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B V Cockeram - One of the best experts on this subject based on the ideXlab platform.
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In-situ Fracture Studies and Modeling of the Toughening Mechanism Present in Wrought LCAC, TZM, and ODS Molybdenum Flat Products In-situ Fracture Studies and Modeling of the Toughening Mechanism Present in Wrought LCAC, TZM, and ODS Molybdenum Flat Produc
2020Co-Authors: B V Cockeram, K S Chan, ) Bechtel-bettisAbstract:Abstract In-situ testing, ultrasonic C-scans, and metallography were used to show that a crackdivider delamination form of thin-sheet toughening occurs in wrought Low Carbon Arc Cast (LCAC) unalloyed molybdenum, Oxide Dispersion Strengthened (ODS) molybdenum, and TZM molybdenum at temperatures ≥ the Ductile to Brittle Transition Temperature (DBTT). Cracking along boundaries relieves mechanical constraint to free ligaments that may plastically stretch to produce toughening. Anisotropy in fracture toughness with lower values in the short-transverse direction is shown to produce the crack divider delaminations at the crack tip in the LT and TL orientations. The delamination zone increases with increasing stress-intensity to sizes significantly larger than the plastic zone, which leads to large increases in fracture toughness by the thin sheet toughening mechanism. Fracture in ODS Mo-alloys proceeds mainly along grain boundaries to produce small ligaments that exhibit ductility for both LT and TL orientations resulting in a lower DBTT and higher toughness values at lower temperatures than observed in LCAC and TZM. A combination of grain boundary fracture and cleavage is prevalent in LCAC molybdenum and TZM. The predominance for microcracking along grain boundaries to leave fine, ductile ligaments in ODS molybdenum can be attributed to a fine-grained microstructure with ≈1-2 μm thickness of sheet-like grains. The presence of mixed grain boundary fracture and cleavage in LCAC and TZM can be attributed to a microstructure with a larger thickness of sheet-like grains (4 -15 μm)
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the mechanical properties and fracture mechanisms of wrought low Carbon Arc cast lcac molybdenum 0 5pct titanium 0 1pct zirconium tzm and oxide dispersion strengthened ods molybdenum flat products
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2006Co-Authors: B V CockeramAbstract:Abstract Molybdenum alloys such as low Carbon Arc cast (LCAC) unalloyed molybdenum, oxide dispersion strengthened (ODS) molybdenum, and molybdenum–0.5pct titanium–0.1pct zirconium (TZM) molybdenum are of interest for structural applications at high temperatures, but these alloys are poorly characterized with respect to fracture toughness and the ductile to brittle transition temperature (DBTT) in the presence of a notch. Both tensile and fracture toughness testing of these flat rolled molybdenum alloys at temperatures above the DBTT are shown to produce a ductile laminate fracture mechanism, where cracks initiate along grain boundaries in the region of triaxial stresses to leave ligaments of sheet-like grains that are stretched to failure with a high degree of plasticity. The DBTT determined from toughness testing is 50–200 °C higher than determined from tensile testing, which shows the constraining effect of the notch. Use of the J -integral test method provided a more consistent and accurate measure of fracture toughness values at temperatures above the DBTT where large amounts of plasticity are observed. A transition was observed from toughness values between 5.8 and 29.6 MPa√m at temperatures below the DBTT to toughness values between 45 and 175 MPa√m for LCAC, 40–215 MPa√m for TZM, and 53–205 MPa√m for ODS. The variation in fracture toughness values at temperatures > DBTT is shown to correlate with size and number density of the ductile laminate features, where high fracture toughness values result from a fine laminate spacing. Since a finer grain size results in a smaller laminate size, the lower DBTT observed for fine grained ODS molybdenum can be understood in terms of the ductile laminate failure mode.
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tensile properties and fracture mode of a wrought ods molybdenum sheet following fast neutron irradiation at temperatures ranging from 300 c to 1000 c
Journal of Nuclear Materials, 2005Co-Authors: B V Cockeram, Richard W Smith, L L SneadAbstract:Abstract A commercially available wrought oxide dispersion strengthened (ODS) molybdenum alloy was irradiated in the high flux isotope reactor (HFIR) at 294–936 °C to neutron fluences between 2.28 and 24.7 × 10 25 n/m 2 ( E > 0.1 MeV) or (1.2–13.1 dpa-Mo). Irradiation of ODS molybdenum at 300 °C and 600 °C results in large increases in strength (57–173%). The DBTT for 300 °C-irradiated ODS Mo was 800 °C, which is the same as observed for low Carbon Arc cast (LCAC) and TZM molybdenum irradiated to the same dose. The DBTT for 600 °C-irradiated ODS Mo was room-temperature, which is a significant improvement over the DBTT values determined for LCAC (300 °C) and TZM (700 °C) and from literature data. The micro-structural feature of small, elongated grains likely enhances the resistance of ODS to irradiation embrittlement. Irradiation of ODS Mo at 870–1000 °C resulted in small increases in yield strength (10–34%) with a post-irradiated DBTT comparable to non-irradiated material (−100 °C).
Manishkumar Chhowalla - One of the best experts on this subject based on the ideXlab platform.
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deposition of smooth tetrahedral amorphous Carbon thin films using a cathodic Arc without a macroparticle filter
Applied Physics Letters, 1995Co-Authors: Manishkumar Chhowalla, G A J Amaratunga, M Weiler, C A Davis, B KleinsorgeAbstract:It is shown that for a cathode consisting of compressed graphite powder, the cathodic Arc discharge is confined within deep erosion holes and the macroparticle emission is greatly reduced. Electron energy loss spectroscopy and scanning electron microscopy show that smooth tetrahedral amorphous Carbon films with up to 85±10% sp3 bonding can be deposited without the use of magnetic filters. The new Carbon Arc discharge process holds potential for deposition of smooth tetrahedral amorphous Carbon films on large area substrates due to the elimination of the magnetic filtering stage.
Mohammad R. Hajaligol - One of the best experts on this subject based on the ideXlab platform.
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characterization of multiwalled Carbon nanotubes prepared by Carbon Arc cathode deposit
Materials Chemistry and Physics, 2003Co-Authors: Donald E. Miser, W G Chan, Mohammad R. HajaligolAbstract:Abstract Multiwalled Carbon nanotubes (MWNTs) prepared by Carbon Arc cathode deposit were characterized by scanning electron microscopy (SEM), high resolution transmission electron microcopy (HRTEM), X-ray diffraction (XRD), and nitrogen gas chromatography BET (Brunauer Emmett Teller) surface measurement. Experimental results indicate that besides Carbon nanotubes, the as-received deposit also contains amorphous Carbon particles, multi-layer polygonal particles (MPPs), and large graphite platelets. Various tube morphologies are revealed by phase contrast HRTEM images, which are probably associated with different catalyst behaviors involved in the tube growth. Within reasonable fluctuation range, fast Fourier transform (FFT) results and the XRD profile provide essentially identical structural information. The tube-containing deposit exhibits a broad pore size distribution range of 2–100 nm, which is consistent with its isotherm profile.
Yevgeny Raitses - One of the best experts on this subject based on the ideXlab platform.
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Detection of nanoparticles in Carbon Arc discharge with laser-induced incandescence
Carbon, 2017Co-Authors: S. Yatom, A. Khrabryi, J. Bak, Yevgeny RaitsesAbstract:Laser-induced incandescence measurements were conducted in the Carbon Arc discharge, used for synthesis of Carbon nanostructures. The results reveal two spatial regions occupied by dominant populations of Carbon particles with different sizes. Close to the axis of the Arc, large micron size particles dominate the incandescence signal. In the Arc periphery, the dominant population of nanoparticles has diameter of 20 nm. Using a heat transfer model between the gas, Arc plasma and the particles, it is shown that such a drastic difference in the particle sizes can be explained by evaporation of the micron-scale particles which move across the Arc plasma towards the Arc periphery. It is also hypothesized that mass evaporated from the micro particles contributes to the Carbon feedstock for the formation of nanostructures.
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Structural variations of the cathode deposit in the Carbon Arc
Carbon, 2016Co-Authors: Yao Wen Yeh, Yevgeny Raitses, Ning YaoAbstract:Synthesis of various Carbon nanostructures, including fullerenes, single-walled and multi-walled nanotubes and nanoparticles, by Arc discharges relies on ablation of the graphite anode and deposition of synthesized Carbonaceous products on the cathode surface and on the reactor chamber walls. For backbone all-Carbon system, the cathode deposit plays a critical role in sustaining the Arc discharge and thereby, the synthesis processes. This deposit usually exhibits spatially distinct structural variations with three different axially symmetrical morphologies. In particular, a rim of pyrolytic Carbon separates the innermost core consisting of multi-walled Carbon nanotubes from the outmost ring with powdery amorphous Carbon soot. Experiments revealed a strong correlation between the current conducting Arc attachment to the cathode deposit and the nanotube forming area in the deposit. Results suggest that particle and heat fluxes from the plasma are responsible for purity of nanotubes in this deposit core area. It appears that a better synthesis selectivity can be obtained in low ablation regime which is characterized by a nearly constant Arc current density independent on the anode diameter.
Stuart W. Staley - One of the best experts on this subject based on the ideXlab platform.
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superparamagnetism in Carbon coated co particles produced by the kratschmer Carbon Arc process
Physical Review B, 1994Co-Authors: Micheal E. Mchenry, J O Artman, M Degraef, Sara A. Majetich, Stuart W. StaleyAbstract:A process based on the Kratschmer-Huffman Carbon Arc method of preparing fullerenes has been used to generate Carbon-coated cobalt and cobalt carbide nanocrystallites. Magnetic nanocrystallites are extracted from the soot with a gradient field technique. For Co/C composites, structural characterization by x-ray diffraction and high-resolution transmission electron microscopy reveals the presence of a fcc Co phase, graphite, and a minority Co2C phase. The majority of Co nanocrystals exists as nominally spherical particles, 0.5 — 5 nm in radius. Hysteretic and temperature-dependent magnetic response, in randomly and magnetically aligned powder samples frozen in epoxy reveals fine-particle magnetism associated with monodomain Co particles. The magnetization exhibits a unique functional dependence on H/T, and hysteresis below a blocking temperature, T& -=160 K. Below T&, the temperature dependence of the coercivity is given by H, =H„[1 — ( T/Ts ) '~ j, with H„-=450Oe.
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preparation and properties of Carbon coated magnetic nanocrystallites
Physical Review B, 1993Co-Authors: Sara A. Majetich, J O Artman, M E Mchenry, N T Nuhfer, Stuart W. StaleyAbstract:Carbon-coated gadolinium carbide nanocrystallites are generated by a process based on the Kratschmer-Huffman Carbon-Arc method of preparing fullerenes, and a novel magnetic-field-gradient separation technique is used to separate them. This separation of nanocrystalline byproducts of the Carbon Arc process is a prerequisite for many of the proposed applications of these materials. While the data presented pertain to ${\mathrm{Gd}}_{2}$${\mathrm{C}}_{3}$, this method is generally applicable to any paramagnetic or ferromagnetic compound. Structural characterization by x-ray and electron diffraction and high-resolution transmission electron microscopy reveal the presence of a single gadolinium-containing (${\mathrm{Gd}}_{2}$${\mathrm{C}}_{3}$) phase and excess Carbon. The carbide phase exists as 10--50 nm spherical particles. SQUID magnetometry shows paramagnetic response attributed to ${\mathrm{Gd}}^{3+}$ ions.