,3 edge of A: pure liquid Al, and B: Al-Si liquid alloy, both obtained at 943 97 K. Figure 8.9: L2,3 edge of pure Al crystal from simulation and experiment [90]. 98 Figure 8.10: Simulated EEL spectra showing the variation in the L2,3 edge of Al as a function of Si concentration 99
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1 INTRODUCTION Interfaces have a profound effect on material properties. This effect arises from fundamentally intertwined aspects such as the change in structural arrangement, composition and/or electronic structure across the interface. Homophase and heterophase interfaces between solid phases have been extensively studied in the past several decades [1]. Their dynamic behavior such as during grain growth [2] and phase transformations [3] has also been widely studied.
In contrast, solid-liquid interfaces has not been extensively investigated due to the experimental difficulties [4] involved in accessing the interface, especially in inorganic engineering materials, which usually have high melting points (typically greater than 800 K) and are optically opaque. However, some progress has been recently made in determining the atomic structure of the solid-liquid interface [5-7].
Overcoming the experimental hurdles in the study of solid-liquid interfaces will facilitate in gaining a deep insight into fundamental phenomena such as the structure and composition of the solid-liquid interface, the partitioning of elements between the solid and liquid phases during crystal growth, critical factors involved in the nucleation and growth of phases and the variation of electronic structure across the interface. These are crucial for a sophisticated understanding of crystallization and melting phenomena [811].
9
1.1 The Al-Si System Al-Si based alloys are of significant technological interest because of their high strength to weight ratio. The binary Al-Si binary phase diagram is shown in Fig. 1.1 below. The Al-Si system has a eutectic composition of 12.5 wt % Si [12] and the eutectic temperature is 850 K. Al-Si alloys are usually modified with minor amounts of other elements such as Cu and Mg to optimize various engineering properties.
Figure 1.1 The bulk Al-Si binary phase diagram at 1 atm [12].
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1.2 Relevant Previous Studies on the Al-Si system 1.2.1 Kinetic Studies in Al-Si Alloys The binary Al-Si system has been extensively studied using ex-situ analysis methods during the past several decades [13]. A brief overview of the previous investigations pertaining to the kinetic analyses of binary hypoeutectic and hypereutectic Al-Si alloys is presented below.
1.2.1.1 Hypoeutectic Al-Si Binary Alloys The major phases present in as-cast microstructure of hypoeutectic Al-Si alloys are primary α-Al and a eutectic mixture of α-Al and Si. The primary Al has been reported to grow in the form of dendrites along [100] [14]. Al in the eutectic mixture has been reported to have mainly the same crystallographic features as the primary α-Al dendrites in unmodified alloys [15]. Addition of elements such as Ca, Na, Sr, Sb to hypoeutectic Al-Si alloys has been reported to modify the morphology and the grain size of the eutectic Si [16]. The morphology of eutectic Si as a function of temperature gradient (G), the growth velocity (V) and alloy chemistry has also been studied in detail [17, 18].
1.2.1.2 Hypereutectic Al-Si Binary Alloys The major phases present in as-cast microstructures of hypereutectic Al-Si alloys are primary Si and a eutectic mixture of α-Al and Si. The nucleation and growth morphologies of primary and eutectic Si in hypereutectic Al-Si alloys have been extensively investigated by ex-situ methods [19-22]. Due to commercial relevance,
11 refinement of the primary Si phase has also been studied using various chemical modifications to the base Al-Si alloys [23].
1.2.1.3 Growth Mechanisms of Si Crystals The growth mechanisms of primary Si are usually determined indirectly by analyzing growth traces [22]. One of the key predictions based on such analyses is that fast growing interfaces such as {113} grows out leaving behind prominent {111} facets. The faceted nature of primary Si is evident from their idiomorphic shape seen in as-cast hypereutectic Al-Si alloys [19]. A common method of Si crystal growth widely reported in the literature is by a twin-plane reentrant edge (TPRE) mechanism [24]. The favorable nucleation site offered by the twin-plane reentrant corner facilitates two-dimensional nucleation of ledges at this point. In-situ high-resolution TEM studies have shown that crystalline Si - liquid Al-Si (111) interfaces are atomically flat and that there is a gradation of ordering in the first several layers of the liquid at the interface [25]. The same studies show that the interface may move by the cooperative organization of many close-packed layers of Si atoms at the (111) interface under the application of a driving force. In-situ studies have also demonstrated the ledge-based growth of Si crystals on (111) from amorphous Si [26]. In-situ observation of the dissolution of primary Si has not been studied for direct comparison between the growth and dissolution morphologies and behavior.
1.2.2 Electronic Structure Studies The electronic structure and plasmon characteristics in solid-state materials have been widely studied [27-32]. The temperature dependence of the plasmon energy in pure
12 crystalline Al has also been reported and shown to decrease with increasing temperature [33]. In contrast, only a few such studies in the liquid state have been reported [34]. Experimental investigations on the effects of solute additions on the electronic structure and properties of alloy liquids have not been previously reported.
More specific background for this research is provided at the beginning of each chapter presenting the main experimental results.
1.3 Motivation for Current Work In-situ studies enable direct observation of the morphological evolution of the phases and determination of the solute partitioning behavior. Direct determination of metastable phase boundaries is also possible. However, due to the experimental difficulties in studying metallic materials using in-situ methods, the instantaneous behavior of the solidliquid interface has not been fully studied.
There are a limited number of in-situ studies on solid-liquid behavior in metallic systems reported in the literature [6, 7, 35, 37]. An in-situ study on hypoeutectic Al-11.6 at % Si alloy was performed by Storaska et al. [35, 36]. Major relevant conclusions from that work were:
1. α-Al/Al-Si liquid interface is diffuse and the interface thickness ranges from 1.4 nm to 1.9 nm.
13 2. Comparison between bright-field and energy-filtered imaging results suggests that the structural and compositional changes across the interface are correlated. 3. Solidification occurs in diffusion controlled regime, most likely by a continuous growth mechanism at undercoolings down to 5.8 x 10-8 K. 4. Morphological evidence suggests the α-Al/Al-Si liquid undergoes a roughening transition at 841 K. 5. The solid-liquid interfacial energy of α-Al/Al-Si system is about 130±35 mJ/m2.
The major motivation for the current work arises from the need to develop a comprehensive understanding of the chemical, structural and electronic behavior in solidliquid metallic systems using in-situ methods to directly study the instantaneous and dynamic events.
The system chosen for this study is an atomized powder of Al-Si-(Cu-Mg) hypereutectic alloy. The main advantage of using an Al based alloy is its relatively low melting temperature, which is within the operational range of the in-situ TEM heating holder available. In addition to this, few nanometers thick alumina film on the surface of the nanoparticle offers a natural shell to contain the liquid phase produced upon heating. The current study provides a strong experimental comparison of the thermodynamic and kinetic behavior between hypoeutectic and hypereutectic alloy systems and the results may be portable to other eutectic systems.
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1.4 Overall Layout of Thesis This dissertation is composed of several self-contained chapters with an overall theme of characterizing the phases and solid-liquid interfaces in an Al-Si based hypereutectic alloy. The experimental procedure is presented in Chapter 2, which discusses various tools and methods used in this study.
Preliminary characterization of the bulk alloy using X-Ray diffraction (XRD), differential thermal analysis (DTA) and differential scanning calorimetry (DSC) were performed to obtain alloy specific information such as the phases present and to determine thermodynamic information such as the incipient melting and liquidus temperature of the alloy. This is discussed in Chapter 3.
Details concerning the development of a thermal shield that was used to increase the temperature limit for acquiring usable energy-dispersive X-ray spectra (EDS) at elevated temperatures and ideas for further improvements are presented in Chapter 4. This has been published as a paper in Microscopy and Microanalysis [38].
Composition profiles across the solid-liquid interface obtained at two different temperatures above the incipient melting temperature are provided in Chapter 5. A significant portion of this chapter is devoted to analysis of the experimentally determined metastable liquidus boundary corresponding to the homogeneous Al-solid solution nucleation and its comparison to computed phase boundaries obtained using JMatPro. This work has been published as a paper in Science [39].
15 Chapter 6 contains comments on the interfacial energies across various phases present during different stages of the solid-liquid phase transformation. The discussion is based on experimental results concerning the morphological evolution of the phases as a function of temperature.
A study on the kinetic aspects such as nucleation, growth and dissolution of primary Si and α-Al from the liquid alloy is presented in Chapter 7.
The temperature dependence of the plasmon energy and the electronic structure variation due to alloy additions in the liquid phase, obtained using electron energy-loss spectroscopy, are presented in Chapter 8.
The major conclusions from this research work are listed in Chapter 9.
Finally, suggestions for future work are presented in Chapter 10.
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2 EXPERIMENTAL PROCEDURES 2.1 Sample Preparation Method
Atomized powders of Al-Si based alloy AA390 were obtained from Valimet Inc [40]. The average alloy powder composition as determined by Valimet through inductively coupled plasma, atomic absorption (ICP, AA) chemical analysis is shown in Table 1 below. The alloy concentration of 17.8 at. % Si is greater than the binary eutectic composition of 12.2 at. % Si and the alloy should begin to melt near the eutectic temperature of 850 K. Above this temperature the particles will contain molten Al-rich liquid with primary solid Si particles, until the liquidus temperature is reached, beyond which all of the Si dissolves.
Element
Al
Cu
Fe
Mg
Mn
Si
Ti, V, Zn
Composition (at %)
Balance
1.8
0.08
0.6
10 µm/s) sometimes leading to atomic stacking errors and causing twin boundaries. Twinning could also occur due to the nature of the heterogeneous oxide site during nucleation. However, during further post-nucleation controlled growth/dissolution using the electron beam, twinning was not commonly observed, as the interface velocity was significantly slower (e.g.,
