6,947 research outputs found
Deformation effects on a dispersion hardened alloys
Creep and creep rupture deformation and tensile tests of nickel alloy
Stress generation in thermally grown oxide films
A three dimensional finite element analysis was conducted, using the ANSYS computer program, of the stress state in a thin oxide film thermally formed on a rectangular piece of NiCrAl alloy. The analytical results indicate a very high compressive stress in the lateral directions of the film (approximately 6200 MPa), and tensile stresses in the metal substrate that ranged from essentially zero to about 55 MPa. It was found further that the intensity of the analytically determined average stresses could be approximated reasonably well by the modification of an equation developed previously by Oxx for stresses induced into bodies by thermal gradients
Deformation and annealing study of Nicraly
Extensive experiments were carried out on the ODS alloy Nicraly, (an alloy prepared by mechanical alloying and consolidating a powder blend consisting of 16% chromium, 4% aluminum, 2-3% yttria, balance nickel), in efforts to develop methods of controlling the grain size and grain shape of the material. The experiments fell into two general categories: variations in the annealing parameters using the as-extruded material as it was received, and various thermomechanical processing schedules (various combinations of cold work and annealing). Success was achieved in gaining grain size and grain shape control by annealing of the as-extruded material. By proper selection of annealing temperature and cooling rates, the grain size of the as-received material was increased almost two orders of magnitude (from an average grain dimension of 0.023 mm to 1.668 mm) while the aspect ratio was increased by some 50% (from 20:1 to 30:1). No success was achieved in gaining significant control of the grain size and shape of the material by thermo-mechanical processing
Influence of composition on the microstructure and mechanical properties of a nickel-base superalloy single crystal
The effects of cobalt, tantalum, and tungsten contents on the microstructure and mechanical properties of single crystal Mar-M247 were investigated. Elevated temperature tensile and creep-rupture properties of 001 oriented single crystals were related to microstructural features of the alloys. Substitution of Ni for Co in the high refractory metal alloys increased the lattice mismatch, which was considered to be the cause of the increases in tensile and creep strength. Substitution of Ni for Ta caused large decreases in tensile strength and creep life, consistent with decreases in gamma prime volume fraction, lattice mismatch, and solid solution hardening. Substitution of W for Ta resulted in decreased life at high stresses, which was related to small decreases in mismatch and volume fraction. However, the W substitution resulted in improved life at low stresses, which was related to solid solution strengthening by W
Elevated temperature deformation of thoria dispersed nickel-chromium
The deformation behavior of thoria nickel-chromium (TD-NiCr) was examined over the temperature range 593 C (1100 F) to 1260 C (2300 F) in tension and compression and at 1093 C (2000 F) in creep. Major emphasis was placed on: (1) the effects of the material and test related variables (grain size, temperature, stress and strain rate) on the deformation process; and (2) the evaluation of single crystal TD-NiCr material produced by a directional recrystallization process. Elevated temperature yield strength levels and creep activation enthalpies were found to increase with increasing grain size reaching maximum values for the single crystal TD-NiCr. Stress exponent of the steady state creep rate was also significantly higher for the single crystal TD-NiCr as compared to that determined for the polycrystalline materials. The elevated temperature deformation of TD-NiCr was analyzed in terms of two concurrent, parallel processes: diffusion controlled grain boundary sliding, and dislocation motion
Factors which influence directional coarsening of Gamma prime during creep in nickel-base superalloy single crystals
Changes in the morphology of the gamma prime precipitate were examined as a function of time during creep at 982 C in 001 oriented single crystals of a Ni-Al-Mo-Ta superalloy. In this alloy, which has a large negative misfit of -0.80 pct., the gamma prime particles link together during creep to form platelets, or rafts, which are aligned with their broad faces perpendicular to the applied tensile axis. The effects of initial microstructure and alloy composition of raft development and creep properties were investigated. Directional coarsening of gamma prime begins during primary creep and continues well after the onset of second state creep. The thickness of the rafts remains constant up through the onset of tertiary creep a clear indication of the stability of the finely-spaced gamma/gamma prime lamellar structure. The thickness of the rafts which formed was equal to the initial gamma prime size which was present prior to testing. The single crystals with the finest gamma prime size exhibited the longest creep lives, because the resultant rafted structure had a larger number of gamma/gamma prime interfaces per unit volume of material. Reducing the Mo content by only 0.73 wt. pct. increased the creep life by a factor of three, because the precipitation of a third phase was eliminated
Resummation Improved Rapidity Spectrum for Gluon Fusion Higgs Production
Gluon-induced processes such as Higgs production typically exhibit large
perturbative corrections. These partially arise from large virtual corrections
to the gluon form factor, which at timelike momentum transfer contains Sudakov
logarithms evaluated at negative arguments . It has been
observed that resumming these terms in the timelike form factor leads to a much
improved perturbative convergence for the total cross section. We discuss how
to consistently incorporate the resummed form factor into the perturbative
predictions for generic cross sections differential in the Born kinematics,
including in particular the Higgs rapidity spectrum. We verify that this indeed
improves the perturbative convergence, leading to smaller and more reliable
perturbative uncertainties, and that this is not affected by cancellations
between resummed and unresummed contributions. Combining both fixed-order and
resummation uncertainties, the perturbative uncertainty for the total cross
section at NLONLL is about a factor of two smaller
than at NLO. The perturbative uncertainty of the rapidity spectrum at
NNLONNLL is similarly reduced compared to NNLO. We also
study the analogous resummation for quark-induced processes, namely Higgs
production through bottom quark annihilation and the Drell-Yan rapidity
spectrum. For the former the resummation leads to a small improvement, while
for the latter it confirms the already small uncertainties of the fixed-order
predictions.Comment: 30 pages + 17 pages in Appendices, 10 figures; v2: journal version;
references added, discussed individual partonic channels for Drell-Ya
Static corrections versus dynamic correlation effects in the valence band Compton profile spectra of Ni
We compute the Compton profile of Ni using the Local Density Approximation of
Density Functional Theory supplemented with electronic correlations treated at
different levels. The total/magnetic Compton profiles show not only
quantitative but also qualitative significant differences depending weather
Hubbard corrections are treated at a mean field +U or in a more sophisticated
dynamic way. Our aim is to discuss the range and capability of electronic
correlations to modify the kinetic energy along specific spatial directions.
The second and the fourth order moments of the difference in the Compton
profiles are discussed as a function of the strength of local Coulomb
interaction .Comment: 10 pages, 7 figs., submitted to PR
Bethe-Salpeter Equation Calculations of Core Excitation Spectra
We present a hybrid approach for GW/Bethe-Salpeter Equation (BSE)
calculations of core excitation spectra, including x-ray absorption (XAS),
electron energy loss spectra (EELS), and non-resonant inelastic x-ray
scattering (NRIXS). The method is based on {\it ab initio} wavefunctions from
the plane-wave pseudopotential code ABINIT; atomic core-level states and
projector augmented wave (PAW) transition matrix elements; the NIST core-level
BSE solver; and a many-pole GW self-energy model to account for final-state
broadening and self-energy shifts. Multiplet effects are also accounted for.
The approach is implemented using an interface dubbed OCEAN (Obtaining Core
Excitations using ABINIT and NBSE). To demonstrate the utility of the code we
present results for the K-edges in LiF as probed by XAS and NRIXS, the K-edges
of KCl as probed by XAS, the Ti L_2,3-edge in SrTiO_3 as probed by XAS, and the
Mg L_2,3-edge in MgO as probed by XAS. We compare the results to experiments
and results obtained using other theoretical approaches
Mesons and diquarks in neutral color superconducting quark matter with -equilibrium
The spectrum of meson and diquark excitations in cold color-superconducting
(2SC) quark matter is investigated under local color and electric neutrality
constraints with -equilibrium. A 2-flavored Nambu--Jona-Lasinio type
model including a baryon , color , and electric chemical
potentials is used. Two relations between coupling constants and in the
diquark- and quark-antiquark channels, correspondingly, are treated,
and . At the gapless- and at the gapped neutral color
superconductivity is realized. It is shown that color and electrical neutrality
together with -equilibrium lead to a strong mass splitting within the
pion isotriplet in the 2SC phase (both gapped and gapless), in contrast with
non--neutral matter. It is also shown that the properties of the physical
-singlet diquark excitation in the 2SC ground state varies for
different parameterization schemes. Thus, for one finds a heavy
resonance with mass 1100 MeV in the non--neutral (gapped) case, whereas,
if neutrality is imposed, a stable diquark with mass 200 MeV
appears in the gapless 2SC phase. For , there is again a resonance (with
the mass 300 MeV) in the neutral gapped 2SC phase. Hence, the existence
of the stable massive SU(2)-singlet diquark excitation is a new peculiarity
of the gapless 2SC.Comment: 18 pages, 9 figures; version accepted for publication in PR
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