Nanowires may be used as building blocks for many future applications ranging from microelectronics and nano-electro-mechanical systems to anode materials in Li ion batteries. This is due to their unique physical properties which are a consequence of their high aspect and surface-to-volume ratio. Particularly, nanowires with typical diameters of 100 nm and below have shown very high strength, which can be of the order of the theoretical strength of the respective material. In this paper, we will introduce a nanomechanical setup that allows for testing nanowires with diameters as small as 30 nm inside a dual beam SEM/FIB. The force measurement is accomplished by a MEMS-based load cell and the strain measurement by digital image correlation of the SEM pictures. Manipulation, transfer and alignment of the samples are performed with the help of a piezoelectric manipulator. Obviously, quantitative measurements with high accuracy are not trivial at this length scale. Therefore, the main sources of experimental errors such as misalignment, determining the sample diameter, or contamination of the sample surface are critically discussed. With this system, Au, Cu, and Si nanowires were tested in tension. For all nanowires, the yield and fracture strengths were found at very high stresses close to the limit of theoretical strength. In particular for the Si nanowires, the fracture strengths do show only a weak size dependence for wire diameters below 200 nm but does exhibit a large scatter in the strength values. This indicates the statistical nature of the fracture process which is analyzed by applying a weakest-link approach. It is shown that the scatter as well as the size dependence can be consistently described by Weibull statistics.

Both theoretical and experimental studies indicate mechanical size effects are related to dislocation mechanisms often influenced by the increased surface to volume ratios at the nanoscale. Mechanical size effects can originate from source limitation (due to source exhaustion and dislocation starvation) or from size-related dislocation mechanisms such as self-multiplication in small BCC volumes and source truncation. To investigate these various mechanisms in small bcc volumes, we have performed in situ transmission electron microscope (TEM) tensile tests of Mo-alloy nanofibers using a “push-to-pull” (PTP) device. By augmenting the in situ testing method with Digital Image Correlation (DIC) we can determine the true stress and strain in local areas of deformation. We found that the nanofibers exhibited clear exhaustion hardening behavior, where the progressive exhaustion of dislocation sources and starvation of dislocations increases the stress required to drive plasticity. Our presentation will focus on recent improvements to our mechanical testing techniques and critically discuss the various size strengthening effects observed during our in situ TEM tensile tests.

For the first time, the mechanical properties of submicron-sized metallic glasses (MGs) were studied through quantitative in situ tensile test inside a transmission electron microscope, which employs high-resolution measurements of the loading forces and accurate strain measurement with deposited markers on the gauge length. The quantitative experiment establishes that the small-volume Cu₅₀Zr₅₀ MG has fracture strength and elongation to failure reaching record high values of about 4 GPa and 6%, respectively. At the same time, the yield stress and elastic stain limit are found to be about twice as large as the already-high elastic limit observed in macroscopic samples, in line with model predictions of the intrinsic limit in the absence of heterogeneous shear band nucleation facilitated by extrinsic factors.

We report a series of in situ and ex situ compression tests of Ti-Al alloys with 6-7 wt.% Al in order to study mechanical size effects in alpha-Ti across several orders of magnitude. In Ti-Al solid solutions (HCP alpha-Ti), high lattice friction and short range ordering produce a planar slip behavior that was presumed to be relatively insensitive to size effects. For the in situ TEM nanocompression tests, Ti-7 wt% Al nanopillars from 280-380 nm in diameter were prepared by FIB from a polycrystalline sample. We analyzed the orientation of the nanopillars in the TEM and selected nanopillars having the (0001) plane inclined 45 - 60 degrees to the long axis (compression axis). In situ TEM nanocompression revealed that the pillars sheared along the (0001) plane as anticipated and that the critical resolved shear stress (CRSS) for basal slip was 450 MPa. Previous results reported that the CRSS for basal slip in bulk Ti-6.6Al is about 230 MPa at room temperature, and microcompression tests of pillars from 1-20 microns in diameter show relatively little size strengthening. Taken together, the in situ and ex situ compression tests show a relatively small size effect in Ti-Al solid solutions due to the predominant effects of the dislocation behavior as compared to the effects of source limitation or source truncation in small volumes. Acknowledgments: This work was supported by the Center for Defect Physics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. ET was supported by a JSPS fellowship. The in situ TEM work was performed at the National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, which is supported by the U.S. Department of Energy under Contract # DE-AC02-05CH11231.

In situ straining experiments in the transmission electron microscope are the unique way to observe the dislocations under stress and so, to determine the strength of the obstacles opposing to the dislocations propagation. This represents an essential step for the determination of what is controlling the plastic deformation. In this paper, two examples will be described. The first case concerns extrinsic obstacles which are present in aluminum and intermetallic TiAl alloys. The strength of this pinning will be determined through the analysis of the critical configuration for the obstacle breaking in the framework of the dislocation theory. From this strength evaluation and from the measurements of the obstacle density, the obstacle overcoming mechanism will be correlated to the macroscopic shear stress. It will be shown that for aluminum alloys the pinning is controlling the dislocation propagation whereas that is not the case for TiAl. The second example is about the interface crossing by dislocations in lamellar TiAl alloys. In these alloys, the rectilinear interfaces are characterized by the different phases and by the orientation relationships between the lamellae. Several situations related to the type of interfaces, the nature of the incident dislocations and/or the orientation of the applied stress will be investigated. The strength of these various interfaces is deduced from the characteristics and the dynamics of the crossing mechanisms. The final aim is to understand the role of these interfaces on the mechanical properties of lamellar TiAl alloys.

Size effects leading to a ‘smaller-is-stronger&’ in micron and sub-micron scale plasticity have been commonly reported in recent literature. Still, less is known regarding the mechanisms responsible for the remarkable strength and hardening as well as the failure behavior of such miniaturized samples. This calls for direct in situ observation of the governing deformation mechanisms. We applied focused ion beam machining to fabricate non-tapered nano-tensile samples with dimensions in the 100 nm - 200 nm regime from single crystal copper. This tensile approach minimizes experimental constraints existing in common pillar compression studies and allows to study the failure behavior. Quantitative in situ tensile tests [1] on single and multiple slip oriented Cu tensile samples were performed in a Jeol 3010 transmission electron microscope operated at 300 keV with a Hysitron Picoindenter. We observe the operation of truncated spiral sources at high stresses as the dominant mechanism carrying plasticity, and both crystal orientations fail by localized shear. While tensile failure occurs after few percent plastic strain and limited hardening in the single slip case, extended homogeneous deformation and necking due to the activation of multiple dislocation sources in conjunction with significant hardening is observed for the multiple slip samples. Moreover, we show that the strain rate sensitivity of such FIB prepared samples is an order of magnitude higher than that of bulk Cu[2]. [1] Kiener D, Minor AM. Nano Lett. 2011;11:3816. [2] Kiener D, Kaufmann P, Minor AM. Adv. Eng. Mater. 2012:online.

Polycrystalline metals demonstrate a clear trend of strengths that scale as a power-law of their size. It has been established long ago in coarse grained materials that the strength is inversely proportional to the square root of the grain size (Hall-Petch law). However about 10 years ago, a breakdown of the Hall-Petch law has been noticed in metals with grain size smaller than 100nm. Because both the mean free path and the probability of multiplication of dislocation are constrained at such a grain scale, a probable change in the plasticity mechanism associated with very high yield stresses is supposed to occur. Exploring these mechanisms in very small crystallites is however complex and the nature of alternate deformation mechanism is still highly debated. In-situ Transmission Electron Microscopy (TEM) experiments have proved in the recent past years to be a powerful tool to probe elementary plasticity mechanisms at the pertinent time and length scale. Down to small scale, these experiments triggered deformation mechanisms to plasticity, including specific dislocation mechanisms (partial dislocation nucleation instead of perfect ones, surface dislocation emissionhellip;) or grain boundary mediated plasticity. In the present paper, I would like to report several observations of both intragranular and intergranular plasticity mechanisms in small grained Al (grain size between 100nm and 1µm) elaborated either by electrodeposition or by severe plastic deformation. Grain boundary (GB) plasticity mechanisms have been observed in specific conditions where usual dislocations mechanisms are prevented. In dislocation free nanocrystalline grains, we have found that stress assisted grain growth can be an efficient process to accommodate deformation. The amount of strain produced during this process can be measured and tentatively modeled. GB sliding due to the motion of interfacial dislocations is another possible mechanism leading to the formation of cavities. In this condition, strain up to several tens of percents has been achieved locally in films deformed uniaxially in tension. Intragranular dislocation mechanisms have been observed systematically in grains containing initially dislocation sources (i.e. more likely in larger grains) or when GB sources can emit dislocations inside grains. The formation of dislocation pile-ups against GB is a direct consequence of intragranular plasticity. The reversal motion of dislocations in the pile-up can explain quantitatively the Bauschinger effect observed during micro-yielding experiments.

Lately, ‘smaller is stronger&’ was widely reported in single crystals with the sample dimensions reduced to sizes less than ~ 20 mu;m in diameter [1-4]. Several new concepts have been brought forward to explain this phenomenon, such as ‘dislocation-starvation&’ [2, 5-7], ‘mechanical annealing&’ [1] and ‘source limited deformation&’ [4], etc. It is agreed that the dislocation starvation model dominated in the face-centered cubic (FCC) micropillars through molecular dynamics and dislocation dynamics simulations. But a body-centered cubic (BCC) micropillar is unlikely to be in a “dislocation-starved” state. The slow moving screw oriented dislocation may be able to generate new dislocations moving in the opposite direction before it exits from the surface, namely self-multiplication mechanism [8-10]. However, “dislocation-starved” state or “mechanical annealing” and size effect were recently reported in BCC Molybdenum pillars when the diameters decrease to hundreds of nanometers [11].Our experiment further demonstrates that “dislocation starvation” does occur in other BCC metals. Also, although the observation of a single dislocation moving through the pillar in real time is difficult, we effectively captured it by changing the testing method from normal bright or dark field TEM image mode to scanning TEM mode (STEM). Here we report our in situ compression test of a BCC single crystal Fe-3% Si pillar in a TEM. We found that the deformation is characterized by series of pronounced pop-ins accompanying stresses increase at these strain bursts. The increasing in stresses was found to be caused by the change from the edge dislocation dominated process to the screw dislocation dominated process. The underlying mechanism might be the change of dislocation sources or dislocation starvation/dislocation exhaustion. References: [1] Z. W. Shan, R. K. Mishira, S. A. S. Asif, O. L. Warren, A. M. Minor, Nature Mater. 7 (2008) 115. [2] M. D. Uchic, D. M. Dimiduk, J. N. Florando, W. D. Nix, Science 305 (2004) 986. [3] D. Kiener, W. Grosinger, G. Dehm, R. Pippan, Acta Mater. 56 (2008) 580. [4] C. A. Volkert, E. T. Lilleodden, Phil. Mag. 86 (2006) 5567. [5] W. D. Nix, J. R. Greer, G. Feng, E. T. Lilleodden, Thin Solid Films 515 (2007) 3152. [6] J. R. Greer, W. C. Oliver, W. D. Nix, Acta Mater. 53 (2005) 1821. [7] J. R. Greer, W. D. Nix, Phys. Rev. B 73 (2006) 245410. [8] C. R. Weinberer, W. Cai, PNAS 105 (2008) 14304. [9] J. R. Greer, C. R. Weinberger, W. Cai, Mater. Sci. Eng. A 493 (2008) 21. [10] S. Brinckmann, J.-Y. Kim, J. R. Greer, Phys. Rev. Lett. 100 (2008) 155502. [11] L. Huang, Q.-J. Li, Z.-W. Shan, J. Li, J. Sun, E. Ma, Nature Communication 2 (2011) 547.

Grain boundary engineering (GBE) is a proven way to increase the resistance of FCC metals to intergranular cracking and corrosion. However, the mechanisms through which engineered microstructures evolve are not well understood. Previous studies have characterized microstructure and grain boundary character distributions before and after GBE processing, providing empirical data from which several mechanisms have been proposed but none verified. The current hypothesis is that Σ3 type grain boundary nuclei form first through annealing twinning and that these nuclei lead to a proliferation of Σ3n boundaries due to geometric constraints. In order to verify this, in situ heating and straining experiments were carried out in a transmission electron microscope (TEM) to replicate GBE thermomechanical processing. High purity copper was used as an analogue model FCC material. The effects of heat and strain on boundary mobility were assessed during cyclic processing. Variations in the mobility of boundaries with different grain boundary character were assessed by utilizing the Nanomegas DIGISTAR/ASTAR orientation image mapping (OIM) method in TEM to assess the character of individual boundaries formed in situ. By isolating the effects of strain and annealing on the formation and mobility of Σ boundaries a better predictive model can be developed for future application of this process in various industries.

The 5XXX series aluminum alloys containing 5% magnesium are commonly used in structural applications requiring good corrosion resistance and weldability. They are a non-heat treatable materials which gain strength through primarily cold work and solid solution hardening with magnesium. Despite the strong characteristics the 5XXX series alloys are susceptible to sensitization. Over extended periods of time at elevated temperatures magnesium segregates from the matrix and forms secondary Al3MG2 (β phase) precipitates along the grain boundaries. This anodic β phase can be very susceptible to corrosive environments and can lead to intergranular stress corrosion cracking. It is well known that β phase forms at medium/high temperatures (150-300°C) but there is still some uncertainty about that actual formation and effect of stress on the kinetics of β phase growth. In-situ heating and straining in Transmission Electron Microscopy (TEM)techniques give promise to understanding the kinetics and mechanisms of β phase growth. In-situ TEM will allow for determination of the time, temperature and strain that is necessary for sensitization to occur. Having a better understanding of these parameters will give a more predictive model for the formation of β phase and ultimately a chance to prevent its formation. The work included in the research will aim to show the earliest stages of β phase growth giving a much better idea of what causes the onset of formation. Finally, orientation imaging will also be coupled with the in-situ experiments to see the dependency of grain boundary type and which are more susceptible to corrosion. Combining these techniques brings us a step forward in understanding the β phase formation in aluminum magnesium alloys.

The use of duplex stainless steels (DSSs), which consist of austenite and ferrite, is rapidly increasing due to their combined advantages of outstanding mechanical and corrosion properties. Generally, it is known that the austenite phase has higher strength comparing with the ferrite, though the difference between mechanical strength depends on the chemical composition in both phases. Recently, however an in-situ tensile test with neutron diffraction reported that the plastic yielding occurs first in austenite phase. In this study, to understand precisely the mechanical behavior of each phase, a dual-scale mechanical behavior of DSS was investigated by a nanoindentation and an in-situ tensile test in a high-resolution electron backscatter diffraction (HR-EBSD). Whereas the nanohardness value of austenite was mostly higher than that of ferrite, the incipient plastic deformation, which is usually measured as pop-in on load-displacement curve, occurred at lower stress in austenite comparing with ferrite. The pop-in stresses for both phases could be understood with the event of dislocation nucleation underneath the indenter. The lower pop-in stress of austenite could be explained as a result of its lower dislocation nucleation energy, which was calculated with the length of Burger&’s vector of dislocation measured by XRD and CBED. With in-situ tensile test in EBSD, the macro-scale tensile behavior including the effect of microstructure was observed. Orientation spread and misorientation distribution in each grain of austenite and ferrite were traced as the deformation proceeded. Based on these results, the mechanical behavior of each phase could be analyzed and compared with the small-scale nanoindentation behavior. According to the results of nanoindentation, hardness of austenite is higher than ferrite but plastic transition occurs as pop-in at lower stress on austenite than ferrite. And maximum shear stress underneath the indenter when pop-in occurs was around theoretical crystal yield strength. Result of XRD and CBED analysis, it was shown that pop-in occurs at lower stress on austenite than ferrite because austenite has lower dislocation nucleation energy in this specimen. And since austenite has higher hardening factor then ferrite, austenite has higher hardness value than austenite. This tendency of deformation was analyzed and also confirmed by in-situ EBSD.

The ALILE process enables fabrication of thin polycrystalline Si films at relatively low temperature (< 450°C) making it highly promising for applications in thin film photovoltaics. While the driving forces for the metal-induced crystallization are rather well understood the details of the materials transport during the layer exchange are largely unknown. In this work the microstructure of stacks of a-Si(100nm)/Al(50nm)/Quartz, annealed at 450°C, has been investigated at different length scales by combining optical microscopy, analytical SEM and analytical TEM [1]. The results indicate that the ALILE and crystallization reaction proceeds by forming 20-50µm wide dendritic “cells” with Al deficient centers. Excessive upward transport of Al by epitaxial growth out of the existing Al grains into the a-Si was observed in a rim of about 10 µm width around the cells and to a smaller extent even a few tens of micrometers away from the reaction front. Using in-situ TEM the lateral and vertical transport of Al at the expanding crystallization front could be directly visualized for the first time. We propose that Coble-type diffusion of Al along the Al grain boundaries and/or the Al/a-Si interface, driven by the compressive stress in the Al layer, is responsible for the massive long range lateral and vertical transport of Al. Our findings shed new light on the redistribution of Al and Si during the ALILE process. [3] [1] B. Birajdar, T. Antesberger, M. Stutzmann, E. Spiecker, pss (RRL) 5, 172 (2011) [2] B. Birajdar, T. Antesberger, B. Butz, M. Stutzmann, E. Spiecker, Scripta Materialia 66, 550 (2012) [3] The authors gratefully acknowledge financial support by the DFG via the Cluster of Excellence Engineering of Advanced Materials.

In-situ mechanical tests of amorphous carbon (a-C)/boron nitride nanotube (BNNT) and a-C/carbon nanotube (CNT) have been conducted to understand the mechanical performance of a-C as a welding material for load transfer between structures in nano-based structural materials. The experiments took place inside the vacuum chamber of a transmission electron microscope with an integrated atomic force microscope system, allowing nanomanipulation simultaneous to real time observation of the hybrid structures. Electron beam induced deposition (EBID) has been used for the deposition of a-C and modification of the structures. The pristine and failed structures were successfully welded with a-C, and a series of tensile, compressive, and lap shear tests were performed on the hybrid structures. The current work presents a-C welding as a viable method for the formation of stable tube-to-tube connections in boron nitride and carbon nanotube bundles and serves as a starting point for the improvement of the mechanical performance of nanotube based materials for space applications, where light-weight/low-budget/high-performance materials are sought after.

Uniform magnetic nanocrystals are synthesized via a templated growth in the presence of the iron-binding recombinant proteins, Mms6 and Mms13, respectively. Use of low-temperature synthetic approach permits control over size, shape, and orientation of the magnetic nanostructured crystals. The protein-templating mechanism is probed via numerous analytical techniques, including Transmission Electron Microscopy with the continuous flow Liquid Cell TEM Holder Platform, and magnetization measurements. Uniform magnetic nanocrystals can be further functionalized to address specific applications. Effect of the protein on the morphology and shape of resultant template magnetic nanometer-sized crystal is investigated.

Noble metal nanoparticles supported on reducible oxides are the top candidates as heterogeneous catalysts in various reactions such as CO oxidation and water gas shift reaction. Such supported catalysts show enhanced activity and durability and lower activation energy due to the metal-support interaction. As the catalytic property of the noble metal depends on the particle size, it is important to design stable catalysts with optimum size which is stable under the operating conditions of the catalysis. The thermal stability of the metals nanoparticles on oxide supports is a basic requirement for the use of these materials as catalysts as the operating temperature for many of the catalysis is high. The stability depends on the nature of the metal, support as well as the interface. In-situ transmission electron microscopy facilitates real time observation of the microstructural changes of the material. In this work, thermal stability of noble metals including Pt and Au nanoparticles on CeO₂ supports is investigated by in-situ heating in transmission electron microscope. CeO₂ with exposed (111) and (100) facets are investigated as supports. The experimental observations lead to the understanding of the kinetics of particle growth of metal nanoparticles on oxide supports and the effect of different crystal planes in stabilizing the noble metal nanoparticles.

Calcium carbonate is one of the most abundant building materials in biomineralization despite being a notably brittle compound. Numerous organisms are capable of constructing sophisticated and functional hybrid structures by the interplay of the mineral with an organic phase - mainly proteins. In order to understand this and to transfer the know-how to biomimetic mineral synthesis, it is important to elucidate the mechanism of interaction between organic and inorganic that lead to these hybrid materials and, in particular, how organics can be incorporated into the mineral. In this work, the negatively charged polyelectrolyte polystyrene sulfonate (PSS) is used to mimic the behavior of the negatively charged protein residues that influence calcium carbonate mineralization in e.g. mollusk shells. In addition, the sulfonate group in PSS is structurally similar to sulfate groups on polysaccharides that have been found to constitute the crystal nucleation site in these shells, therefore making PSS a suitable organic model. Wang et al. showed that the calcite crystallization utilizing the ammonium carbonate diffusion method in the presence of PSS yielded a family of well-defined mesocrystals — i.e. regular but porous scaffolds composed of well-separated, but almost perfectly 3D aligned calcite nanocrystals — on a glass substrate. Although the CaCO3 mesocrystal formation mechanism was suggested to proceed via an amorphous precursor, it remains unclear if this is the true pathway or if a different transformation process is followed through e.g. oriented attachment of nanocrystals or secondary nucleation on primary nanoparticles. Here we use both in situ TEM and in situ AFM to follow the formation process in detail. For in situ TEM a custom designed fluid cell was utilized, where the fluid containing CaCl2 and PSS was inserted via an access port between two Si3N4/Si(100)/Si3N4 wafers with 50-by-100 µm electron transparent Si3N4 membranes (~100 nm thickness). These two wafers were then bonded to and separated by a Si3N4 spacer nominally 300-400nm in thickness. A second access port was filled with solid ammonium carbonate, to slowly increase supersaturation by the decomposition of CO2 into the solution thereby inducing crystallization of CaCO3. Preliminary results using in situ AFM showed immediate adsorption of ~60-100nm PSS complexes on the Si3N4 wafer of the TEM fluid cell. In situ TEM typically revealed a core-shell structure of what appeared to be initial liquid-like or amorphous particles which, upon beam exposure after an hour-long reaction, nucleated an identical particle in direct contact with the original one. These results suggest that the initial mesocrystal formation might occur through a secondary nucleation of nanoscopic particles that build up into the eventual micron-sized mesocrystal obtained ex-situ. We detail the structural evolution and draw inferences about the formation mechanism of the resulting CaCO3 mesocrystals.

Abnormal grain growth is characterized by the development of a bimodal distribution of grain sizes during growth. The underlying mechanisms of abnormal grain growth caused by thermal annealing have been contributed to solute segregation, inclusions, and precipitation of new phases in multi-species materials. Arguably, in single component materials, the underlying mechanisms that contribute to abnormal grain growth are less understood. For these cases, the grain boundary character and its associated energy is considered to dominate which grains grow the fastest. In the present work, a series of elemental Cu, Ni, Fe and W thin films have been sputter-deposited and in situ annealed up to 750 deg. C in the TEM. The onset of abnormal grain growth has been quantified using precession enhanced diffraction based orientation mapping. By measuring the grain boundary misorientation and texture change of the film at different incremental temperature steps, the evolution of specific grains are tracked to reveal favorable near-neighbor grain boundary alignments. The results will be discussed in terms of comparing the two FCC and two BCC structures with and between each other. In addition, the experimental findings are used to refine Monte Carlo predictions of grain boundary misorientation distributions leading to a better understanding of the mechanisms which contribute to deviations from normal grain growth.

An o-ring sealed transmission electron microscopy (TEM) wet-cell with silicon nitride (Si₃N₄) windows was used to enclose a chlorauric acid (HAuCl₄) solution in the vacuum for the in-situ study. Gold nano structures were in-situ observed to develop in the HAuCl₄ aqueous solution under electron beam (EB) irradiation through wet-cell TEM. The nucleation, growth and coalesce of nanoparticles were observed in the EB irradiated region. The diameter of the nanoparticles ranges from 14 to 72 nm. Bubble-like materials from 6 nm to around 160 nm in diameter were also observed. Nanoparticles developed outside the EB irradiated region were found being larger than that inside the irradiated region. The Si₃N₄ windows were later separated to check the deposited structures in more detail. Electron diffraction from the nano features can be indexed by Au crystal. Gold was also observed by energy dispersive X-ray spectrum. More interestingly, nano liquid droplets were observed to fluctuate inside the bubble-like feature on the Si₃N₄ window under EB irradiation. Acknowledgments: The help from Prof. S. Dillon is greatly acknowledged. This work was supported by Shanghai Nano Project (11nm0507000) , Shanghai Leading Academic Discipline Project (B502), and Shanghai Key Laboratory Project (08DZ2230500).

Understanding the mechanical properties of nanomaterials is a subject of major interest in different fields of nanosciences and nanotechnologies. Particularly in microelectronics, metallic thin films used in interconnections are subjected to high current density and consequently high thermal loads. Thermal loads generate stresses through the differences in coefficients of thermal expansion with silicon substrate and oxides. Hence, determining the elastic to plastic transitions and the deformation modes becomes a critical issue to warrant device integrity. With the advent of in situ transmission electron microscopy (TEM)-nanoindentation, the mechanical behavior of small-scale samples can be investigated at critical interfaces, as plastic behavior can be triggered locally and the associated physical processes followed dynamically in the microscope. Here, we investigate the plastic behavior of passivated Al thin films. A dedicated TEM sample holder (Nanofactory Instruments), equipped with an actuator and a diamond mounted on a force-sensing MEMS device, allows indenting cross-sectional specimens that has been milled in a H-bar configuration using a focused ion beam (FIB). To study the dislocation behavior at the Al/SiO2 interfaces, the upper layer of silica, which is designed to distribute the stress imposed by the indenter and also to function as a protective layer during the FIB-milling process, is pressed into the diamond indenter. The in situ nanoindentation experiment was recorded on video with correlated force values. Finite element modeling helped converting the applied external load, into an estimation of the stress reaching the Al layer. The stress in the Al film of the specimen has also been estimated from the radius of curvature of dislocations. Both ways of estimating the stress in the Al film gave comparable results and served to estimate the yield stress of the Al film could. Especially, it was shown that dislocations tend to be captured by the Al/SiO2 interface. Pushed into this interface in compression, the dislocations did not go backwards. The result is comparable with in situ thermal cycling of similar samples, where the stress originates from difference in thermal expansion coefficients [1], except that here, thermal diffusion is reduced. To support the discussions, finite element models are built to calculate the transmitted stresses into the Al film by the indentation process, as well as to estimate the thermal stresses in the heating experiments [2,3]. References [1] M Legros et al, Acta Materialia 50 (2002), p. 3435. [2] P Mullner and E Arzt, MRS Proceedings 505 (1998), p. 149. [3] This work was supported by the EU through the ESTEEM project (Grant No. IP3: 0260019), and by the French National Agency for Research (ANR PNANO/HD STRAIN Project No. ANR-08- NANO-0 32).

Ostwald ripening that large particles grow bigger by consuming the smaller species is a key coarsening mechanism for a variety of physical and chemical processes. It plays an important role in the synthesis of monodisperse nanoparticles, geological rock texture formation and a variety of industrial reactions. However, Ostwald ripening often leads to undesirable consequences in many applications of nanoparticles thus it is desirable to limit this process by controlling the kinetic parameters. We study the growth of Bi nanoparticles in an engineered precursor-scarce environment in a liquid cell at an elevated temperature (180 degree C) using transmission electron microscopy (TEM). Observation reveals dynamics of oscillatory growth of individual nanoparticles, pairwise Ostwald ripening and anti-Ostwald ripening and a global collective oscillation. We achieved quantitative tracking of the volume trajectories of more than 30 nanoparticles with subsecond time resolution. It allows us to quantify the growth and dissolution rates of each individual nanoparticle, and build distance-dependent correlations between each pair of particles. By analyzing the pair correlation, we identified that a mass-transport zone is present around each particle, which couples to the observed growth kinetics. This study shed light on a new route for system engineering to reverse particle coursing by Ostwald Ripening. HZ thanks the funding support from U.S. DOE Office of Science Early Career Research Program.

Nanostructured surfaces with special wetting properties have potential to transform number of industries, including power generation [1], water desalination [2], gas and oil production [3], and microelectronics thermal management. The special wetting properties of these surfaces stem from the interaction of solid, liquid, and gas phases in the volume linking the droplet/bubble to the underlying substrate [4]. However, experimental determination of the geometry these complex interfaces has been limited by lack of appropriate imaging techniques. Here we demonstrate that imaging of such interfaces can be achieved using sample plunge freezing in liquid nitrogen slush, cryogenic temperature selective Focused Ion Beam (FIB) milling and SEM imaging. Plunge freezing is an established technique for preserving geometry of hydrated biological [5] and geological [6] specimens as well as colloidal suspensions and emulsions [7] for electron microscopy. We show that microscale water droplets condensed on a variety of substrates preserve their morphology during the plunge freezing process. After transfer into the microscope chamber, the frozen droplets can be selectively sectioned using FIB milling and imaged using SEM. We show examples of composite interfaces involving solid, liquid, and gas phases below water droplets condensed on superhydrophobic surfaces consisting of various nanostructures [8] as well as hybrid liquid-solid substrates [9]. We also discuss imaging artifacts and opportunities for 3D “destructive tomography”, elemental, and phase imaging of the interfaces. References: 1.(a) Dietz, C., et al., Appl. Phys. Lett. 2010, 97,033104; (b) Chen, R., et al., Nano Lett. 2009, 9 548. 2.Humplik, T., et al., Nanotech. 2011, 22,292001. 3.Smith, J. D., et al., Phys. Chem. Chem. Phys. 2012, 14, 6013. 4.(a) Cassie, A. B. D., et al., Trans.Farad.Soc. 1944, 40, 546; (b) Wenzel, R. N., Ind. Eng. Chem. 1936, 28, 988; (c) Patankar, N. A., Soft Matter 2010, 6, 1613; (d) Israelachvili, J. N., Intermolecular and Surface Forces. 3rd ed.; Elsevier: San Diego, 2011. 5.Dobro, M. J., et al., Meth. Enzymol. 2010, 481,63. 6.(a) Mikula, R. J., et al., Coll. Surf. A-Physicochem. 2000, 174, 23; (b) Boassen, T., et al., In Int. Symp. SCA, Vol. SCA2006-43 pp 1-6. 7.Binks, B. P., et al., Phys. Chem. Chem. Phy. 2002, 4, 3727. 8.Rykaczewski, K., et al., Soft Matter 2012. 9.(a) Wong, T.-S., et al., Nature 2011, 477 443; (b) Lafuma, A., et al., Europhys. Lett. 2011, 96, 56001.

As the device dimensions are scaled down, the current density in the metal interconnects increases. Under a high current density of ~106 A/cm2, voids and hillocks may form in the interconnectors due to the mass transport induced by electromigration, which leads to device failure. It is one of great technological importance to understand electromigration failure in thin film interconnects. Electromigration induced mass transport, in principle, can take place along different paths, such as lattices, grain boundaries, surfaces, and interfaces, which all have distinct activation energies of electromigration. Although most of the studies have shown that the Cu/Si3N4 interface is the dominant electromigration path, some have reported dominant path is the grain boundary, where it is proposed that the electromigration on Cu interconnects happens by coupling between grain boundary and interface diffusion. In order to understand the electromigration mechanism, in-situ electromigration sequence was observed under transmission electron microscopy (TEM). Single-level Cu lines with/without the Si3N4 capping layer on Si3N4 membrane were used as electromigration test structures. Firstly, a 50 nm-thick Si3N4 membrane was deposited onto the SiO2/Si substrate by a low pressure chemical vapor deposition. And then KOH preferred etching of Si substrate was carried out to make self-supporting Si3N4 membrane, with dimension of 2400mu;m×1200mu;m. On the Si3N4 membrane, Cu film of 50nm thickness was deposited by a ultrahigh vacuum DC magnetron sputtering, and patterned into a line width of the 3mu;m wide by photolithography and wet etching. To understand the effects of capping layer on electromigration mechanism, Si3N4 films of thickness 20nm was deposited as a dielectric capping layer on 3mu;m wide patterned Cu lines. Using a specially designed in-situ TEM stage, electric current was applied to the both ends of Cu line. Electromigration induced microstructural changes in surface and grain boundary of Cu line were recorded by in-situ TEM in real time under high current density of 1×106 A/cm2. Dominant path of electromigration of a single-level Cu line with/without the Si3N4 capping layer will be presented. This research was supported by the Nano-Material Technology Development Program (the Green Nano Technology Development Program) through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (2011-0019984).

Transmission electron microscope (TEM) is usually taken as a tool to characterize the microstructures of the materials before/after deformation test. By incorporating a miniaturized load/depth transducer into a TEM holder, Hysitron has recently developed a novel and unique nanomechanical apparatus which enables one to acquire quantitative mechanical data while simultaneously recording the microstructure evolution of the materials during deformation, developing a one-to-one relationship between imposed stress and individual deformation event. This presentation will report the current progress of this unique technology as well as some of its applications. It was found that prior to the compression tests, the nickel pillars fabricated through Focused Ion Beam (FIB) were observed to contain a high density of defects. However, quite unexpectedly, the dislocation density was observed to decrease dramatically during the deformation process and, in some cases, even resulted in a dislocation-free crystal. The phenomena, which we termed as “mechanical annealing”, is the first direct observation of the dislocation starvation mechanism and sheds new light on the unusual mechanical properties associated with submicron- and nano- scale structures. However, due to the dislocation core structure difference between the screw and edge dislocations in body centered cubic (BCC) metals, it was believed that BCC metals are incapable of mechanical annealing and the sample size strengthening behavior should be less that in FCC metals. By in situ compression of nanopillars inside a transmission electron microscope, we demonstrate that with the pillar diameter decreasing to hundreds of nanometers, significant mechanical annealing does occur in BCC Mo. In addition, there exists a critical size (DC ~ 200 nm for Mo at room-temperature) below which the strengthening exponent in Hall-Petch like regression increases dramatically to that similar to FCC metals. We attribute the observed phenomena to the diminishing importance of lattice friction at high stresses, when the size-enhanced flow stress exceeds a single screw dislocation&’s lattice friction.

Microscale bending experiments were conducted inside scanning electron microscope using a nanoindentation device (PI 85 PicoIndenter, Hysitron, Inc.) to understand fracture properties of diffusion aluminide bond coatings (PtNiAl). The compositional gradient as well as the microstructural variation along the thickness direction makes diffusion aluminide bond coats an ideal candidate material for microscale fracture toughness study. Two different microbeam geometries, double cantilever beam and doubly clamped beam, were manufactured from the bond coat region using a focused ion beam. The crack initiation and propagation characteristics with varying Pt content and changing Ni:Al ratio were observed and the corresponding load-displacement data were obtained. While the clamped microbeams are used to obtain initiation fracture toughness of each individual zone of interest, the double cantilever beam specimen has the potential to determine propagation toughness and R-curve behavior across the entire coating thickness in one single test. Also, the double cantilever specimen is known to be stable under displacement control, whereas the doubly clamped beam is shown to be stable even under load control beyond a critical crack length to width (a/W) ratio for the beam dimensions used. The coating exhibits an R-curve behavior even though the bulk (Pt,Ni)Al itself is inherently brittle.

Polymeric electrospun scaffolds have been used in tissue engineering due to their microstructures, which mimic the fibrous networks in natural biological materials such as cartilage and blood vessels. An understanding of the failure of fibrous networks can not only facilitate the production of electrospun scaffolds with improved toughness, but also provides insight for treatment of diseases and conditions that involve soft tissue failure. Despite the importance, an understanding of the toughening mechanisms in fibrous materials, particularly at small scales, is still incomplete. The failure of polycaprolactone (PCL) electrospun scaffolds under mode I loading was experimentally studied at both macroscopic and microscopic length scales and compared with that of other nonwoven polymeric materials. Uniaxial tensile tests and fracture tests were first performed on PCL electrospun scaffolds. The detailed toughening mechanisms at the notch front were then examined by performing in-situ fracture testing of PCL scaffold in the scanning electron microscope (SEM). In the fracture tests, the scaffolds stretched more than a hundred percent without significant crack propagation. Blunting and necking occurred in the vicinity of the notch root, forming a region of intense deformation ahead of the notch. The examination of this notch region in the SEM showed that the randomly oriented fibers rearranged and formed parallel fiber bundles. The formation of fiber bundles increased during necking. These fiber bundles, which aligned parallel to the loading direction, resisted crack propagation. The understanding of toughening mechanisms presented here is to not only for the particular case of PCL electrospun scaffolds, but also for other fibrous materials, including ductile polymers. This study offers guidelines for the production of fibrous materials with enhanced toughness.

Magnesium is a lightweight metal that would find widespread use for structural applications if its room temperature formability can be enhanced. Basal slip has much lower CRSS than prismatic and pyramidal slip in Mg, but the two independent basal slip systems cannot satisfy the arbitrary shape change during complex deformation encountered in the shaping of any component. Therefore, deformation twinning often becomes important deformation modes. Recently we have run a systematic series of in situ TEM mechanical tests on pure Mg oriented for basal slip and deformation twinning, respectively, where we have quantitatively measured and characterized the basal slip behavior and the deformation twinning behavior. Strong crystal size effects on both deformation mechanisms were found. More importantly, the surface nucleation mechanism became increasingly significant at extremely small scales. Both the dislocation and deformation twinning behavior changed during plastic deformation in small-scale samples, resulting in high strength and also high ductility. The mechanism of external dimension refinement was further compared with internal dimension refinement where grain boundaries might be engineered to reduce plastic anisotropy and enhance both the strength and ductility in Mg. [Nano Lett. 12 (2012) 887]

Deformation twinning is an important plastic deformation mechanism in Hexagonal Close Packed materials (HCP) due to the lack of sufficient number of slip systems to accommodate general plastic flow by slip alone. It is evident from the recent literature that twinning can enhance the ductility of HCP materials and also has significant influence on the texture evolution in polycrystalline Magnesium (Mg) and its alloys. Hence, it is important to understand the propagation and growth of twins during plastic deformation. Most of the studies conducted hitherto are ex-situ and focused on understanding the influence of twinning on the stress-strain behavior. However, a detailed characterization of twinning during deformation is still lacking. We conduct in-situ micro tensile experiments on a polycrystalline Mg to understand the deformation twinning. Dog-bone shaped, micro-tensile specimens are made from pre-twinned samples. Preliminary results from in-situ experiments indicate that a large amount of twins are generated at the grain boundaries. A significant amount of offset, in the thickness direction, is noticed at the grain boundaries whose surrounding grains deform by twinning. The local stress-strain behavior is estimated using digital image correlation technique and correlated to the microstructural changes that occur during deformation.

Material processes subject to extreme driving forces and conditions far from equilibrium inherently occur on very short time scales, ranging from the femtosecond-scale processes occurring from non-equilibrated electron states to microsecond transient events of phase transformations and deformation processes. Typically, we are confined in the laboratory to conduct experiments near equilibrium or observe the material&’s state post process due to the resolution limitations of conventional analytical techniques. Though insight about material&’s behavior is gained from these observations, much of coupled and convoluted events of complex processes on short time scales not well understood that require technique that both high spatial and temporal resolution to observe nanoscale microstructural features evolving on short timescales. In effort to meet the need for studying fast dynamics and transient states in material processes, we have constructed a nanosecond dynamic transmission electron microscope (DTEM) at Lawrence Livermore National Laboratory to improve the temporal resolution of in-situ TEM observations. Prior DTEM hardware only allowed single-pump/single-probe operation, building up a process's typical time history by repeating an experiment with varying time delays at different sample locations. Movie Mode DTEM upgrade now enables single-pump/multi-probe operation. These technical improvement provide the ability to track the creation, motion, and interaction of individual defects, phase fronts, and chemical reaction fronts, providing invaluable information of the chemical, microstructural and atomic level features that influence the dynamics and kinetics of rapid material processes. For example, the potency of a nucleation site is governed by many factors related to defects, local chemistry, etc. While a single pump-probe snapshot provides statistical data about these factors, a multi-frame movie of a unique event allows all of the factors to be identified and the progress of nucleation and growth processes can be explored in detail. It provides unprecedented insight into the physics of rapid material processes from their early stages (e.g. nucleation) to completion, giving direct, unambiguous information regarding the dynamics of complex processes. This presentation will discuss the technical aspects of the Movie Mode DTEM technology in the context of recent material science studies using the novel in situ TEM capability. Work preformed at LLNL under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and supported in part by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering.

The melting and solidification of metals and alloys is a ubiquitous manufacturing process used to fabricate components for an enormous number of applications. The properties and performance of these components is dictated by the phases and final microstructure of the solidified material, which is determined in turn by the thermal and compositional conditions that exist during solidification as well as thermodynamic and kinetic constraints of the materials system. During rapid solidification, pronounced deviations from equilibrium are known to occur [1-3], leading to the formation of unique microstructures and metastable phases with potentially useful properties. In-situ experiments that directly image the evolving microstructure and advancing solid-liquid interface during the solidification process can provide insight into the mechanisms and kinetics governing rapid solidification, leading to eventual improved control over component properties. Using the dynamic transmission electron microscope (DTEM), Al-Cu thin films were pulsed-laser-melted and the solidification of the alloys was investigated with in-situ time-resolved imaging. The Al-Cu system has been widely investigated and its thermodynamic and physical properties as well as regular eutectic (Al-Al₂Cu) are well defined [4], providing a model system for alloy solidification studies. Here, we present results from both time-resolved DTEM experiments and post-solidification characterization of multiple compositions of Al-Cu thin films (hypo-, hyper-, and eutectic alloys), and the effects of Cu content on the resulting microstructures, phase formation, and kinetics will be discussed [5]. References [1] Zimmermann, M. et al. Acta Metall., 37 (1989) 3305. [2] Jones, H. Mater. Sci. Eng. A, 137 (1991) 77. [3] Kurz, W. and Gilgien, P. Mater. Sci. Eng. A, 178 (1994) 171. [4] Murray, J. L. Int. Metals Rev., 30 (1985) 211. [5] This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory and supported by the Office of Science, Office of Basic Energy Sciences, Division of Materials Science and Engineering of the U.S. Department of Energy under Contract No. DE-AC52-07NA27344. Work performed at the University of Pittsburgh was supported by the National Science Foundation, Division of Materials Research, Metals & Metallic Nanostructures program through Grant No. DMR 1105757.

Sintering is usually considered as a key-step in the preparation of ceramics as it drives both density and microstructure of the final materials. Since such parameters could strongly influence the physico-chemical properties of materials during their life-cycle, the sintering of CeO₂ and ThO₂ pellets, as model compounds for the next generation of nuclear fuels, was investigated through an original approach based on the combination of dilatometric measurements and in situ HT-ESEM observations. On the one hand, the use of environmental microscope allowed observing in situ the behaviour of the compact during heat treatments between 1000°C and 1400°C. The subsequent image analysis led to the determination of original local kinetics parameters (i.e. at the grain scale), including grain boundaries mobility and individual grain growth rates. Such observations confirmed the general behaviour expected from theoretical models and numeric simulations but also evidenced unusual phenomena such as closed porosity elimination through the surface. When considering a larger population of grains, global kinetics parameters (i.e. at the pellet scale) were assessed. Particularly, the average grain size was plotted versus the holding time for the different temperatures considered. In these conditions, the nature of the mechanism controlling the global sintering rate was pointed out and the energy of activation corresponding to grain growth processes was evaluated, in good agreement with the data previously reported in literature. On the other hand, analogous experiments were performed by dilatometry and allowed monitoring both linear shrinkage and density of the samples. First, the value of the energy of activation obtained by HT-ESEM was confirmed by the use of the Dorn's method. Also, in the case of ThO₂, the combination of the two sets of data led us to establish for the first time a sintering map of this compound. This latter clearly evidenced two different zones in the evolution of the pellet microstructure driven by densification (d/d_calc. le; 92%) then by grain growth (d/d_calc. ge; 92%). The combination of HT-ESEM observations and of dilatometric measurements then appeared as a very rapid and reliable way of investigation of ceramics sintering which can be used efficiently to monitor the final microstructure of dense materials.

With electron beams smaller than the bond length of hydrogen, aberration-corrected scanning transmission electron microscopes (STEM) can currently image materials with resolutions below the shortest bond length in nature. However, these atomic resolution images are only 2D projections of a specimen. In order to determine the full 3D structure, one must acquire a series of STEM images over a range of specimen tilts. Unfortunately, the resolution of a 3D STEM tomogram is diminished by missing information from the restricted specimen tilt range and finite tilt increments. This typically limits volumetric resolutions of electron tomography to roughly 1 nm—twenty times worse than the best resolution in 2D projections. In addition, high-resolution (< 1nm) tomography of extended objects, to date, has not been possible. The sub-Ångstrom resolution of aberration-corrected electron microscopes is accompanied by a strongly diminished depth of focus, causing regions of “large” specimens (> 5nm) to appear blurred or missing. High-resolution 3D reconstruction of extended objects requires collecting information beyond a traditional tilt series. Here we present through-focal tomography that combines depth sectioning and traditional tilt-tomography to reconstruct extended objects, with high-resolution in three-dimensions. This novel technique fills in the missing 3D information by acquiring a through-focal image series at each specimen tilt. A through-focal tomographic reconstruction of porous PtCu nanoparticles used in the proton exchange membrane fuel cells (PEMFCs) revealed a nano-pore network throughout many particles on an extended Vulcan carbon support (~390nm). Experimental and simulated comparisons of through-focal tomography with traditional tomography methods reveal improved contrast and resolution over large fields of view.

Graphene, which is a single atomic layer of carbon atoms bonded in a hexagonal lattice, is among the few materials that are stable in two dimensions. Controlling the structure of such a thin material is difficult during synthesis or exfoliation so a post treatment may be the most viable option. In our studies, we show that graphene can be modified using a high temperature anneal and electron beam irradiation. For edge modification, a graphene flake was annealed from 400 to 1200 degrees Celsius while being examined under an electron microscope. As the temperature was stepped up to higher values, contaminants disappeared from the graphene lattice. At the highest temperatures, the graphene edges were observed to reconstruct into atomically straight bi-layer edges. This only occurred in areas where there was a concurrent electron beam irradiation. Without the sputtering of the electron beam, the annealed graphene sees little to no reconstruction. Our studies also show that the same mechanisms can be used on nanopores within the lattice. By focusing the electron beam onto the graphene, nano-pores or -holes were drilled into its structure. Using temperatures between 400 and 1200 degrees Celsius, the nano-pores were expanded or shrunk. Small diameter nano-holes were able to shrink completely, closing off the nano-pore, when being both annealed and bombarded by electrons. This gives a mechanism for controlling the final pore diameter by removing or blocking the beam exposure at the desired size. These simple post-synthesis treatments pave the way for scientists to modify the structure of graphene which can lead to tailoring their device characteristics. This work was supported by SWAN (GRC-NRI), AOARD-AFOSR (FA2385-10-1-4066) and the World Class University Program (by MEST through NRF (R31-10026)).

Detecting nano-scale temperature and temperature distributions is important for studies of heat generation and transfer in a wide range of engineering systems, such as microelectronic, optoelectronic and micromechanical systems. While in the past few decades, much progress has been made in the area of nano-scale temperature mapping techniques, no current method adequately combines high spatial resolution, high temperature sensitivity, and an ability to work in non-contact mode (such that the local temperature distribution is not perturbed by heat diffusion between the contact probe and the sample surface). In this presentation, we introduce a new nano-scale resolution non-contact in-situ temperature measurement technique (which we call thermal scanning electron microscopy, ThSEM). This technique is based on temperature dependent thermal diffuse scattering in electron backscatter diffraction (EBSD) in a scanning electron microscope (SEM). Unlike scanning thermal microscopy (SThM), which uses a contacting probe, ThSEM is a non-contact method and in contrast to optical temperature mapping techniques, ThSEM doesn&’t have the spatial resolution limitation that arises from the optical wavelength. While in the current work, a spatial resolution of less than 100 nm and a temperature resolution better than 10 °C are attained using a thermionic source SEM, it is possible for ThSEM to reach much higher temperature resolution and spatial resolution (< 10 nm) using field emission SEMs operating at lower beam energy. ThSEM preserves many of the merits of SEM and can zoom over a broad range of fields of view. Also, the hardware setup is very similar to the EBSD system in an SEM, which makes the integration of temperature mapping into SEM relatively straightforward. Moreover, multiple signals or contrast mechanisms, such as temperature distributions, grain orientation maps, topographic images, and elemental maps could be obtained from the same sample area simultaneously depending on the specific SEM capability. This technique thus adds a new channel - the temperature signal - to the collection of existing SEM signals. In this presentation, the feasibility of ThSEM for nano-scale temperature measurement is examined, the temperature dependencies of Kikuchi line intensities for a Si sample are presented and explained quantitatively using the Debye-Waller factor (DWF), parameters affecting temperature sensitivity are analyzed and optimized, and the spatial resolution of this technique is examined.

The mechanical deformation of CNT arrays (also called forests) is a complex, multi-scale phenomena and may be directly coupled to the performance of many applications including strain sensing and highly efficient interfaces. Numerous nanoindentation and compression studies have provided quantitative characterization of the mechanical properties of CNT arrays such as modulus and yield strength. The value of these techniques, however, are restricted in scope as they provide limited insight into CNT interactions and deformation mechanisms. Recently, in situ SEM compression testing has facilitated the direct observation of CNT arrays deformation and has provided valuable insight into the initiation and propagation of coordinated buckling. Even with the significant advance in capability afforded by in situ testing, the knowledge gained has been largely phenomenological. As a result, the in situ method alone is incomplete as a tool for describing CNT array mechanics and for quantitatively validating mechanical models. We demonstrate the use of 2-D digital image correlation (DIC) to spatially resolve strain and displacement of patterned CNT array columns based on in situ SEM compression sequences. The technique utilizes the native grayscale pattern established by the individual CNTs and CNT bundles within an array as a traceable speckle pattern for DIC analysis. In such a way, spatially resolved strain maps of CNT columns are generated based upon the motion of constituent CNTs for column widths of up to 100 mu;m and lengths of up to 75 mu;m. Upon validation of the DIC technique, we evaluated the compression of CNT columns with square cross sections having aspect ratios (defined as column length / column width) ranging from approximately 0.25 to 6.0. The columns deform in one of three distinct modes, which vary as a function of column aspect ratio. These modes include crushing, bending, and bottom-up buckle accumulation. In spite of the significantly different appearance of these deformation modes, DIC analysis reveals a consistent CNT array yield criterion of 5% local compressive strain for each column. Local strain in excess of 5% initiates coordinated buckling. Strain maps show that strain is highly non-uniformly distributed throughout the compression sequence. Rather, lateral bands of enhanced compressive strain form at relatively low global strain and grow in magnitude and dimension with increased global strain. Buckle formation generally initiates from within these bands. Outside of the localized bands, strain is significantly lower than the global applied strain by as much as an order of magnitude. Further application of in situ SEM DIC to CNT arrays and similar material systems is expected to significantly expand understanding of the mechanics of nanoscale hierarchical materials and provide a quantitative foundation for comparison to mechanical models.

Investigation of the mechanics of natural materials, such as spider silk, abalone shells, and bone, provided great insight into the design of materials that can simultaneously achieve high specific strength and toughness. Research has shown that their emergent mechanical properties are owed in part to their specific self-organization in hierarchical molecular structures, from nanoscale to macroscale, as well as their mixing and bonding. Following this inspiration we have addressed the design of carbon nanotube (CNT) based fibers and yarns by applying lessons learned from mulitscale experiments and simulations across multiple lengths scales. Carbon nanotubes (CNTs) are envisioned to be ideal building blocks in hierarchical macroscopic composite fibers due to their extraordinary strength and stiffness. Macroscopic materials based on CNTs, however, have been limited by weak shear interfaces between adjacent CNT shells and matrix elements. Initial studies have demonstrated that double-walled CNTs (DWNT) are very attractive building blocks for macroscopic high CNT density fibers. Here we present experimental-computational studies of shear interactions within hierarchical DWNT yarns that have furthered the understanding of a number of key mechanical mechanisms which contribute to the ultimate behavior of yarns. At the individual bundle level we have studied the shear behavior within DWNT bundles through in-situ SEM/TEM mechanical testing coupled with Molecular Mechanics (MM) and Density Functional Theory (DFT) modeling. In-situ SEM pullout experiments conducted on DWNT bundles revealed the typical sword-in-sheath failure mechanism and allowed quantification of the force required to shear a small inner bundle of DWNTs out from an outer sheath of DWNTs. In this study a normalized pullout force of 1.7 +/- 1.0 nN/CNT-CNT interaction for sliding of a smaller inner bundle of DWNTs out of a larger outer shell of DWNTs. Through comparison with MM and DFT simulations of sliding between adjacent CNTs in bundles it was identified that factors contributing to the pullout force included the creation of new CNT surfaces, carbonyl functional groups terminating the free ends, corrugation of the CNT-CNT interaction, and polygonilization of the CNTs in the bundle. In addition a top down analysis of the experimental results revealed that greater than one half of the pullout force was due to dissipative forces. This finding of behavior at the CNT bundle level significantly differed from the behavior of pullout in individual MWNTs for which dissipation is found to be negligible.

Understanding the interfacial stress transfer between carbon nanotubes (CNTs) and polymer matrices is of great importance to the development of CNT-reinforced light-weight and high-strength polymer nanocomposites. In this talk, we present our recent experimental work on studying the interfacial strength between Poly(methyl methacrylate) (PMMA) and the embedded individual CNTs. The interfacial strength of the CNT-PMMA interface is characterized using in-situ nanomechanical single-tube pull-out techniques. By pulling out individual tubes from the polymer matrix using atomic force microscopic force sensors inside a high resolution scanning electron microscope, the pull-out force and the embedded tube length are measured with resolutions of sub-nN and nm, respectively. Our results reveal the dependence of the pull-out force on the embedded tube length and identify the critical embedded tube length that leads to the maximum pull-out force. Our results show a mean interfacial fracture energy of ~0.175 J/m2 and interfacial shear stress of ~ 45 MPa for the CNT-PMMA interface. Our data on the CNT-PMMA interfacial strength are in reasonably good agreement with both the theoretical predictions by molecular dynamics (MD) simulations and those experimental data reported on other similar types of non-bonded nanotube-polymer interfaces (e.g. CNT-epoxy). Our results clearly demonstrate the shear lag effect in pulling out polymer-embedded nanotubes. Our in-situ single-tube pull-out experimental methodology can be readily extended to study the interfacial strength of both non-bonded and bonded interfaces formed by individual nanotubes and a variety of polymer matrices.

Grain boundaries are well known to influence the mechanical behaviour of materials. For polycrystalline metals strengthening by the Hall-Petch mechanism is a well-established concept. However, the strength of individual grain boundaries against dislocation plasticity, i.e. their barrier strength, remains still obscure. Recent advances in micro- and nanomechanical testing permit now to select individual boundaries and to probe their strength with a high spatial resolution. Additionally, the electron microscopy observations provide insights into the deformation mechanisms. In the present talk recent developments for miniaturized deformation experiments at room temperature and up to 400°C will be reported and results addressing the mechanical properties and deformation structure of individual grain boundaries will be presented.

Understanding the deformation mechanisms in nanocrystalline metals and alloys is crucial for improving their performance and stability as needed for technical applications. During the last couple years our understanding of the deformation mechanisms of nanocrystalline metals has improved a lot mostly based on in-situ deformation experiments using XRD. However, it is difficult to see and understand the local processes based on these bulk measurements. The local processes are typically investigated using classical BF/DF-TEM. However, varying contrast contributions due to local orientation, bending and defects make an accurate interpretation for nanometer sized grains difficult. This becomes particularly challenging during in-situ straining experiments as the grains are deforming and reorienting. Here we show a novel approach, combining modern TEM techniques with in-situ straining to follow the full crystallographic orientation of each individual grain in the field of view with nanometer resolution through the steps of increasing strain or cycles of straining. Hysitron&’s TEM Picoindenter is used to strain TEM lamella or thin films with nanometer precision, providing the corresponding stress-strain curves. Together with the Nanomegas orientation mapping system [1] operating on a Tecnai F20 in µP-STEM mode, it opens the possibility to obtain orientation maps with nanometer resolution at selected states during the straining and at the same time, follow the deformation continuously using fast STEM imaging of the area of interest. Data processing using the Mtex Toolbox for quantitative texture, grain size and orientation analysis is the next step towards data interpretation [2]. It enables good identification of the crystallographic orientation of all grains and sub-grains and the quantification of twins and other special boundaries. First investigations were conducted on magnetron sputtered Au and Pd samples. To avoid straining during handling and preparation of the TEM lamellas, the metals were directly sputtered onto TEM holey carbon grids with a 2 nm carbon layer and transferred to a Hysitron push to pull device using a DualBeam FIB. Straining of this film inside the TEM allowed us to follow the deformation processes in ultra-fine grained Au up to 9.5% strain before rapture of the sample. The analysis reveals a complex mixture of processes taking place during straining including growth of individual large grains by slowly ‘eating&’ smaller ones, twining/detwining and grain rotation (Figure 1). The results will be discussed in terms of the deformation mechanism and how it is related to the local grain constellation. References: [1] Rauch, E.F. et al., Zeitschrift für Kristallographie, pp. 103 225 (2010). [2] Bachmann, F. et al., Solid State Phenomena, pp 63, 160 (2010). [3] Support by the Deutsche Forschungsgemeinschaft (FOR714) is gratefully acknowledged.

Nanoindentation at elevated temperature is an increasingly popular area of research [1-3] with a several custom systems at various institutions and variety of manufacturers offering standard options for elevated temperature testing. These encompass a range of technical solutions for performing tests: indenter/sample heating, water cooling, heat shields, inert gas shrouding, vacuum chambers, etc. These all attempt to circumvent the challenges of thermal displacement drift and indenter/sample oxidation. A brief summary of these technical solutions and how they address these challenges will be made, specifically with regards to the modifications made to an in situ nanoindentation system for testing at elevated temperatures up to 500°C within an SEM. Procedures for the calibration of the tip temperature via Raman spectroscopy and precision thermocouple measurements will be discussed, and the mechanisms of thermal drift will be discussed as revealed via careful contact thermometry. Thermal drift is shown to be a minimum when the indenter and sample are at an isotherm, which is validated by direct thermal measurements. A wide variety of materials display interesting mechanical behaviour at elevated temperatures. By using an in situ SEM elevated temperature indenter, a unique ability of coupling observation and measurement of mechanical deformation has been achieved. In addition to standard reference materials such as fused silica, pure aluminium, and tungsten, results from several classes of materials will be shown as case studies: silicon, GaN nano-prisms and lead-free solders. References [1] N.M. Everitt et al. Phil. Mag. 91, 1221-1244 (2011). [2] Trenkle, Packard, and Schuh. Rev. Sci. Instrum. 81, 073901 (2010). [3] Zhi Chao Duan, A. M. Hodge, JOM, 61, p32-36 (2009).

Thin film components of conventional and flexible solid-state devices experience mechanical strain during fabrication and operation. At the bulk scale, small values of strain do not affect thermal conductivity, but this may not true for grain sizes comparable with the electron and phonon mean free paths and for higher volume fraction of grain boundaries. To investigate this hypothesis, we developed a MEMS based experimental thermo-electro-mechanical characterization platform. Using this setup in-situ TEM characterization of thermal and electrical conductivity of nominally 100-nm-thick aluminum films (average grain size 40 nm) is carried out as a function of tensile strain, using a 3-omega; technique. The analysis shows that mechanical strain decreases the mean free path of the thermal conduction electrons, primarily through enhanced scattering at the moving grain boundaries. This conclusion is supported by similar effects of mechanical loading observed on the electrical conduction in the nanoscale aluminum specimens.

High strength low alloy steels (HSLAs) are used in many applications, such as ship and automotive construction, energy production and transmission and structural applications. These alloys utilize small additions of alloying elements to modify recrystallization kinetics and control the final microstructure. The effects of many of these elements have been studied singly, but their synergistic interactions within more complex alloys are not well understood. Niobium (Nb) containing microalloyed HSLAS alloyed with molybdenum (Mo) and vanadium (V) are both known to retain strength at high temperature better than other HSLAS, but Mo is significantly more potent in this role. In this work, in situ TEM tempering (annealing) of Nb-based HSLAs containing either Mo or V additions is presented.. These experiments provide a direct measurement of the effects of Mo and V on Nb alloyed HSLAs, allowing microstructural evolution to be connected to thermal processing schedules . This data allows for the quantification of the effect of Mo and V on precipitation, grain growth and phase transformation kinetics. The in situ experiments are compared to theoretical calculations of the kinetic processes during annealing. This combinatory approach of in situ TEM processing and theoretical calculation yields a deeper understanding of phase transformation phenomena critical to the microstructural control and final mechanical properties in new HSLAs alloys.

Although nanograined materials are known to exhibit many enhanced properties relative to their coarse-grained counterparts, a reliable prediction of their behavior under external influences such as thermal energy requires an understanding of their unique microstructure. For the case of pulsed laser deposited (PLD) nanograined Ni films, previous work using in-situ heating transmission electron microscope (TEM) experiments revealed abnormal grain growth, complex defect structures, and evidence of metastable hcp grains that compete with stable fcc phase grains. A detailed unde