739 research outputs found
Analysis of Temporal Features of Gamma Ray Bursts in the Internal Shock Model
In a recent paper we have calculated the power density spectrum of Gamma-Ray
Bursts arising from multiple shocks in a relativistic wind. The wind optical
thickness is one of the factors to which the power spectrum is most sensitive,
therefore we have further developed our model by taking into account the photon
down-scattering on the cold electrons in the wind. For an almost optically
thick wind we identify a combination of ejection features and wind parameters
that yield bursts with an average power spectrum in agreement with the
observations, and with an efficiency of converting the wind kinetic energy in
50-300 keV emission of order 1%. For the same set of model features the
interval time between peaks and pulse fluences have distributions consistent
with the log-normal distribution observed in real bursts.Comment: ApJ in press, 2000; with slight revisions; 12 pag, 6 fi
On the effect of Ti on Oxidation Behaviour of a Polycrystalline Nickel-based Superalloy
Titanium is commonly added to nickel superalloys but has a well-documented
detrimental effect on oxidation resistance. The present work constitutes the
first atomistic-scale quantitative measurements of grain boundary and bulk
compositions in the oxide scale of a current generation polycrystalline nickel
superalloy performed through atom probe tomography. Titanium was found to be
particularly detrimental to oxide scale growth through grain boundary
diffusion
Effect of microsegregation and heat treatment on localised γ and γ’ compositions in single crystal Ni-based superalloys
The present work investigates the impact of residual segregation on the underlying microstructure of a 3rd generation single crystal, nickel-based superalloy to understand potential variation in mechanical behaviour between dendrite cores and interdendritic regions. Despite the applied heat-treatments, chemical variation between dendrite cores and interdendritic regions persisted particularly for elements Re, Nb and Ta. Atom probe tomography (APT) was utilized for its nanoscale capability to map site-specific chemical changes in the γ matrix, γ’ precipitates and across the γ/γ’ interface. Greater interfacial segregation of Re, matched by a corresponding depletion of Ni were observed within dendrite cores, with the extent found to increase following heat treatment. Differences in lattice parameters between dendrite cores and interdendritic regions were identified, with larger lattice misfits associated with interdendritic regions
A gas-phase reaction cell for modern Atom Probe systems
In this work, we demonstrate a new system for the examination of gas
interactions with surfaces via Atom Probe Tomography. This system provides the
capability to examine the surface and subsurface interactions of gases with a
wide range of specimens, as well as a selection of input gas types. This system
has been primarily developed to aid the investigation of hydrogen interactions
with metallurgical samples, to better understand the phenomenon of hydrogen
embrittlement. In its current form, it is able to operate at pressures from
10^-6 to 1000 mbar (abs), can operate using a variety of gasses, and is
equipped with heating and cryogenic quenching capabilities. We use this system
to examine the interaction of hydrogen with Pd, as well as the interaction of
water vapour and oxygen in Mg samples
Atomic-scale Studies of Uranium Oxidation and Corrosion by Water Vapour
Understanding the corrosion of uranium is important for its safe, long-term storage. Uranium metal corrodes rapidly in air, but the exact mechanism remains subject to debate. Atom Probe Tomography was used to investigate the surface microstructure of metallic depleted uranium specimens following polishing and exposure to moist air. A complex, corrugated metal-oxide interface was observed, with approximately 60 at.% oxygen content within the oxide. Interestingly, a very thin (~5 nm) interfacial layer of uranium hydride was observed at the oxide-metal interface. Exposure to deuterated water vapour produced an equivalent deuteride signal at the metal-oxide interface, confirming the hydride as originating via the water vapour oxidation mechanism. Hydroxide ions were detected uniformly throughout the oxide, yet showed reduced prominence at the metal interface. These results support a proposed mechanism for the oxidation of uranium in water vapour environments where the transport of hydroxyl species and the formation of hydride are key to understanding the observed behaviour
Atom probe characterisation of segregation driven Cu and Mn-Ni-Si co-precipitation in neutron irradiated T91 tempered-martensitic steel
The T91 grade and similar 9Cr tempered-martensitic steels (also known as
ferritic-martensitic) are leading candidate structural alloys for fast fission
nuclear and fusion power reactors. At low temperatures (300 to 400 C)
neutron irradiation hardens and embrittles these steels, therefore it is
important to investigate the origin of this mode of life limiting property
degradation. T91 steel specimens were separately neutron irradiated to 2.14 dpa
at 327 C and 8.82 dpa at 377 C in the Idaho National Laboratory
Advanced Test Reactor. Atom probe tomography was used to investigate the
segregation driven formation of Mn-Ni-Si-rich (MNSPs) and Cu-rich (CRP)
co-precipitates. The precipitates increase in size and, slightly, in volume
fraction at the higher irradiation temperature and dose, while their
corresponding compositions were very similar, falling near the Si(Mn,Ni) phase
field in the Mn-Ni-Si projection of the Fe-based quaternary phase diagram.
While the structure of the precipitates has not been characterized, this
composition range is distinctly different than that of the typically cited
G-phase. The precipitates are composed of CRP with MNSP appendages. Such
features are often observed in neutron irradiated reactor pressure vessel (RPV)
steels. However, the Si, Ni, Mn, P and Cu solutes concentrations are lower in
the T91 than in typical RPV steels. Thus, in T91 precipitation primarily takes
place in solute segregated regions of line and loop dislocations. These results
are consistent with the model for radiation induced segregation driven
precipitation of MNSPs proposed by Ke et al. Cr-rich alpha prime (')
phase formation was not observed.Comment: Pre-print (not peer reviewed
- …