Dislocation loop rafting and dislocation decoration have been previously observed in neutron and heavy ion irradiated materials. Understanding the fundamental aspects of these phenomena assist in ...evaluating irradiation damage of nuclear materials. Multiple different mechanisms have been suggested to explain loop rafting. Here, we performed a detailed transmission electron microscopy study on dislocation loop rafts in heavy ion irradiated tungsten. Different imaging conditions showed that the rafts are of Burgers vector type and specifically the same Burgers vector variant (<11¯1>) in the particular grain analyzed. Some rafts were associated with dislocation lines while some form as a result of alignment of dislocation loops. They were shown to form at both room temperature (RT) and high temperature with stronger rafts forming at RT. These observations confirm the mechanism previously suggested by Wen et al. which explains raft formation due to loop glide, cluster-cluster and grown-in dislocation-cluster interaction with subsequent Burgers vector rotation. Similar irradiation studies on nanocrystalline tungsten showed that these materials are more resistant to raft formation at RT irradiations.
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Cyclone-driven marine rafting Waters, Jonathan M.; Craw, Dave
Marine ecology. Progress series (Halstenbek),
08/2018, Letnik:
602
Journal Article
Recenzirano
Storm events can have strong effects on marine biological dispersal. Here, we used geological evidence to assess the speed of biological rafting driven by a major cyclonic storm (Cyclone Gita) in ...southern New Zealand during February 2018. We undertook a comparative analysis of coastal rock types to assign beach-cast macroalgal specimens to their geological source regions. Approximately 10% (23 of 250) of Lepas-encrusted beach-cast kelp (Durvillaea) specimens sampled from Dunedin beaches in the immediate aftermath of Cyclone Gita (22 and 23 February 2018) bore holdfast-clasts consistent with origins in the distant Fiordland region of southwestern South Island. Based on Lepas barnacle growth data, we estimated that this distinctive ‘pulse’ of rafted material achieved the 350 to 500 km journey from Fiordland to Dunedin in as little as 10 d, with many of these rafts thus inferred to have travelled 30 to 50 km d−1. These rapid rafting events were facilitated by westerly gales in Foveaux Strait (12 to 17 February), followed by cyclonic southeasterly winds off eastern South Island (19 to 21 February). The rafting-speed estimates from this study are substantially faster than mean current-flow estimates for the same coastal region (12 to 20 km d−1), and also exceed the speeds of previously documented kelp-rafting events in this region (estimated 10 to 15 km d−1) and elsewhere. Together, these data highlight the key role of storms in driving ‘wind slippage’ of buoyant material, facilitating pulses of dispersal at speeds far exceeding the predictions of oceanographic models.
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•Crystal plasticity model is integrated into phase-field framework.•3D creep simulations are performed to study the N-type rafting in Ni-based superalloys.•Plasticity driven coherency ...loss between matrix and precipitates is described.•Correctly modelled the coalescence of precipitates for real rafted microstructure.
The role of coherency loss on rafting of superalloys under high temperature low stress creep conditions is investigated by phase-field crystal plasticity simulations. It is demonstrated that coalescence, critically depending on the state of coherency between precipitate and matrix is crucial to understand the rafting behavior of superalloys. An explicit mechanisms is developed predicting coherency loss based on the plastic activity in the matrix. The simulations are verified using experimental creep test results.
The creep deformation of three single crystal Ni-based superalloys with various Ru contents has been investigated to clarify the effect of Ru additions at very high temperatures and low stresses ...conditions. The creep properties were significantly improved by Ru additions under the conditions of 1150°C/100MPa and 1180°C/70MPa. It is noted that obvious γ′ phase dissolution occurs during the entire creep deformation at very high temperatures. It differs from the typical high-temperature creep curves that a short incubation period occurs before the primary creep stage during the creep deformation at very high temperatures. Ru additions are able to produce denser interfacial dislocation networks and improve the stability and homogeneous configuration of interfacial dislocations; and promote the high-temperature stability of γ′ phase and formation of perfect γ′ rafting. It is thus shown that Ru additions reduce the minimum creep rate, and prolong the secondary creep stage accordingly. The topological inversion of γ/γ′ microstructure did not occur under both conditions. The origin of the rapid increase of creep rate is closely relevant to the unstable propagation of micro-cracks in the vicinity of porosity in necked regions.
Floating seaweeds are important dispersal vectors in marine ecosystems. However, the relationship between benthic populations and stranded seaweeds has received little attention. After detachment, a ...fraction of floating specimens returns to the shore, resulting in strandings that fluctuate in space and time. It has been hypothesized that the availability of stranded seaweeds is related to their benthic abundance on adjacent coasts. Using the large fucoid Durvillaea antarctica, we tested whether stranded biomasses are higher at sites with dense adjacent benthic populations. Benthic abundance of D. antarctica along the continental coast of Chile was estimated using three approximations: (i) availability of potentially suitable habitat (PSH), (ii) categorical visual abundance estimates in the field, and (iii) abundance measurements in the intertidal zone. Higher PSH for D. antarctica was observed between 31° S–32° S and 40° S–42° S than between 33° S and 39° S. Lowest benthic biomasses were estimated for the northern latitudes (31° S–32° S). Regression models showed that the association between stranded biomass and PSH was highest when only the extent of rocky shore 10 km to the south of each beach was included, suggesting relatively short‐distance dispersal and asymmetrical transport of floating kelps, which is further supported by low proportions of rafts with Lepas spp. (indicator of rafting). The results indicate that stranded biomasses are mostly subsidized by nearby benthic populations, which can partly explain the low genetic connectivity among populations in the study region. Future studies should also incorporate other local factors (e.g., winds, currents, wave‐exposure) that influence stranding dynamics.
Oceanic rafting through driftwood has been hypothesized to be a mechanism underlying the transoceanic distribution of termites (Order: Blattodea). To test this hypothesis, we examined the salinity ...tolerance of 12 termite species from eight genera and three families, namely Archotermopsidae, Kalotermitidae, and Rhinotermitidae.
Our results revealed that the survival rates of Incisitermes inamurai (Oshima), Cryptotermes domesticus (Haviland), Prorhinotermes flavus (Bugnion and Popoff), P. simplex (Hagen), and Heterotermes sp. are not affected by treatment with 3.5% saline water, strongly supported that oceanic rafting may be a mechanism underlying the insects' transoceanic distribution.
By contrast, Hodotermopsis sjostedti Holmgren, Glyptotermes satsumensis (Matsumura), Coptotermes formosanus Shiraki, Co. gestroi (Wasmann), Reticulitermes virginicus (Banks), R. leptomandibularis Hsia and Fan, and R. kanmonensis Takematsu treated with saline water showed dehydration symptoms, and their mortality rates increased significantly within 12 days after treatment. These results indicate that these species are less likely to be dispersed through oceanic driftwood and that their transoceanic dispersal is rather more likely to occur through human transportation or land bridges.
Phylogenetic positions and habitats of salinity‐tolerant termites implied that the acquisition of salinity tolerance may relate to both phylogeny and coastal living.
Many termite species are transoceanic distributed and proposed to disperse through rafting, and examining their salinity tolerance is crucial to test such hypothesis.
This study proved that many Kalotermitidae termites and some primitive Rhinotermitidae termites are tolerant to saline water; they showed very low salinity‐induced mortality and no symptoms of dehydration.
All salinity‐tolerant termites inhabit coastal areas, less likely to access freshwater.
•The effects of rafting extent, type and orientation on the monotonic and cyclic deformation behaviours are investigated.•The deformed γ'/γ microstructure and dislocation configurations under ...different rafting states are characterized.•A microstructural state-based constitutive model to account for the role of rafting extent and type is proposed.•The rafting morphology-related Bauschinger effect during cyclic deformation is well captured by a micromechanics-based back stress model.
Monotonic tensile and cyclic deformation behaviours are investigated under different microstructural rafting states of a SC Ni-based superalloy, with emphasis on the influences of the rafting extent, type and loading orientation. The deformed microstructures and the dislocation configurations are characterized to give a micro-based understanding on the varying of deformation behaviours due to rafting. It is found that the decreases in the initial yield point and cyclic stress amplitude are only related to the rafting extent. Nevertheless, the rafting type (namely, the plate-like and needle-like morphology) has an undeniable contribution to the shape of hysteresis loops, where the plate-like rafting morphology results in more significant Bauschinger effect than needle-like rafting morphology. The variation of monotonic and cyclic deformation induced by rafting shares affinity with the alteration of internal stress and the movement of dislocations. Afterwards, a microstructure-sensitive constitutive model with two-phase flow rules has been developed. The effect of rafting on the monotonic and cyclic stress-strain responses is captured by introduce a series of microscopic mechanisms and a micromechanics-based back stress model that considers the morphology and size of the γ'/γ two-phase structures. The developed model is used to simulate the macroscopic stress-strain responses of the SC Ni-based superalloy under different rafting states. Model predictions are in good agreement with tests, capturing the reduction of cyclic stress amplitudes and the change in hysteresis loops. Finally, the impacts of the two-phase flow rules and the micromechanics-based back stress on the simulation capability have been discussed.
Ocean circulation driving macro-algal rafting is believed to serve as an important mode of dispersal for many marine organisms, leading to predictions on population-level genetic connectivity and the ...directionality of effective dispersal. Here, we use genome-wide single nucleotide polymorphism data to investigate whether gene flow directionality in two seahorses (
) and three pipefishes (
) follows the predominant ocean circulation patterns in the Gulf of Mexico and northwestern Atlantic. In addition, we explore whether gene flow magnitudes are predicted by traits related to active dispersal ability and habitat preference. We inferred demographic histories of these co-distributed syngnathid species, and coalescent model-based estimates indicate that gene flow directionality is in agreement with ocean circulation data that predicts eastward and northward macro-algal transport. However, the magnitude to which ocean currents influence this pattern appears strongly dependent on the species-specific traits related to rafting propensity and habitat preferences. Higher levels of gene flow and stronger directionality are observed in
,
and
, which closely associated with the pelagic macro-algae
spp., compared to
and the
/
sister-species pair, which prefer near shore habitats and are weakly associated with pelagic
. This study highlights how the combination of population genomic inference together with ocean circulation data can help explain patterns of population structure and diversity in marine ecosystems.
In this work, the remarkable high-temperature strength properties of monocrystalline gamma '-hardened Ni-base superalloys are reviewed with the goal of understanding better the important role played ...by the gamma / gamma ' lattice misfit delta with respect to microstructural stability and high-temperature creep and fatigue resistance. In particular, attention is focussed on the phenomenon of microstructural degradation by directional coarsening in the form of gamma / gamma ' rafting which also occurs in turbine blades in service. The role played by the lattice misfit (which is negative for all currently used Ni-base superalloys) is elucidated with respect to its sign, magnitude and temperature dependence. Based on a detailed discussion of high-temperature deformation involving gamma / gamma ' rafting, it is argued that superalloys with a positive gamma / gamma ' lattice misfit should exhibit better high-temperature strength properties than otherwise similar superalloys with a negative misfit. In this context, the new class of gamma '-hardened Co-base superalloys is of special interest. These alloys have promising high-temperature strength properties and possess a positive lattice mismatch. It is suggested that this positive mismatch is one of the beneficial factors that contribute to the promising properties of these alloys and that this aspect should be explored further.
The existence of floating marine litter in marine environments enhances the potential for the transport of fouling organisms using these substrates as vectors. In this study, we examined the fouling ...organisms on different types of litter stranded on two beaches of the Moroccan Mediterranean. The study revealed 13 fouling species belonging to 8 phyla (Arthropoda, Bryozoa, Annelida, Mollusca, Cnidaria, Echinodermata, Chlorophyta, and Ochrophyta) on marine litter. Rafting vectors were almost exclusively made up of plastics and could mainly be attributed to land-based sources. The most common fouling species were the crustacean Lepas pectinata, Lepas anatifera, Perforatus perforatus, and bryozoan species. More taxa were found on large litter than on small litter. Relative substratum coverage was highest for bryozoan sp. (31.0 %), green algae (29.0 %), Lepas anatifera (21.42 %), Lepas pectinata (17.8 %), and Perforatus perforatus (17.46 %). Our results suggest that the growing generation of plastic litter may enhance the probability of the introduction of non-native species into the Moroccan Mediterranean. Therefore, monitoring efforts are needed to identify vectors and the arrival of novel invasive species in this area.
•Barnacles and Bryozoans were the main biofouling groups.•A total of 13 fouling species were found on the litter along the coast of Morocco.•The barnacle Lepas pectinata and Lepas anatifera occurred most frequently.•Rafting species reported on plastic litter was higher than that on other litter.•Plastic may increase the chance of introduction of invasive species in Morocco.
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