Symposium NM10 : Nano-Metallic Materials by Design

The immiscible nanocrystalline Cu-Ta produced by cryogenic mechanical alloying demonstrate an extraordinary high-temperature stability of the microstructure and a high strength under both tension and compression. This talk highlights the stabilization and mechanical properties of the nanocrystalline Cu-Ta family of alloys, which in many cases demonstrate divergent behaviors, wherein the inferred long-established mechanisms governing no longer apply. For instance, contrary to the conventional believe and hitherto, we discover that a truly stabilized nanocrystalline material will not exhibit inverse Hall–Petch strengthening or superplastic behavior even when subjected to temperature as high as 80% of its melting point. Many of the unusually properties of such alloys are due to the precipitation of nanometer-scale Ta based clusters coherent with the Cu matrix. These clusters impose strong resistance to dislocation glide, pin grain boundaries in place, and effectively block grain boundary motion and grain rotation. Finally, this talk will also review the recent progress made through experimental studies (APT, TEM/SEM, DSC, mechanical testing) and atomistic computer simulations (MD and Monte Carlo) aimed to understand other remarkable properties of the fully stabilized nanocrystalline material, including the unusually small strain rate sensitivity (in dynamic regime), the limited strain hardening and extraordinary creep behavior.

Nanoporous coatings offer many exciting properties (e.g., for catalysis or structural colors) due to their exceptional internal geometry. An established way to prepare such coatings is by dealloying of co-deposited thin films made by physical vapor deposition. In contrast, electrodeposition is not typically employed for the deposition of the precursor films, although it is a simple and extensively used coating technique in both research and industry.
The reason for neglecting electrodeposition is straightforward: chemical dealloying asks for two alloying partners with very dissimilar nobility. In contrast, the electrochemical deposition of an alloy via co-deposition requires the two alloying elements to have similar reduction potentials. These contradictory premises make electrodeposition a less than ideal synthesis approach. Nevertheless, an electrochemical route would be favorable for some applications of nanoporous films, e.g., for the conformal coating of complex surfaces or low-cost applications.
Here, we demonstrate the electrochemical co-deposition of CuZn alloy films from a pyrophosphate bath [1] and their subsequent dealloying to fabricate nanoporous Cu films with pore and ligament sizes on the order of 50 – 100 nm. Films containing 0 – 50 at.% Zn can be synthesized from a single electrolyte bath, with the Zn percentage being a function of the deposition potential. This enables simple tuning of the Zn fraction during synthesis. We show that films with gradual or sharp gradients of Zn content along the growth direction can easily be achieved. These Zn-gradients translate in pore gradients upon dealloying. The microstructures and mechanical properties of the as deposited and de-alloyed thin films are characterized.
Interestingly, we observe a strong dependency of the dealloying behavior on the composition of the deposition solution, which is possibly related to the microstructure of the films. A closer study of the microstructures of these films will be presented. This study could help to elucidate generic dealloying principles of electroplated films and help define universal strategies for the electroplating of precursor coatings. Furthermore, we envision to utilize this fundamental understanding for microscale electrochemical additive manufacturing. Controlling the Zn content with voxel-by-voxel resolution could enable the synthesis of microstructures with 3D architectured porosity.

[1] L. F. Senna, S. L. Díaz, L. Sathler, J. Appl. Electrochem. 2003, 33, 1155.

I was shown recently that thin nanoporous dealloyed layers on a binary AgAu alloy could be efficiently filled by electrodeposition of Cu, giving mechanical stabilization prior to the use of atom-probe tomography for atomic-scale analysis [1]. In those analyses, it was shown for the first time that it is possible to capture the early-stage dealloying of AgAu in aqueous perchloric acid, with ligaments of only a few nm width and ligament cores at or near the bulk alloy composition. More recently, similar analyses have been done on very thin (sub-micron) dealloyed layers on ternary AgAuPt alloys with [Pt] from 1 to 3 at%. A great deal of effort has gone into the optimization of the electrodeposition method and the inclusion of the diffuse layer-substrate interface in the APT sample. Beyond that, it has been necessary to quantify and correct for the ion trajectory aberrations that may exist in the data, in order to gain new information on the concentration and distribution of Pt on ligament surfaces. These analyses of the raw data are ongoing, and are giving new insights into the mechanism of Pt redistribution. The electrodeposition process is of fundamental interest in its own right, and our studies have now been extended to other metals, and to electrodeposited oxides. Generally speaking, the success of the electrodeposition method is ascribed to a combination of small dealloyed layer thickness (and thus short characteristic diffusion time to the dealloying front), simple deposition mechanism, and favourable kinetics. Various templating methods based on these procedures are being studied. Similar studies on dealloyed layers subjected to post-treatments, such as heating in oxidizing or reducing environments, are also under way, supported by in situ environmental TEM.

[1] A.A. El-Zoka, B. Langelier, G.A. Botton, and R.C. Newman (2017), Enhanced analysis of nanoporous gold by atom probe tomography. Materials Characterization 128: 269-277.

Dimethyl ether (DME) is a clean-burning alternative to diesel fuel that meets strict emissions standards in combustion engines. DME is currently derived from natural gas but can also be synthesized from biomass on-site and is thus a sustainable and highly distributed energy source. DME has properties similar to propane and can utilize established fueling infrastructure and handling procedures. In addition to its use in internal combustion engines, the convenience of DME generation and transport can be exploited to overcome the high-pressure infrastructure requirements of hydrogen fuel in more efficient electrochemical fuel cells.

We here describe the synthesis of monodisperse spherical Pt₂Bi alloy nanoparticles ca. 5 nm as well as larger non-uniform Pt₂Bi alloy nanoplatelets in electrostatically stabilized aqueous suspensions for use in direct-DME fuel cells. Synthesis involves the use of stannous chloride as an inorganic autocatalytic reducing and stabilizing agent and is unique to nanoparticle (NP) synthesis. The method avoids the problems of system contamination due to in-situ electrochemical reduction as well as the unsustainable use of organic solvents and hazardous reducing agents. Furthermore, a stable Pt₂Bi alloy (confirmed through HAADF-STEM, XRD, XAS, and EDS) is obtained without the need for subsequent high temperature annealing in reducing environments. The dealloyed nanoparticles exhibit substantial enhancement in dimethyl ether (DME) electro-oxidation activity relative to Pt-C with minimal surface poisoning. ECSA peak deconvolution assisted by Bi and Ge adatom analyses indicate the proportion of Pt(100) to Pt(110) facets on the dealloyed Pt₂Binanoplatelets is substantially greater than on Pt-C. These changes in surface atom coordination are consistent in the substantial reduction of poisoning by adsorbed intermediates observed on the dealloyed PtBi₂ nanoparticles versus Pt-C, as predicted by theoretical analyses and single crystal studies with pure Pt.

Nanostructures, self-organized into periodic concentration modulations, have been reported with radically different two-phase morphologies, including vertical and lateral striations. In order to understand the origin of these morphologies, we study the organizing mechanisms of these architectures via phase decomposition during elevated temperature co-sputtering of immiscible metals such as Cu-Mo using analytical electron microscopy. Based on structural and chemical analysis results, an evolution in self-organized, nano-metallic morphologies was observed according to the direction of phase separation. This was the result of the phase separation kinetics relative to the deposition rate during growth. Depending on the comparison between the rate of phase separation and the deposition rate, novel lateral, vertical, and randomly oriented concentration modulations in three-dimensions were obtained. A predictive capability of these self-organizing nano-architectures will allow for synthesis by design for advanced functionalities.

Joining of nanostructured materials by conventional processes, such as welding destroys the functionality of the material by disrupting its microstructure within the heat-affected zone. One of the weaknesses of nanometallic materials is the lack of knowledge on how to joining them. We present an approach to joining of magnetron-sputtered, nanolayered Cu/Nb composites without compromising the integrity of the nanolayered architecture by using a microstructure-preserving lap joint. The two parent materials are interconnected by a nanolayered composite section of the same total thickness and same nanolayered architecture as the parent material. The gap between the sections of parent material is kept constant while the overlap of the ends of the lap joint may be varied in length. The pristine metal joint is characterized by using scanning electron microscopy and nanoindentation, exploring potential variations in structure and mechanical properties within the parent material, overlap area, and gap infill region. Annealing testing of the metal joint and subsequent nanoindentation gives insights into the structural characteristics of the metal joint at high temperature levels. Further, we conduct tensile testing of free-standing samples and explore potential failure modes of the nanometallic materials joint when subjected to uniaxial monotonic tensile stress. Our work advances future applications of nanocomposite materials by paving the way toward practical, microstructure-preserving methods of joining them.

Nanocrystalline materials have exceptional mechanical properties (e.g., high strength, excellent wear resistance), making them ideal for structural applications. However, nanocrystalline materials cannot be joined by fusion-based welding methods because the high temperatures required by these methods can activate grain growth. Ultrasonic welding is an attractive approach to bonding nanocrystalline materials because it is a low-temperature solid-phase process. In ultrasonic welding, the work is clamped between a stationary anvil and a sonotrode that oscillates at ultrasonic frequencies. The workpieces form a metallurgical bond when surface oxides are sufficiently dispersed and intimate metal-metal contact is made. Although ultrasonic welding is a solid-phase process, the ultrasonic vibrations cause some frictional heating at the weld interface. In this work, we first describe the effect of frictional heating on the structure of nanocrystalline feedstock by relating the process parameters to the instantaneous grain size using the Burke-Turnbull grain growth equation. Next, to minimize the thermal excursion and thus grain growth, but maintain weld strength, we develop a junction growth model that predicts the real contact area throughout the bond formation process. This junction growth model can predict the process parameters required to completely bond the foils, without over-welding them and causing unnecessary heating. The structural evolution and junction growth models, when integrated, can predict parameter sets that optimize both nanostructure preservation and bond strength in ultrasonically welded nanocrystalline materials.

The most efficient and widely used transparent conducting material (TCM) is currently indium tin oxide (ITO). However the indium scarcity associated to the lack of flexibility of ITO as well as relatively high cost of fabrication has prompted the search for alternative low cost and flexible materials. Among emerging transparent electrodes (TEs), silver nanowire (AgNW) networks appear as a promising substitute to ITO since these percolating networks exhibit high flexibility and excellent optoelectronic properties [1], with sheet resistance of a few Ω/sq and optical transparency of 90%, fulfilling the requirements for many applications such as solar cells, OLED displays, transparent heaters, or radio-frequency (RF) antennas and transparent shielding [2]. In addition, the fabrication of these electrodes involves low-temperature process steps and upscaling methods, thus making them very appropriate for future use as TE for flexible devices.
Our research is focused on the fundamental understanding of the physical phenomena taking place at the scales of both the network (macroscale) and the NW-to-NW junctions (nanoscale), and on the ability of AgNW networks to be integrated as transparent electrodes for flexible optoelectronic and RF devices. In-situ electrical measurements performed during optimisation process such as thermal annealing and/or chemical treatments provide useful information regarding the activation process of the junctions [3]. Besides, nano-characterisation techniques such as Transmission Electron Microscopy (TEM) and ultramicrotomy help visualizing the physical phenomena involved in the diffusion of silver atoms to create well-sintered junctions. At the network’s scale, our ability to distinguish the nanowires taking part in the electrical conduction (“electrical percolating pathways”) from the inactive nanowires is a critical issue for the applications. By combining experimental and simulation studies, a discrete activation process of efficient percolating pathways through the network was evidenced. In the case where the network density is close to the percolation threshold and when low voltage is applied, individual “illuminated” pathways can be detected through the network while new branches get activated as soon as the voltage is increased.[4]
Here we will present our results on the study of AgNW networks at the macro and nano scales described above and will correlate it with the overall performance/characteristics of the networks. We will also present results on the integration of optimized AgNW networks into functional devices.
[1] T. Sannicolo et al. Small, 12, 6052–6075, (2016).
[2] C. Celle, et al. Nano Res. 5, 427, (2012).
[3] M. Lagrange, et al. Nanoscale 7, 17410, (2015).
[4] T.Sannicolo et al. Nano Lett., 16, 7046–7053, (2016).

Avalanche-like discontinuous plasticity, called intermittent plasticity, is the emergence of unpredictable sudden large-scale plastic deformations, which are characterized by peculiar statistical distributions, i.e., power-law distributions [1]. A remarkable feature of the avalanche plasticity is that the intermittency becomes pronounced strongly in micro-scale samples [2-4]. Thus, suppressing this intermittent behavior will be significant in synthesizing and designing the nano-scale structural materials. Atomistic structures such as grain boundaries (GBs), stacking faults (SFs), vacancies, and other lattice defects might inhibit the intermittency. Actually, it has been demonstrated that GBs play a role as an obstacle to the propagation of the avalanche plasticity [5]. This role might be closely related to mechanical properties of polycrystalline materials. However, theoretical approaches are still quite preliminary, because the role is fully atomistic scale behaviors. To investigate the behaviors, e.g., the dislocation-grain boundary interaction, defect nucleation, dislocation entanglements and so on, molecular dynamics (MD) simulations with realistic interatomic interaction are indispensable.
In our previous MD simulation study, avalanche behaviors of dislocations in single metallic crystals have been successfully reproduced by molecular dynamics simulations of constant strain rate uniaxial tensile deformation [6]. We also applied the numerical method to metallic poly-crystal models simplified by symmetric tilt GBs [7] and metallic glasses. The numerical results show that avalanche-like motions of dislocations and those of shear transformations of atoms in local areas. The statistical distributions of plastic deformations in our simulations follow power-law distributions which are qualitatively consistent with the previous experimental studies. Our MD simulations also demonstrate that GBs suppress the instability of the plasticity; GBs reduce frequency of system-spanning large scale slips. This fact implies that designs of atomistic structures, such as GBs or SFs, will be significant in synthesizing of nano-scale structural materials. In addition to the plastic manner in bulk solids, a potential of MD simulations for nano metallic materials cooperating to 3D tomography observations by transmission electron microscopes is discussed.

[1] M. C. Miguel, et al., Nature, 410, 667, (2001).
[2] D. M. Dimiduk, et al., Science, 312, 1188-1190 (2006).
[3] F. F. Csikor, et al., Science, 318, 251 (2007).
[4] P. D. Ispánovity, et al., Phys. Rev. Lett., 112, 235501 (2014).
[5] T. Richeton, J. Weiss, and F. Louchet, Nature Mater., 4, 465 (2005).
[6] T. Niiyama and T. Shimokawa, Phys. Rev. E, 91, 022401 (2015).
[7] T. Niiyama and T. Shimokawa, Phys. Rev. B, 94, 140102 (2016).

Hydrogen-lattice defect interactions play a key role in designing materials that can efficiently store the hydrogen, or materials that can efficiently avoid its embrittling influences. Since the interactions act as primary factors causing embrittlement, many scientific and technical efforts have been dedicated to observe them directly. For example, the most important evidence for the occurrence of hydrogen-enhanced localized plasticity was captured by in-situ environmental transmission electron microscopy experiments with a hydrogen gaseous environment. This imaging technique visually elucidated hydrogen-assisted dislocation motion and a reduction in dislocation-dislocation repulsive interactions by hydrogen uptake in thin foils. In our work, by using Electron Channeling Contrast Imaging (ECCI), surface dislocation patterns in a bulk specimen were visualized in a scanning electron microscope. In addition, due to the easiness and wide view field of the technique, statistical distribution of plasticity-related parameters such as dislocation density was quantified. On the other hand, the recent highly-efficient Grand Canonical Monte Carlo (GCMC) method hybridized with Molecular Dynamics (MD) simulations were utilized and the hydrogen-dislocation atomic interactions were simulated. Hydrogen atoms were dynamically exchanged between the ideal gas reservoir and the interstitial sites during GCMC steps, and the sample was relaxed in MD intervals. Our novel designed experimental procedure coordinated with the numerical studies revealed that hydrogen diffusion can have a much higher influence on dislocation network than shown in previous studies. The origins of this observation were numerically investigated and will be presented in this talk.

Polycrystalline materials’ mechanical properties and failure modes depend on many factors that include diffusion and segregation of different alloying elements and solutes as well as the structure of its grain boundaries (GBs). Segregated solute atoms to GB can alter the properties of steel alloys. Segregation of certain impurities to GBs is the main cause for intergranular fracture of steel among various other mechanisms. As a consequence, the presence of these impurities mainly defines the strength of steel. Some of these elements lead to enhancing the strength of steel, on the other hand others can degrade the toughness of steel significantly. It is well known that carbon increases the cohesion at grain boundary. While the presence of hydrogen in steel have a drastic effects including blistering, flaking and embrittlement of steel.
In practice during forming processes, the coincidence site lattice (CSL) grain boundaries GBs are experiencing deviations from their ideal configurations. Consequently, this will change the atomic structural integrity by superposition of sub-boundary dislocation networks on the ideal CSL interfaces. For this study, the ideal ∑3 and ∑5 GB structures and its angular deviations in BCC iron within the range of Brandon criterion are studied comprehensively using molecular statics simulations. The clean GB energy will be quantified, followed by the GB and free surface segregation energies calculations for hydrogen and carbon atoms. Rice-Wang model is used to assess the embrittlement impact variation over the deviation angles.

Nanoporous gold (npg), a nanostructured bicontinuous network structure fabricated via electrochemical dealloying offers a unique combination of tailorable strength and functionality. The mechanical behavior of npg has been shown to be strongly dependent on its average ligament width, with local stresses in the as-dealloyed material approaching the theoretical strength of gold, underscoring the “smaller is stronger” paradigm. Yet strong deviations from classical laws for cellular structures have been found for npg, pointing to the need for a more detailed investigation of the 3D network structure, and a better understanding of the evolution of structural characteristics resulting from coarsening. Here we present and discuss results from high-resolution tomographic characterization, in situ micromechanical testing, and microLaue diffraction in terms of the underlying structure-property relations and mechanisms of deformation of this unique hybrid material.

The mechanical behavior of nanostructured metallic multilayers has been extensively investigated using nano-indentation or micro-pillar compression tests in recent years. However, relatively few studies have examined the mechanical behavior of multilayers via tensile testing, which provides the most accurate measure of mechanical properties such as modulus, yield strength and ductility. In this study, we examined the mechanical properties of a Cu/Co nanocomposite multilayer film (4 nm layer thickness of Cu and Co), sputter-deposited at room temperature, using MEMS-based tensile testing. XRD analysis of the film showed a strong (111) texture, with the peak position corresponding to a d-spacing that was the average of the d-spacing of FCC Co and FCC Cu. The film showed a high yield and fracture strength of 1.15 GPa and 1.4 GPa, respectively and also exhibited notable inelastic strain recovery, both during and after unloading. We also co-deposited a Cu-Co film with the same composition at room temperature using magnetron sputtering. XRD analysis of the co-deposited film revealed a FCC solid solution with a (111) texture, even though Co and Cu are expected to have little miscibility at room temperature. Interestingly, no phase separation was seen in the co-deposited film even when it was annealed at 400 ^oC. However, phase separation occurred at 600 ^oC, with the formation of FCC Co and FCC Cu. Tensile testing was performed on these co-deposited, solid solution and phase separated films to compare their properties with those of the nanocomposite multilayers.

Architectured microstructures, for example with grain size gradients, can significantly improve a metal’s performance. [1] Yet, the complexity of such microstructures is limited, because the local control over the microstructure during synthesis is very basic. For example, we can produce grain size gradients or layered structures with nanometer resolution in thin-films by changing the deposition condition during growth. However, the microstructural control over these gradients is limited to a single dimension: the growth direction.
Additive manufacturing (AM) could expand our abilities to fabricate architectured, metallic microstructures to all three dimensions. There are two main prerequisites to enable “microstructure-printing”: first, the resolution of the printing process should be comparable to typical microstructural features: ~10 µm. Secondly, precise control is needed over the synthesized microstructure at voxel-level.
Multiple metal micro-AM techniques are available [2], which all satisfy the resolution-criterion. Yet, many of them lack the ability to actively control the microstructure of the deposited material. Exceptions are electrochemical and electron-beam based techniques, and, to some extent, approaches that employ in-situ sintering, but the electrochemical methods undoubtedly promise the most elaborate control. This is owed to the fact that electrodeposited microstructures can be a strong function of the deposition potential. Because the in-situ manipulation of this potential is straightforward, the manipulation of the synthesized microstructure is possible at voxel-level.
Here, we demonstrate this basic approach using meniscus-confined electroplating of an electrochemical model-system: electrodeposited CuZn-alloys. In these alloys, the Zn-content is determined by the deposition potential in the range of 0 – 50 at.%. Controlling the local Zn-percentage enables manipulation of the local grain size, as well as the local solid-solution level. Additionally, dealloying of the CuZn-alloy yields nanoporous structures whose porosity replicates the initial Zn distribution. We show printed microscale objects with both grain-size and pore-size gradients and a characterization of their mechanical behavior.
Future possibilities for manipulating microstructures are powerful: nano-twin density and orientation, grain-size, alloy composition or incorporation of reinforcement particles can be changed at will by adjusting the electrode potential. The combination of this strategy with the high resolution of microscale AM could enable the ultimate control over architectured microstructures.

[1] K. Lu, Science (80-. ). 2014, 345, 1455.
[2] L. Hirt, A. Reiser, R. Spolenak, T. Zambelli, Adv. Mater. 2017, 201604211, 1604211.

Next generation Metallic materials and Alloy with higher energy devices, low cost, long lifetime, and reduced rare earth content or rare earth free, require the investigation of unknown alloy systems and a vast amount of experiments. High throughput and combinatorial experimental technologies are finding their applications in novel amorphous materials and alloy research. MTI Corporation’s effort on developing a high throughput bulk materials production line for novel metallic material and alloy research are reviewed and discussed.
MTI Corporation’s high throughput bulk materials production line is designed for novel amorphous materials and alloy research with 32 samples up to 10 gram in one automatic run. The first step is an automatic dispensing of solid powders. Four or more powder dispensing heads and balances, with one dispensing head for each material component, is integrated with a carousel-type sample changer for automatic powder dispensing with varying ratios between different material components. The second step is the ball milling of the as-dispensed powder samples. With 4 sets of 4 cavity milling jars, totally 16 parallel ball milling experiments of 10 g samples can be carried out in a planetary ball mill machine. The mixed powder samples are then pellet pressed by a 10-ton electric hydraulic press with a carrousel type sample changer. Next, there are 3 different methods for the melting processing: Melt Spinning, Arc Melting, and Induction Heating. In the melt spinning, metal ingots are melted via induction heating method and then propelled by over pressurizing the crucible. A thin stream of molten is then dropped onto a fast moving surface of a cold copper drum. This causes rapid quenching & solidification of the molten and thus form the liquid metal alloy. For Arc melting, the pellet samples are loaded onto the 32-cavity copper crucible of an automatic high throughput arc melting system integrated with a glove box for preparing the bulk alloy samples. However, induction melting is slower. The pellet samples are loaded onto the sixteen 6 mm diameter gravities high purity graphite crucible. Three-layer special design refractory liner could withstand the 2200°C high temperature. Then, a compact 16-channel tube furnace up to 1100 °C is employed for sintering, annealing, or quenching. Later on, the high precision diamond wire saw with the swing rotary sample fixture and microscope monitor provides the low-stress cut and the 16 samples automatic polishing machine is used for lapping and polishing. Finally, the elemental compositions of bulk samples are high throughput analyzed by an X-ray fluorescence (XRF) scanning system for providing instant feedback to the synthesis/processing steps
High-throughput experimental solutions increase productivity by 16 times.
Existing experimental techniques are modified for high-throughput application.
Novel melt spinning and arc melting processes enable reduced processing time and low production cost.

Nanomaterials exhibit a number of specific properties which differ them from conventional microcrystalline counterparts. As an example, in bulk solids, melting temperature is considered as a constant, specific for each material. However, at the nanoscale, it decreases rapidly with the size of nano-elements. Although melting point depression has been experimentally demonstrated, its mechanism is not fully understood, in particular little is known about microstructural changes before melting in two-component materials. The specific goal of this study was to give a better insight into these phenomena based on observations using high resolution scanning transmission electron microscopy (HRSTEM).
Ag/AlN and Ag-Cu/AlN nanomultilayers (NMLs) were deposited by magnetron sputtering at room temperature. The Cu content in Ag-Cu layers corresponded to the bulk eutectic composition Cu_40at.%Ag. The metallic and ceramic layers were deposited alternatively with 10 repetitions of such bi-layer on Al₂O₃ substrate. The thickness of both individual layers was 10 nm. NMLs were next annealed at a constant temperature in the range from 400 to 900^oC.
In as-deposited state, the NMLs are of uniform thickness. and the confined alloy nanolayers consist of a highly supersaturated solid solution Cu_40at.%Ag phase. The supersaturated solid solution is thermally unstable and therefore heating induces a phase separation into Ag-reach and Cu-reach regions prior to melting First droplets on the top surface of NMLs can be seen after annealing at 450^oC much below the bulk eutectic melting temperature of this alloy of 778^oC. The surface droplets consist of pure Cu, which indicates off-eutectic melting behaviour. At higher annealing temperatures, the NML structure collapses and intensive outflow of brazing material is observed. The microstructural evolution of Ag/AlN and Ag-Cu/AlN NMLs during heating is compared.

Fabrication of small hollow metal-oxide nanostructures has a broad application in the biomedical field with a great potential in the diagnosis and treatment of diseases like cancer. This work is focused on the synthesis and characterization of TiO₂ hollow nanoparticles (TiO₂ HNP). These type of nanoparticles are important in the biomedical field due to their metallic and size properties. First, with sizes smaller than cells and viruses, TiO₂ HNP can be used for targeted drug delivery. The hollow nanoparticles can potentially be loaded with different therapeutic drugs, and functionalized with labels to attach and treat only damaged cells. Secondly, by exposing TiO₂ HNP to a magnetic field that varies in time, they can be used for magnetic resonance imaging (MRI). Thirdly, by combining targeting treatment and MRI, TiO₂ HNP can be used for theranostics.
In this work, the synthesis of TiO₂ HNP was carried out using a template mediated method. Carbon spheres (CS), derived from sustainable precursors, act as a template and are synthesized by a hydrothermal process. After mixing the CS in an ethanolic solution and utilizing titanium tetraisopropoxide as the precursor, the CS are removed and the TiO₂HNPs are formed and by a post- thermal treatment. The crystal structure was analyzed using X-ray diffraction (XRD). Size distribution and morphology were characterized by scanning electron microscopy (SEM). The HNP structure was verified by transmission electron microscopy (TEM).

Rare-earth metals are of general interest to basic chemistry and application (e.g. catalysis, magnetism, thermoelectrics). Aiming at nanosized rare-earth metals, the synthesis becomes highly callenging due to the high reactivity of the nanoparticles, which is drastically higher than for the bulk metals due to the small size and the high specific surface. Such high reactivity, however, can be also very interesting in view of follow-up reactions that – in contrast to the bulk metals – can be performed at room temperature.^1,2

As a straightforward access to reactive base-metal nanoparticles, we could recently establish a sodium-naphthalenide-driven synthesis in ethereal solvents.^3,4 The mild reaction conditions and the absence of any capping agents result in oxide-free nanoparticles with quasi-naked surfaces and narrow size distribution (3-5 nm). The nanoparticles can likewise be obtained as powder samples or as colloidal suspensions.^3,4

Based on our previous studies, we here present the preparation of Sm(0) nanoparticles with a mean diameter of 1.7±0.2 nm. The highly uniform nanoparticles were characterized in detail by advanced electron microscopy as well as by spectroscopic methods. The as-prepared Sm(0) nanoparticles are crystalline, not impurified by oxygen, and they are highly reactive (i.e. pyrophoric). Besides synthesis and characterization, this presentation will illustrate the reactivity in view of room-temperature reactions in order to obtain, for instance, nanosized Sm₂O₃, Sm₂S₃, SmI₃, Sm-Co alloys, as well as metalorganic compounds.⁵ These follow-up reactions and the respective materials can be highly interesting as catalysts, semiconductors, or heavy-fermion systems.


References
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[3] C. Schöttle, P. Bockstaller, R. Popescu, D. Gerthsen, C. Feldmann, Angew.
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Many metal oxides have a ubiquitous presence in modern life. The most popular examples, silicon dioxide (SiO₂), iron oxide (Fe₃O₄), aluminium oxide (Al₂O₃) and titanium dioxide (TiO₂), are earth-abundant and have a wide range of applications, ranging from energy storage to catalysis. Despite their importance, the capability to tune the properties of these materials through mild and operationally straightforward methods still remains challenging, as there exist a limited number of techniques available to do so. For example, TiO₂ has attracted enormous attention in the field of renewable energy due to its potential as a photocatalyst. However, its wide optical bandgap (~3.2 eV) makes it only capture ultraviolet (UV) light, which makes up ~7 % of the solar spectrum and exclude visible light. As a result, a particular area of interest is to increase the range of sunlight that TiO₂ can absorb. Most efforts have focused on the use of molecular organic and inorganic dyes, elemental doping with light elements or defect engineering.
In this work, we introduce a new approach, we refer to as “molecular cross-linking”, whereby a hybrid material is formed by cross-linking polyhedral boron cluster precursors, [B₁₂(OH)₁₂]^2-, to the network of TiO₂ using a simple solution-based synthesis. This new approach is enabled by the inherent robustness of the molecular boron clusters, which is compatible with harsh conditions required for the synthesis of metal oxides. The combined comprehensive structural characterization of this hybrid material from X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) provides evidence for the presence of crystalline TiO₂ in the anatase phase with X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES) confirming Ti⁴⁺. The intact boron clusters were further probed via solid state NMR spectroscopies and pair distribution function (PDF) analysis. The excellent light absorption property of this cross-linked material renders significantly enhanced photocatalytic activity compared to pristine TiO₂. Experimentally this property manifests in much faster photodegradation of organic pollutants under a low power red LED. The electrochemically-active nature and high electrical conductivity of the cross-linked material allowed us to explore it for energy storage. A pouch-cell supercapacitor containing cross-linked TiO₂ exhibited superior performances in comparison to both forms of TiO_2. The successful modification of metal oxides demonstrates the value of molecular cross-linking as a new and previously unattainable strategy to induce change in the properties of metal oxide materials. The simplicity and generality of the route, where cross-linking molecular clusters to a metal oxide via a facile reaction proceeding at room temperature using readily available precious metal-free precursors, makes this potentially applicable to a wide range of other metals and associated applications.

Solid state phase transformations are used for the design of advanced Mn-containing steels and Ti alloys with nano-scale microstructures, where the mechanical properties are controlled by the properties, morphology and interactions of the different microstructural constituents. Specifically the interplay of local defect structures and Gibbs-driven chemical decoration of these defects is an essential vehicle for locally stimulating site specific transformation phenomena.
To this aim we utilize appropriate mechanical and thermal treatments to induce segregation and partitioning phenomena on the nano-scale causing local allotropic and martensitic transformations. By applying multi-scale characterization using scanning electron microscopy, transmission electron microscopy and atomprobe tomography, we study the local elemental partitioning and phase transformations as well as the thermodynamical and structural driving forces causing these local phase transformations at crystal defects and their effect on microstructure and mechanical properties.
We show that through this approach the mechanical properties can be tailored covering a broad range from low yield strength and high ductility to high yield strength and high ductility.

Nanostructured alloys are currently of much interest for their potential applications in extreme environments. Most research has focused on their high strength and ductility, however, for high temperature applications, their susceptibility to creep damage is also important, and much less work has been performed on this topic. Creep may be significant in these materials as diffusional creep scales inversely with grain size, e.g., Coble creep varies as 1/r³, and creep has indeed been observed in some pure metals even at room temperature. Nanocrystalline alloys, moreover, are subject to grain growth, possibly also limiting their range of operating temperatures.
In the current work we have examined grain growth and creep behavior in dilute Al-Sc alloys. Al-Sc is an age hardening alloy, which has been well studied for coarsening and strength in the past. We prepared our alloys as thin films using magnetron sputtering with a concentrations of 1.1 at. %. These films were stabilized at a grain size ~ 150 nm by first annealing them at 250°C. Creep measurement were performed using a bulge test (JJ Vlassak and WD Nix, J. Mats. Res. 7, 3242, (1992)); the grain size and precipitate structure were examined by transmission electron microscopy. Nanohardness was also measured. The creep measurements revealed that the creep rates increased linearly with applied stress, while the temperature dependence showed approximately Arrhenius behavior with an activation enthalpy of ~ 0.5 eV, which is somewhat lower than that of grain boundary diffusion in large grained Al. Hardness of these samples was ~ 1.2 GPa. The results are compared with large grained Al-Sc alloys.

A thermodynamic approach can govern thermal stability of nano-scale grains in Nanocrystalline Alloys. Solute additions can segregate to grain boundaries and contribute to total net change of grain boundaries energy. This reduces the driving force for grain growth and maintains the grains within a nano-scale grain size. This thermodynamic modeling is readily able to predict grain boundary segregation in multicomponent nanocrystalline alloys. A semi-regular solution model was developed, and minimized through a Lagrange Multiplier method. This model will give equilibrium atomic concentrations of solvent and each solute inside the grain and at the grain boundary. Moreover, the equilibrium volume fraction of grain boundary obtained by this model could give an approximation of grain size at a given temperature. In this approach, we maintain the solute content within the solubility limit while we introduce multiple solutes into alloy system. This can offer a new research agenda in the discourse of thermodynamic stabilization in Nanocrystalline Alloys.

The alloy composition Ti-6Al-4V, also known as Ti6Al4V is the workhorse alloy of the titanium industry. Its uses span aerospace airframes, engine components and also major non-aerospace applications in the marine, offshore and power generation industries due to its low density, high strength and excellent corrosion resistance. However, the properties of the alloy changes with oxygen content which remains dissolved at high temperatures and precipitate out to form oxides at low temperatures. Oxygen is detrimental to ductility of Ti-6Al-4V especially when it exceeds 0.33 wt.%, above which ductility drops rapidly and can be much lower than the corresponding ASTM specification of a minimum value of 10% [1]. The oxygen in the alloy powder could be contributed from dissolved oxygen or adsorbed oxygen. Adsorbed oxygen is a function of the particle size and scales with decreasing dimensions of individual particles. Commercially available Ti6Al4V alloy powder size is large with size ranging from several tens to hundreds of microns. For increasing the overall strength of the alloy there is a need for decreasing oxygen content and particle size which in turn rises the cost significantly. Simultaneous improvements in mechanical properties of the alloy arising from the dispersion of hard particles, removal of oxygen, and grain size reduction can be achieved through the process of mechanical alloying of the alloy with oxygen scavenger element such as yttrium (Y). Use of Y to remove oxygen from Ti alloys have been demonstrated in prior work on consolidation of γTiAl dispersed with Y metal by ball milling [2].

In this work, Ti6AL4V-Y composite material was developed by consolidation of ball milled Ti6AL4V and Y powders. Ti based milling medium was used to avoid metallic elemental contamination and the powders were consolidated using spark plasma sintering (SPS). Structural characterization using Scanning Electron Microscope/Energy Dispersive Spectroscopy (SEM/EDX) and X-ray diffraction indicated the formation of Y₄Al₂O₉. This shows the oxygen scavenging effect of Y to form stable oxide by removing oxygen from Ti. Mechanical properties evaluation using nanoindentation and microhardness tests showed improved hardness in Y added composites.

References

1. Miura, H., Itoh, Y., Ueamtsu, T., & Sato, K. ‘The influence of density and oxygen content on the mechanical properties of injection molded Ti-6Al-4V alloys. Advances in Powder Metallurgy and Particulate Materials, (2010), 46-53.
2. P.B. Trivedi, , E.G. Baburaj, A. GenC, L. Ovecoglu, S.N. Patankar, and F.H. Froes, , “Grain size control in Ti-48Al-2Cr-2Nb with yittrium additions”, Met. Mater. Trans. 33A (2002) 2729-2736.

The desirable properties of nanocrystalline metals including high strength and hardness, low friction and wear, and resistance to fatigue crack initiation can be lost if the metal undergoes mechanically-induced grain growth. In prior work, mechanically-induced grain growth has been shown to directly cause fatigue crack initiation and wear-induced delamination in nanocrystalline metals. In-situ TEM and synchrotron x-ray high-cycle fatigue experiments reveal the incipient stages of damage, where grain growth provides the precursor state that precedes the formation of nanometer-scale cracks. Yet grain growth can also be beneficial- in cases such as monotonic tension, the boundary migration process is thought to be an additional deformation mechanism enabling enhanced ductility. For this reason, it is necessary to develop strategies to control boundary stability under mechanical driving forces. Recently there has been a strong interest in the stabilization of nanocrystalline grain boundaries against thermal evolution by lowering the energetic cost of the grain boundary via alloying. Specific binary alloys have been shown to preferentially segregate the solute species to grain boundaries and lower the driving force for boundary migration. The question remains as to whether the resulting thermal stability also gives rise to enhanced stability under mechanical driving forces. Emerging results on the fatigue and wear resistance of several binary nanocrystalline alloys, including the noble metal system Pt-Au, appear to suggest that the thermally stabilized boundaries are indeed resistant to fatigue and wear-induced microstructural evolution, resulting in exceptional performance.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525.

Nanoporous materials such as gold (np-Au) exhibit intriguing properties related to their nanoscale ligament size and high surface-area-to-volume ratio, such as elevated levels of equivalent strength and the ability to shed radiation damage to ligament surfaces. This talk will cover the behavior of np metals subjected to irradiation and to deformation, including in-situ observations of how these materials respond to such stimuli in an electron microscope.

Nanoporous metals are potentially advantageous in radiation environments, due to the high amount of ligament surface area that can act as a sink for defects produced during irradiation. Additionally, the nanoscale ligament sizes of np-Au and np-Nb lead to size effects in mechanical behavior, due to the confined deformation volumes within ligaments. In-situ TEM experiments were performed on np-Au and np-Nb samples during ion irradiation at different energies and varying total ion dose. The resulting changes in defect structure within ligaments, as well as changes in the overall porous network structure, were observed. Effects of ion irradiation were also investigated by nanoindentation of samples before and after irradiation, providing a link to mechanical properties.

The deformation of np-Au is complicated, due to the interconnected nanoscale ligaments that constitute the network configuration. Structural models have been developed to describe the deformation of materials with micron-scale porosity, but these have met limited success describing np-Au, as they assume simple modes of loading and deformation of individual ligaments. To better understand the deformation of networked np-Au, we employed scanning nanobeam diffraction to track the deformation of ligaments within a region comprising multiple ligament-pore cells, during in-situ tension testing in the TEM. Post-processing of deformation videos allowed the determination of strain fields during tensile loading and crack propagation, shedding light on the coordinated deformation of connected ligaments within a single-layer np-Au thin film.

As a three dimensional network of nanoscale Au struts (or ligaments), the nanoporous gold (NPG) often combines some properties of the porous materials and that of the nanoscale crystals. But one property, i.e., the plateau plastic yielding in compression, which is often observed in both porous materials and the nanoscale solids, has yet been conclusively achieved in nanoporous metals. Some small scale testing and simulation studies showed that the plateau plastic yielding may exist in compression of NPG. However, as far as the authors know, all macroscopic scale mechanical tests of previous NPG samples revealed an apparent "strain hardening" after yielding during compression, irrespective of the structure size and the relative density. The lack of the plateau plastic yielding and the presence of “strain hardening” in compression of NPG may be attributed to the densification of porous structure and thus the increase in network connectivity during compression, and/or arguably the strain hardening in the (bended) individual nano-ligaments. Here we report that by properly modifying the structure of a dealloying-made NPG, the plateau plastic yielding can appear in the compression stress-strain curve of the macroscopic NPG samples. A comparative study between the NPG samples with and without the plateau plastic yielding, with respect to their structure and deformation mechanisms, will be presented.

We present a molecular dynamics-based study of the mechanical properties of Nano-porous Au foams under compression and tension with identical morphologies, but varying ligament diameters. Compression and tension tests of these porous microstructures are compared to similar tests conducted for gold nanowires, and the validity of Gibson-Ashby scaling at small ligament sizes is investigated. A significant tension/compression asymmetry in the mechanical response of the foam is found that strongly depends on the ligament diameter. A model is presented for this asymmetry based on the contribution of the free surfaces present in the foams.

In recent years, access to and use of focused ion beam (FIB) techniques have increased, leading to new research opportunities in the field of nanoporous (np) metals. Direct observation of sub surfaces and 3D reconstructions are now possible, refining our knowledge of the np structure and its behavior. Because np metals in bulk form are typically challenging to fabricate and classic mechanical testing techniques (tension/compression tests) difficult to implement, several studies used nanoindentation to obtain mechanical properties. However, the deformation behavior of network structures of this scale (pores and ligaments in tens of nm) has yet to be fully understood. During this talk, we will show how the np structure accommodates the imposed deformation, by presenting cross section images and 3D reconstructions of np-Au after nanoindentation testing.

We have used np-Au as a model material to directly characterize the mechanical response of a np metal structure during nanoindentation. We have prepare bulk np-Au specimens and imaged the sub surface under indents, after FIB cross sectioning. In addition, we have automated FIB milling and imaging steps and were able to reconstruct indented regions, in 3D. Results from this study indicated that np-Au behaves more like a dense metal during nanoindentation, with the deformation propagating far ahead of the indenter, without full densification of the np structure. This observation and its implications will be discussed and compared with results previously obtained from millimeter-scale tensile and compression testing on np-Au.

When metallic alloys are exposed to a corrosive environment, porous nanoscale morphologies spontaneously form that can adversely affect the mechanical integrity of engineered structures. Nanoporous gold (NPG) is the prototypical example of the morphology that evolves as a result of such a dealloying process. I will discuss results of experiments exploring the tensile and dynamic fracture properties of NPG. Tensile properties were examined as a function of ligament size and sample density. The statistical distribution of the ligament diameters in these samples was determined and fit to the Weibull distribution. The Young’s modulus was found to obey a power law, but with an exponent larger than that predicted by Gibson-Ashby scaling. The fracture behavior showed a brittle-ductile transition as a function of increasing ligament size. These results are interpreted within the framework of extreme value statistics. The dynamic fracture properties of NPG were examined using and an experimental realization of the “infinite strip” sample and high-speed photography at frame rates of 1 million frames per second. This sample geometry allows for crack growth to occur under essentially fixed values of the stress intensity factor by controlling the strain energy in the system. The crack speed was found to linearly increase with the imposed strain energy up to ~70% of the Rayleigh velocity, at which point crack bifurcations evolve. Finally, I will describe how these results connect to stress-corrosion cracking processes in important engineering alloys such as stainless steel.

Nanoporous materials —also known as nanofoams— fabricated by dealloying methods exhibit unique properties and a bi-continuous network, leading to various applications as functional materials. A dealloying method utilizing a metal as a dealloying agent instead of an aqueous solution has led to an alternative route for fabricating nanoporous metals, for example, iron-based and titanium-based alloys. This opens new opportunities for material design, while preserving the unique bi-continuous morphology. It is, however, critical to establish the correlation between the processes, morphology (2D & 3D) and chemical heterogeneity of these new nanoporous metallic materials for suitable applications.

We utilized full-field nano-tomography via transmission X-ray microscopy to study the evolution of the morphology of nano-/meso-porous iron-based alloys by liquid metal dealloying. The 3D morphology of the materials was quantified as a function of processing conditions, including precursor compositions, dealloying temperature and time. A systematic trend of spontaneous coarsening as a result of prolonged dealloying time and increased temperature was observed; the precursor compositions significantly impact not just the porosity of the nanoporous materials, but also the surface shape of the nanofoams. A gradient of the ligament size and compositional change along the dealloying direction were also observed. In addition, spectroscopic imaging at synchrotron sources is a powerful technique for spatially resolving chemical and elemental distributions in these materials. X-ray fluorescence microscopy provides additional elemental and chemical information of the nano-porous materials. We also drew comparison between the structures fabricated by the liquid metal dealloying with the aqueous solution dealloying to shed light on the dealloying mechanism for future material design.

Experiments have demonstrated a rich variety of self-organized nanoscale concentration modulations in physical vapor deposited films of phase separating binary alloys. However, no comprehensive model capable of predicting the entire spectrum of these self-organized nanostructures as a function of material and processing parameters has yet been formulated. As the first step in this direction, we adopt a 3D phase-field approach to numerically investigate the role of deposition rates and separation kinetics on the morphological self-structuring in representative binary vapor-deposited alloys. Film coarsening characteristics are explored by comparing computational results with corresponding annealing studies. We propose new strategies for morphology control based on insights gained from experimental and computational studies.

Bimetallic interfaces are ubiquitous features in metal alloys, composites and machines. It remains challenging to design bimetallic interfaces with high strength and damage tolerance because of the diversity of interfacial features (e.g. atomic bonding, geometry, chemical composition) and their competing mechanical effects and complex interactions. In particular, lattice mismatch between the two metals determines whether the interface has perfect atomic registry (coherent interface) or contains defects such as misfit dislocations (semicoherent interface). The effect of atomic coherency on interfacial strength and plasticity is well established for arrays of randomly dispersed precipitates in metallic alloys, but this knowledge cannot be readily applied to nanostructured multilayers or nanostructures due to differences in geometry, dislocation density and proximity to other interfaces. To address this issue, it is necessary to nondestructively image coherent versus semicoherent bimetallic interfaces in nanostructures, and quantify their mechanical response.
Here, we study deformation at individual bimetallic interfaces by compressing colloidal core-shell nanocubes inside of a scanning electron microscope. Bimetallic nanocubes are ~50 nm in size. Interfacial coherency is manipulated by varying lattice mismatch at the core-shell interface. We have synthesized Au@Ag nanocubes with a lattice mismatch of 0.2%, Au@Cu nanocubes with a lattice mismatch of 12%, and monometallic Ag nanocubes. Transmission electron microscopy and X-ray diffraction are used to characterize nanocube structure and composition. We find that Au@Ag nanocubes have low lattice strain, and a coherent core-shell interface. The Au@Cu nanocubes have higher strain, and a semicoherent interface that contains dislocations. Nanocubes are compressed at 0.1 s^-1 under load control. The strength of the Au@Ag nanocubes is found to be ~800 MPa. The Au@Ag stress-strain curve is serrated with large, discrete stress drops, similar to the stress-strain behavior of single crystal metallic nanostructures. This indicates that dislocations cut across the coherent interface in Au@Ag nanocubes. The mechanical response of the core-shell nanocubes is compared to that of monometallic Ag nanocubes in order to differentiate the effect of the bimetallic interface and sample size.

Metal nanoparticles (NPs) specifically Al NPs, have been prominently featured over the years as energetic nanomaterials in the development of solid-state propellants, explosives and pyrotechnics. The promise of such materials has always resided on the kinetically controlled ignition processes in nanoscale regimes due to large specific surface areas, and small diffusion length scales at fuel-oxidizer interfaces. Yet, the reality of a rapidly growing passive oxide shells on NP surfaces has resulted in their performance being largely diffusion-limited thereby eluding the much-anticipated high detonation rates in the past. To overcome this challenge, herein we report the synthesis and characterization of metallic nanoparticles and specifically, Al NPs coated with graphitic shells in our effort to preserve the solid fuel surface from excessive oxidation while allowing heat conduction through the graphitic shells. Specifically, we use our recently developed laser ablation synthesis in solution (LASiS) technique to synthesize Al NP/Graphite core-shell structures by ablating an Al target in various organic solvents (acetone, toluene, ethanol). The structural and crystalline properties of the synthesized metallic Al NP core were measured and compared using Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD). Finally, using Laser-induced Air Shock from Energetic Materials (LASEM) method, the detonation performance of these materials were measured and compared to commercially available solid fuels in order to tailor the interfacial functionalities of these shell-core nanometallic energetic materials.

Chemical synthesis of base metals is the more challenging the smaller the particles are and the lower the electrochemical potential of the respective metal is. The annual number of publications addressing metal nanoparticles, to this concern, is a useful indication. Thus, the synthesis of Au(0) nanoparticles (E₀ = +1.5 V) was addressed by about 4000 publications in 2016, about 700 papers were related to Co(0) nanoparticles (E₀ = –0.3 V), whereas only 8 publications addressed Ti(0) (E₀ = –1.9 V), and none publication addressed the synthesis of Gd(0) nanoparticles (E₀ = –2.4 V) [1]. Here, it must be noted that the electrochemical potential only reflects the reactivity of bulk metals. Due to the absence of any passivation layer, high surface areas, and the great number of surface atoms, nanosized base metals can be expected to be significantly more reactive.

Via sodium-naphthalenide-driven reduction in ethers (e.g. 1,2-dimethoxyethane, tetrahydrofuran), we could now obtain very uniform Mo(0), W(0), Fe(0), Zn(0), Ti(0), Gd(0), and U(0) nanoparticles with diameters £10 nm [2-5]. Simple metal chlorides were used as the starting materials. The reactive metal nanoparticles can be easily obtained with yields of 95-99%. It is to be noted that Gd(0) and U(0) nanoparticles were made via liquid-phase methods for the first time [5]. Whereas suspensions of the base metal nanoparticles are comparably inert, powder samples are highly reactive and show spontaneous ignition when in contact to air.

The synthesis strategy allows a reproducible synthesis of Mo(0), W(0), Fe(0), Zn(0), Ti(0), Gd(0), and U(0)⁰ nanoparticles with high purity and with large quantities. Such dependable and comparably uncomplex synthesis is even more relevant since Gd⁰ and U⁰ stand as representatives for further lanthanide and actinide metals. All in all, the synthesis has the potential to become a general and reliable strategy for base metal nanoparticles and metal compounds (e.g. alloys, intermetallics, bimetallic heterostructures, metal nitrides) with applications ranging from catalysis, magnetic and hard materials, to batteries and solar cells.

This presentation will summarize the current status regarding the synthesis, the reactivity and specific follow-up reactions of base metal nanoparticles.

References
[1] The American Chemical Society, Program Package Scifinder, Washington 2017.
[2] C. Schöttle, P. Bockstaller, D. Gerthsen, C. Feldmann, Chem. Commun. 2014, 50, 4547-4550
[3] C. Schöttle, P. Bockstaller, R. Popescu, D. Gerthsen, C. Feldmann, Angew. Chem. Int. Ed. 2015, 54, 9866-9870.
[4] C. Schöttle, D. Doronkin, R. Popescu, D. Gerthsen, J.-D. Grunwaldt, C. Feldmann, Chem. Commun. 2016, 52, 6316-6319.
[5] C. Schöttle, S. Rudel, R. Popescu, D. Gerthsen, F. Kraus, C. Feldmann, 2017, submitted.

The highly-controlled formation of radial silicon/NiSi core-shell heterostructures has been demonstrated for the first time. Here, we investigated the radial diffusion of nickel atoms into silicon nanopillar cores, followed by nickel silicide phase formation and the creation of a well-defined shell. The described approach is based on a two-step thermal process, which involves metal diffusion at low temperatures in the range of 200-400C^o, followed by a thermal curing step at a higher temperature of 400C^o. In-depth crystallographic analysis was obtained by nanosectioning the resulting silicide-shelled silicon nanopillar heterostructures, giving us the ability to study in detail the silicide shell structure. Interestingly, it was observed that the resulting silicide shell thickness has a self-limiting behavior, and can be tightly controlled by the modulation of the initial diffusion-step annealing temperature. In addition, electrical measurements of the core-shell structures revealed that the resulting shells can serve as an embedded conductive layer. The silicide shell phase and structure were examined under high-temperature conditions (T>600C^o), where the silicide phase forms within the whole nanopillar cross-sectional area, accompanied with the appearance of an additional NiSi₂ phase.

The eradication of tenacious dental plaque is of paramount importance, however, their early diagnosis can be a daunting task in dental clinics due to the limitations of current instrumental methods, especially X-ray-based techniques. We approach this problem by integrating antibacterial properties and X-ray contrast enhancement in a single nanoplatform specific to periodontal bacterial colonies. Herein, the synthesis of inherently therapeutic hafnium oxide nanoparticles (HfO_x NPs) was achieved, conjugated with cationic macromolecules. These particles could be utilized as X-ray contrast media for dental X-ray radiography and as a bactericidal agent against cariogenic pathogens. Ex vivo studies using extracted human tooth demonstrated striking X-ray attenuation of NPs vs. tooth and could accurately detect the biofilm. Moreover, HfO_x NPs exhibited significant antibacterial properties. Electron microscopy revealed that the antibacterial activity occurred via a ‘latch and kill’ mechanism. Mechanistic studies determined that these NPs fragmented bacterial DNA components to exert their antimicrobial effect. CT imaging of NP-treated bacteria verified the contrast enhancement in bacteria-rich regions. Importantly, HfO_x NPs effectively inhibited the growth of a mature biofilm on an ex vivo human tooth model.
Prompted by the in vitro and ex vivo results, we evaluated the efficacy of the NPs in an in vivo rodent model (Sprague Dawley rat) of dental biofilm. Mature biofilm grown for a week was treated topically daily for two weeks with 0.02 mg.ml^-1 of HfO_X NPs. There was a significant inhibition of biofilm compared with chlorohexidine and water treated group. This result was also confirmed by an S.mutans detection kit. Furthermore, the Hf concentration of the collected tissue revealed minute accumulation (<12 ppm) in the gum, whereas in other organs it was down to ppb range. This concentration is in the safe range as the MTT results on the NIH 3T3 fibroblast cells indicated prior to conducting in vivo experiments.
This report is the first to demonstrate that HfO_x NPs can be used for simultaneous diagnosis and antibacterial treatment without requiring an additional drug.

Crystalline-amorphous nanolaminates represent a unique class of hierarchically structured materials where deformation is governed by a confluence of mechanisms deriving from defect interactions with both phase and grain boundaries. While a number of pioneering studies have shown that the amorphous layers act as both a source and sink for dislocations operating within the crystalline regions, design principles for simultaneously optimizing the multiple inherent structural length scales to tune plasticity at the nanoscale have yet to be established. Using molecular dynamics simulations, we first explore the influence of structural length scales including the crystalline-to-amorphous layer thickness ratio and nanocrystalline grain size on the underlying deformation mechanisms. Illustrative compound deformation mechanism maps capturing contributions from the three dominant mechanisms– shear transformation zone, dislocation, and grain boundary plasticity– are constructed to provide new insights into mechanistic transitions as a function of phase and interfacial volume fraction. Microstructural design windows are identified based on an Ashby plot analysis combining ductility-limiting localization factors with flow stresses calculated from the simulated stress-strain curves. Guided by the insights from atomistic simulations, nanocrystalline modulated Ni-W alloy nanolaminates are synthesized via electrodeposition and used to map mechanical properties including hardness and activation volume through nanoindentation. With the activation volume serving as a signature for the underlying deformation mechanisms, we correlate the measured mechanical properties to the deformation mechanism maps from atomistic simulations and identify microstructural conditions that produce a crossover to shear band dominated plasticity.

In this study, the mechanical behavior of metallic nanocomposites with bicontinuous structures has been studied using in situ nano-pillar compression testing in SEM. The nanocomposites with ligament sizes ranging from 2.5 nm to 15 nm were prepared by co-sputtering Cu and Mo with a nominal 50/50 atomic ratio at high temperatures. By varying the deposition temperature from 600^oC to 800^oC, the morphology of the nanocomposite changed from an anisotropic structure with alternating vertical Cu/Mo nanolayers to a more isotropic structure with intertwined Cu/Mo phases. During the compression tests, both morphologies exhibited large compressive strains (> 30%) at extremely high stresses (> 2 GPa) with maximum flow stresses approaching 3 GPa. Kink bands were observed in the anisotropic samples through post-mortem TEM investigation, whereas uniform deformation was observed in the isotropic samples. Slip along the aligned interfaces that are weak in shear is proposed to account for the strain localization in the kink bands.

Nanostructured metals often fail after minimal uniform deformation due to flow localization. I will describe an effort to create metal nanocomposites that resist flow localization by engineering the morphology of their microstructure via solid metal dealloying. By varying the compositions and thermal processing histories of these materials, we generate a range of complex nanocomposites with different microstructural length scales, microstructure morphologies, and phase connectivities. We then investigate the effect of these features on the hardness and flow localization in these nanocomposites using nanoindentation. This work screens a wide range of microstructures, providing design criteria for nanostructured materials that undergo large-scale, uniform plastic deformation.

Nanolaminated metallic multilayers can show extreme strengths due to the ability to induce a strengthening mechanism, confined layer slip, which is only possible at length scales on the order of 10’s of nm’s. Similar performance can be found in materials which are formed in a metastable structure and then subsequently thermally treated to precipitate extremely fine arrays of second phase particles. This current study explores combining these two strengthening mechanisms to precipitate hard particles within a metallic multilayer. The relatively immiscible system of Cu/Cr was selected as the multilayer structure. Three systems were formed using sputter deposition: a Cu-5%Cr/Cr bilayer which was then subsequently heat treated to for Cr precipitates within the Cu layer, a Cu/Cr with W particles deposited on the interface between the FCC and BCC layer, and a Cu-W particle / Cr system where W particles were co-deposited during the Cu deposition. All layer thicknesses were 30 nm, and the particles ranged from 5 to 15 nm in size. All systems were then tested using nanoindentation to assess the hardness, and two different tips (Berkovich and cube corner) used to explore strain hardening behavior. The Cu-5%Cr system softened by approximately 5% when heat treated at 373K, but then increased in hardness by 10% when annealed at 573K. However, it was not possible to determine if the Cr precipitated at the Cu-Cr interface, or within the Cu layer itself. The W system was used to explore the difference in the impact of the effects of particles at the interface versus within the ductile FCC layer, and these results were compared to computational simulations using a combination of DFT and MD to explain the impact that intra-, rather than inter-layer deposition had on strength.

Metallic nanowires (NWs) have attracted much attention in recent years for a wide range of applications including flexible and stretchable electronics, where their mechanical behaviors are of important relevance. In this talk, we will present our recent work on in-situ transmission electron microscope (TEM) mechanical testing of crystalline metallic NWs, closely integrated with atomistic simulations. Using a unique nanomechanical testing stage based on the MEMS technology, we measure the stress-strain curves while simultaneously observing the microstructural evolutions. We identify dislocation nucleation from free surfaces as the dominant deformation mechanism. For single crystalline NWs, large plastic deformation is observed as a result of coherent twin propagation. In addition, we find that twin boundaries in the NWs play a critical role in the mechanical behaviors. In the case of bitwinned NWs that have a single twin boundary along the nW length direction, we observe a transition of deformation mechanism from localized dislocation slip to delocalized plasticity via an anomalous tensile detwinning mechanism.

Grain refinement is a useful method to achieve material strengthening following Hall-Petch relationship, especially when the grain size drops to nano-scale. However, nano-grained metals exhibit limited tension ductility and poor work-hardening ability due to the suppressed dislocations slip in confined space of smaller grains. How to break the mismatch of strength and ductility is a perplexing issue. Recently, gradient nano-grained (GNG) materials with a gradient structure in which the grain size ranges from tens of nanometers at the surface to tens of micrometers in the core have been proposed to overcome this long-standing dilemma. Constitutive modeling and simulation is crucial to understand the physical deformation mechanism controlling the strength and ductility, and to promote the microstructure optimization for industrial application. Here, we developed a dislocation mechanism based crystal plasticity model, where multiple dislocation evolution mechanisms are considered. Furthermore, damage evolution and mechanically driven grain growth during the deformation of GNG materials are incorporated to reveal the significant role of GNG layer on the extraordinary plasticity response. The developed constitutive model was implemented in crystal plastic finite element method (CPFEM), and successfully predicted the tensile mechanical behavior of GNG copper, such as yield stress, work-hardening and ductility. Meanwhile, the modeling and simulation clearly revealed underlying deformation mechanism controlling the ductility and strengthening with the detailed spatial distribution and temporal evolution of microstructure and damage. Finally, the constitutive model was tentatively employed to optimize the balance of strength and ductility of GNG copper by manipulating the microstructure of gradient region, showing incredible consistence with the existing optimization results conducted experimentally.

Charge-induced reversible straining was recently observed in nanoporous noble metals, such as Pt, Au, and Au-Pt alloys, which are becoming a promising type of electrochemical actuators for potential applications such as artificial muscles. These nano-porous metals, however, are expensive noble metals made from costly processes such as de-alloying. In this talk, we report an electrochemical actuating property of nanoporous nickel, with the actuation mechanism mainly due to a pseudocapacitive behavior by means of reversible faradic redox reactions. By using a dual-template synthesis method, a bi-layered cantilever, comprising a nanoporous layer backed by a solid layer of the same metal, was fabricated. Reversible bending of the cantilever upon cyclic potential triggering was observed. The strain of the cantilever increases nonlinearly with both potential and charge due to redox reactions. Benefiting from the stable Ni(II)/Ni(III) redox couples at the electrode surface, the reversible actuation is very stable in hydroxide solutions. Also, by conditioning the nanostructure of the actuating Ni into a dual-scale nanowire network that facilitates ion transport, record high strain response time in the order of 0.1 second was obtained, which is more than two orders faster than current metallic based actuators.

To understand the mechanism, a multi-scale, multi-field simulation approach is used to model the above electrochemical actuation behavior. Specifically, molecular dynamics simulations with reactive force-field potentials and a modified charge-equilibrium (QEq) method are used to calculate the surface stress built up in Ni(100) surface in contact with water electrolyte due to a voltage applied across the interface, as a result of capacitive charging of the double layer in the contacting electrolyte as well as redox reaction of the Ni surface. The calculated surface stress is then used as input in a meso-scale finite-element (FE) model to compute the actuating stress set up in a single hexagonal unit cell of a Ni nanohoneycomb structure. The single-unit actuating stress is eventually used in a continuum FE model at a larger scale, to calculate the bending of an entire bilayered cantilever which replicates experimental conditions. The actuation deflection of the bilayered nanohoneycomb nickel is predicted to be 41.4 μm at 0.43 V vs the point of zero charge (PZC), which corresponds to ~0.48 V vs saturated calomel electrode (SCE), and this is in excellent agreement with the experimental value of 45-62 μm at a similar voltage vs SCE. This is the first successful attempt to simulate the electrochemical actuation of a real-sized, nano-porous metallic structure in an electrolytic environment.

Nanoporous (NP) metallic materials exhibit microscale plasticity, but macroscopically fail in a relatively brittle-like manner. In this study, Molecular dynamics (MD) simulations are employed to investigate governing deformation mechanisms responsible for ductile behavior of the constituent ligaments of NP structure (nanowires) how it can be related to their triple nodal network. Shear strain tensor analysis is used to differentiate deformation mechanisms accommodating strain among nanowires and triple nodes during stretching. In addition, a computational analysis method is used to quantify plastic and elastic deformation. In general, dislocation activity accommodates 10% to 20% of total plastic deformation while most of the plastic strain generates within bulk and surface atoms. We also studied the deformation behaviors of triple nodes and found out that yield strength of triple node structure is quite close to the strength of NP structure. This further means that the brittle behavior of the NP system can be represented as a function of nodal deformation rather than ductile deformation of only single ligaments. Our preliminary results also suggest that core-shell ligaments can increase both ductility and strength of the NP structure due to the increased activity of twins while nucleation of partials are prominent in monolithic ligaments with no shell layer.

Optically thin silver and copper films are attracting growing attention for a variety of emerging applications, particularly for flexible optoelectronics. For large area, cost sensitive applications such as photovoltaics, copper is an attractive alternative to silver because it offers comparable electrical conductivity at much lower cost. The drawback of copper is its higher susceptibility to oxidation in air and its higher absorption of blue light, and so its potential for applications has been sparsely investigated. This talk will describe a new, scalable approach to improving the transparency of optically thin copper films by patterning with an array of ~ 100 million apertures cm^-2, based on polymer blend lithography. Additionally a new way to dramatically retard oxidation of optically thin copper films without electrically isolating the metal or degrading its optical properties will be described based on an ultra-thin hybrid organic-inorganic adhesion layer. The potential utility of these nanostructured electrodes as window electrodes in high performance inverted organic photovoltaic devices will also be presented. Taken together it is anticipated these developments will stimulate interest in the utility of copper window electrodes for a variety of emerging applications.

Although nanocrystalline thin films have interesting mechanical properties, they usually have low thermal stability due to their high density of interfaces which act as channels for diffusion and drive grain growth. The application of nanocrystalline coatings is usually limited to temperatures below half the melting point of the constituent metals. At higher temperatures, several processes cause deterioration of the nanograin structure, which results in degradation of the exceptional properties of the film. Therefore, to expand the usage of nanocrystalline coatings, it is imperative to enhance their thermal stability by controlling the microstructural transformations that could lead to grain growth. Here we proposed that nanometallic multilayers (NMMs) can be used as a model systems for the study of thermal phenomena in nanomaterials.
The potential of NMMs as a route to synthesize nanostructures is explored by examining the microstructural evolution of Mo-Au, Hf-Ti and Ta-Hf NMMs through differential scanning calorimetry (DSC) and heat-treatments at different temperatures. Transitions occurring between room temperature and 1000 °C are identified using DSC scans. The microstructures before and after transitions are characterized by TEM and EDS techniques to measure changes in composition and grain size. Understanding the evolution of NNMs will allow tailoring nanomaterials with increased thermal stability.

Micrographs of materials contain microstructural information that is quantifiable in principle, but difficult to extract in practice. A quantitative and statistical understanding of microstructure features is necessary for establishing microstructure-property relationships, and especially helpful for nanometallic materials which have grain or domain dimensions less than 1 µm. We developed Automatic QUantitative Analysis of Microscopy Images (AQUAMI): an open source Python package that can automatically analyze micrographs of materials and extract quantitative information to characterize microstructure. In this presentation, we discuss the application of this digital image analysis method for obtaining essential nanoscale characteristics, such as the mean feature dimensions, size distribution, and phase area fraction while considering several potentially error causing experimental parameters. All measurements were fully objectively performed thus results are repeatable and can be directly compared between research groups, for example, taking images from published articles. We describe the working principle of the software and demonstrate it on micrographs of nanoporous and nanocomposite metals.

Rationally designed metallic nanostructures exhibit extreme combinations of properties to approach the limits of multiphase thermal transport. For more than a century, porous media for heat transfer applications have consisted of disordered morphologies with stochastic properties, minimal microstructural control, and inadequate performance metrics. In this talk, we employ a materials-by-design approach to define the design rules and experimentally demonstrate multifunctional metal nanoarchitectures capable of extreme rates of heat transfer. Beginning with structure-property relations and design rules, we perform a structural optimization for thermophysical properties and synthesize the optimized metal nanoarchitectures and nanocomposites using templated electrodeposition. These methods are applied to two case studies in multiphase thermal transport: thermal batteries and capacitors (solid-liquid heat transfer) and evaporation surfaces for thermal management (liquid-vapor heat transfer).

The ideal thermal battery utilizes a solid-liquid phase change material (PCM) capable of isothermal energy absorption to buffer high intensity transient heating loads. However, the thermal power density (i.e. the charge/discharge rate) is limited by both the diffusion length in the PCM and the thermal conductivity of the metal. We establish the design space and identify the mathematical performance limits for metal/PCM nanocomposites and find that copper inverse opals (CuIOs) possess ideal structural properties for high rate thermal batteries. We measure the CuIO thermal conductivity as the pore size is reduced to the nanoscale and observe the transition from diffusive to sub-continuum electron transport [1]. We integrate paraffin-infiltrated CuIO nanocomposites over hotspot devices to demonstrate a fourfold increase in heat absorption compared to solid copper during transient heating >1 kW cm^-2. This same combination of thermal properties also enables porous CuIOs to be effective evaporation surfaces due to hydraulic permeability from the open-celled structure. In thermofluidic applications, the design rules incorporate fluid transport instead of heat capacity and favor material architectures with minimal tortuosity. Using capillary rise measurements, we observe that the dominant hydraulic resistance in CuIOs is derived from grain boundaries and defects in the crystalline porosity, and in the single crystal limit is limited by tortuosity and geometric constrictions [2]. In response, we introduce the use of copper woodpiles as a platform for optimizing capillary-driven fluid delivery due to direct thermal and fluidic lines of sight.

[1] Barako et al. Nano Letters (2016)
[2] Pham et al. Sci. Rep. (2017)

Strengthening metals through grain boundary (GB) and twin boundary (TB) interfaces into the nanoscale region is manifested by a maximum strength, a phenomenon that is known as Hall-Petch limit, followed by softening behavior. The underlying mechanisms of this observation have been studied extensively by atomistic simulations. Yet, experimental validations of these mechanisms have been challenging, primarily due to the technological difficulty in synthesizing nanotwinned FCC metals in the truly nanocrystalline regime (<100 nm) that could match grain sizes in atomistic models. In this talk, we present experimental and atomic simulation investigations on the effects of microalloying on microstructure stability and Hall-Petch strengthening in sputter-deposited nanotwinned Ag containing trace concentrations of Cu solute atoms (<1.0 wt. %). First, we discovered that annealing of nanotwinned Ag at a Cu concentration level of 0.81 wt. % results in grain size (d = 49 nm) and TB spacing (λ = 3.5 nm) that were well below those previously obtained in nanotwinned Ag, leading to a record hardness up to 39% above values reported for this metal without alloying. Second, large-scale hybrid Monte-Carlo and Molecular Dynamics simulations, and density-functional-theory-based calculations, were deployed to study the atomic-scale processes associated with Cu solute atom segregation, yielding and plastic flow in nanotwinned Ag, as a function of TB spacing. It was found that Cu atoms are segregated concurrently to GBs and intrinsic kink-like TB defects during thermal annealing, resulting in dramatic improvements of both twin stability and yield strength under applied stress in micro-alloyed nanocrystalline-nanotwinned Ag. Furthermore, we present the plastic deformation mechanisms underpinning the Hall-Petch strength limit in nanocrystalline-nanotwinned Ag models, for which the simulated microstructure is comparable to that of micro-alloyed experiments.

The recent synthesis of twin interfaces in two covalent-bonding materials, cBN and diamond, has improved their thermal stability and fracture toughness and meantime introduced a new record for material's hardness. The continuous hardening of these materials by decreasing the twin-spacing even lower than the critical thickness (about 15 nm), which was a turning point to softening behavior in Cu, is mysterious. Here, we show a similar observation of hardening in nanotwinned Pd polycrystalline samples at room temperature and reveal that there exists a softening temperature for materials, below that the softening will be replaced by hardening behavior. Our large molecular dynamics and finite element simulations show that below this transition temperature, thermally-activated source-controlled plasticity will be substituted by the stress-driven one. Since the stress-concentration at grain boundary-twin intersections for nucleation of partial dislocations gets a higher value by increasing the twin spacing, twinning migrations are progressively observed in grains with thicker twin interfaces by decreasing the temperature. The Higher amount of stress concentration is caused by lower elastic-field interactions of close intersections when twin interfaces become far from each other. The higher the bond's strength, the higher transition temperature is captured by our simulations and predicted by our theoretical modeling.These results give an insight for observing hardening in covalent-bond materials in which the transition temperature is anticipated to occur at values higher than the room temperature similar to Pd.

The goal of the work presented is to gain an understanding of the deformation mechanisms underlying the deformation of nanolayered composites containing either bcc and hcp Mg phases. Nanolayered composites comprised of 50% volume fraction of Mg and Nb were synthesized using physical vapor deposition with individual layer thicknesses h of 5 nm, 6.7 nm, and 50 nm. At the lower layer thicknesses of h = 5 nm and 6.7 nm, the Mg was found to have undergone a phase transition from HCP to BCC, such that it formed a coherent interface with the adjoining Nb phase. Micropillar compression testing normal and parallel to the interface plane showed that the BCC Mg composite is much stronger and can sustain higher strains to failure. Transmission electron microscopy and density functional theory calculations for the relative barriers to shear on crystallographic slip systems and Mg/Nb interface together suggest that the deformation is predominantly mediated by slip in the layers. A crystal plasticity model with the h-dependent critical resolved shear stresses was developed and applied to understand the linkage between the observed deformation response and underlying mechanisms. Calculations from the model predict that the stress-strain response results from dislocation mediated plasticity on the {110}and {112} slip systems.

Nanoporous metals such as nanoporous gold (NPG) exhibit nanoscale ductility but appear macroscopically brittle due to significant strain localization and subsequent fracture. Quantifying the morphology evolution and location of strain during deformation of this three-dimensional networked material may give further insight into the cause of strain localization and also validate similar simulation experiments through comparison. We investigate the microstructure response to tensile stress of NPG films consisting of ligament and pore sizes of approximately 10 nm through in-situ scanning electron microscope (SEM) and transmission electron microscope (TEM) tensile tests. SEM datasets of these tests were used to track and quantify the evolution of ligament and pore morphology during deformation. The change in diameter, length, orientation, aspect ratio, and area fraction was calculated for the pores and ligaments on a frame-to-frame basis using automatic image analysis. The location and extent of strain was also observed. TEM datasets revealed the dislocation and strain field activity in NPG during deformation. Thus, the combination of SEM and TEM in-situ experiments allowed for a direct comparison of lattice deformation mechanisms and broader nanoscale ligament and pore morphology evolution. These experiments form a baseline for quantifiable comparison with simulated mechanical testing and future improvements to experimental samples.

The demand for smaller and faster microelectronic devices necessitates thinner diffusion barrier layers between the silicon semiconductor and copper conductor layers to prevent them from diffusing into each other and increasing resistivity along conducting pathways. To avoid the blurred silicon/copper interface, transition metals are being studied as ultrathin conductive barrier layers. Transition metal oxides usually have high dielectric constants and can be deposited in ultrathin films on silicon without diffusing into it as copper does, making it an ideal candidate for this application.
In our study, the ultra-thin film (5 nm) of cobalt metal with high purity (96.5% at cobalt) is successfully deposited by photo-assisted atomic layer deposition with remote plasma system. A 60 nm thick cobalt metal film grown after annealing at 250 °C were analyzed by X-ray diffraction and XPS. Reflections confirmed crystalline cobalt metal, with an average crystallite size of 33.5 ± 4.0 nm from the scherer equation. And continued sputtering resulted in ionizations that a film composition consisting of 96.5% cobalt metal after 5 min. The remainder of the film was oxygen, with carbon and nitrogen levels below the detection limits (< 1.0%). The top-down AFM images show as-deposited Co film at 200 °C with RMS of 1.1 nm and Co film at 180 °C with RMS of 0.6nm. And A 50± 5nm thick cobalt metal film was grown at 200 °C with 1.0E-5 μΩ.cm which are close to that of bulk cobalt (6.24 μΩ.cm at 20 °C).

Ductile iron (DI) is a unique engineering material due to its low production costs and attractive mechanical properties. However, compared with forged steel, the relatively poor toughness still imposes limitations on its widespread use. Inspired by a quenching and partitioning process (Q&P), and the formation of a nano super bainite at low temperatures in very high strength steels, a commercial unalloyed ductile iron has been heat treated to produce a multiphase matrix microstructure consisting of lenticular prior martensite (PM), feathery upper bainite (UB) and a nano super bainite composed mainly of nano-scaled lath bainitic ferrite (BF) and a carbon-enriched austenite (RA) film. Multi-step thermal treatments composed of heating the DI to 890°C for 20min followed by rapidly quenching in a patented water-based liquid at 190°C, which is a slighter lower than the starting temperature of martensite transformation (Ms) for controlling amount of PM, have been developed. The DI is then reheated to 220°C for times between 5 and 240min in an electric furnace, rather than a typical salt bath treatment used for most high strength DI, and subsequently air-cooled to room temperature. A very high tensile strength of more than 1.6 GPa, a hardness of HRC54, and an elongation in excess of 5%, are achieved. This is attributed to a synergistic multi-phase strengthening effect. The developed nano super bainite exhibits a good balance between strength and toughness. The presence of martensite formed during the quenching process prior to the isothermal treatment, accelerates the kinetics of subsequent nano super bainitic transformation by bainitic laths nucleating quickly at the martensite-austenite interfaces. This design methodology potentially broadens the application of DI to components that experience in a more demanding service environments such as heavy loads.

Lightweight Al matrix nanocomposite received numerous attention due to their high specific strength, high electrical conductivity and high thermal conductivity. This talk will highlight two distinctive Al matrix nanocomposites reinforced by calcium metal and carbon nanotubes, respectively. For Al matrix calcium composite, a complete processing-structure-property relation will be discussed with the aid of analytic modeling to help design the optimal microstructure for strength and electrical conductivity. For the Al matrix carbon nanotube composite, the focus would be on resolving the processing challenge of carbon nanotubes reinforced metal matrix composite, such as the dispersion, the oxidative and reactive damage control. The successful processing of Al carbon nanotube composite can be measured by the amount of electrical conductivity or thermal conductivity increase over that for pure Al.

Accumulative roll bonding (ARB) is one of the severe plastic deformation (SPD) processes which can produce bulk ultrafine grained (UFG) metallic materials. Thus, the ARB process has been applied to fabricate various nanocrystalline metallic sheets and multi- layered composites owing to the relatively simple processing. In this study, the ARB process was used to make multi-layered metal sheets by using 1mm thick metal sheets having a different chemical composition as sandwich layers. The microstructure and mechanical properties of the fabricated sheets were investigated in detail. Multilayered Al/Cu and Cu/Cu sheets was fabricated by ARB process with combination of Al and Cu sheets. The conductivity and strength of the multilayered sheets increased with increasing the number of layers within same thickness as long as the continuity of the metal layer was maintained. Also, The fabricated 1mm thick multilayered Cu/Cu alloy sheets consist of 64 layers with the grain size of below 170nm. The sheets have very high strength and high electrical conductivity, that is, the tensile strength and electrical conductivity of the sheets are 500MPa and 85%IACS, respectively. The multilayered sheets have a good formability during ARB and subsequent rolling process, thus, layered sheet with the thickness of 0.2mm fabricated by additional cold rolling for lead frame application, the tensile strength of the final sheet increased to 610MPa with same electrical conductivity. In order to evaluate the productivity in commercial scale production, continuous ARB were designed. By optimizing the process condition, finally, 1mm thick nanocrystalline multilayered Cu strip with the width of 200 mm can be successfully fabricated by pilot clad rolling mill in industrial scale.

It remains a challenge to synthesize Ag nanocubes with sharp corners and edges while retaining a compact size below 20 nm. Here we demonstrate the use of site-selective growth to sharpen the corners and edges of truncated Ag nanocubes with sizes down to 18 nm, followed by their elongation into nanobars with aspect ratios up to 2. The key to the success of this synthesis is the site-selective deposition at corners and edges, as enabled by cetyltrimethylammonium chloride (CTAC). While CTA⁺ is an effective colloidal stabilizer, Cl⁻ can react with Ag⁺ to generate AgCl precipitates, slowing down the reduction kinetics. In addition, Cl⁻ can serve as a facet-selective capping agent towards the {100} side faces and thereby confine the growth mainly to corners and edges. Interestingly, once all the corners and edges have been sharpened, the growth is switched to an asymmetric mode to favor deposition on one of the six side faces only, leading to the formation of Ag nanobars with controllable aspect ratios. The symmetry reduction takes place as a result of the limited supply of Ag atoms, the strong capping of Cl⁻ ions towards the {100} facets, and the possible involvement of localized oxidative etching caused by Cl⁻/O₂. We also demonstrate that the Ag nanocubes with sharp corners and edges can serve as a better sacrificial template than their truncated counterparts in generating Au hollow nanostructures with ultrathin walls.

The metastable beta phase of tantalum is of interest due to the recent discovery of a giant Hall spin effect in this material, which enable much higher magnetic information storage densities. Both this phase and the stable BCC alpha phase can be made by sputter deposition. However, despite fifty years of study, the mechanism of phase selection remains unknown. We prepared a series of films under varying Ar sputter pressures while holding all other parameters constant and minimizing the effect of impurities. Measurements of film stress as a function of sputter gas pressure allow us to unambiguously index diffraction peaks to determine phase and texture. We find only the beta phase in the form of a dominant (002) beta Ta fiber component that becomes broader as the pressure increases. Based on calculations of the energy of incident Ta atoms and Ar neutrals, we show that resputtering could account for the changes in texture distribution. By comparing these results with a detailed review of the literature, we are able to propose a reproducible phase selection mechanism that is consistent with the vast majority of published results.

Controlling the micro/nanostructure of thin films would enable us to explicitly tailor their mechanical behavior. Here, we describe a new process to synthesize thin films with precise microstructural control via systematic, in-situ seeding of nanocrystals, and subsequent crystallization of amorphous precursor films. Using this process, we synthesized an austenitic NiTi film with a mean grain size of around 100 nm. We then co-fabricated freestanding samples of the film with MEMS testing stages and performed a series of cyclic tensile load-unload experiments. The film showed a high phase transformation stress (> 700 MPa) during the first cycle that increased even further during subsequent cycles. Furthermore, the film exhibited significant inelastic strain recovery during unloading, which was characterized by a continuous decrease in stress-strain slope rather than the pseudoelastic behavior typically observed in microcrystalline NiTi. Interestingly, the strain recovery continued even after the film was fully unloaded (macroscopically free of stress), and accelerated when the temperature was increased, with full recovery occurring at 60 ^oC. Preliminary in-situ TEM straining studies suggest that this unusual unloading/post-unloading behavior is caused by the heterogeneous deformation of the nanocrystalline microstructure. While some of the grains accommodate the inelastic deformation during loading by phase transformation, others accommodate it via plasticity. Hence, when the film is unloaded, inelastic strain recovery occurs through a combination of reverse phase transformation and reverse plasticity, leading to a divergence from the conventional pseudoelastic/shape memory behavior.