Showing papers in "Journal of Materials Research in 2007"
TL;DR: In this article, the authors review the processing, microstructure development, and resulting mechanical properties of Zr-, Ti-, Cu-, Mg-, Fe-, and Ni-based glassy alloys and also consider the superiority of composite materials containing different phases for enhancing the strength, ductility, and toughness.
Abstract: The development of bulk metallic glasses and composites for improving the mechanical properties has occurred with the discovery of many ductile metallic glasses and glass matrix composites with second phase dispersions with different length scales. This article reviews the processing, microstructure development, and resulting mechanical properties of Zr-, Ti-, Cu-, Mg-, Fe-, and Ni-based glassy alloys and also considers the superiority of composite materials containing different phases for enhancing the strength, ductility, and toughness, even leading to a “work-hardening-like” behavior. The morphology, shape, and length scale of the second phase dispersions are crucial for the delocalization of shear bands. The article concludes with some comments regarding future directions of the investigations of spatially inhomogeneous metallic glasses.
373 citations
TL;DR: In this paper, the transport properties of yttrium-doped barium zirconate (BYZ) have been explored, with the aim of attaining reproducible proton conductivity in well-densified samples.
Abstract: The factors governing the transport properties of yttrium-doped barium zirconate (BYZ) have been explored, with the aim of attaining reproducible proton conductivity in well-densified samples. It was found that a small initial particle size (50–100 nm) and high-temperature sintering (1600 °C) in the presence of excess barium were essential. By this procedure, BaZr0.8Y0.2O3−δ with 93% to 99% theoretical density and total (bulk plus grain boundary) conductivity of 7.9 × 10−3 S/cm at 600 °C [as measured by alternating current (ac) impedance spectroscopy under humidified nitrogen] could be reliably prepared. Samples sintered in the absence of excess barium displayed yttria-like precipitates and a bulk conductivity that was reduced by more than 2 orders of magnitude.
353 citations
TL;DR: In this article, a review of corrosion performance of a number of alloy systems documents several metallic glasses with corrosion resistance superior to that of crystalline metals, but the metallic glasses do not have superior corrosion resistance.
Abstract: The review of corrosion performance of a number of alloy systems documents several metallic glasses with corrosion resistance superior to that of crystalline metals. In other cases, the metallic glasses do not have superior corrosion resistance. The nature of corrosion resistance of the metallic glasses is often directly related to the development of a passive film (protective layer) on the reactive alloy substrate, increased durability of the passive film, or enhanced resistance to localized corrosion where the passive film is broken or damaged. Potential mechanical/environmental degradation processes include stress-corrosion cracking, corrosion fatigue, various forms of hydrogen damage, wear, and abrasion. The availability of bulk metallic glasses in significant three-dimensional sizes will stimulate important work in these areas that will enhance the fundamental understanding of the corrosion behavior and mechanical interactions and develop design guidelines and materials properties database for designers and engineers.
260 citations
TL;DR: In this article, the authors used a variety of techniques to assess changes in the grain size, chemical ordering, grainboundary segregation, and grain-boundary structure during the heat treatment of Ni-W specimens synthesized over a wide range of grain sizes from 3 to 70 nm.
Abstract: Nanocrystalline alloys often show exceptional thermal stability as a consequence of kinetic and thermodynamic impediments to grain growth. However, evaluating the various contributions to stability requires detailed investigation of the solute distribution, which is challenging within the fine structural-length-scales of nanocrystalline materials. In the present work, we use a variety of techniques to assess changes in the grain size, chemical ordering, grain-boundary segregation, and grain-boundary structure during the heat treatment of Ni–W specimens synthesized over a wide range of grain sizes from 3 to 70 nm. A schematic microstructural evolution map is also developed based on analytical models of the various processes activated during annealing, highlighting the effects of alloying in nanocrystalline materials.
201 citations
TL;DR: In this article, the degradation rate after immersion in simulated body fluids (SBF) for seven days was calculated from both the weight loss and hydrogen evolution methods, and the corresponding electrode controlled processes and evolution of the corrosion products layer were discussed.
Abstract: Fast degradation rates in the physiological environment constitute the main limitation for magnesium alloys used in biodegradable hard tissue implants. In this work, the corrosion behavior of AZ91 magnesium alloy in simulated body fluids (SBF) was systematically investigated to determine its performance in a physiological environment. The influence of the main constituent phases on the corrosion behavior was studied by in situ visual observation and scanning electron microscopy. Energy dispersive x-ray spectrometry and Fourier transfer infrared spectroscopy revealed that both calcium and magnesium phosphates are present in the corroded products besides magnesium oxide. Electrochemical methods including open circuit potential evolution and electrochemical impedance spectroscopy were used to investigate the mechanism. The corresponding electrode controlled processes and evolution of the corrosion products layer were discussed. The degradation rate after immersion in SBF for seven days was calculated from both the weight loss and hydrogen evolution methods.
198 citations
TL;DR: The photophysical properties of MSnO3 including optical absorption, photoluminescence, and energy band structure including band edge positions were investigated experimentally and theoretically in association with their photocatalytic properties as discussed by the authors.
Abstract: The photophysical properties of MSnO3 (M = Ca, Sr, and Ba) including optical absorption, photoluminescence, and energy band structure including band edge positions were investigated experimentally and theoretically in association with their photocatalytic properties. Photocatalytic reactions for H2 and O2 evolution in the case of sacrificial reagents were performed under ultraviolet (UV) light irradiation. The order of the activities of H2 evolution was CaSnO3 > SrSnO3 > BaSnO3, agreeing not only with that of the conduction-band edges (or band gaps) but also with that of the transferred excitation energy, while that of O2 evolution was CaSnO3 < SrSnO3 < BaSnO3, consistent with that of the angle of the Sn–O–Sn bonds as well as the delocalization of excited energy. When loaded with RuO2 cocatalyst, both CaSnO3 and SrSnO3 can efficiently split pure water into hydrogen and oxygen in a stoichiometric ratio under UV light irradiation. In addition, RuO2-loaded SrSnO3 showed higher water splitting activity than RuO2-loaded CaSnO3 did. This is attributed to the suitable conduction and valence band edges and to high mobility of the photogenerated charge carriers caused by the proper distortion of SnO6 connection in SrSnO3. The RuO2-loaded BaSnO3 photocatalyst cannot split pure water, which might be because of a high concentration of defect centers such as Sn2+ ions and the probability of radiative recombination in BaSnO3.
186 citations
TL;DR: Poon et al. as mentioned in this paper investigated compositional effects on plasticity and elastic properties in amorphous steels and found that the relationship between the elastic moduli of the alloys and those of the alloying elements must be considered in designing ductile Fe-based BMGs.
Abstract: Iron-based bulk metallic glasses (BMGs) are characterized by high fracture strengths and elastic moduli, with some exhibiting fracture strengths near 4 GPa, 2–3 times those of conventional high-strength steels. Among the Fe-based BMGs, the non-ferromagnetic ones, designated “non-ferromagnetic amorphous steel alloys” by two of the present authors [S.J. Poon et al.: Appl. Phys. Lett. 83, 1131 (2003)], have glass-forming ability high enough to form single-phase glassy rods with diameters reaching 16 mm. Fe-based BMGs designed for structural applications must exhibit some plasticity under compression. However, the role of alloy composition on plastic and brittle failures in metallic glasses is largely unknown. In view of a recently observed correlation that exists between plasticity and Poisson’s ratio for BMGs, compositional effects on plasticity and elastic properties in amorphous steels were investigated. For the new amorphous steels, fracture strengths as high as 4.4 GPa and plastic strains reaching ∼0.8% were measured. Plastic failure instead of brittle failure was observed as the Poisson’s ratio approached 0.32 from below. Investigation of the relationship between the elastic moduli of the alloys and those of the alloying elements revealed that interatomic interactions in addition to the elastic moduli of the alloying elements must be considered in designing ductile Fe-based BMGs. The prospects for attaining high fracture toughness in Fe-based BMGs are discussed in this article.
167 citations
TL;DR: In this paper, the properties of high-purity nano-polycrystalline diamonds synthesized by direct conversion from graphite and various non-graphitic carbons under static high pressures and high temperatures were investigated by microindentation testing with a Knoop indenter and observation of microstructures around the indentations.
Abstract: Mechanical properties of high-purity nano-polycrystalline diamonds synthesized by direct conversion from graphite and various non-graphitic carbons under static high pressures and high temperatures were investigated by microindentation testing with a Knoop indenter and observation of microstructures around the indentations. Results of indentation hardness tests using a superhard synthetic diamond Knoop indenter showed that the polycrystalline diamond synthesized from graphite at ⩾15 GPa and 2300–2500 °C (consisting of fine grains 10–30 nm in size and layered crystals) has very high Knoop hardness (Hk ⩾ 110 GPa), whereas the hardness of polycrystalline diamonds synthesized from non-graphitic carbons at ⩾15 GPa and below 2000 °C (consisting only of single-nano grains 5–10 nm in size) are significantly lower (Hk = 70 to 90 GPa). Microstructure observations beneath the indentations of these nano-polycrystalline diamonds suggest that the existence of a lamellar structure and the bonding strength of the grain boundary play important roles in controlling the hardness of the polycrystalline diamond.
159 citations
TL;DR: In this article, a method for in situ scanning electron microscope (SEM) micro-compression tests is presented, which allows for very efficient positioning and assessment of the failure mechanism.
Abstract: A novel method for in situ scanning electron microscope (SEM) micro-compression tests is presented. The direct SEM observation during the instrumented compression testing allows for very efficient positioning and assessment of the failure mechanism. Compression tests on micromachined Si pillars with volumes down to 2 μm3 are performed inside the SEM, and the results demonstrate the potential of the method. In situ observation shows that small diameter pillars tend to buckle while larger ones tend to crack before failure. Compressive strength increases with decreasing pillar diameter and reaches almost 9 GPa for submicrometer diameter pillars. This result is in agreement with earlier bending experiments on Si. Difficulties associated with precise strain measurements are discussed.
128 citations
TL;DR: In this article, the authors investigated the corrosion susceptibility of surgical AZ91 magnesium alloys in simulated body fluids (SBFs) consisting of bovine serum albumin (BSA) and acidic SBFs (pH 5) using electrochemical methods.
Abstract: The objective of this study is to investigate the corrosion susceptibility of surgical AZ91 magnesium alloys in simulated body fluids (SBFs) consisting of bovine serum albumin (BSA) and acidic SBFs (pH 5) using electrochemical methods. The addition of BSA significantly moves the open-circuit potential toward a more positive value and suppresses the corrosion reaction. The corrosion resistance under the open-circuit conditions in the SBFs with 1 g/L BSA is approximately twice that in the SBFs. A higher BSA concentration decreases the corrosion susceptibility. In addition, the acidic SBF results in a higher alloy dissolution rate. The possible mechanisms are discussed.
128 citations
TL;DR: In this paper, the glass transition temperature Tg within 10 K of the crystallization temperature at 40 K/min was studied by differential scanning calorimetry, which depends on the thermal history of the sample.
Abstract: Sputtered amorphous Ge4Sb1Te5, Ge1Sb2Te4, Ge2Sb2Te5, and Ag0.055In0.065Sb0.59Te0.29 thin films were studied by differential scanning calorimetry. Upon continuous heating, heat release due to structural relaxation of the amorphous phase between 0.5 and 1.0 kJ/mol was observed. This value depends on the thermal history of the sample. Preannealing of the amorphous phase revealed the glass transition temperature Tg within 10 K of the crystallization temperature upon continuous heating at 40 K/min.
TL;DR: In this paper, the photovoltaic response of colloidal, infrared-sensitive, PbSe nanocrystals (NCs) of various sizes and conjugated polymers of either regioregular poly (3-hexylthiophene) (RR-P3HT) or poly- (2-methoxy-5-(2-ethylhexoxy)-1,4-phenylene vinylene) (MEH-PPV).
Abstract: We investigated the photovoltaic response of nanocomposites made of colloidal, infrared-sensitive, PbSe nanocrystals (NCs) of various sizes and conjugated polymers of either regioregular poly (3-hexylthiophene) (RR-P3HT) or poly- (2-methoxy-5-(2-ethylhexoxy)-1,4-phenylene vinylene) (MEH-PPV). The conduction and valence energy levels of PbSe NCs were determined by cyclic voltammetry and revealed type II heterojunction alignment with respect to energy levels in RR-P3HT for smaller NC sizes. Devices composed of NCs and RR-P3HT show good diode characteristics and sizable photovoltaic response in a spectral range from the ultraviolet to the infrared. Using these materials, we have observed photovoltaic response at wavelengths as far to the infrared as 2 μm (0.6 eV), which is desirable due to potential benefits of carrier multiplication (or multi-exciton generation) from a single junction photovoltaic. Under reverse bias, the devices also exhibit good photodiode responses over the same spectral region.
TL;DR: In this article, high-resolution electron microscopy was used to study the local crystal structure and characteristic defects of bis(triisopropylsilylethynyl) (TIPS) polycrystalline films.
Abstract: As a modification to the insoluble and herringbone-structured pentacene, bis(triisopropylsilylethynyl) (TIPS) pentacene has two bulky side groups, leading to good solubility in common organic solvents and regular π–π stacking arrangements in the crystalline state. Solution processing of TIPS–pentacene thin films was investigated as a function of various process parameters in this work. Electron diffraction results suggested that TIPS–pentacene molecules tended to align with the acene unit edge on to the substrate, touching down with their bulky side groups. In a TIPS–pentacene polycrystalline film, the long axis of individual crystallite is [2 1 0], while the shorter axis is [1 2 0]. High-resolution electron microscopy was used to study the local crystal structure and characteristic defects of TIPS–pentacene thin films. Due to the nonaromatic side groups, TIPS–pentacene was found to be significantly more sensitive to the electron beam (critical dose Jc = 0.05 C/cm2 at 300 kV) than pentacene itself (Jc = 0.2 C/cm2 at 100 kV).
TL;DR: In this paper, high-density uniform silver nanoparticles were prepared by reducing silver diamine ions with D-glucose in the presence of a stabilizing agent, along with the nature of the dispersing agent, the pH and temperature of the reaction had the most pronounced effect on the reduction rate, the nucleation of the metallic phase, and ultimately the size and dispersion of the resulting particles.
Abstract: Highly dispersed uniform silver nanoparticles were prepared by reducing silver diamine ions [Ag(NH3)2]+ with D-glucose in the presence of a stabilizing agent. Along with the nature of the dispersing agent, the pH and the temperature of the reaction had the most pronounced effect on the reduction rate, the nucleation of the metallic phase, and ultimately the size and dispersion of the resulting particles. Through suitable manipulations of these parameters, it was possible to prepare uniform Ag nanoparticles ranging in size from 30 to 120 nm. A rapid and complete reduction of the silver species was possible only at elevated pH and temperatures above 50 °C. The reduction of silver diamine ions in these conditions caused the complete cleavage of the C–C bond, resulting in the release of 12 electrons per molecule of D-glucose. It was also found that the addition of ammonia to an already acidified silver nitrate solution leads to the formation of a much more stable and safe-to-handle diamine complex.
TL;DR: In this article, the origin of the plasticity in amorphous alloys considering characteristics of the inherent atomic-scale structure as defined by short-range ordered (SRO) clusters is examined.
Abstract: Unlike the dislocation-based plasticity in crystalline metals, which can be readily explained by their crystal structure and the presence of defects, the nature of the plasticity in amorphous alloys is not completely understood. Experiments have shown that the plasticity in amorphous alloys is strongly dependent on their atomic packing density. This study, based on the combination of experimental and computational techniques, examines the origin of the plasticity in amorphous alloys considering characteristics of the inherent atomic-scale structure as defined by short-range ordered (SRO) clusters. The role of various SRO atomic clusters in creating free volume during shear deformation is discussed. We report that the plasticity exhibited by amorphous alloys is very sensitive to the characteristics of the atomic packing state, which can be described by various SRO atomic structures and quantified by the effective activation energy for crystallization.
TL;DR: A new Ti-based bulk-metallic glassy (BMG) alloy without Ni was developed in various forms such as melt-spun ribbon and cylindrical rods as discussed by the authors.
Abstract: A new Ti-based bulk-metallic glassy (BMG) alloy without Ni was developed in various forms such as melt-spun ribbon and cylindrical rods. Ti metal and Ti-based alloys are well known as biomaterials because Ti has good biocompatibility with the human body. We examined mechanical and chemical properties of a newly developed Ti-based BMG alloy in comparison with pure Ti metal and Ti–6Al–4V alloy, which are used for biomaterials. The new Ti-based BMG (Ti45Zr10Pd10Cu31Sn4) alloy does not contain Ni, Al, and Be elements, which are known to be toxic. The Ti45Zr10Pd10Cu31Sn4 BMG alloy rod with a diameter of 3 mm, which is produced by copper mold casting, exhibits a compressive strength of 1970 MPa and a Young’s modulus of 95 GPa. In addition, the Ti45Zr10Pd10Cu31Sn4 BMG alloy shows a supercooled liquid region of 56 K and a reduced glass-transition temperature, Trg(=Tg/Tl), of 0.56. The high thermal stability of supercooled liquid has enabled the fabrication of a cylindrical rod specimen with a diameter of 4 mm. This alloy exhibits precipitation of a primary nanoscale icosahedral phase upon devitrification followed by the formation of a metastable unidentified phase. Ti2Cu and Ti3Sn are stable phases formed in this alloy. The Ti45Zr10Pd10Cu31Sn4 BMG alloy has a high corrosion resistance and is passivated at a lower passive current density of approximately 10−2 A/m2 compared to those of pure titanium and the Ti–6Al–4V alloy in 1 mass% lactic acid and phosphate-buffered saline solutions at 310 K.
TL;DR: In this paper, the minimum shear offsets necessary to detect local heating were determined and it was shown that the dependence of heat content on offset is consistent with frictional heating in the band and the effective stress on the band undergoing shear is 50-70% of the macroscopic shear stress.
Abstract: Under ambient conditions, plastic flow in metallic glasses is sharply localized into shear bands The heat content of, and consequent temperature rise at, shear bands in three bulk metallic glasses are compared using a recently reported fusible coating method The minimum shear offsets necessary to detect local heating are determined It is shown that the dependence of heat content on offset is consistent with frictional heating in the band The effective stress on the band undergoing shear is 50-70% of the macroscopic shear stress, a ratio compared with simulations of shear-band initiation and operation It is also noted that frictional heating can occur not only at shear bands, but also at mixed-mode cracks
TL;DR: Ce3+-activated lanthanide silicon oxynitride (La5Si3O12N, La4SiO7N2, LaSiO2N, and La3Si8O4N11) phosphors were prepared by firing the mixture of La2O3, Si3N4, SiO2, and CeO2 at 1500-1600 °C under a 0.5 MPa nitrogen atmosphere.
Abstract: Ce3+-activated lanthanide silicon oxynitride (La5Si3O12N, La4SiO7N2, LaSiO2N, and La3Si8O4N11) phosphors were prepared by firing the mixture of La2O3, Si3N4, SiO2, and CeO2 at 1500–1600 °C under a 0.5 MPa nitrogen atmosphere. Diffuse reflection spectrum, photoluminescence spectra, and temperature-dependent luminescence of these phosphors are presented in this work. A blue emission of Ce3+ in all lanthanide silicon oxynitrides was observed under ultraviolet irradiation, which is strongest in La3Si8O4N11:Ce3+. The concentration quenching and thermal quenching of the samples were discussed.
TL;DR: In this article, the onset of sintering, extent of densification, and final grain size can be controlled by either the size of the initial nanosilver particles or the binder systems with different burnout characteristics.
Abstract: Control of the low-temperature sintering of nanosilver particles was attained by dispersing and stabilizing nanosilver particles into a paste form using the selected organic binder systems. As demonstrated by scanning electron microscopy (SEM) and thermogravimetric analysis (TGA), with the existing binder systems, undesirable premature coalescence of nanosilver particles was prevented and the metastable structure was retained until the binder burned out at relatively higher temperatures. Enhanced densification was achieved upon the binder burnout because at the relatively higher temperatures the densification mechanisms, e.g., grain-boundary or lattice diffusion, become more dominant. We propose that the onset of sintering, extent of densification, and final grain size can be controlled by either the size of the initial nanosilver particles or the binder systems with different burnout characteristics.
TL;DR: An iron-based amorphous metal, Fe49.7Cr17.7Mn1.9Mo7.4W1.6B15.2C3.8Si2.4 (SAM2X5), with very good corrosion resistance has been developed as discussed by the authors.
Abstract: An iron-based amorphous metal, Fe49.7Cr17.7Mn1.9Mo7.4W1.6B15.2C3.8Si2.4 (SAM2X5), with very good corrosion resistance has been developed. This material was prepared as a melt-spun ribbon, as well as gas atomized powder and a thermal-spray coating. During electrochemical testing in several environments, including seawater at 90 °C, the passive film stability was found to be comparable to that of high-performance nickel-based alloys and superior to that of stainless steels, based on electrochemical measurements of the passive film breakdown potential and general corrosion rates. This material also performed very well in standard salt fog tests. Chromium (Cr), molybdenum (Mo), and tungsten (W) provided corrosion resistance, and boron (B) enabled glass formation. The high boron content of this particular amorphous metal made it an effective neutron absorber and suitable for criticality control applications. This material and its parent alloy maintained corrosion resistance up to the glass transition temperature and remained in the amorphous state during exposure to relatively high neutron doses.
TL;DR: The morphology, structure, and nucleation mechanisms of OPBs in epitaxial films of layered complex oxides are presented with a review of published studies and new work.
Abstract: Out-of-phase boundaries (OPBs) are translation boundary defects characterized by a misregistry of a fraction of a unit cell dimension in neighboring regions of a crystal. Although rarely observed in the bulk, they are common in epitaxial films of complex crystals due to the physical constraint of the underlying substrate and a low degree of structural rearrangement during growth. OPBs can strongly affect properties, but no extensive studies of them are available. The morphology, structure, and nucleation mechanisms of OPBs in epitaxial films of layered complex oxides are presented with a review of published studies and new work. Morphological trends in two families of layered oxide phases are described. The atomic structure at OPBs is presented. OPBs may be introduced into a film during growth via the primary mechanisms that occur at film nucleation (steric, nucleation layer, a-b misfit, and inclined-c misfit) or after growth via the secondary nucleation mechanism (crystallographic shear in response to loss of a volatile component). Mechanism descriptions are accompanied by experimental examples. Alternative methods to the direct imaging of OPBs are also presented.
TL;DR: In this paper, the lead-free piezoelectric ceramic solution [Na 0.5K0.5NbO3] was produced using a mixed-oxide process.
Abstract: Powders of the solid lead-free piezoelectric ceramic solution [Na0.5K0.5NbO3]1−x–[LiTaO3]x, x = 0.06, were produced using a mixed-oxide process. Phase analysis indicated the formation of an orthorhombic solid solution at 800 °C, which coexisted with intermediate binary niobate and tantalate phases. A tetragonal main-phase solid solution was formed at ⩾950 °C, along with minor quantities of a tungsten bronze phase. Addition of 3 wt% excess alkali carbonates to the starting powders allowed the orthorhombic solid solution to be retained to 1100 °C and prevented formation of the secondary tungsten bronze phase. Elemental chemical analysis confirmed changes in alkali oxide composition, consistent with volatilization losses, particularly of potassium and lithium oxides. Phase stability near the reported morphotropic phase boundary is shown to be sensitive to alkali oxide content.
TL;DR: In this paper, an experimental study of the elastic and plastic properties of single crystals of sucrose was carried out using nanoindentation, which can be considered to be a model material for both pharmaceutical excipients and explosives.
Abstract: An experimental study of the elastic and plastic properties of sucrose single crystals, which can be considered to be a model material for both pharmaceutical excipients and explosives, has been carried out using nanoindentation. Instrumented indentation was used to characterize the properties of both habit and cleavage planes on the (100) and (001) orientations; the elastic modulus on the (100) is 38 GPa, while the modulus on the (001) is 33 GPa. The hardness of sucrose is approximately 1.5 GPa. Nanoindentation enables assessment of the onset of plastic deformation on cleaved surfaces, and a maximum shear stress of 1 GPa can be supported prior to plastic deformation. The deformation in this material is crystallographically dependent, with pileup around residual indentation impressions showing evidence of preferential slip system activity.
TL;DR: In this paper, the ductility of bulk metallic glass (BMG) alloy was investigated at different cooling rates and the effect of cooling rate on the BMG alloy was interpreted in terms of the variation in amorphous nature and free volume of the as-cast materials.
Abstract: Compressive deformation was experimentally investigated for Ti41.5Cu42.5Zr2.5Hf5Ni7.5Si1 bulk metallic glass (BMG) fabricated at different cooling rates. It was found that the ductility of the BMG alloy increased with increasing of the cooling rate in solidification. The alloy with a monolithic amorphous structure exhibited a large ductility, up to 12%. The effect of cooling rate on the ductility of the BMG alloy is interpreted in terms of the variation in amorphous nature and free volume of the as-cast materials.
TL;DR: In this paper, an elasto-plastic extension of the higher order elasticity model was proposed to provide a micromechanically motivated model for the indentation size effect in polymers.
Abstract: Similar to metals, the hardness of many polymers increases with decreasing indentation depths at depth ranges from several microns down to several nanometers. While for metals such phenomena are commonly attributed to geometrically necessary dislocation densities, such an explanation cannot be applied to polymers. To provide a micromechanically motivated model for the indentation size effect in polymers, here we propose an elasto-plastic extension of the higher order elasticity model recently developed by the authors. In this model, size effects in polymers (as well as in nematic liquid crystals) are related to Frank elasticity arising from bending distortions of the polymer chains and their interactions. On the basis of this theory, we derive a simple model for indentation size effects in polymers. Unlike other models, our model includes only elastic size effects due to rotational gradients. It is shown that the proposed model can explain the experimentally observed size effects in polymers. Together with the existing experimental data mentioned here, new experimental data for silicon rubber are also presented and discussed.
TL;DR: In this article, the sintering temperature of elemental W powders was reported to be reduced from the conventional temperature of ≥2500 °C to the modest temperature range of 1700-1790 °C by making the W powder nanostructured through high-energy mechanical milling (MM) prior to sinter.
Abstract: The present investigation reports for the first time a dramatic decrease in the sintering temperature of elemental W from the conventional temperature of ≥2500 °C to the modest temperature range of 1700–1790 °C by making the W powder nanostructured through high-energy mechanical milling (MM) prior to sintering. The crystallite size of the initial W powder charge with a particle size of 3–4 μm could be brought down to 8 nm by MM for 5 h in WC grinding media. Further milling resulted in a high level of WC contamination, which apparently was due to work hardening and the grain refinement of W. A sintered density as high as 97.4% was achieved by sintering cold, isostatically pressed nanocrystalline (8 nm) W powder at 1790 °C for 900 min. The microstructure of the sintered rods showed the presence of deformation bands, but no cracks, within a large number of W grains. The mechanical properties, when compared with the hardness and elastic modulus, of the sintered nano-W specimen were somewhat superior to those reported for the conventional sintered W.
TL;DR: In this article, a simple operating map for nanoparticle synthesis is presented based on the experimental results and theoretical model of surface free energy and undercooling as a function of evaporation temperature and inert gas pressure, particle formation is analyzed.
Abstract: Silver nanoparticles were synthesized by an inert gas condensation method using flowing helium in the process chamber. Nucleation, growth mechanism, and the kinetics of nanoparticle formation in vapor phase are studied. Effect of process parameters, such as evaporation temperature and inert gas pressure, on the particle crystallinity, morphology, and size distribution are examined. Particles were synthesized at evaporation temperatures of 1123, 1273, and 1423 K and at helium pressures of 0.5, 1, 5, 50, and 100 Torr. Synthesized silver nanoparticles were characterized by x-ray diffraction (XRD) and transmission electron microscopy (TEM). The particle size ranged from 9 to 32 nm, depending on the growth conditions. At lower evaporation temperature and inert gas pressure, smaller particles with spherical shape showing less agglomeration are formed. Based on the experimental results and theoretical model of surface free energy and undercooling as a function of evaporation temperature and inert gas pressure, particle formation is analyzed. A simple operating map for nanoparticle synthesis is presented. The theoretical model is well supported by the experimental data.
TL;DR: In this article, the effect of Zn on the drop resistance of Sn-3.5Ag solder was investigated using scanning electron microscopy, energy dispersive x-ray, and xray diffraction, which were subsequently correlated to the results of microhardness and drop tests.
Abstract: Varying amounts of Zn (1, 3, and 7 wt%) were added to Sn–3.5Ag solder on a Cu pad, and the resultant solder joint microstructures after a reflow and isothermal aging (150 °C, up to 500 h) were investigated using scanning electron microscopy, energy dispersive x-ray, and x-ray diffraction, which were subsequently correlated to the results of microhardness and drop tests. Zinc was effective in improving the drop resistance of Sn–3.5Ag solder on the Cu pad, and an addition of 3 wt% Zn nearly doubled the number of drops-to-failure (Nf). The beneficial role of Zn was ascribed to suppression of Cu6Sn5 and precipitation of Zn-containing intermetallic compounds (IMCs). However, the Zn effect was reduced as Cu6Sn5 and Ag3Sn precipitated in a joint IMC layer after prolonged aging. The interface between Ag5Zn8 and Cu5Zn8 was resistant to drop impact, but two other layered IMC structures of Cu6Sn5/Cu3Sn and Cu5Zn8/Cu6Sn5 were not.
TL;DR: In this paper, a method of severe plastic deformation for the creation of bulk nanostructured materials is presented, which combines inherent advantages afforded by large strain deformation in chip formation by machining, with simultaneous dimensional control of extrusion in a single step of deformation.
Abstract: Large strain extrusion machining (LSEM) is presented as a method of severe plastic deformation for the creation of bulk nanostructured materials. This method combines inherent advantages afforded by large strain deformation in chip formation by machining, with simultaneous dimensional control of extrusion in a single step of deformation. Bulk nanostructured materials in the form of foils, plates, and bars of controlled dimensions are shown to result by appropriately controlling the geometric parameters of the deformation in large strain extrusion machining.
TL;DR: In this article, the magnetic phase diagrams of RMnO3 (R = Er, Yb, Tm, Ho) were investigated up to 14 T via magnetic and dielectric measurements.
Abstract: The magnetic phase diagrams of RMnO3 (R = Er, Yb, Tm, Ho) are investigated up to 14 T via magnetic and dielectric measurements. The stability range of the atomic force microscopy order below the Neel temperature of the studied RMnO3 extends to far higher magnetic fields than previously assumed. Magnetic irreversibility indicating the presence of a spontaneous magnetic moment is found near 50 K for R = Er, Yb, and Tm. At very low temperatures and low magnetic fields, the phase boundary defined by the ordering of the rare-earth moments is resolved. The sizable dielectric anomalies observed along all phase boundaries are evidence for strong spin-lattice coupling in the hexagonal RMnO3. In HoMnO3, the strong magnetoelastic distortions are investigated in more detail via magnetostriction experiments up to 14 T. The results are discussed based on existing data on magnetic symmetries and the interactions among the Mn-spins, the rare-earth moments, and the lattice.