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Following the continually increasing demand for high-density interconnection and multilayer packaging for chips, solder bump size has decreased significantly over the years, this has led to some challenges in the reliability of interconnects. This thesis presents research into the resulting effects of miniaturization on the interconnection with Sn-solder, especially focusing on the full intermetallics (IMCs) micro-joints which appear in the 3D IC stacking packaging. Thereby, systematic studies have been conducted to study the microstructural evolution and reliability issues of Cu-Sn and Cu-Sn-Ni IMCs micro-joints. (1) Phenomenon of IMCs planar growth: The planar IMCs interlayer was asymmetric and composed of (Cu,Ni)6Sn5 mainly in Ni/Sn (2.5~5 μm)/Cu interconnect. Meanwhile, it was symmetric two-layer structure in Cu/Sn (2.5~5 μm)/Cu interconnect with the Cu3Sn fine grains underneath Cu6Sn5 cobblestone-shape-like grains for each IMCs layer. Besides, it is worth noticing that the appearance of Cu-rich whiskers (the mixture of Cu/Cu2O/SnOx/Cu6Sn5) could potentially lead to short-circuit in the cases of ultra-fine ( < 10 μm pitch) interconnects for the miniaturization of electronics devices. (2) Microstructural evolution process of Cu-Sn IMCs micro-joint: The simultaneous solidification of IMCs interlayer supressed the scalloped growth of Cu6Sn5 grains in Cu/Sn (2.5 μm)/Cu interconnect during the transient liquid phase (TLP) soldering process. The growth factor of Cu3Sn was in the range of 0.29~0.48 in Cu-Cu6Sn5 diffusion couple at 240~290 °C, which was impacted significantly by the type of substrates. And the subsequent homogenization process of Cu3Sn grains was found to be consistent with the description of flux-driven ripening (FDR) theory. Moreover, Kirkendall voids appeared only in the Cu3Sn layer adjacent to Cu-plated substrate, and this porous Cu3Sn micro-joint was mechanically robust during the shear test. (3) Microstructural evolution of Cu-Sn-Ni IMCs micro-joint: There was obvious inter-reaction between the interfacial reactions in Ni/Sn (1.5 μm)/Cu interconnect. The growth factor of (Cu,Ni)3Sn on Cu side was about 0.36 at 240 °C, and the reaction product on Ni side was changed from Ni3Sn4 into (Cu,Ni)6Sn5 with the increase of soldering temperature. In particular, the segregation of Ni atoms occurred along with phase transformation at 290 °C and thereby stabilized the (Cu,Ni)6Sn5 phase for the high Ni content of 20 at.%. (4) Micro-mechanical characteristics of Cu-Sn-Ni IMCs micro-joint: The Young s modulus and hardness of Cu-Sn-Ni IMCs were measured by nanoindentation test, such as 160.6±3.1 GPa/ 7.34±0.14 GPa for (Cu,Ni)6Sn5 and 183.7±4.0 GPa/ 7.38±0.46 GPa for (Cu,Ni)3Sn, respectively. Besides, in-situ nano-compression tests have been conducted on IMCs micro-cantilevers, the fracture strength turns out to be 2.46 GPa. And also, the ultimate tensile stress was calculated to be 2.3±0.7 GPa from in-situ micro-bending tests, which is not sensitive with the microstructural change of IMCs after dwelling at 290 °C. more...

Currently, there is not enough information available on the effect of inclusions on extrusion alloys. Theoretical calculations in the past demonstrated the probable role of oxides in Fe-intermetallic phase selection (Cao et al., 03). However, no concrete evidence can be found in the literature to support this argument. This study investigates the role of in-situ oxides in intermetallic phase selection. Various Mg oxides (spinel and MgO) were formed in-situ by adding different levels of Mg. A special intermetallic extraction process was used for 3D analysis. SEM, EDS and XRD analysis were used for qualitative and quantitative analysis. Dry and wet surfaces of the oxide bi-films were observed with the wet surfaces highly associated with MgO and spinel particles. MgO particles had spherical morphology and there average diameter was observed to be in the range 200nm-400nm, whereas spinel particles had octahedral morphology with average length of the side in the range 1-2μm. MgO was found in locations which appear to be the most probable nucleation points of α-AlFeSi intermetallics and Mg2Si. These results provide a new and more distinctive perspective on the actual morphology of Fe-rich intermetallics and Mg oxides than the ones that exist in the literature. It also provides direct evidence of the role of inclusions (oxides) in intermetallic phase nucleation. This information can be utilised to improve the surface properties in 6xxx extrusion alloys. more...

The transportation sector is responsible for a significant contribution of global carbon emissions. Recent efforts to reduce carbon emissions have been focused on reducing the weight of all types of vehicles to improve efficiency. Aluminum alloys are widely used due to a combination of low density and excellent mechanical and corrosion properties. However, aluminum alloys contain high embodied emissions due to the energy intensive refinement process from bauxite ore. Recycled aluminum, called secondary aluminum, only requires around 5% of the energy needed to produce aluminum from bauxite ore (primary aluminum). Current recycling practices result in significant amounts of elemental impurities in secondary aluminum alloys, significantly decreasing their mechanical properties and limiting their uses in the transportation industry. The control of impurities during solidification ultimately controls the achievable mechanical properties of casting alloys, which are the most widely used. Thermodynamic calculations using the CALPHAD (calculation of phase diagrams) method were used to predict the effect of common impurities during solidification of secondary aluminum alloys. CALPHAD was also used to predict the effect of potential modifying elements. Lab-scale casting experiments were performed to evaluate the thermodynamic predictions via microstructure observations using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM) techniques. Control methods for favorable microstructures were developed. From the results, a secondary structural alloy composition was chosen for production-scale experiments using high pressure die casting (HPDC). The results showed that Al-Si based alloys with Fe impurity can be effectively modified by Mn and Sr additions, and the effects of Mg and Cu impurities can be mitigated by Ce additions, leading to property improvements. The investigations also showed novel Al-Ce based alloys can be manufactured using scrap material with high impurity contents without a loss in mechanical properties. more...

For microelectronics used in the low-temperature applications, the understanding of their reliability and performance has become an important research subject characterised as electronics to serve under the severe or extreme service conditions. Along with the impact from the increased miniaturization of devices, the various properties and the relevant thermo-mechanical response of the interconnection materials to temperature excursion at micro-scale become a critical factor which can affect the reliable performance of microelectronics in various applications. Pure indium as an excellent interconnection material has been used in pixellated detector systems, which are required to be functional at cryogenic temperatures. This thesis presents an extensive investigation into the thermo-mechanical properties of indium joints as a function of microstructure, strain (loading histories-dependent) and temperature (service condition-sensitive), specifically in the areas as follows: (i) the interfacial reactions and evolution between indium and substrate during the reflow process (liquid-solid) and thermal aging (solid-solid) stages by taking low-temperature cycling into account; (ii) determination of the effects of joint thickness and the types of substrate (e.g. Cu or Ni) on the mechanical properties of indium joints, and the stress- and temperature-dependent creep behaviour of indium joints; (iii) the establishment of a constitutive relationship for indium interconnects under a wide range of homologous temperature changes that was subsequently implemented into an FE model to allow the analysis of the evolution of thermally-induced stresses and strains associated with a hybrid pixel detector. more...

Greater demands are being placed on the separable connector to perform with higher reliability in harsher automotive environments. Corrosion in its various forms is a major mechanism which affects contact reliability and this current work focuses on surface oxidation and the related phenomenon of fretting corrosion, from which hot dipped tin (HDT), a common automotive connector coating, is known to suffer. For an in-depth study of high contact resistance, in both static conditions and when subjected to relative micromovement, an interdisciplinary approach was necessary, drawing on the results of published work carried out in the fields of contact and surface science, corrosion and tribology. more...

The influence on the as-cast grain size of a range of alloying additions to some TiAI-based alloys has been assessed using optical microscopy, analytical scanning and analytical transmission electron microscopy. It has been found that additions of boron, either as TiB2 or AlB 12,resulted in significant grain refinement, but of the other additives (yttrium, yttria, silicon and sulphur) only sulphur had any significant effect. Measurements of tensile properties of the grain refined alloys showed a significant improvement, and creep rates were not degraded. Failure of the fully lamellar structure during tests was found to occur by the splitting of grains, in the hard orientation, parallel to the lamellae. Determination of the nature of B-containing precipitates in the alloys, formed at high cooling rates, showed that they were a eutectic structure of either TiB and B2 or TiB2 and B2 depending upon alloy composition. At lower cooling rates TiB2 usually formed, unless Ta was present in the alloy in which case a complex structure of TiB and TaB formed. In all cases, irrespective of the alloy or boron addition method, a minimum concentration of about 0.6 at.% B was required to achieve an equiaxed grain structure in plasma-melted ingots. At lower concentrations the dendrite arm spacing and columnar grain width are reduced. In Ta and W-containing alloys there is a marked tendency for the borides to form large (400J.1m) clusters. The size and frequency of these clusters can be influenced by the sequence and the form in which the various alloying additions are made. The mechanism by which grain refinement occurs is the rejection of boron from the liquid during solidification, leading to local high concentrations of boron at the roots of the secondary dendrite arms, which under normal casting conditions then break off and act as nuclei for equiaxed grains. The boron is then precipitated, as eutectic phase, in the interdendritic regions. Under quiescent casting conditions grain refinement does not occur with concentrations of boron sufficient to refine plasma-melted material, although again the boron precipitates interdendritically. Grain refined samples forge with very little cracking under conditions which result in extensive cracking in non grain refined samples. Zener pinning of the grains by the borides ensures that grain growth in the forged and forged plus heat treated material is very slow. These results are discussed in terms of the importance of grain refinement in TiAI-based alloys. more...

The literature on the processing, microstructure and properties of MoSi2 was briefly reviewed together with earlier work on the alloying behaviour of MoSi2 based alloys. Solidification microstructures and phase selection in ingots and melt-spun ribbons of binary, ternary and quaternary MoSi2 based alloys with additions X = W, Ta, Ti and Y = Al that substitute for Mo and Si respectively were studied. Low levels of interstitials (< 200wppm O2, < 20wppm H2) were achieved using clean melting based on cold hearth non-consumable tungsten arc melting. The following alloys were prepared: Mo-66.7Si, Mo-2.1W-66.7Si, Mo-2.1W-2.1Ta-66.7Si, Mo-2.1W-6.4Ta-66.7Si Mo-22.6Al-44.1Si, Mo-30Al-36.7Si and Mo-6Ti-30Al-36.7Si (at%). Evaluation of microstructure and properties was done using a number of experimental techniques which complement each other. These includedoptical microscopy, XRD, SEM, EPMA, TEM, DSC/TG, high temperature DTA, isothermal oxidation, T-o-F SIMS, room temperature hardness, microhardness and high temperature microhardness. The ingots of the alloys with Al or Al+Ti additions exhibited a smaller number and size of cracks and lower melt viscosity compared to the other alloys. Tungsten exhibited complete solid solubility in MoSi2. The C11b-MoSi2 structure was modified to the hexagonal C40 and/or orthorhombic C54 structures via Ta and Al or Al+Ti additions respectively. The C40 structure formed with Al addition exhibited two specific compositions with 20 at% and 24 at% Al depending on the extent of substitution of Si atoms in it. Rapid solidification refined the microstructures of the alloys by two orders of magnitude. Suppression of Mo5Si3 formation was also possible by rapid solidification. The C40 structure obtained by addition of 6.4 at% Ta in Mo-2.1W-6.4Ta-66.7Si was favoured by rapid solidification over the C11b structure. In the ribbons the solubility of Ta in (Mo,W)Si2 (C11b structure) was extended to 2 at%, while the solubility of Ti was extended up to 6 at% in Mo(Al,Si)2 (C40 structure). Segregation free microstructure with 24 at% and 30 at% Al were formed in the zone A (C40 structure) of the Mo-22.Al-44.1Al and Mo-30Al-36.7Si alloys. At low temperatures (T = 773 K) the PEST oxidation phenomenon was observed in alloys with W and/or Ta addition, while suppression of PEST was achieved in alloys with Al and Al+Ti additions where a mixed Al/Si oxide was formed with traces of Al/Mo oxide. In the former alloys a silica scale (2 to 7 μm thick) was formed at high temperatures (T > 1600 K) and there was no evidence of Mo5Si3 formation beneath the silica scale. In the latter alloys an alumina scale (10 to 20 μm thick) was formed at 1623 K, while at 1873 K the scale consisted of two oxide layers, the inner being alumina and the outer mullite and silica with dissolved alumina. Formation of Mo5Si3 beneath the alumina scale depended on the Al content of the alloy. Addition of W increased the microhardness of MoSi2 and the hardness of the alloys. Addition of Ta decreased the microhardness of MoSi2 while the hardness depended on Ta content and therefore on the volume fraction of the harder C40 phase. Addition of Al and Al+Ti decreased the hardness of MoSi2. Increase in the retained microhardness of MoSi2 up to 1273 K was observed with addition of Al (C40 structure). more...

Abstract: This thesis describes the synthesis, structures, and properties of two broad classes of inorganic solids, chalcohalides and intermetallics, with the common theme that they contain rare-earth (RE) elements, which belong to the set of critical minerals essential for many technological applications. Quaternary rare-earth chalcohalides RE–M–Ch–X (M = d- or p-block metal or metalloid, Ch = chalcogen, X = halogen) represent a growing family of mixed-anion compounds, which remain relatively scarce compared to conventional “single-anion” compounds such as oxides, but offer the possibility of greater control over physical properties. Several series of compounds RE3Tt2Ch8I (RE = La, Ce, Pr; Tt = Si, Ge; Ch = S, Se) were prepared by direct combination of the elements at high temperatures. By gradual substitution of different proportions of Tt or Ch components in solid solutions of these semiconducting compounds, the band gap can be tuned within a range of 1.9 to 3.6 eV. The Ce-containing members exhibit blue luminescence, making them suitable candidates for applications in phosphor-converted white light emitting diodes. The emission behaviour can be controlled by changing the crystal field environment of Ch vs. I anions around the luminescent Ce3+ centres within these compounds. Ternary rare-earth intermetallics RE–M–X (M = d-block metal; X = p-block metal or metalloid) have long been a rich source of quantum materials with exotic electrical and magnetic properties. Nevertheless, many aspects of their chemistry, including synthesis, phase stability, crystal structures, and chemical bonding remain poorly systematized and understood. Because reliable physical measurements depend on the availability of large single crystals or pure phase samples, efforts were made to apply flux methods for crystal growth of these compounds. In particular, ternary gallides RE−M−Ga (RE = La, Ce, Yb) were prepared in a gallium self-flux, and they were examined against a previously proposed stability diagram which predicts formation of ternary intermetallic phases. The new compound YbCu3Ga8 was discovered in this process, and the structure of YbNi3Ga9 was redetermined to resolve ambiguities in the literature. As examples of polar intermetallics, these compounds were confirmed by electronic structure calculations to exhibit multicenter covalent bonding within the anionic Ga networks. Similarly, crystal growth of ternary germanides REMGe (RE = Ce, Eu, Yb) was attempted in an indium flux to test a classification model. Crystals of RECo2Ge2 and CeIr2Ge2 were obtained, the latter exhibiting polymorphism with structures in two different space groups. The Yb-containing compounds Yb5Ir4Ge10 and Yb4Ir7Ge6 were also identified from the Yb−Ir−In−Ge reaction. more...

Laser powder bed fusion is a manufacturing process where parts are fabricated layer-by-layer from computer model data. This additive process contrasts with traditional methods such as casting or powder metallurgy, which require a mould to be created and impart geometric part design constraints. Since parts are fabricated directly from a computer model with little or no part-specific preparation, laser powder bed fusion is ideal for rapid prototyping, one-off part fabrication, and individually customized implants for medical industry. Nevertheless, a limited number of alloys can be laser powder bed fusion fabricated without defects due to rapid solidification and high thermal gradient. Furthermore, Al-Si system is currently popular for laser powder bed fusion since it offers defect-free fabrication and rapid solidification-induced solid solution strengthening. However, Al-Si system is not thermally stable. Elevated temperature exposure initiates Si diffusion out of Al matrix, resulting in coarse Si agglomerate formation and mechanical properties reduction. For top value applications in automotive and aerospace industries, it is necessary to explore alloy systems that result in not only defect free fabrication, but also formation of a coarsening resistant microstructure. Previous research on heat tolerant Al alloys fabricated via casting determined that the Al-Ce and Al-Ce-Si system results in formation of coarsening resistant intermetallic precipitates. Despite these alloys exhibiting excellent strength retention at elevated temperatures, room temperature ductility was low, and this appeared to be due to coarse needle-like precipitate morphology formed due to low cooling rate during casting. This research investigates application of AlSi10Mg in-situ alloyed with 4 wt.% Ce and Al6061 in-situ alloyed with 1-7 wt.% Ce for laser powder bed fusion. Defect formation mechanisms are investigated, microstructure development is evaluated, mechanical properties are characterized, and coarsening resistance is studied. It is established that Ce addition is effective in forming a coarsening resistant microstructure. Furthermore, laser powder bed fusion results in homogeneous distribution of nano-scale precipitates, resulting in high ductility. Findings in this research increase understanding of laser powder bed fusion and rapid solidification of Al-Ce-Si alloy system and are valuable for propagation of heat-tolerant alloys suitable for additive manufacturing. more...

Modern engineering applications use alloys with up to 10 different elements to meet performance requirements. Designing new alloys in such a vast multicomponent space is difficult. Even with first-principles methods to expedite the quest for new alloys, improvements to materials search algorithms are required to fully sift through the multidimensional chemical space. One possible search strategy is to exploit the natural hierarchy between crystal structures. Many crystal structures are related to a high symmetry parent crystal structure via a group/subgroup symmetry relationship. While many ordered structures are traditionally viewed as distinct crystals, they can be thought of as derivative structures of a parent crystal. The first half of this dissertation describes a framework for alloy design that is based on a parent-derivative crystal structure hierarchy. Chemical trends in this hierarchy are illuminated and new structural transformation pathways between important parent crystals are described. The second half of this dissertation uses a similar philosophy to explore a different set of materials; it examines the effects of chemical trends on the electrochemical properties of materials used for energy storage. To meet rising energy demands with intermittent renewable energy sources such as solar and wind power, society needs to create high performance secondary batteries for energy storage. Li-ion batteries have quickly risen to be a prime candidate to meet energy storage needs, and new advances are being made to promote sustainable earth-abundant materials, higher charge rates, and larger capacities. Some of these new advances include the investigation of Na-ion and Mg-ion batteries. Spinel intercalation compounds are well-known to facilitate high rate and high voltage Li-ion batteries, but less is known about their Na-ion and Mg-ion counterparts. When exploring new materials, it helps to have a strong fundamental understanding of how changes in chemistry or structure affect electrochemical performance of battery materials. This dissertation uses a combination of electronic structure calculations and statistical mechanics methods to study the thermodynamic and kinetic properties of a wide range of spinel intercalation compounds.The identification of a crystal structure hierarchy was achieved by analyzing existing crystal structure databases using a crystallographic mapping algorithm to find representative parent crystal structures from which many derivative structures are formed. We found that 73% of binary intermetallic compounds found on the ICSD to be derived from only 20 unique parent crystal structures. There are important crystallographical relationships, such as the Bain path and the Burgers path, that connect parent crystals within our hierarchy. These crystallographic relationships facilitate structural phase transformations which are important for shape-memory alloys, magnetocalorics, or self-assembling block co-polymers. A crystal structure mapping algorithm was used to show the existence of 7 new structural transformation pathways between orderings on simple parent crystal structures to others belonging to the top 20 most common parent crystal structures. We use high-throughput density functional theory (DFT) to probe which elemental combinations are most likely to cause spontaneous structural transformations without an energy barrier. We find multiple chemical combinations that lead to barrierless transformation pathways. These barrierless transformations suggest a nucleation mechanism that does not require large structural fluctuations that are often energetically costly. In our investigation of electrochemical properties of energy storage materials, we examine the spinel structure which has a general formula AMX2 where A is a guest cation, M is a transition metal and X is a chalcogenide. The electrochemical properties of spinel can be affected by many factors including (i) ionicity of the MX2 framework, (ii) guest cation radius, and (iii) guest cation oxidation state. We conducted a systematic study and determined that guest cation radius and MX2 ionicity play a significant role in guest cation site preference which, in turn, affects the electrochemical properties of spinel. The insights of this study suggest that large cations in an oxide spinel creates a desirable energy landscape for high-rate capable batteries. Using kinetic Monte Carlo simulations and a model cluster expansion Hamiltonian, we identify a topological pitfall that makes spinel prone to highly correlated cation diffusion at intermediate to high guest cation concentrations. This undesirable property is dependent on the strength of the nearest-neighbor repulsion of diffusing cations within spinel. A strong repulsion results in a dependence on large vacancy clusters to mediate diffusion. This dependence indicates that a percolating network of large vacancy clusters is necessary to facilitate long range diffusion. Our results show that the detrimental effects can be mitigated by reducing the strength of the repulsion or increasing the connectivity of the percolating network. Although our focus is on the fully-ordered spinel structure, the conclusions drawn from this study apply to other close-packed anion hosts such as disordered rocksalt electrodes and partially ordered spinels. more...

FeAl-based Metallic-Intermetallic Laminate (MIL) composites and FeAl/FeAl2 eutectoid MIL composites of various iron alloys were fabricated with an innovative “multiple-thin-foil” configuration and “two-stage reaction” strategy. Alternating stacked metal foils were reactive sintered via spark plasma sintering (SPS) (a.k.a. field assisted sintering) to grow intermetallics. The “multiple-thin-foil” configuration reduces reaction time, enables local chemical composition control and allows metal/intermetallic combinations, which cannot be produced via the conventional methods. Fe-FeAl, 430SS-FeAl, and 304SS-FeAl MIL composites can be synthesized with desired metallic/intermetallic ratios, where FeAl is the single intermetallic phase present in the composites. The deformation and fracture evolution of the FeAl-based MIL composites is investigated here via incremental compression testing. Geometrically necessary dislocation (GND) analysis indicates the FeAl regions deform in similar manners for the three MIL composites, and each fails in a similar mode. Single-phase intermetallic FeAl layered material is also synthesized using a similar approach to study the fracture mechanisms of FeAl-based MIL composites. Mesoscale hetero-deformation induced (HDI) stress, which is tensile on the FeAl layers of MIL composites, accelerates crack nucleation and crack propagation, eventually inducing failure. The HDI stress evaluated via finite element analysis (FEA) simulation explains the difference between 430SS-FeAl and 304SS-FeAl MIL composites, which possess similar microstructure and composition, but very different strengths. Meanwhile, the electromigration effect in SPS is quantitively analyzed in the Fe-Al diffusion couple system. In SPS, the samples are heated by the applied voltage and a high electric current, which can lead to an electromigration effect. FEA simulation is utilized to determine the voltage applied to the Fe-Al diffusion couple, which is found to be extremely small to induce any voltage effect. Additionally, the simulation suggests the temperature and current density distribution is uniform across the metallic diffusion couple, which makes quantitative measurement feasible. For the first time, a mathematic algorithm, which allows diffusivity and electromigration coefficients to be solved, is developed for the system with multiple reactive layers. more...

Three intermetallic series with increasing rare earth dilution are examined and their magnetic and physical properties are determined. Of the first series, RNiAl₄, five materials are examined. For the TbNiAl₄ case, the incommensurate magnetic structure has been determined and then used to rationalise results from previous low temperature nuclear orientation (LTNO) studies. Similarly for NdNiAl₄, a model incommensurate structure has been used to simulate an LTNO experiment. For PrNiAl₄, the presence of an incommensurate magnetic phase has been confirmed and the magnetic structure determined. For ErNiAl₄, the magnetic structure has been charted from well below the ordering temperature to well above it. In DyNiAl₄, two magnetic structures have been identified: a higher temperature incommensurate phase and a lower temperature commensurate phase and these coexist in zero applied field, a first for the series. A reanalysis of existing TbNiAl₄ data has also identified similar phase coexistence. The collective results are used to examine trends across the series. Of the RM₂Al₁₀ series, TbRu₂Al₁₀ has been examined in depth. The material possesses two phase transitions as a function of temperature at TN = 15.0(3) K and TN' = 6.5(3) K and three more as a function of applied field at 2 K, at 1.30(3) T, 1.85(3) T and 3.20(3) T. These metamagnetic transitions transform the low temperature incommensurate square wave structure into two identifiably different 'pulse wave' structures and then to a para/ferromagnetic structure respectively. A magnetic phase diagram has been created and comparisons between TbRu₂Al₁₀ and other RM₂Al₁₀ compounds have been made as well as comparisons with the RNiAl₄ series. For the RM₂Al₂₀ series, three materials were analysed. For NdTi₂Al₂₀, a neutron diffraction experiment was performed at very low temperatures (70 mK) however no magnetic diffraction peaks were observed. DyTi₂Al₂₀ was subject to a similar experiment at 30 mK, where a commensurate antiferromagnetic structure was found. For TmV₂Al₂₀, inelastic neutron scattering was used to determine the crystal field parameters of the Tm³⁺ ion. Improvements were made to the original parameters obtained from bulk measurements, with W = 0.43(1) K and x = -0.63(1) using the Lea, Leask and Wolf formalism. more...

This thesis presents a study on the formation mechanism of porous Cu3Sn intermetallic compounds in microbumps for 3D-IC packaging, which are issues of importance to the packaging technology in microelectronic industry. Experimental emphases are on the formation processes of porous Cu3Sn under current stressing and by solid state aging. Theoretical interpretations of the observations are also presented. Following a background introduction, we first report the results on electromigration tests of SnAg solder bump samples with 15 �m bump height and Cu under-bump-metallization (UBM). The test conditions were 1.45 �104 A/cm2 at 185 oC and 1.20 �104 A/cm2 at 0 oC. A porous Cu3Sn intermetallic compound (IMC) structure was observed to form within the bumps after several hundred hours of current stressing. In direct comparison, annealing alone at 185 oC will take more than 1000 h for porous Cu3Sn to form, and it will not form at 170 oC even after 2000 h. A mechanism is proposed to explain the formation of this porous structure assisted by electromigration. The results show that the SnAg bump with low bump height will become porous-type Cu3Sn when stressing with high current density and high temperature. Polarity effects on porous Cu3Sn formation is also examined and interpreted.The second part of the experimental study is on the growth competition between the co-existing layer-type and porous-type Cu3Sn in solder microbumps of Cu/SnAg/Cu. The thickness of the SnAg solder is about 14 μm and the Cu column on both sides is 20 μm. Upon wetting-reflow, the solder is reacted completely to form Cu-Sn intermetallic compounds in a multi-layered structure of Cu/Cu3Sn/Cu6Sn5/Cu3Sn/Cu. Upon further annealing at 220 �C and 260 �C, we obtain Cu/Cu3Sn/porous Cu3Sn/Cu3Sn/Cu, in which both types of Cu3Sn co-exist and form an interface. In the layer-type growth, we assume Cu to be the dominant diffusing species, coming from the Cu column. The Cu reacts with Cu6Sn5 to grow the Cu3Sn layer. In the porous-type growth, we assume Sn to be the dominant diffusing species, coming from the depletion of Sn in Cu6Sn5. The depleted Cu6Sn5 transforms to the porous-type Cu3Sn. At the same time, the Sn diffuses to the side-wall of Cu column to form a coating of Cu3Sn. Experimental observations of 3-dimensional distribution of voids in the porous-type Cu3Sn are performed by synchrotron radiation tomography; the voids are interconnected for the out-diffusion of Sn. The competing growth between the layer-type and the porous-type Cu3Sn is analyzed.Lastly, for a theoretical understanding, we discuss the mechanism of this unique porous void formation in microbumps, including the necessary and sufficient conditions of porous Cu3Sn formation and a comparison between incomplete and complete cellular precipitations. more...

As conventional lead solders are being replaced by Pb-free solders in electronic devices, the reliability of solder joints in integrated circuits (ICs) has become a high concern. Due to the miniaturization of ICs and consequently solder joints, the current density through the solder interconnects has increased causing electrical damage known as electromigration. Electromigration, atomic and mass migration due to high electron currents, is one of the most urgent reliability issues delaying the implementation of Pb-free solder materials in electronic devices. The research on Pb-free solders has mainly focused on the qualitative understanding of failure by electromigration. There has been little progress however, on the quantitative analysis of electromigration because of the lack of available material parameters, such as the effective charge, (z*), the driving force for electromigration. The research herein uses idealized interconnects to measure the z* of electromigration of Cu in Sn-3.0Ag-0.5Cu (SAC305) alloy under different experimental conditions. Planar SAC 305 interconnects were sandwiched between two Cu pads and subject to uniaxial current. The crystallographic orientation of Sn in these samples were characterized with electron backscatter diffraction (EBSD) and wavelength dispersive spectroscopy (WDS) before and after electromigration testing. Results indicate that samples with the c-axis aligned perpendicular to current flow, polycrystalline, and those with a diffusion barrier on the cathode side all inhibit the growth of intermetallic compounds (IMC). The effective charge values of Cu in SAC 305 under the different conditions tested were quantified for the first time and included in this dissertation. The following research is expected to help verify and improve the electromigration model and identify the desirable conditions to inhibit damage by electromigration in Pb-free solder joints. more...

What drives the stability of complex intermetallic compounds? Many, if not most, metals and alloys crystallize in one of the familiar body-centered cubic (bcc), face-centered cubic (fcc), or hexagonal close-packed (hcp) crystal structures. But what of Cu5 Zn8 with 54 atoms in its unit cell, or Ti2 Ni with its 96? Or larger still Fe11 Zn39 , with 408 atoms in the repeat unit? Chemically and physically straight-forward semi-empirical calculations can ¨ lend us insight, and we review one such technique-the extended Huckel method-with new examples. Along the way, we solve the crystal structures for the new family of ternary compounds Ir x Ru1[-] x Zn10 and see what clues the Ir site preferences can give us about the connection between atomic structure and electronic structure via the Mott-Jones method of constructing electronic wavefunctions from the intensity of Bragg peaks. Finally, we investigate the connection between intermetallic structures that possess pseudo five-fold diffraction patterns. We find these peaks corresponding to features of several different types of tetrahedral close-packing, which in turn can all be described as three-dimensional projections of a four-dimensional perfect packing of tetrahedra: the 600-cell. We further show that the Bragg peaks comprising the pseudo five-fold diffraction pattern are linked to structural stability, and hence the higher-dimensional geometry. more...

Magnesium (Mg) is a light-weight metal that has extraordinary physical and chemical properties for many potential applications in automobile, military, and electronics. Aluminum alloys, because of its light-weight, high strength and corrosion resistance have a wide range of commercial applications. Given these two, sometime competing, alloy systems, there are now many applications where the metallurgical compatibility of Mg- and Al-alloys are required for engineering applications. One such case is the development of diffusion barrier for U-Mo metallic fuel in Al-alloy cladding, where Mg, with its complete immiscibility with U and Mo is being considered as the diffusion barrier. While negligible diffusional interaction between Mg and U-Mo alloys have been reported, diffusional interaction between the Mg and Al-alloy cladding has not been investigated. In this study, solid-to-solid diffusion couples were assembled using discs of pure Mg (99.999 %) and AA6061 Al-alloy. After preparation, Mg was diffusion bonded to AA6061 in sealed quartz capsule at 300°, 350°, and 400°C for 720, 360, and 240 hours, respectively. Scanning electron microscopy was used to inspect the interdiffusion zone, while phase identification was performed using X-ray energy dispersive spectroscopy. One specific phase that exists in the binary Mg-Al system, labeled “ε” was observed and characterized by transmission electron microscopy. From the preceding data, the growth rates as well as interdiffusion coefficients of the intermetallic phases were extracted and compared to previous investigations using pure Mg and Al. more...

U-Zr metallic fuels cladded in Fe-alloys are being considered for application in an advanced Sodium-Cooled Fast Reactor (SFR) that can recycle the U-Zr fuels and minimize the long-lived actinide waste. To understand the complex fuel-cladding chemical interaction of the U-Zr metallic fuel with Fe-alloys, a systematic multicomponent diffusion study was carried out using solid-to-solid diffusion couples. The U-10 wt.% Zr vs. pure Fe diffusion couples were assembled and annealed at temperatures, 630, 650 and 680°C for 96 hours. Development of microstructure, phase constituents, and compositions developed during the thermal anneals were examined by scanning electron microscopy, transmission electron microscopy and X-ray energy dispersive spectroscopy. A complex microstructure consisting of several layers that include phases such as U6Fe, UFe2, ZrFe2, α-U, β-U, Zr-precipitates, χ, ε, and λ was observed. Multiphase layers were grouped based on phase constituents and microstructure, and the layer thicknesses were measured to calculate the growth constant and activation energy. The local average compositions through the interaction layer were systematically determined, and employed to construct semi-quantitative diffusion paths on isothermal U-Zr-Fe ternary phase diagrams at respective temperatures. The diffusion paths were examined to qualitatively estimate the diffusional behavior of individual components and their interactions. Furthermore, selected area diffraction analyses were carried out to determine, for the first time, the exact crystal structure and composition of the χ, ε and λ-phases. The χ, ε and λ-phases were identified as Pnma(62) Fe(Zr,U), I4/mcm(140) Fe(Zr,U)2, and I4/mcm(140) U3(Zr,Fe), respectively. more...

Shape memory alloys (SMAs) have the potential to power small, lightweight,solid-state actuators through a thermally-induced phase transformation. Components of these materials can be “trained" via thermomechanical cycling to exhibita shape memory effect, whereby the SMA acquires one geometric configuration inits high-temperature, austenite phase and another in its low-temperature, martensitephase. This transformation can be used to power an actuator. Nickel-Titanium (NiTi)is a shape memory alloy which is of interest because it can generate high stresses andrelatively large strains through these thermomechanical processes. However, use ofthis alloy is hindered by the difficulty of integrating components made of it into structures and mechanisms made of other materials. Machining traditional joining featuressuch as bolt holes and splines into NiTi pieces is difficult and expensive.The objective of this research is to investigate processes for joining NiTi to structural materials such as aluminum and steel alloys. The processes presented here areultrasonic soldering, adhesive bonding, laser welding, and tungsten inert gas (TIG)welding. Tubes of NiTi were joined to 304 stainless steel tubes and to aluminum 2024blocks and tested in shear using a custom-made torsional testing machine.There are two main sections to this thesis. The first section describes the development of a high-temperature torsional testing machine for characterization ofthe joints. This apparatus is used to determine the ultimate strength of the jointsand assess their resistance to creep and their performance during thermomechanicalcycling.The second section of this thesis describes the design and testing of the joints.Strength data, mechanical behavior plots, optical micrographs and hardness mapsare presented. Lastly, the four joining methods are compared both qualitativelyand quantitatively, and design guidelines are provided which can help establish thesuitability of each process for different applications. more...

Abstract: This thesis focuses on the synthesis, structures, and physical properties of ternary rare-earth antimonides and germanides. These ternary compounds exhibit diverse polyanionic substructures with classical and non-classical Sb–Sb bonding, Ge–Ge bonding, or both. The Zintl-Klemm concept and band structure calculations were applied to understand their structures and bonding. Electrical resistivities and magnetic properties were measured for these compounds. The compounds RE2Ti7Sb12 (RE = La–Nd) and RE2Ti11–xSb14+x (RE = Sm, Gd, Tb, Yb), which were synthesized by arc-melting, adopt different structures depending on the size of the RE atoms. Both consist of a complex arrangement of TiSbn polyhedra, linked to form a 3D framework with large cavities in which the RE atoms reside. Hypervalent Sb–Sb bonds are manifested in disordered Sb fragments in RE2Ti7Sb12, and 1D linear chains, zig-zag chains, and pairs in RE2Ti11–xSb14+x. A series of compounds, RECrGe3 (RE = La–Nd, Sm), was synthesized by the Sn-flux method. They adopt a hexagonal perovskite structure type, in which chains of face-sharing Cr-centred octahedra are linked by triangular Ge3 clusters. These unusual single-bonded Ge3 substructures can be rationalized simply by the Zintl-Klemm concept. Electrical resistivity measurements show metallic behaviour with prominent transitions coincident with ferromagnetic transitions (Tc ranging from 62 to 155 K) found in magnetic measurements. Band structure calculations show the presence of a narrow, partially filled band with high DOS at Ef, in agreement with the observation that LaCrSb3 is an itinerant ferromagnet. With a different number of d-electrons in the M site, the isostructural REVGe3 compounds exhibit antiferromagnetic behaviour. The doped quaternary compounds LaCr1-xVxGe3 and LaCr1-xMnxGe3 exhibit depressed Curie temperatures. The structures of RECrxGe2 compounds (RE = Sm, Gd–Er) are built up by inserting transition-metal atoms into the square pyramidal sites of a hypothetical “REGe2” host structure (ZrSi2-type). The presence of extensive anionic Ge substructures in the form of 1D zigzag chains and 2D square sheets can be explained by the Zintl-Klemm concept. Magnetic measurements indicated antiferromagnetic ordering with low TN ranging from 3 to 17 K. Compounds involving a p-block element as the second component were prepared. In RE12Ge7–xSb21 (RE = La–Pr), a complex 3D polyanionic framework with Ge pairs, five-atom-wide Sb ribbons, and 2D Ge/Sb layers is present. The bonding exemplifies the competition of valence electron transfer from the RE atoms to metalloids with similar electronegativities. Full electron transfer from the RE atoms to the anionic substructure cannot be assumed. Magnetic measurements on Ce12Ge6.5Sb21 indicate antiferromagnetic coupling. The metal-rich compounds RE5TtxSb3–x (Tt = Si, Ge) adopt the orthorhombic beta-Yb5Sb3-type structure with a range of solid solubility between 0.9 and 1.6. They are not electron-precise and do not obey the Zintl-Klemm concept. more...

The intermetallic compound TiAl possesses a unique complex of properties that include sufficiently low material density, high values of the strength-to-ductility ratio, high elastic moduli, high oxidation resistance, low creep rate, and improved fatigue characteristics. These properties make TiAl alloys very attractive, particularly for structural applications for aerospace and aeronautic industries, where, at certain temperatures, they might be capable of replacing heavy nickel-based superalloys. However, so far applications of TiAl alloys have been limited by their poor ductility. Many of the recent studies have focused on the source of this limited ductility and on methods to improve this property. It has been found out experimentally that the strength and ductility of $gamma$-TiAl alloys can be affected by many different parameters, including alloy stoichiometry, heat treatment, deformation temperature, impurity content, grain size, and ternary element additions. In this thesis we present the results of our computer simulations of deformation and fracture in TiAl. In contrast to many previous studies our simulations include the interaction of the crack with point defects in the lattice. We use the molecular statics technique with atomic interactions described in terms of the embedded atom method. We simulate the crack propagation along (100), (001), (110) and (111) planes in TiAl. The cleavage along (100) and (001) planes shows purely brittle behavior, whereas the cleavage along (110) and (111) planes is accompanied by extensive dislocation emission. Our studies of the crack interaction with point defects reveal that vacancies and antisites near the crack tip can influence the amount of plastic deformation. Another important observation is that the antisite formation energy near the crack tip is generally lower than in the perfect lattice. This observation suggests the formation of relatively disordered zones near the crack tip at high temperatures, and leads us to a formulation of a new mechanism of a brittle-to-ductile transition in TiAl. more...

Ordered intermetallics based on transition metal aluminides have been proposed as structural materials for advanced aerospace applications. The development of these materials, which have the advantages of low density and high operating temperatures, have been focused on the aluminides of titanium, nickel and iron. Though these materials exhibit attractive properties at elevated temperatures, their utilization is limited due to their propensity for low temperature fracture and susceptibility to decreased ductility due to environmental effects. A major embrittlement mechanism at ambient temperatures in these aluminides has been by the loss of cohesive strength at the interfaces (intergranular failure). This study focuses on this mechanism of failure, by undertaking a systematic study of the energies and structures of specific grain boundaries in some of these compounds. The relaxed atomistic grain boundary structures in B2 aluminides, FeAl, NiAl and CoAl and L1₀ γ-TiAl were investigated using molecular statics and embedded atom potentials in order to explore general trends for a series of B2 compounds as well as TiAl. The potentials used correctly predict the proper mechanism of compositional disorder of these compounds. Using these potentials, point defects, free surface energies and various grain boundary structures of similar energies in three B2 compounds, FeAl, NiAl and CoAl were studied. These B2 alloys exhibited increasing anti-phase boundary energies respectively. The misorientations chosen for detailed study correspond to the Σ5(310) and Σ5(210) boundaries. These boundaries were investigated with consideration given to possible variations in the local chemical composition. The effects of both boundary stoichiometry and bulk stoichiometry on grain boundary energetics were also considered. Defect energies were calculated for boundaries contained in both stoichiometric and off-stoichiometric bulk. The surface energies for these aluminides were also calculated so that trends concerning the cohesive energy of the boundaries could be studied. The implications of stoichiometry, the multiplicity of the boundary structures and possible transformations between them for grain boundary brittleness are also discussed. more...

The United States Air Force is in the process of developing magnetic bearings as well as an aircraft Integrated Power Unit and an Internal Starter/Generator for main propulsion engines. These developments are the driving force behind a new emphasis on high temperature, high strength magnetic materials for power applications. Analytical work, utilizing elasticity theory, in conjunction with design requirements, indicates a need for magnetic materials to have strengths in excess of 80 ksi up to about 1000 degrees F. It is this combination of desired material characteristics that is the motivation for this effort to measure, model, and predict the creep behavior of such advanced magnetic materials. Hiperco® Alloy 50HS, manufactured by Carpenter Technology Corporation, is one of the leading candidates for application and is studied in this effort by subjecting mechanical test specimens to a battery of tensile and creep tests. The tensile tests provide stress versus strain behaviors that clearly indicate: a yield point, a heterogeneous deformation described as LuÌ ders elongation, the Portevin-LeChatelier effect at elevated temperatures, and, most often, a section of homogeneous deformation that concluded with necking and fracture. Creep testing indicated two distinct types of behavior. The first was a traditional response with primary, secondary and tertiary stages, while the second type could be characterized by an abrupt increase in strain rate that acted as a transition from one steady state behavior to another. This second linear region was then followed by the tertiary stage. The relationship between the tensile response and the creep responses is discussed. Analyses of the mechanical behavior includes double linear regression of empirically modeled data, scanning electron microscopy for microstructural investigations, isochronous stress-strain relations, and constant strain rate testing to relate the tensile and creep test parameters. Also, elastic and creep deformation analyses are done, which incorporate material property data and material constants determined along with stress and displacement profiles for a specific Air Force design configuration. more...