Investigation of anomalous hardness in sub-stoichiometric transition metal carbides using ab-initio simulations

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Diffusional and microstructural profiles in metallic-to-UHTC conversion by carbonization

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A computational investigation of the phase and microstructural stability in transition metal carbides and nitrides

The group IVB and VB transition metal carbides and nitrides represent one of the major classes of ultrahigh temperature ceramics (UHTCs). Here, we investigate the stability of these compound at low temperature for a wide range of stoichiometries using electronic structure density functional theory (DFT). This, combined with intelligent search algorithms, have been able to suggest potential stable phases in these materials. The results of which have highlight a competition between vacancy ordering in the carbon/nitrogen depleted rocksalt matrix with other stacking fault derived structures, such as the nanolamellar zeta phase (M4C3 or M4N3). Using this DFT phase stability information, a model has been constructed that provides direct insight into how phase stability controls microstructure. Through this model, we have found that a barrier-free state can exist for the nucleation of the stacking fault phases, which now can describe the propensity of faulting in specific types of carbides and nitrides. Even more intriguing is the consequence in microstructure formation between the meta-stable and stable versions of the zeta phase in the carbides and nitrides respectively. Through these computational tools and models, we are able to elucidate the underlying physics that gives rise to phase and microstructure stability for this particular class of UHTCs. Please click Additional Files below to see the full abstract

Phase evolution in thermally annealed metallic-UHTC composites

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The zeta phase in the transition metal carbides and nitrides: Structure, microstructure and properties

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Reactive Chlorine Species Reversibly Inhibit DnaB Protein Splicing in Mycobacteria

Intervening proteins, or inteins, are mobile genetic elements that are translated within host polypeptides and removed at the protein level by splicing. In protein splicing, a self-mediated reaction removes the intein, leaving a peptide bond in place. While protein splicing can proceed in the absence of external cofactors, several examples of conditional protein splicing (CPS) have emerged. In CPS, the rate and accuracy of splicing are highly dependent on environmental conditions. Because the activity of the intein-containing host protein is compromised prior to splicing and inteins are highly abundant in the microbial world, CPS represents an emerging form of posttranslational regulation that is potentially widespread in microbes. Reactive chlorine species (RCS) are highly potent oxidants encountered by bacteria in a variety of natural environments, including within cells of the mammalian innate immune system. Here, we demonstrate that two naturally occurring RCS, namely, hypochlorous acid (the active compound in bleach) and N-chlorotaurine, can reversibly block splicing of DnaB inteins from Mycobacterium leprae and Mycobacterium smegmatis in vitro. Further, using a reporter that monitors DnaB intein activity within M. smegmatis, we show that DnaB protein splicing is inhibited by RCS in the native host. DnaB, an essential replicative helicase, is the most common intein-housing protein in bacteria. These results add to the growing list of environmental conditions that are relevant to the survival of the intein-containing host and influence protein splicing, as well as suggesting a novel mycobacterial response to RCS. We propose a model in which DnaB splicing, and therefore replication, is paused when these mycobacteria encounter RCS

Comparing the Strength of FCC and BCC Sub-micron Pillars: Compression Experiments and Dislocation Dynamics Simulations

Abstract We compare mechanical strength of FCC Gold and BCC Molybdenum single crystal pillars of sub-micron diameter in compression tests. Both crystals show an increase of flow stress with decreasing diameter, but the change is more pronounced in Au compared with Mo. The ratio between the observed maximum flow stress and the theoretical strength is much larger in Au pillars than in Mo pillars. Dislocation Dynamics (DD) simulations also reveal different dislocation behavior in these two metals. While in an FCC crystal a dislocation loop nucleated from the surface simply moves on its glide plane and exits the pillar, in a BCC crystal it can generate multiple new dislocations due to the ease of screw dislocations to change slip planes. We postulate that this difference in dislocation behavior is the fundamental reason for the observed difference in the plastic deformation behavior of FCC and BCC pillars

Tailoring hardness and deformation slip mechanisms in Hf-Ta-C

Transition metal carbides (TMCs) compose a class of ultra-high temperature ceramics characterized by their exceptionally high melting temperatures and hardness. The most refractory of these carbides are B1 phase HfC and TaC with melting temperatures in excess of 3800 ˚C. In low temperature indentation studies, HfC has been shown to slip on the {110} planes whereas TaC slips on {111}. This difference has been contributed to an intrinsic stacking fault present in TaC and absent in HfC. In the present work, we have expanded those studies to investigate how alloying the B1 crystal structure with mixtures of Ta and Hf concentrations can alter the slip behavior as well as the hardness. The experimental slip systems were quantified by dynamical electron diffraction. The results of which were then compared to a series of density functional theory (DFT) calculations of the stability of the intrinsic stacking fault. Furthermore, the elastic constants of the mixed transition metal carbides were computed and the theoretical hardness was compared to experimental values. We noted the highest hardness was predicted for the Hf3TaC4 composition. Finally, we will discuss the asymmetry of transition metal mixing of TaC and HfC powders to produce a single phase B1 structure. The Hf-rich compositions were found to be more difficult to yield this single B1 phase because of the associated higher metal vacancy formation energy. Please click Additional Files below to see the full abstract

Superelasticity and micaceous plasticity of the novel intermetallic compound CaFe2As2 at small length scales

Shape memory materials have the capability to recover their original shape after plastic deformation when they are subjected to certain stimulus. Shape recovery usually occurs through a reversible phase transformation and, in general, has limited performance with 10% maximum strain. Here, we report the first discovery of superelastic and shape memory behavior with 12% recoverable strain in a novel intermetallic compound CaFe2As2, and discuss its unique elastic and plastic deformation behaviors in terms of a collapsed tetragonal phase transition and anisotropic stacking fault energy, respectively, with solution growth of the single crystal, in-situ micropillar compression, and density functional theory (DFT) calculations. Single crystals of CaFe2As2 were grown out from Sn flux and contains mirror-like clean facets of {0 0 1} and {3 0 1} type planes. We fabricated micropillars on these two planes, and conducted in-situ micropillar compression testing in a scanning electron microscope. The [0 0 1] CaFe2As2 micropillar exhibits unprecedented superelasticity: over 12% recoverable strain without negligible residual fatigue damage under cyclic deformation. Due to its high yield strength (2.6 GPa) and large elastic strain, it is possible to absorb and release a large amount of elastic strain energy. Also, it has potential to show one-dimensional shape memory effects at low temperatures (near 0 K) by the reversible phase transformation between the tetragonal/orthorhombic to the collapsed tetragonal phase. Furthermore, this material exhibits strong anisotropy in plasticity. For the [3 0 -1] CaFe2As2 micropillar, we found easy, preferential slip in the [1 0 0]/(0 0 1) slip system which we termed micaceous plasticity. Superelasticity and micaceous plasticity was quantitatively investigated through measuring the uni-axial stress-strain data and comparing our results to DFT calculations. DFT calculations revealed that making and breaking As-As bonds is responsible for superelasticity. A composite model was developed to monitor the volume fraction evolution of the two different phases under compression testing and successfully reproduced the experimental stress-strain curve we measured. In addition, DFT results showed a significantly low energy barrier for the [1 0 0]/(0 0 1) slip between Ca and As layers, which agrees with our experimental observation. We believe that our efforts in both experimental and computational analysis allow us to gain a fundamental understanding of the unique deformation behavior of CaFe2As2 Please click Additional Files below to see the full abstract

A Search for Exozodiacal Clouds with Kepler

Planets embedded within dust disks may drive the formation of large scale clumpy dust structures by trapping dust into resonant orbits. Detection and subsequent modeling of the dust structures would help constrain the mass and orbit of the planet and the disk architecture, give clues to the history of the planetary system, and provide a statistical estimate of disk asymmetry for future exoEarth-imaging missions. Here we present the first search for these resonant structures in the inner regions of planetary systems by analyzing the light curves of hot Jupiter planetary candidates identified by the Kepler mission. We detect only one candidate disk structure associated with KOI 838.01 at the 3-sigma confidence level, but subsequent radial velocity measurements reveal that KOI 838.01 is a grazing eclipsing binary and the candidate disk structure is a false positive. Using our null result, we place an upper limit on the frequency of dense exozodi structures created by hot Jupiters. We find that at the 90% confidence level, less than 21% of Kepler hot Jupiters create resonant dust clumps that lead and trail the planet by ~90 degrees with optical depths >~5*10^-6, which corresponds to the resonant structure expected for a lone hot Jupiter perturbing a dynamically cold dust disk 50 times as dense as the zodiacal cloud.Comment: 22 pages, 6 figures, Accepted for publication in Ap