bluuebunny/crossref_metadata_2025_split huggingface dataset:
Phase Stability and Role of Ternary Additions on Electronic and Mechanical Properties of Aluminum Intermetallics
Authors: Freeman A.J. , Hong T. , Lin W. , Xu Jian-Hua 
Published: March 2011

ABSTRACTFirst principles total energy local density method have addressed the problems of (i) bonding, cohesion and phase stability and (ii) the role of ternary additions, anti-phase boundaries (APB's) and other faults in determining the structural, electronic and mechanical properties of aluminum intermetallic alloys. A key goal has been to attempt to understand, at the electronic level, fundamental quantities that may be related to the crucial brittleness vs. ductility issue in high temperature Ni and Ti and other aluminides. Other contrasts between observed ductility properties of related systems (e.g., NiAl and RuAl) are related to their differing electronic and bonding properties, particularly the nature of p-d hybridization and the directional properties of their electronic charge distrubutions - especially for states near the Fermi energy.
Bonding Energies and Long-Range Order in the Trialuminides
Authors: Sparks C. J. , Specht E. D. , Ice G. E. , Zschack P. , Schneibel J. 
Published: March 2011

ABSTRACTThe degree of long-range order in the trialuminides is determined by X-ray powder diffraction techniques. Long-range order exists to their melting points. For the binary trialuminides Al3Ti, Al73Ti27, and Al3Sc, the degree of long-range order is nearly perfect and is a measure of the lack of mixing of the aluminum atoms onto the sublattice occupied by either Ti or Sc. A calculation of the bond energy between neighboring pairs of atoms from the ordering (melting) temperature is made following the Bragg-Williams mean field theory approach. These bond energies compare favorably with more sophisticated calculations. Bond energies are found to be larger than the energy difference between the crystal structure forms DO22, Ll2, and DO23, and therefore, more relevant to understanding the mechanical and chemical behavior of the trialuminides. Ordering or melting temperatures of these intermetallics reflect the strong Al-metal near-neighbor pair potentials and may provide insights to their brittle properties.
X-Ray Determination of Site Occupation Parameters in Ordered Ternaries Cu(Aux.M1x.)M = Ni,Pd
Authors: Kumar R. , Sparks C. J. , Shiraishi T. , Specht E.D. , Zschack P. , Ice G. E. , Hisatsune K. 
Published: March 2011

ABSTRACTX-ray scattering data obtained for multiple wavelengths with synchrotron radiation were analyzed by the Rietveld method to determine Ni and Pd distributions on the Cu(000) and Au(½½½) sites in the CuAuI tetragonal P4/mmm structure. Alloys of CuAuxM1-x containing 6 at. % Ni or 10 and 25 at. % Pd were processed to obtain maximum ordering. Nickel is predominantly found on the Cu site and most all the Pd is found on the Au site. The uncertainty in site occupation parameters is discussed for various contributions which affect powder intensity measurements. For highly absorbing materials, an observed powder roughness effect decreases the low angle (2θ) intensities relative to the high 2θ intensities. This effect reduces the reliability of the thermal parameters and obscures a proper description of the thermal motion of the two sublattices. Corrections to the X-ray intensity data for surface roughness/porosity effects reduce uncertainties to about ±1 at. % on the refined value of the site occupations. This use of variable wavelength X-rays with simultaneous refinement of the corresponding data is capable of distinguishing site occupations even between two elements of almost equal scattering factor as, for example, Cu and Ni atoms in this investigation. Chemical phase stability is related to the site occupation parameters.
The Structure and Properties of Boundaries in Bicrystals of Boron-Doped Ni3(Al,l at% Ta)
Authors: Mills M. J. , Goods S. H. , Foiles S. M. 
Published: March 2011

ABSTRACTThe effect of boron on the structure and macroscopic properties of an isolated grain boundary in bicrystals of a non-stoichiometric Ni3Al alloy (76 at% Ni, 23 at% Al, 1 at%Ta) has been studied. The room temperature tensile ductility and fracture mode of the bicrystals varies dramatically with the rate of cooling after elevated temperature heat treatment. In the absence of significant segregation of boron to the boundary, the bicrystals fail via brittle interfacial fracture with little or no ductility. When the segregation of boron to the boundary is maximized, the bicrystals are highly ductile. High resolution transmission electron microscopy reveals that this ductile state is achieved without the formation of a detectable region of compositional disorder at the boundary. Atomistic calculations using a Monte Carlo scheme predict that only partial disordering of the planes immediately adjacent to the boundary should occur for Ni-rich alloys both with and without boron. These results suggest that the presence of boron causes an increase in the cohesive energy of the boundaries rather than a change in the local compositional ordering.
Stress-Assisted Discontinuous Precipitation during Creep of Ti3Al-Nb Alloys
Authors: Rowe R.G. , Hall E.L. 
Published: March 2011

ABSTRACTStress-assisted discontinuous precipitation was observed during creep of Ti- 25Al-12.5Nb at.% and associated with microstructures in which large primary creep strains were observed [1]. It was found that a large shift between the equilibrium βo (B2) phase composition at the heat treatment temperature and disordered β(bcc) phase at the creep temperature provided a driving force for discontinuous precipitation of disordered beta phase. Applied stress accelerated the growth of discontinuous β phase at grain boundaries perpendicular to the principal stress axis, but did not produce a significant shift in composition. The difference between beta and ordered beta phase boundaries in the Ti-Al-Nb system at 650°C and 1040°C suggests that discontinuous precipitation or related dissolution should occur in all Ti3Al-Nb alloys.
Deformation Mechanisms and Solid-Solution Strengthening in Ordered Alloys
Authors: Dimiduk Dennis M. , Rao Satish 
Published: March 2011

ABSTRACTFundamental to understanding the results of alloy design studies, is the need for understanding the intrinsic role of solutes in a particular compound. For many compounds such an understanding must be built from a systematic exploration of the role of deviations from the stoichiometric composition as well as the role of ternary solute additions on the variation of flow behavior. Within most intermetallic systems the problem is complicated since the fundamental mechanisms of flow are not well established and, in those systems where these mechanisms are known, thermal activation can lead to dislocation-core transformations and changes in the operative slip systems with temperature. In general, flow may be governed by more than one dislocation process at a given temperature and deformation twinning may be a major contributing deformation mechanism. The problem of isolating the mechanisms of solid-solution hardening may, therefore, require treatment as a problem of combined strengthening mechanisms operating in parallel. This paper reviews the key aspects of deformation mechanisms and solute strengthening in intermetallic alloys. Classical elastic theories of solute hardening serve as an origin, from which, the progress made to date in isolating the mechanisms of solute hardening in ordered alloys is discussed.
Cleavage Fracture and Dislocation Structures of Ductile L12 Type Ni3Ge Single Crystals
Authors: Inoue H.R.P. , Wayman C.M. , Saburi T. 
Published: March 2011

ABSTRACTFracture and dislocation structures of single crystals of the L12-type compound Ni3Ge deformed in tension-compression at room temperature are investigated by optical and electron microscopy. After considerable plastic deformation, cracks nucleate along primary (111) planes of [245]-oriented single crystals and grow in zigzag manner by connecting one octahedral plane to another. Fracture occurs by brittle cleavage. Similar cleavage fracture also occurs when thin films (whose plane is (111)) prepared from cyclically deformed specimens are further deformed at room temperature. These observations are different from those reported in a previous study where the cleavage plane is claimed to be {001} for single crystals deformed in tensio at room temperature. A number of piled-up dislocations are emitted due to stress concentration at and ahead of crack tips, evidencing that this compound is intrinsically ductile. Superlattice intrinsic stacking faults are observed in front of crack tips along with a moire structure. Although there is argument that such moire structures are formed in persistent slip bands during cyclic deformation of Ni3Al, this is unlikely in the present case. The moire structure observed is believed to be formed by deformation of the thin (111) film.
First-Principles Calculations of Stacking Faults and Grain Boundaries in Metals
Authors: Hampel K. , Vvedensky D.D. , Crampin S. 
Published: March 2011

ABSTRACTA detailed understanding of planar defects plays an important role in the search for a comprehensive description of the mechanical behaviour of metals and alloys. We present calculations for isolated stacking faults and grain boundaries using the layer Korringa-Kohn-Rostoker method including an assessment of the force theorem, which has already proven itself in evaluating defect energies for elemental close-packed metals. These ab initio total energy calculations will be supplemented by a study of the changes in bonding and local magnetic properties near a symmetric Σ5 (310) grain boundary in Fe
Hydrogen Embrittlement of FeAl and Fe3Al
Authors: Shea M. , Castagna A. , Stoloff N. S. 
Published: March 2011

ABSTRACTThis paper deals with the role of charging conditions and strain rate on the embrittlement of several Fe.Al and Fe3Al alloys. Fracture paths are transgranular or mixed mode in both air and after precharging of the iron aluminides, unlike the intergranular cracking typical of Ll2 superlattices exposed to hydrogen. The fractographic observations are consistent with a decohesion mechanism. The role of long range order in transport of hydrogen will be discussed.
Alloying of Al3Ti With Mn and Cr to Form Cubic L12 Phases
Authors: Nic J.P. , Zhang S. , Mikkola D.E. 
Published: March 2011

ABSTRACTBased on studies of several binary trialuminides, it has been concluded that those with the L12 structure offer the best prospects for developing useful mechanical properties. Further, evaluation of a series of cubic ternary alloys produced by alloying Al3Ti with fourth period transition elements from Cr to Cu has shown that the mechanical properties and lattice parameters of the cubic alloys vary in a consistent manner with the location of the ternary alloying element in the periodic table. These structure/property relationships have been interpreted in terms of the number of d-shell electrons available for bonding. The Cr- and Mn-modified alloys have been shown to be the most attractive in terms of hightemperature structural applications. Finally, the work has resulted in a suggested mechanism for the D022 to Ll2 transformation on alloying.
Microcracking Development in the L12 Ternary Titanium Trialuminide
Authors: Winnicka M.B. , Varin R.A. 
Published: March 2011

ABSTRACTDevelopment of microcracking in a ternary titanium trialuminide modified with copper (Al5CuTi2) has been investigated. It has been observed that indenting the material by diamond pyramid indenter (Vickers) causes two types of cracking: (a) on the faces of the indentation and (b) on the surface around the indentation. Compression tests carried up to various levels of permanent deformation have shown a different response of low porosity (2.6±0.5%) and high porosity (5.3±1.5%) material to the formation of microcracks.
L12 Ordering in Disordered NiAIFe Alloys
Authors: Yavari A.R , Baro M.D. , Fillion G. , Surinach S. , Gialanella S. , Clavaguera-Mora M.T. , Desriè P. , Cahn R.W. 
Published: March 2011

ABSTRACT:Ordering of the disordered state (obtained by cold-working or by meltspinning) of L12 type Ni,AI,Fe alloys has been monitored by calorimetric, magnetic and X-ray diffraction measurements .It is found that the magnetic moment per atom of the Ni,Al,Fe alloys in the γ-disordered state is about half of that of the LI2 -γ'state. Magnetic measurements such as ours constitute a new tool for studyingthe γ→γ' transition below the Curie temperature.
Effect Of Ceramic Dispersoids On The High Temperature Strength Of Mechanically Alloyed MoSi2
Authors: Kim Y.S. , Johnson M.R. , Abbaschian R. , Kaufman M.J. 
Published: March 2011

ABSTRACTMechanical alloying (MA) has been used to synthesize MoSi2 from elemental Mo and Si powders. SiC and A12O3 dispersoids were also incorporated as a means of increasing high temperature strength. After synthesis, the powders were consolidated using vacuum hot pressing. It is shown that the size and volume fraction of porosity increases anomalously with increasing hot pressing temperature over the range 1200 to 1700°C. The strengths of the A12O3 composite are, in most instances, considerably higher than the monolithic material produced under similar conditions.
Deformation Mechanisms Associated with Annealed Structures in a Ti-24Al-Nb Alloy
Authors: Morris Maria A. , Morris David G. 
Published: March 2011

ABSTRACTCrystallographic and chemical analysis of the microstructures obtained by annealing a heavily deformed Ti-24Al-11 Nb alloy have been performed. The influence of the niobium distribution within the different phases present and the variation of degree of long range order caused by this distribution have been related to the deformation mechanisms leading to strain hardening at the early stages of deformation. In particular, slip band formation and twinning have been observed to occur within the (α2 phase according to the different degree of order measured.
Electronic Structure of Ordered and Disordered Cu50Pd50 and Cu75Pd25 Alloys
Authors: Bose S.K. , KudrnovskÝ J. , Jepsen O. , Andersen O.K. 
Published: March 2011

ABSTRACTWe present the electronic density of states (DOS) in ordered and disordered Cu50Pd50 and Cu75Pd25 alloys calculated by using the linear muffin-tin orbitals (LMTO) method. For ordered alloys the results are obtained via self-consistent LMTO method. We then show how the LMTO parameters for pure Cu and pure Pd can be appropriately transferred to the ordered alloy calculation to produce results in excellent agreement with the self-consistent calculation. For disordered alloys we present the electronic DOS calculated via LMTO-recursion and LMTO-CPA methods, and study the effect of the deviation from the ideal lattice structure, due to different sizes of the Cu and the Pd atoms, on the DOS of these alloys. We also discuss how the scheme of transferring parameters from the pure components to the alloy can be used in the calculation for the disordered phase.
The Effect of Yttrium Ion Implantation on the High Temperature Oxidation Properties of NiAl
Authors: Pint B.A. , Jain A. , Hobbs L.W. 
Published: March 2011

ABSTRACTNiAl was ion implanted with yttrium (2×1016 cm−2) in order to study its effect on the very high temperature (1000–1500°C) oxidation properties. At 1000°C, implanted Y stabilizes the faster growing θ-Al2O3 phase, thus slightly increasing the oxidation rate. At 1200°C, where predominantly α-Al2 O3 is formed with and without Y, there is a factor of 4 reduction in the oxidation rate. However at higher than 1200°C, there is little effect by Y on the oxidation rate. Cyclic testing showed that the Y implant had an imperfect and short-lived improvement on adherence relative to other Y-containing alloys. Variations in aluminum content from 23.5 to 36.0wt%(40–55at%) had little effect on the oxidation properties. Initial experiments at 1500°C with a novel Rh marker indicate that alumina grows at least partially by outward cation diffusion both with and without Y.
Nonlinear Optical Properties of PBT in Nematic Solutions
Authors: Mattoussi Hedi , Kaatz Philip G. , Patterson Gary D. , Berry Guy C. 
Published: March 2011

ABSTRACTThird-order nonlinear optical properties of lyotropic liquid crystal poly(1,4-phenylene-2,6-benzobisthiazole), PBT, solutions are studied by third harmonic generation measurements. Besides the enhancement observed for this coefficient with respect to the pure PBT, coupling is observed between the mean filed nematic director n and the incident fundamental polarization B0. Different geometries, with respect to the relative orientation of B0 and n, provided different harmonic Maker Fringe Patterns. These data are compared using refractive index measurements, independently achieved on these materials, and necessary for Maker Fringe analysis.
Insoluble, Low Dielectric Polyimides
Authors: Stoakleya Diane M. , Clair Anne K. St. 
Published: March 2011

ABSTRACTStudies have been conducted at NASA Langley establishing structure-property relationships in polyimides that provide a means of lowering dielectric constants by reducing chain-chain electronic interactions and by incorporation of fluorine into the polymer molecular structure. The resulting polyimides exhibit excellent thermal stability and mechanical strength, improved resistance to moisture and enhanced solubility. Most polyimides containing fluorine in only one monomer are at least partially soluble in common organic solvents, and polyimides containing fluorine in both the dianhydride and the diamine are fully soluble. Although this enhanced solubility aides processability and is desirable for many applications, it is a severe disadvantage for aircraft applications where exposure to hydraulic fluids and other chemicals is likely to occur. This work involves the development of insoluble, low dielectric polymers prepared from 2, 2-bis[4-(3, 4-dicarboxyphenoxy)phenyl]hexafluoropropane dianhydride (BFDA) and selected fluorine-containing diamines.BFDA-containing polyimide films had dielectric constants that ranged from 2.44 to 2.53 at 10 GHz and exhibited insolubility in solvents including N, N-dimethylacetamide, chloroform, diglyme and N-methylpyrrolidinone.
Polyanilines: In Situ Radiation and Thermal Induced Doping
Authors: Angelopoulos Marie , Shaw Jane M. , Lee Kam-Leung 
Published: March 2011

ABSTRACTOnium salts and amine triflate salts decompose upon exposure to radiation or thermal treatment respectively to generate protonic acids. These salts can be blended with polyaniline and upon decomposition the resulting acids act as insitu dopants for the polymer. These novel doping techniques eliminate the need for external dopant solutions and thereby simplify the processing of conducting polyaniline.
Rheo-Optical Properties of Ion Irradiated Polystyrene Films
Authors: Calcagno L. , Compagnini G. , Foti G. , Mezzasalma A. , Mondio G. 
Published: March 2011

ABSTRACTRheological and optical properties of polystyrene are modified by 200 keV He irradiation because of new molecular bonds induced by ion energy deposition. Total amount of bonds, as calculated by optical measurements, increases linearly with ion fluence without any dependence on the initial molecular weight. Density of interchain bonds or crosslinks, as calculated by rheological measurements, increases linearly at low fluence (∼1013 ions/cm2), while for high values shows a saturation behaviour.Correlation between optical and rheological measurements allows us to calculate the fractions of interchain and intrachain bonds of ion irradiated 9000, 35000 and 100000 initial molecular weight samples.
A Modified Extended Hückel Calculations For Q1D-Graphites
Authors: Raković D. , Kostić R. , Krstić S. , Davidova I. , Fayfel B. L. , Gribov L. A. 
Published: March 2011

ABSTRACTIn this paper we have computed electronic density of states for several Q1D graphites: polyacene (PA), polyacenacene (PAA), polyphenanthrene (PP), polyphenanthrophenanthrene (PPhP), and polyperinaphthalene (PPN). The modified extended Hiickel method for finite Q1D chains has been adopted. The change of the electronic properties due to the growth of the Q1D-graphites toward the two-dimensional direction, starting from trans-polyacetylene, cis-polyacetylene or poly(p-phenylene), is discussed. Our calculations show that PA, PAA, and PPN are intrinsic conductors, while PP and PPh are semiconductors with energy gaps of 1,4 eV and 0,8 eV, respectively. The comparison with other computational results is presented.
Morphology and Conductivity of Polymer Blends Filled with Carbon Black
Authors: Geuskens G. , Kezel E. De , Blacher S. , Brouers F. 
Published: March 2011

ABSTRACTIt has been shown that the electrical conductivity of polymer blends filled with carbon black (CB) can be much higher than that of each constituent for the same level of loading. It is very difficult to understand this phenomenon due to the complexity of the three phases system. In particular it has been argued that the results could not be explained if the morphology of the blend was independent of the amount of CB. Indeed, an image analysis of the TEM and SEM micrographs of polymer blends filled with Ketjenblack (KB), reveals that the size distribution of the polystyrene component of the blend changes with the concentration of KB. One observes that an increase of 5% in KB results in a reduction of the average size of the PS component by 50% Since the KB dispersion seems to be linked to the dispersion of the polystyrene component our findings could explain why the dispersion is improved in the blend and gives rise to a lower percolation threshold.
Free-Volume Properties of Amorphous Polymers Probed by Positron Annihilation Spectroscopy
Authors: Jean Y. C. , Zandiehnadem F. , Deng Q. 
Published: March 2011

AbstractPositron annihilation spectroscopy (PAS) has been developed to characterize the free-volume properties of polymers. Positron annihilation lifetime measurements give direct information about the dimension, content, and hole-size distributions of free-volume in amorphous materials. The angular correlation of positron annihilation radiation measurements give additional information about the shape of the free-volume holes in oriented polymeric materials. The unique capability of PAS to probe free-volume properties is from the fact that positronium atom is preferentially trapped in the atomic-scale holes which have a size ranging from 1 to 10 Å.
Dependence of Segmental Mobility in Polycarbonate on Time and Deformation
Authors: Smith Thor L. , Haidar Bassel 
Published: March 2011

AbstractStudies are discussed which show that the segmental mobility In polycarbonate (Lexan from General Electric Co.) increases when a static deformation in either extension or simple compression (uniaxial stress) is applied to a specimen, but thereafter the mobility decreases progressively. These changes can be termed erasure (partial) of physical aging (strain softening) and physical aging (hardening), respectively. Because the volume of a specimen deformed in simple compression decreases, reversal of aging (de-aging) cannot be attributed to an increase in free volume, as usually defined. A proposed explanation will be mentioned. A study was also made of the yield stress modified by unconventional methods at several temperatures. The results can be explained In terms of the rate of physical aging and its temperature dependence determined on specimens at 2.6% extension over a broad range of temperatures. These results show that yielding results from progressive de-aging. The yield phenomena obtained by special methods results from both physical aging and de-aging.