bluuebunny/crossref_metadata_2025_split huggingface dataset:
Additivity of Direct and Indirect Strengthening in TiB2-Reinforced Near-γ Titanium Aluminides
Authors: Kampe S. L. , Sadler P. , Christodoulou L. , Larsen D. E. 
Published: April 2011

AbstractAn experimentally-based description of strengthening in a series of TiB2-reinforced near-y titanium aluminides is presented. The analysis provides a means to examine the strengthening contributions which are individually attributable to compositional variants, to indirect sources which are present due to the influence of the particulate on the underlying microstructure, and to those directly evolving from the presence of the reinforcement and its interaction with dislocations. Once the indirect strengthening contributions are isolated and arithmetically removed from the magnitude of the experimentally determined composite strength, the resulting direct strengthening increments are found to obey a linear relationship with the inverse-squareroot of the edge-to-edge interparticle spacing. The description of the dependence on spacing is found to be compatible with other studies where similar relationships have been reported.
Ductile Phase Toughening of MoSi2: Effect of Reinforcement Morphology
Authors: Chen Long-Ching , Bahtishi Nedhal , Lederich Richard , Soboyejo Wolé 
Published: April 2011

AbstractThe effects of ductile reinforcement morphology on the mechanical behavior of MoSi2 reinforced with 20 vol.% Nb are presented. While all the composites exhibit improved fracture toughness relative to the monolithic MoSi2, the most significant improvement is obtained in the composite with laminate reinforcement, followed by coarse fiber, fine fiber and particulate reinforcements. Bend strength measurements indicate a ductile-brittle-transition-temperature of ∼1250°C, and highlight the attractive properties of laminate reinforced composite. Preliminary studies of room temperature fatigue crack growth in particulate reinforced composites show a stable fatigue crack growth, which is not achieved in monolithic MoSi2. Possible ways of achieving a better combination of mechanical properties are also discussed.
Thermal Fatigue of MoSi2 Particulate and Short Fiber Composites
Authors: Kush M. T. , Holmes J. W. , Gibala R. 
Published: April 2011

AbstractInduction heating of disk shaped specimens was used to compare and contrast the thermal fatigue behavior of MoSi2 and MoSi2-based composites. Specimens were subjected to 5 s heating and cooling cycles between temperature limits of 700°C and 1200°C. The monolithic material and a MoSi2- 10 vol% TiC composite exhibited poor thermal shock resistance and could not be thermally cycled according to this temperature-time profile. A 30 vol% TiC composite exhibited much better thermal shock and thermal fatigue resistance as compared to the monolithic material, but exhibited undesirable oxidation. MoSi2-10 and 30 vol% SiC particulate composites exhibited excellent thermal shock and thermal fatigue resistance compared to that of the monolithic material. A MoSi2-10 vol% SiC whisker composite did not show improved thermal fatigue resistance due to the initial processing defects present in the material. The monolithic material and the 10 vol% TiC composite were also subjected to 30 s heating and cooling cycles between temperature limits of 700°C and 1200°C. Both of these materials exhibited better thermal fatigue resistance at this temperature-time profile, but the 10 vol% TiC composite also exhibited undesirable oxidation. The fatigue results are discussed with reference to the initial microstructure of the specimens and the stress-strain history of the specimens which was obtained by a thermoelastic finite element analysis.
Microstructure and Deformation of Ti-22Al-23Nb Orthorhombic-Based Monolithic and Composite Titanium Aluminides
Authors: dary François-charles , Woodard Shiela R. , Pollock Tresa M. 
Published: April 2011

AbstractA new class of intermetallic matrix composites (IMC's) based on orthorhombic titanium aluminides offer attractive properties for high-temperature structural components at temperatures up to 760°C. Results from an ongoing study on the microstructural stability and mechanical properties of the orthorhombic-based alloy Ti-22Al–23Nb (at%), in both monolithic and composite forms, are discussed. Oxygen acquired during processing or as a result of high-temperature exposure in air or vacuum has a pronounced influence on the microstructure of the monolithic and composite materials. Two-phase lath microstructures of ordered beta (βo) + orthorhombic (O) phases produced by processing low oxygen material above the beta transus are morphologically stable at 760°C. Conversely, in higher-oxygen three-phase microstructures containing O+βo+ α2(Ti3Al), lath coarsening and additional precipitation of α2 in oxygen-enriched sheet surface regions is observed. At 760°C/69MPa the two-phase lath microstructure has a higher creep resistance and lower tensile strength compared to the three-phase α2- containing microstructures of the higher oxygen material.
Processing and Properties of Nb5Si3/Nb Laminates
Authors: Short John , Kajuch Jan , Lewandowski John J. 
Published: April 2011

AbstractMechanical alloying(MA) and Reactive Sintering(RS) techniques were successfully used to produce Nb5Si3. Model laminates of mechanically alloyed Nb5Si3 and commercially pure niobium were prepared via vacuum hot pressing. Tensile properties for the as-received, vacuum heat treated and niobium with a solid solution of silicon were obtained at 298K and 77K. 298K bend tests were conducted in an in situ deformation stage in a SEM. 77K bend tests were also conducted. Toughness values are correlated to the ligament properties and effect of constraint.
Transformation Toughening Simulated at Different Length Scales
Authors: Clapp P. C. , Zhao Y. , Rifkin J. A. 
Published: April 2011

AbstractThe microscopic mechanism of “transformation toughening” is thought to be the stress reduction at a crack tip resulting from a displacive phase transformation induced by the stress field of a crack under external loading. Whether transformation toughening or “transformation embrittlement” is the result depends on many different characteristics of the displacive transformation, as well as the geometry of the stress field of the crack. Since both crack and displacive transformation dynamics are sufficiently rapid to be suitably simulated in a molecular dynamics scheme we have explored this approach with the ordered intermetallic NiAl, employing Embedded Atom Method (EAM) potentials. These potentials, in turn, have allowed the construction of a Ginzburg-Landau strain free energy functional (with all the material dependent parameters determined from molecular dynamics simulations) which may then be used to carry out Monte-Carlo simulations of the crack-transformation zone interaction on a substantially larger spatial scale. The simulations reported here show the complex microstructure involving self-accommodating martensite variants which result from the stress induced martensitic transformation near a crack tip in NiAl, and also measure the resulting reduction of stress intensity factor due to the transformation. It is concluded that current continuum mechanics models of transformation toughening need to be substantially revised if they are to adequately model the size, shape and microstructure of the transformation zone and produce accurate predictions of transformation toughening.
Combustion Synthesis of Niobium Aluminide Matrix Composites
Authors: Kachelmyer C. R. , Varma A. 
Published: April 2011

AbstractCombustion synthesis of NbAl3-matrix composites with Al2O3 and B additions was studied using the thermal explosion mode. The addition of B to the reaction mixture resulted in the formation of NbB2, small amounts of NbB and unreacted Al. The Al2O3 addition did not affect the NbAl3-matrix reaction completion but the final product density increased with increasing Al2O3 loading. In both NbAl3-matrix composites, the reaction was initiated above the melting point of Al.
Polymer Derived, Stoichiometric SiC Fibers
Authors: Lipowitz J. , Rabe J. A. , Orr L. D. , Androl R. R 
Published: April 2011

AbstractPolycrystalline, stoichiometric β- silicon carbide fiber tow suitable for ceramic or metallic matrix composite use has been prepared using a polymer precursor route to obtain textile grade, weavable fibers. Individual filaments have a diameter of 10 μm, an average tensile strength of up to 3.4 GPa (500 ksi), an elastic modulus up to 430 GPa (62 msi) and a density > 3.1 g/cm3. Crystallites average about 60 nm by x-ray line broadening. The SiC fiber shows improved mechanical and thermal stability properties, especially in an inert atmosphere, as compared to commercial polymer-derived SiC ceramic fibers. Thermochemical and mechanical properties, as well as fractography and microstructure will be presented.
Self Assembling Nanostructured Delivery Vehicles for Biochemically Reactive Pairs
Authors: Kossovsky Nir , Gelman A. , Hnatyszyn H.J. , Sponsler E. , Chow G.-M. 
Published: April 2011

ABSTRACTIntrigued by the deceptive simplicity and beauty of macromolecular self-assembly, our laboratory began studying models of self-assembly using solids, glasses, and colloidal substrates. These studies have defined a fundamental new colloidal material for supporting members of a biochemically reactive pair.The technology, a molecular transportation assembly, is based on preformed carbon ceramic nanoparticles and self assembled calcium-phosphate dihydrate particles to which glassy carbohydrates are then applied as a nanometer thick surface coating. This carbohydrate coated core functions as a dehydroprotectant and stabilizes surface immobilized members of a biochemically reactive pair. The final product, therefore, consists of three layers. The core is comprised of the ceramic, the second layer is the dehydroprotectant carbohydrate adhesive, and the surface layer is the biochemically reactive molecule for which delivery is desired.We have characterized many of the physical properties of this system and have evaluated the utility of this delivery technology in vitro and in animal models. Physical characterization has included standard and high resolution transmission electron microscopy, electron and x-ray diffraction and ζ potential analysis. Functional assays of the ability of the system to act as a nanoscale dehydroprotecting delivery vehicle have been performed on viral antigens, hemoglobin, and insulin. By all measures at present, the favorable physical properties and biological behavior of the molecular transportation assembly point to an exciting new interdisciplinary area of technology development in materials science, chemistry and biology.
Electronic Structure Calculations of Pure and Oxidized Copper Clusters Using Jellium and MO - LCAO Models
Authors: Gronbeck Henrik , Andersson Mats , Rosen Arne 
Published: April 2011

ABSTRACTA self consistent jellium approach to the chemisorption of molecular oxygen on copper clusters is investigated and compared with local density MO - LCAO calculations. The jellium model is found to be well suited for chemisorption studies and the results explain the main trends in the measured chemisorption properties of O2 on copper clusters.
Nanostructural Investigation of Silica Aerogels
Authors: Wang Jue , Shen Jun , Wu Weidong , Zhou Bin 
Published: April 2011

ABSTRACTSilica aerogels are sol-gel derived nanostructured materials with high porosities up to 99.8%. The nanostructures of silica aerogels are investigated by BET, SEM, SAXS techniques. A typical fractal structure is found especially for base-catalyzed aerogels. That is the aerogels are made of particles of size a, these particles form the branching and interconnecting networks up to an average cluster size ξ, At length scales above ξ, the material is an homogeneous assembly of such clusters; between a and ξ, the structure is fractal. Meanwhile, the effects of catalysis and aging on the structures are investigated and discussed. The reasons why SCD technique should be used are also deeply discussed.
Studies on Nanostructured M50 Type Steel Using X-Ray Absorption Spectroscopy and NMR
Authors: Balasubramanian Mahalingam , Zhang Y.D. , Budnick J.I. , Gonsalves K.E. , Xiao T.D. 
Published: April 2011

ABSTRACTWe have investigated the local structural and magnetic properties of Fe in nanostructured Fe-Cr-Mo-V-C M50 type steel. Experimental techniques which we used include XAFS and NMR. NMR results on as-synthesized M50 type steel shows that the material contains a small amount of bcc Fe plus some other highly disordered phase(s). XAFS measurements indicate that the near neighbor coordinations around Fe are not simply well ordered bcc or fcc Fe and that the sample doesn't have long range crystalline order. XAFS measurements on the consolidated sample show that Fe is predominantly in the form of bcc Fe with good long range order.
Chemical Synthesis and Characterization of Nanostructured Titanium Aluminide
Authors: Schwab S.T. , Paul P.P. , Pan Y-M. 
Published: April 2011

ABSTRACTAlthough plagued by a lack of ductility, titanium aluminides are among the most promising intermetallics under development. Significant improvements in ductility may be obtained from nanostructured intermetallics. Nanosize particles of TiAl3 have been prepared by heat-treatment of chemically synthesized precursor (compound 1). Nanosized TiAl3 has been characterized by chemical analysis, XRD, EDS, NMR, and HREM.
HRTEM Analysis and Magnetic Properties Study of Surface Oxidizing and Nitriding of Nanocrystalline Particles
Authors: Hsu Chi-Ming , Lin Hong-Ming , Tsai Kuen-Rong , Lee Pee-Yew 
Published: April 2011

ABSTRACTNanocrystalline iron powders were produced by means of the gas condensation. Pure nitrogen, or oxygen, or air, was introduced into the as-made powders before they were taken from the chamber. Various atmospheres produced varied layer structures around the iron particles. A high resolution transmission electron microscope was used to analyze the crystal structure and vibrating-sample magnetometer was used to measure the magnetic properties of nanocrystalline iron particles. The results showed that a layer of amorphous or nanocrystalline structure was formed initially on the surface of iron particles. This thin amorphous layer crystallized into Fe304 after annealing under electron irradiation when the introduced atmosphere was pure oxygen or air. However, it formed a passivated layer of ζ-Fe2N when nitrogen was introduced. The exchange anisotropy between the iron core and the superficial iron oxide or nitride shell was observed as the sample was cooled in an applied field of 10 kOe from room temperature to 5 K. The shift of the hysteresis loop caused by exchange anisotropy was 16% for the nitride shell and 8% for the oxide shell.
Metal Cluster Oxidation: Sticking Probabilities and Ionization Potential Shiffs
Authors: Andersson Mats , Holmgren Lotta , Persson John L. , Åklint ThorbjÖrn , RosÉn Arne 
Published: April 2011

ABSTRACTThis study deals with the initial oxidation of some 3d metal clusters in gas phase. In the experiments neutral metal clusters in a supersonic beam are made to collide with one or a few oxygen molecules in a reaction cell. The reaction products are detected with photoionization and mass spectrometry. By comparing the abundance of unreacted and reacted clusters, absolute sticking probabilities (S) can be determined. Transition metals with open d shells, e.g. Ni, have high, almost constant S for O2 on clusters containing more than 20 atoms. The size evolution of S for Cu, on the other hand, shows repeated minima and maxima. Cluster sizes which have high ionization potentials (IPs) and closed electronic shells, according to the spherical jellium model, are the ones with the lowest S. The IPs of CunO2 clusters have also been investigated. Compared with the IPs of the pure Cu clusters, the IPs of the oxidized clusters are generally higher and the large drops at the shell openings are less distinct.
Magnetic Properties of Nanosize Iron Clusters
Authors: Venturini E. L. , Wilcoxon J. P. , Newcomer P. P. 
Published: April 2011

ABSTRACTIsolated, monodisperse α-Fe clusters between 1.4 and 15 nm in diameter were prepared inside inverse micelles using an oil-continuous, nonaqueous system. The magnetic properties of these clusters were studied in a SQUID magnetometer as a function of cluster size, temperature and applied magnetic field. The blocking temperature, coercive field and remanent moment of 12.5 nm Fe clusters in inverse micelles are significantly lower than those reported for clusters of similar α-Fe core size but with a surface oxide. The novel synthesis technique may yield metallic clusters with essentially intrinsic magnetic properties.
Grain Boundaries in Nanophase Materials and Conventional Polycrystals – Are They Distinct?
Authors: Mehta S. C. , Smith D. A. , Erb U. 
Published: April 2011

ABSTRACTNanograined materials, with grain sizes in the range of 1–20 nm, exhibit significant enhancement of grain boundary dependent properties such as yield strength, intergranular fracture toughness, grain boundary diffusivity, specific heat and thermal expansion coefficient. Measurements by indirect techniques suggest that the grain boundaries in nanophase materials are structurally different from the boundaries in their conventional polycrystal counterparts. Exploratory HRTEM observations, on the other hand, indicate that the grain boundary structure in nanophase materials is the same as that found in grain boundaries in conventional polycrystals. This paper reports an HRTEM investigation of the microstructure in electrodeposited nanocrystalline (nc) Ni1wt.%P alloy. These observations reveal the presence of about 8-10 vol. % porosity in the microstructure. There is also evidence for the presence of an amorphous phase at some grain boundaries and triple junctions. A comparison of grain boundary structures with boundaries in conventional materials suggests that grain boundaries in the nc Ni-P alloy are, for the most part, normal.
Characterization of Nanoscale Particles Produced by Laser Vaporization / Condensation in a Diffusion Cloud Chamber
Authors: El-Shall M. Samy , Slack W. , Hanley D. , Kane D. 
Published: April 2011

ABSTRACTNanoscale metal oxide particles have been synthesized by using a novel method which combines laser vaporization of metal targets with controlled condensation in a diffusion cloud chamber. The following oxides have been synthesized: ZnO, SiO2, Fe2O3, Bi2O3, PdO, NiO, AgO, TeO, Sb2O3, TiO2, ZrO2, A12O3, CuO, In203, SnO2, V2O5 and MgO. With this method, the size of the particles can be conveniently controlled by careful control of the degree of supersaturation which is accomplished by adjusting the temperature gradient, total pressure, and partial pressure of the metal vapor generated by laser vaporization in a diffusion cloud chamber. The microscale structures of the SiO2 and A1203 particles exhibit interesting web-like matrices with a significant volume of vacancies. These materials may have special applications in catalysis and as reinforcing agents for liquid polymers.
Molecular Sieve Thin Films via Laser Ablation
Authors: Balkus Kenneth J. , Riley Scott J. , Gnade Bruce E. 
Published: April 2011

ABSTRACTLaser ablation has become widely recognized as an effective technique for the preparation of thin solid films. We have employed an excimer laser (KrF, 248 nm) to deposit well dispersed thin films of aluminum phosphate molecular sieves on a titanium nitride substrate. Results for the ablation of AIPO4-5, AIPO4-H3 and AIPO4-H1 molecular sieve targets are presented. The laser power and repetition rate as well as substrate distance and temperature affect the thin film formation. A subsequent hydrothermal post treatment of the ablated films was found to enhance the surface crystallinity. The molecular sieve thin films were characterized by XRD, SEM, XRF, and FT-IR spectroscopy.
Quantum Confinement in GaP Nanoclusters
Authors: Laurich Bernhard K. , Smith David C. , Healy Matthew D. 
Published: April 2011

ABSTRACTWe have prepared GaP and GaAs nanoclusters from organometallic condensation reactions of E[Si(CH3)3]3 (E = P, As) and GaCl3. The size of the as synthesized clusters is 10 Å to 15 Å. Larger clusters of 20 Å to 30 Å size were obtained by thermal annealing of the as grown material. X-ray diffraction and transmission electron microscopy confirm the high crystalline quality. A lattice contraction of 6.7% could be seen for 10 Å sized GaAs clusters. The clusters are nearly spherical in shape. Optical absorption spectra show a distinct line which can be assigned to the fundamental transition of the quantum confined electronic state. The measured blue shift, with respect to the GaP bulk absorption edge is 0.53 eV. As the cluster is smaller than the exciton radius, we can calculate the cluster size from this blue shift and obtain 20.2 Å, consistent with the results from X-ray diffraction of 19.5 Å for the same sample.
Spontaneous Formation of Ag and Au Particles in Alcohols
Authors: Huang Z.-Y. , Quinn M. , Mills G. , Galef W. 
Published: April 2011

ABSTRACTMetal particles are generated via the spontaneous reduction of Ag+ and AuCl4− ions by solvent molecules that takes place in air-saturated alcoholic solutions containing hydroxide ions. Changes in the plasmon band of the Ag particles are observed when the metal particles are in contact with Ag20 particles. The optical changes are explained in terms of surface effects of the metal particles. Larger shifts of the Au plasmon band and light scattering were observed at the initial stages of the particle formation process. These effects are explained in terms of formation of networks consisting of small metal particles. It is proposed that generation of small Au particles occurs mainly on silica surfaces, and that particle-networks are formed when small particles desorb from the surfaces.
Sorption Behavior of Neptunium on Bentonite-Effect of Calcium Ion on the Sorption -
Authors: Kozai Naofumi , Ohnuki Toshihiko , Muraoka Susumu 
Published: April 2011

AbstractThe sorption behavior of neptunium on bentonite was studied with batch type sorption and desorption experiments over a pH range of 2 to 8. A series of parallel studies using Na-smectite, Ca-smectite and admixtures of Na-smectite and calcite quantified the capacity of Ca2+ (which occurs in bentonite as an exchangeable cation of smectite and as a component of calcite) to inhibit the sorption of neptunium. The distribution coefficient (Kd) of neptunium for bentonite was constant from pH 2 to 7, while for pure Na-smectite Kd increased below pH 5 due to “specific sorption” of neptunium on Na-smectite. Specific sorption was defined as occurring when neptunium could be desorbed by a strong acid (1 M HCl) but was stable in the presence of 1 M KCl. It was found that the quantity of neptunium sorbed on Na-smectite was inversely proportional to the concentration of Ca2+ in solution, an effect most pronounced at pH <5. These results show that Ca2+ limits the specific sorption capacity of Na-smectite for neptunium. Similarly, in the mixture of Na-smectite and calcite, sufficient Ca2+ was solubilized to depress neptunium sorption. This investigation demonstrates that Ca2+ contained in bentonite as exchangeable cation and released from calcite reduces the specific sorption of neptunium.