Deformation-induced microstructural banding in TRIP steels

Microstructure inhomogeneities can strongly influence the mechanical properties of advanced high-strength steels in a detrimental manner. This study of a transformation-induced plasticity (TRIP) steel investigates the effect of pre-existing contiguous grain boundary networks (CGBNs) of hard second-phases and shows how these develop into bands during tensile testing using in situ observations in conjunction with digital image correlation (DIC). The bands form by the lateral contraction of the soft ferrite matrix, which rotates and displaces the CGBNs of second-phases and the individual features within them to become aligned with the loading direction. The more extensive pre-existing CGBNs that were before the deformation already aligned with the loading direction are the most critical microstructural feature for damage initiation and propagation. They induce micro-void formation between the hard second-phases along them, which coalesce and develop into long macroscopic fissures. The hard phases, retained austenite and martensite, were not differentiated as it was found that the individual phases do not play a role in the formation of these bands. It is suggested that minimizing the presence of CGBNs of hard second-phases in the initial microstructure will increase the formability

Eolian transport of glycerol dialkyl glycerol tetraethers(GDGTs) off northwest Africa

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Biotic and abiotic degradation of alkenones and implications for UK'37 paleoproxy applications. A review.

International audienceLipid biomarkers in sediments are widely used to infer environmental conditions that have occurred in the geological past, but these reconstructions require a careful consideration of the biotic and abiotic processes that degrade and alter the lipid biomarker compositions before and after deposition. In this paper, we use alkenones produced by haptophyte microalgae to explore the range of effects of these degradative processes. Alkenones are now perhaps the best studied of all biomarkers with several hundred papers on their occurrence in organisms, seawater and sediments. Much information has been obtained on their degradation from laboratory incubations and inferences from changes in their distribution in aquatic environments. Although alkenones are often considered as more stable than many other lipid classes, it is now clear that their distributions can be affected by processes such as prolonged oxygen exposure, aerobic bacterial degradation and thiyl radical-induced stereomutation processes which, in some cases, can lead to changes in the proportions of the alkenones used in the temperature proxy. The same set of chemical and biological processes act on all lipids in aquatic environments and, in cases where there is a marked difference in reactivity, this may lead to significant changes in the biomarker distributions and relative proportions of different lipid classes

Silicic acid biogeochemistry in the Gulf of California: Insights from sedimentary Si isotopes

Iron is considered to play a large role in the cycling of Si in Fe limited regions of the ocean but little is known about its role in Si biogeochemistry outside these areas. Here, we present published sediment trap data, new nutrient profiles and high resolution sedimentary records (Si isotopes, Biogenic silica%, N% and C%) from the Gulf of California, a non-Fe limited region, to investigate the history of Si cycling in this highly productive basin. Modern nutrient profiles show that silicic acid in subsurface waters is in excess relative to nitrate and therefore incompletely utilised during moderate winter upwelling events. Modern data however suggest that during intense upwelling episodes, silicic acid is preferentially utilised relative to nitrate by the biota, which we suggest reflects transient iron limitation. Our new δ30Si record from the Guaymas Basin shows dramatic variations at millennial time-scales. Low δ30Si values synchronous with Heinrich events are interpreted as resulting from the decline in Si(OH)4 utilisation at times of decreased upwelling strength, while nearly complete Si(OH)4 utilisation was observed at times of invigorated upwelling and increased opal burial during the Holocene, the Bølling-Allerød and the last glacial period. We attribute the complete utilisation of Si(OH)4 to the occurrence of transient Fe limitation at these times. Our study highlights the importance of Fe limitation on Si and C cycling in coastal upwelling regions and suggests that upwelling dynamics, in combination with Fe availability, have the potential to modulate marine Si distribution and opal burial even at short time-scales

Small phytoplankton drive high summertime carbon and nutrient export in the Gulf of California and Eastern Tropical North Pacific

Summertime carbon, nitrogen, and biogenic silica export was examined using 234Th:238U disequilibria combined with free floating sediment traps and fine scale water column sampling with in situ pumps (ISP) within the Eastern Tropical North Pacific and the Gulf of California. Fine scale ISP sampling provides evidence that in this system, particulate carbon (PC) and particulate nitrogen (PN) concentrations were more rapidly attenuated relative to 234Th activities in small particles compared to large particles, converging to 1-5 μmol dpm-1 by 100 m. Comparison of elemental particle composition, coupled with particle size distribution analysis, suggests that small particles are major contributors to particle flux. While absolute PC and PN export rates were dependent on the method used to obtain the element/234Th ratio, regional trends were consistent across measurement techniques. The highest C fixation rates were associated with diatom-dominated surface waters. Yet, the highest export efficiencies occurred in picoplankton-dominated surface waters, where relative concentrations of diazotrophs were also elevated. Our results add to the increasing body of literature that picoplankton- and diazotroph-dominated food webs in subtropical regions can be characterized by enhanced export efficiencies relative to food webs dominated by larger phytoplankton, e.g., diatoms, in low productivity pico/nanoplankton-dominated regions, where small particles are major contributors to particle export. Findings from this region are compared globally and provide insights into the efficiency of downward particle transport of carbon and associated nutrients in a warmer ocean where picoplankton and diazotrophs may dominate. Therefore, we argue the necessity of collecting multiple particle sizes used to convert 234Th fluxes into carbon or other elemental fluxes, includingμm, since they can play an important role in vertical fluxes, especially in oligotrophic environments. Our results further underscore the necessity of using multiple techniques to quantify particle flux given the uncertainties associated with each collection method