Sarath Menon

Enhanced cellular preservation by clay minerals in 1 billion-year-old lakes

Organic-walled microfossils provide the best insights into the composition and evolution of the biosphere through the first 80 percent of Earth history. The mechanism of microfossil preservation affects the quality of biological information retained and informs understanding of early Earth palaeo-environments. We here show that 1 billion-year-old microfossils from the non-marine Torridon Group are remarkably preserved by a combination of clay minerals and phosphate, with clay minerals providing the highest fidelity of preservation. Fe-rich clay mostly occurs in narrow zones in contact with cellular material and is interpreted as an early microbially-mediated phase enclosing and replacing the most labile biological material. K-rich clay occurs within and exterior to cell envelopes, forming where the supply of Fe had been exhausted. Clay minerals inter-finger with calcium phosphate that co-precipitated with the clays in the sub-oxic zone of the lake sediments. This type of preservation was favoured in sulfate-poor environments where Fe-silicate precipitation could outcompete Fe-sulfide formation. This work shows that clay minerals can provide an exceptionally high fidelity of microfossil preservation and extends the known geological range of this fossilization style by almost 500 Ma. It also suggests that the best-preserved microfossils of this time may be found in low-sulfate environments.

Active eutectoid decomposition in a near-eutectoid zirconium-copper alloy

The decomposition of the beta-phase into the alpha- and the Zr2Cu-phases has been investigated in a Zr-1.6 wt pct Cu alloy. It has been observed that the decomposition process is extremely rapid and cannot be suppressed even by rapid beta quenching. The product structure obtained on fast cooling consists of a lamellar aggregate of the alpha-phase and a metastable phase which does not have the stoichiometry of the Zr2Cu-phase. Structurewise, this phase is similar but not identical to the equilibrium phase. A mechanism for this rapid decomposition of the beta-phase has been proposed. It has also been found that the equilibrium Zr2Cu-phase emerges after short aging treatments at sufficiently high temperatures in the α+Zr2Cu field.

Strengthening Mechanisms in NiAl Bronze: Hot Deformation by Rolling and Friction-Stir Processing

Microstructures produced by isothermal hot rolling of a NiAl bronze material were evaluated by quantitative microscopy methods and parameters describing the contributions of precipitate dispersions, grain size, solute content, and dislocation density to the yield strengths of the individual constituents of microstructure were determined. Models for the strengths of the individual constituents were combined to predict the temperature dependence of the yield strength as a function of hot rolling temperature, and the prediction was found to be in good agreement with measured yield strengths. The models were applied to microstructures in a stir zone produced by multipass friction-stir processing (FSP) and, again, found to predict measured yield strengths with high accuracy. Such models may aid in assessing the role of microstructure gradients produced during FSP and other processes.

Electron microscopy reveals unique microfossil preservation in 1 billion-year-old lakes

Electron microscopy was applied to the study of 1 billion-year-old microfossils from northwest Scotland in order to investigate their 3D morphology and mode of fossilization. 3D-FIB-SEM revealed high quality preservation of organic cell walls with only minor amounts of post-mortem decomposition, followed by variable degrees of morphological alteration (folding and compression of cell walls) during sediment compaction. EFTEM mapping plus SAED revealed a diverse fossilizing mineral assemblage including K-rich clay, Fe-Mg-rich clay and calcium phosphate, with each mineral occupying specific microenvironments in proximity to carbonaceous microfossil cell walls.

Microstructure and Properties of Friction Stir Processed HY80 Steel

Microstructures and mechanical property changes associated with Friction Stir Processing (FSP) of HY-80 steel both dry and under seawater were examined. FSP on HY-80 plates employed a PCBN / tungsten rhenium tool operating at 400 rpm and 2 ipm. Micro structural characterization of the as-received HY-80 plate emphasized the differences in the distribution of constituent phases and inclusions in the rolling, transverse and normal planes using optical and scanning electron microscopy. Micro structural details of transverse sections as well as the plan section at the tool extraction sites of processed plates were also evaluated. Mechanical properties of these samples were evaluated by tensile tests, microhardness tests and Charpy V-notch impact tests. Residual hydrogen content was also evaluated. Stir zones exhibited untempered bainitic / martensitic micro structure s with minimal hydrogen pick up but distinct property gradients from stir zone to base metal. The influence of a post-FSP tempering treatment also is summarized.

Corrosion of femtosecond laser surface textured aluminium alloy

ABSTRACT With superhydrophobic properties being extended to a variety of metallic substrates through the process of ablation due to femtosecond laser surface processing (FLSP), it is important to understand corrosion behaviour of such a processed material. The material was tested through the use of an accelerated corrosion fog chamber using both treated and untreated aluminium alloy samples. During the accelerated corrosion testing, the FLSP-treated sample suffered from pitting corrosion at a rate faster than the untreated sample, effectively removing the surface treatment. While there are significant hydrodynamic benefits to this material, the elevated corrosion rates raise concerns about the resiliency of this surface treatment.

Inclusions and the Color of Obsidian

Obsidian is a volcanic glass formed by the rapid quenching of the lava, is chemically similar to rhyolite or granite, and is composed mainly of silica with many other elements present in various oxide or silicate forms. Obsidian appears in a wide variety of colors though the jet-black variety is the most common. The color of obsidian depends on the wide variety of crystalline inclusions present in the glass with magnetite and some others providing a jet-black color [1, 2]. When crushed into fine powder, the black obsidian appears nearly transparent white or with a slight gray tinge. This effect maybe due to the separation of the larger inclusions from the glassy powder produced when a piece of obsidian is crushed. Thus, when analyzing obsidian specimens, characterization of the inclusions in them is of prime importance.

Microstructure Refinement and Homogenization of Non-Deforming Constituent Distributions During FSW/P

Weld nugget / stir zone microstructures often exhibit highly homogeneous distributions of non-deforming constituent phases as well as refinement of such phases and grain structures. The role of recrystallization in grain refinement during FSW/P is well understood. However, the processes involved in refinement of particle size and homogenization of particle distributions during FSW/P are less well understood. FS of an as-cast AA356 alloy has been conducted over a wide range of RPM values using both threaded and smooth tools. The conversion of the primary — eutectic microstructure to a highly refined and homogeneous dispersion of Si particles in a finegrained Al matrix will be documented. The roles of advective mixing, diffusional processes, and discontinuous shearing of material will be delineated and quantitative measures of homogeneity will be discussed. The potential roles of tool and process conditions in homogenization are also considered.