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The effects of oxygen and nitrogen on the mechanical properties of Czochralski (CZ) and float zone silicon have been studied using nano-indentation. Nitrogen free FZ Si exhibited low hardness of 6.49 GPa and elastic modulus of 104 GPa. When doped with 2×1015cm−3 nitrogen, FZ Si hardness and elastic modulus increased to 8.2 and 182 GPa, respectively. In the near-surface denuded zone of N-doped CZ Si (N-CZ) the hardness correlates well with the O and N profiles. Distinct high hardness points, found in the O- and N- rich subsurface region, were attributed to precipitates. Nano-scratch tests of N-CZ Si confirmed the existence of hard phases, mostly small precipitates, whose density, estimated to be 2×1013cm−3, is in the range of previously suggested nuclei density in as-grown N-CZ silicon.
The evolution of elastic strain caused by melting and solidification of small inclusions in aluminum was investigated by in-situ transmission electron microscopy. The appearance and subsequent decay of elastic strain during phase transformation of inclusions around 100nm in size were observed directly, and the decay rate was determined as a function of temperature. The mechanism of strain accommodation was studied by determining the activation energy of the process using alloy composition and inclusion size to control the transformation temperature.
The mechanisms associated with cycle-by-cycle damage accumulation resulting in fatigue crack propagation between a highly constrained polymer layer and an adjacent elastic substrate are explored. Specifically, cyclic fatigue-induced crack growth between a bisphenol F model epoxy system and a passivated silicon substrate under Mode I loading is reported. Preliminary findings regarding the effects of fatigue load ratio on interfacial crack growth rates are presented. While intermediate crack growth rates were significantly accelerated under cyclic loading, the near-threshold crack growth behavior under cyclic and monotonic loading was surprisingly similar.
Nanostructured alloys have great potential as soft magnetic materials. In particular, nanocrystalline Fe-Co based alloys are believed to be good candidates for imparting improved magnetic behavior in terms of higher permeability, lower coercivity, reduced hysteresis losses and higher Curie temperatures. In the present work, Fe-50at.%Co alloy powders have been prepared using mechanical alloying (MA) in a planetary ball mill under controlled environment. The particle size and morphology of MA powders was investigated using scanning electron microscopy. The crystal size and internal strain was measured using X-ray diffraction. It has been shown that the crystal size could be reduced down to less than 15 nm in these alloys. Finally, the influence of grain size and internal strain on the magnetic properties has been discussed.
We present the results of large-scale NonEquilibrium Molecular Dynamics (NEMD) simulations for Cu/Ag interfaces sliding in the velocity regime 0≤v≤1Km/sec. System sizes of 2.8 × 106 atoms are considered using Embedded Atom Method (EAM) potentials. Single crystals with 010 interfaces sliding along the <100> direction are considered. We discuss the observed velocity weakening in the tangential force at high velocities, and its connection with the observed dislocation structure and nanostructure that are nucleated during dry sliding.
The relative surface contact stiffness of SnO2 nanobelts has been investigated via ultrasonic force microscopy (UFM). The nanobelt crystal structure, as determined via transmission electron microscopy, was indexed to the tetragonal rutile structure (with lattice constants identical to those of bulk SnO2) as reported previously. The atomic Sn:O composition of the nanobelts studied was at or near 1:2. Topographic imaging studies revealed the nanobelt surface to be atomically flat with the exception of surface nanodots, assumed to be local SnO2 crystallites. Preliminary reduced modulus measurements were carried out via differential UFM on both the flat and nanodot regions of the nanobelt. Using the underlying Si substrate as a calibration standard the SnO2 modulus was estimated at 157±12 GPa, significantly lower than corresponding bulk values for any of the observed crystal orientations. We speculate this discrepancy is due in part to a combination of the aspherical probe tip and unknown adhesive properties of nanobelt. An intrinsic reduction of the SnO2 nanobelt modulus cannot be ruled out.
The tensile behavior of bimodal nanocrystalline Al-7.5Mg alloys was investigated using experiments and two-dimensional axisymmetric elastic-plastic finite element method (FEM). Cryomilled nanocrystalline powders blended with 15% and 30% unmilled coarse-grained powders were consolidated by hot isostatic pressing followed by extrusion to produce bulk bimodal nanocrystalline Al-7.5Mg alloys, which were comprised of nanocrystalline grains separated by coarse-grain regions. The calculated stress-strain curves have acceptable agreement with experimental curves of the bimodal structures. The bimodal Al-7.5Mg alloys show reasonable ductility while retaining enhanced strength compared to conventional alloys and nanocrystalline metals.
Thin gold films and coatings on metal have long constituted an important technology for the microelectronics industry and will continue to be important for microdevices such as contact springs. The properties of these materials may be highly processing dependent, particularly when the gold is deposited by electrochemical means. In this study, we characterize gold electrodeposited on Ni substrates from two bath chemistries: hard Au sulfite with proprietary hardening additive and soft Au cyanide. TEM and SEM show that the bath chemistry alters the microstructure and the resulting surface of the electrodeposits. Nanoindentation techniques were used to determine the elastic and plastic properties of the Au electrodeposits as a function of the specifics of processing. Soft Au electrodeposits have a grain size of on the order of 300 nm and a hardness of about 1 GPa. Hard Au electrodeposits produced from the sulfite bath feature grain sizes as small as 30 nm, some twinning, and fine porosity uniformly distributed both within the grains and at grain boundaries. The hardness is about 2 GPa, approaching the hardest values reported for sputtered gold films. The effect of the hardening agent on the microstructure of electrodeposits from the Au sulfite bath was also investigated and found to significantly refine the grain size at concentrations of at least 4 mL/L, although little additional refinement was found at higher concentrations.
The formation of tribomaterial during sliding is well documented for a wide range of materials. Transfer, adhesion, mechanical mixing and formation of nanocrystals are commonly reported, but the mechanisms involved have not been well understood. Recently, molecular dynamics (MD) simulations have been performed to obtain new information on the response of materials to sliding interactions. The results help to explain experimental observations on simple metals such as copper, bulk metallic glasses and multi-component nanocomposite coatings. When the sliding speed is sufficiently high, the strain rate allows vorticity to develop. It is suggested that the resulting eddies are largely responsible for frictional energy dissipation and mechanical mixing in both crystalline and amorphous materials and for the shear mixing of soft embedded particles in nanocomposite coatings. Similarities of flow behavior with that of fluids are noted.
In a recent study of diffusional creep in polycrystalline thin films deposited on substrates, we have discovered a new class of defects called the grain boundary diffusion wedges (Gao et al., Acta Mat. 47, pp. 2865-2878, 1999). These diffusion wedges are formed by stress driven mass transport between the free surface of the film and the grain boundaries during the process of substrate-constrained grain boundary diffusion. The mathematical modeling involves solution of integro-differential equations representing a strong coupling between elasticity and diffusion. The solution can be decomposed into diffusional eigenmodes reminiscent of crack-like opening displacement along the grain boundary which leads to a singular stress field at the root of the grain boundary. We find that the theoretical analysis successfully explains the difference between the mechanical behaviors of passivated and unpassivated copper films during thermal cycling on a silicon substrate. An important implication of our theoretical analysis is that dislocations with Burgers vector parallel to the interface can be nucleated at the root of the grain boundary. This is a new dislocation mechanism in thin films which contrasts to the well known Mathews-Freund-Nix mechanism of threading dislocation propagation. Recent TEM experiments at the Max Planck Institute for Metals Research have shown that, while threading dislocations dominate in passivated metal films, parallel glide dislocations begin to dominate in unpassivated copper films with thickness below 400 nm. This is consistent with our theoretical predictions. We have developed large scale molecular dynamics simulations of grain boundary diffusion wedges to clarify the nucleation mechanisms of parallel glide in thin films. Such atomic scale simulations of thin film diffusion not only show results which are consistent with both continuum theoretical and experimental studies, but also revealed the atomic processes of dislocation nucleation, climb, glide and storage in grain boundaries. The study should have far reaching implications for modeling deformation and diffusion in micro- and nanostructured materials.
A study has been made of the effect of solute (Mn, Al, Ni) additions on microstructure refinement due to large strain deformation in single phase, copper solid solutions. The solutes were specifically selected for their influence on stacking fault energy (SFE) of copper, and the large strain deformation was imposed by chip formation in machining. The microstructure of Cu- 0.7at%Ni chip consists of elongated, sub-micrometer sized grains while Cu-7at%Al chip is made up of long, thin microbands with twins. The microstructure of the chip changes as the SFE of the material varies. With all of the solid solutions studied, the hardness of the chips is found to be significantly greater than that of the bulk material. Recrystallization temperature of solid solution chips is found to be higher than those of pure copper chips.
Bilayer structures consisting of ZrO2-3mol% Y2O3(TZ-3Y) and zirconia-alumina composites as inner (substrate) and outer (coating) layers, respectively, are fabricated using gel-casting for the inner layer and dip-coating for the outer layer in aqueous system. The relatively tough TZ-3Y is used as the inner layer for damage absorption due to its mechanical properties. The mixture ratio of alumina/zirconia slips for dip-coating is 1:9, 2:8, and 3:7 as mole ratio. The processing additives for gel casting, such as dispersant, monomer, dimer, and initiator, are adjusted and optimized by measuring viscosity. From which the solid loading of starting material (TZ-3Y) is determined. The optimum amount of dispersant (D-3019; anionic dispersant agent) for TZ-3Y is 0.7 wt%. The slip pH affects the electric double layer in sols, which causes the different rheological behaviors and solid loadings. The castable solid loading of TZ-3Y is 37 vol%, showing a pseudoplastic rheological behavior. The effect of slip type (different mixture ratio) on sintered body is investigated through hardness (Vickers indentation), microstructure (SEM), and strength (4-point bending tests). Strength of sintered bodies after dip-coating into the slips is higher than that before dip-coating, but hardness is not much different among cases. The effects of thickness in the outer layer on damage resistance and mechanical properties of the bilayer structures are discussed extensively.
We analyze a large-scale molecular dynamics simulation of work hardening in a ductile model material comprising of 500 million atoms interacting with a Lennard-Jones pair potential within a classical molecular dynamics scheme. With tensile loading, we observe emission of thousands of dislocations from two sharp cracks. The dislocations interact in a complex way, revealing three fundamental mechanisms of work-hardening. These are (1) dislocation cutting processes, jog formation and generation of point defects; (2) activation of secondary slip systems by cross-slip; and (3) formation of sessile Lomer-Cottrell locks. The dislocations self-organize into a complex sessile defect topology. Our analysis illustrates mechanisms formerly only known from textbooks and observed indirectly in experiment. It is the first time that such a rich set of fundamental phenomena has been seen in a single computer simulation.
Monazite (LaPO4) was indented at room temperature. Deformation twin boundaries and stacking faults were characterized by high resolution transmission electron microscopy. Kinked deformation twins were also characterized and analyzed. Three types of stacking faults associated with climb-dissociated partial dislocations were observed. Two were found on twin boundaries, and a third in the lattice. Formation mechanisms are discussed. The superimposition of stacking faults along twin boundaries during deformation twinning and the glide of climb-dissociated partial dislocations allowed by stacking fault migration are discussed. The possible relationship between the formation mechanisms for these defects and the low- temperature recrystallization and self-annealing of defects in monazite is considered.
In this work, a low level birefringence detection system was employed to study the stress distribution in Si substrate induced by thermally grown ultra-thin SiO2 film. According to traditional bi-metallic strip theory, it is expected that the stress should show a linear dependence on depth with the zero stress plane located at the position of two third of the thickness of the substrate from the SiO2/Si interface. The linear dependence of stress on depth in accordance with the bi-metallic strip theory was observed only in part of the substrate. For the region below the SiO2/Si interface extending to a depth of about 1/5 of the thickness of the substrate, the magnitude of the stress was found to be significantly smaller than expected. The position of the zero stress plane was found to depend on the thickness of the SiO2 film and the oxidation conditions. The zero stress plane seemed to move towards the bottom of the Si substrate as the thickness of the SiO2 film became thinner and no zero stress plane was observed in the Si substrate when the SiO2 films became sufficiently thin.
W incorporated diamond-like carbon (W-DLC) films were deposited on silicon (100) wafers by a hybrid deposition method combining ion beam deposition of carbon with DC magnetron sputtering of tungsten. During the films deposition, a wide range of negative bias voltage from 0 to -600 V was applied. W concentration in the film could be controlled by varying the Ar/C6H6 ratio in the supplying gas. In the present experimental condition, WC1−x nano-sized particles were not observed in the amorphous carbon matrix. Regardless of the W concentration in the film, it was found that the G-peak position of the Raman spectra had a lowest value at a bias voltage of - 200 V, which represents the highest sp3 bond fraction in the film. The highest residual stress, hardness and Young's modulus were also observed when the bias voltage was -200 V. This result shows that the mechanical properties of W-DLC films were mainly dependent on the atomic bond structure of carbon. On the other hand, the electrical resistivity significantly decreased by the W incorporation.
This study deals with the long-term reliability of a high precision pressure sensor using bellows mainly made of electroplated Ni. Thermomechanical properties of this deposit are obtained by several experiments and compared to theoretical models, computations and other authors' results. Bellows are expected to stay in service for many decades, thus their high cycle fatigue behavior has to be known. Stress-life fatigue curve for crack initiation and fatigue crack growth in the electroplated Ni are measured and identified using numerical computations. Results are compared with other results obtained on similar Ni electrodeposits. Normalized stress-life fatigue curve shows no specific nanosize effects.
Equal-channel angular pressing (ECAP) processed ultrafine grained (UFG) and coarse grained (CG) 7075 Al alloys were treated by natural aging and T651 temper (annealed at 120 °C for 48 h in Ar atmosphere), respectively. Mechanical tests showed that for the UFG sample, the natural aging resulted in the highest strength (the ultimate tensile strength is 720 MPa). In contrast, for the CG sample, the T651 treatment resulted in a higher strength (the ultimate strength is 590 MPa) than the natural aging (530 MPa). Microstructural analyses indicated that the enhanced strength of the T651 treated CG sample was mainly caused by high densities of G- P zones and metastable η' precipitates. The enhanced strength of the naturally aged UFG sample was mainly caused by the high densities of G-P zones and dislocations. Upon T651 treatment, the dislocation density of the UFG sample deceased significantly, overcompensating the precipitation strengthening.
Metallic polycrystalline thin films are used in a plethora of applications, including especially metallic interconnects for the microelectronics industry. Despite the numerous approaches at modeling and simulating processing of these films, there have not been any simulation tools, which can accurately predict texture and microstructure evolution in these films. We have developed a novel 2D model called FACET for the simulation of polycrystalline thin film growth at realistic spatial and time scales. The basic idea is to use the results of smaller-scale atomic simulations (density functional theory, molecular dynamics, and lattice Monte Carlo) to provide input and guidance on the evolution of grain structure and texture on a micron scale. The feature scale model is based on describing grains in terms of two-dimensional faceted surfaces and grain boundaries. The model includes the major phenomena involved in film growth, including deposition, nucleation, surface diffusion (on the substrate and on the growing film), inter-facet diffusion, and grain growth and coarsening. In addition, the texture of each grain is treated individually, so that the texture evolution of the system can be simulated. Predictions of the FACET code are compared with previous experimental studies of texture and microstructure in Silver films.
In this paper, the results of experiments on irradiation of the singlewall (SWNT) and multiwall carbon nanotube (MWNT) samples by argon ions are presented. They were obtained by reflection electron energy loss spectroscopy and Auger electron spectroscopy.
Results indicate the π-plasmon energy Eπ and the full width at half maximum (FWHM) of the plasma peak were sensitive to the dose of ion irradiation. In particular, the π-plasmon energy Eπ decreases and the plasma peak broadens with the increase of the dose of Ar+irradiation.
The π-plasmon peak broadening is associated with damage of carbon nanotubes under ion irradiation. Possible causes of the carbon nanotubes deformation and influence of deformation on π-band structure of carbon nanotubes are discussed.