Kinetics of Twinning and Dislocation Slip During Cyclic Deformation of ZK30 Magnesium Alloy

The present study clarifies the anisotropy of tension-compression behaviour during the cyclic deformation of ZK30 magnesium alloy. Some details of mechanical twinning and dislocation slip are studied by acoustic emission technique and direct video observations. Through the combination of these methods, the overall effect of deformation mechanisms is determined for each loading direction and their effect on the cyclic deformation is highlighted

Application of acoustic emission method for investigation of hydrogen embrittlement mechanism in the low-carbon steel

© 2014 Elsevier B.V. All rights reserved. The hydrogen embrittlement (HE) phenomenon in the electrolytically charged mild low-carbon steel was investigated during tensile testing coupled with in situ acoustic emission (AE) measurements and post mortem fractographic analysis. It was demonstrated that the ductility reduction correlated with the formation of HE-induced quasi-cleavage regions known as "fisheyes" which formed during the necking stage under the influence of mobile hydrogen. Based on the AE analysis we conclude that formation of "fisheyes" in the mild steel under load cannot be explained by a decohesion mechanism and the alternative mechanisms are discussed

The Applicability of Taylor’s Model to the Drilling of CFRP Using Uncoated WC-Co Tools: The Influence of Cutting Speed on Tool Wear

This work investigates the applicability of Taylor’s model on the drilling of CFRP using uncoated WC-Co tools, by assessing the influence of cutting speed (Vc) on tool wear. Two different resins, possessing low and high glass transition temperatures (Tg), and two different reinforcements, high strength and high modulus woven fabrics, were combined into three different systems. Flank wear rate gradient exhibited to be more reinforcement dependent, while the actual flank wear rate showed to be sensible also to the type of resin. In terms of tool life, all CFRP systems adjusted into a power-type tool life vs. cutting speed curve through the full range of cutting speeds (1,000 RPM to 18,000 RPM). Therefore, Taylor’s model can be applied to estimate the tool life in the drilling of CFRP using uncoated WC-Co tools with good accuracy

Nanodiamonds with silicon vacancy defects for nontoxic photostable fluorescent labeling of neural precursor cells

Nanodiamonds (NDs) containing silicon vacancy (SiV) defects were evaluated as a potential biomarker for the labeling and fluorescent imaging of neural precursor cells (NPCs). SiV-containing NDs were synthesized using chemical vapor deposition and silicon ion implantation. Spectrally, SiV-containing NDs exhibited extremely stable fluorescence and narrow bandwidth emission with an excellent signal to noise ratio exceeding that of NDs containing nitrogen-vacancy centers. NPCs labeled with NDs exhibited normal cell viability and proliferative properties consistent with biocompatibility. We conclude that SiV-containing NDs are a promising biomedical research tool for cellular labeling and optical imaging in stem cell research

Quantitative characterization of cleavage and hydrogen-assisted quasi-cleavage fracture surfaces with the use of confocal laser scanning microscopy

© 2016 Elsevier B.V."True" cleavage (TC) and quasi-cleavage (QC) fracture surfaces of low-carbon steel specimens tested in liquid nitrogen and after hydrogen charging respectively were investigated by quantitative confocal laser scanning microscopy (CLSM) and conventional scanning electron microscopy (SEM) with electron-backscattered diffraction (EBSD). Topological and crystallographic features of the TC fracture surface are found in good agreement with the generally accepted cleavage mechanism: TC facets diameters correspond to those of grains; the crack path strictly follows the crystallographic orientation of grains and the most of the cleavage cracks are parallel to [100] planes. On the 2D SEM images, the QC facets appeared resembling the TC ones in terms of river line patterns, shapes and sizes. However, the substantial differences between the topography of these two kinds of fracture surfaces were revealed by 3D CLSM: the average misorientation angle between QC facets and the roughness of the QC fracture surface were much lower than those measured for TC. It is demonstrated that all these features are attributed to the specific fracture mechanism operating during hydrogen-assisted cracking

Strain rate sensitivity of mechanical properties of the ZK60 alloy with the high degree of corrosion damage

There is a strong belief that hydrogen absorbed by magnesium alloys during corrosion can cause their stress corrosion cracking. One of the characteristic markers indicating the involvement of diffusible hydrogen into the fracture mechanism of metals is the negative strain rate dependence of the embrittlement degree. Recent studies show that the loss of ductility of the ZK60 alloy specimens subjected to a short-term (1.5 h) pre-exposure in a corrosive medium actually decreases with the increasing strain rate. However, after the removal of corrosion products from the surface of the specimens, the strain rate dependence of the ductility loss becomes positive, which indicates the absence of hydrogen in the bulk of the metal. At short-term exposure in a corrosive environment, the deep penetration of hydrogen into a metal could be limited due to the insufficient time for hydrogen diffusion. The paper studies the mechanical behavior of the ZK60 alloy subjected to a longer (12 h) pre-exposure in a corrosive medium followed by tensile testing in air at various strain rates. The authors consider the effect of strain rate, long-term pre-exposure in a corrosive medium, and subsequent removal of corrosion products on the strength, ductility, stages of work hardening, and localized deformation, as well as on the state of the side and fracture surfaces of specimens. It is established that the ductility loss of the specimens pre-exposed in a corrosive medium for 12 h decreases with the increasing strain rate, regardless of whether the corrosion products have been removed from their surface or not. It is shown that in this case, the negative strain rate dependence of the ductility loss is associated not with hydrogen dissolved in the bulk of a metal but with the presence of severe corrosion damage of the specimens’ surface. An explanation for the effect of corrosion damage on the mechanical properties and their strain rate sensitivity is proposed

Enhancing seedling survival on former floodplain grazing land in the Capertee Valley, Australia

© 2017 Ecological Society of Australia and John Wiley & Sons Australia, Ltd Active revegetation is an essential component of biodiversity conservation for fragmented ecosystems and the species that depend on them. However, key knowledge gaps exist around the most cost-effective revegetation strategies to employ in different contexts. This article reports on a revegetation trial undertaken in the Capertee Valley of New South Wales, Australia, to assist the conservation of the critically endangered bird, the Regent Honeyeater (Anthochaera phrygia). Seven treatments were compared to assess their cost-effectiveness for enhancing plant survival at a floodplain site with a history of grazing on introduced pastures. While overall survival rates were low, treatments involving tree guards had higher survival rates and were more cost-effective than treatments without guards. Weed growth, animal activity and water stress all appeared to play a role in the low survival rates at this site, with enhanced weed control emerging as a priority for future trials at similar sites

The effect of strain rate on mechanical properties and fracture mode of the AZ31 alloy and commercially pure magnesium pre-exposed in a corrosive medium

Magnesium alloys are promising materials for aviation, automotive engineering, and medicine, however, due to the low resistance to stress corrosion cracking (SCC), their wide application is limited. To create alloys with high resistance to SCC, a comprehensive study of this phenomenon nature is required. Previously, it was suggested that diffusible hydrogen and corrosion products formed on the magnesium surface can play an important role in the SCC mechanism. However, the contribution of each of these factors to the SCC-induced embrittlement of magnesium and its alloys is understudied. Since the influence of diffusible hydrogen on the mechanical properties of metals increases with the strain rate decrease, the study of the strain rate sensitivity of the SCC-susceptibility of magnesium alloys is a critical task. In this work, the authors studied the effect of the strain rate in the range from 5•10−6 to 5•10−4 s−1 on the mechanical properties, the state of the side and fracture surfaces of the as-cast commercially pure magnesium and the AZ31 alloy before and after exposure to a corrosive environment and after removal of corrosion products. The study identified that the preliminary exposure to a corrosive medium leads to the AZ31 alloy embrittlement, but does not affect the mechanical properties and the fracture mode of pure magnesium. The authors found that the AZ31 alloy embrittlement caused by the preliminary exposure to a corrosive medium appears extensively only at the low strain rate and only if the layer of corrosion products is present on the specimens’ surface. The study shows that a change in the strain rate has little effect on the mechanical properties of pure magnesium. The authors concluded that the main cause of the AZ31 alloy embrittlement after soaking in a corrosive medium is the corrosion products layer, which presumably contains the embrittling agents such as hydrogen and residual corrosive medium

The dependence of the biodegradable ZX10 alloy corrosion process on the structural factors and local pH level

Magnesium biodegradable alloys are a promising material for self-dissolving surgical implants. Magnesium is known to be sensitive to electrochemical corrosion due to the galvanic effect between the matrix and particles of secondary phases and inclusions. Another important factor is the pH level. The behavior of certain chemical reactions depends on the pH level, so one can assume that the pH level of a corrosive medium at the material surface is a factor determining what chemical reactions can occur there. Finally, there is evidence that variability of the crystallographic orientation of the grains may be a cause of anisotropy of corrosion properties. The purpose of this work is to reveal the influence of the electrode potential of the microstructural elements, the crystallographic orientation of the grains, and the pH level of the near-surface volume of the corrosion solution on the corrosion process. In the study, sections of 2×1.5 mm were marked on the ZX10 alloy samples, for which maps of the distribution of crystallographic orientations and chemical composition were drawn. To assess the influence of the electrode potential of the particles, the authors carried out a Kelvin probe mapping in the 90×90 µm area. Next, corrosion tests were carried out with video filming of the surface on the marked area. To determine the pH level influence, the solution circulation in the cell was varied. Upon completion of the tests, corrosion products and corrosion damage were examined in detail. According to the results, the pH level in the liquid near-surface micro-volumes has a greater influence than the electrode potential of the particles as it provokes the formation of corrosion products of a different composition, which leads to passivation of the surface areas around the particles. The authors identified two different types of filiform corrosion. For filiform corrosion, a correlation between the corrosion direction and the crystallographic orientation of the grains was established

p75 Neurotrophin receptor expression defines a population of BDNF-responsive neurogenic precursor cells

Although our understanding of adult neurogenesis has increased dramatically over the last decade, confusion still exists regarding both the identity of the stem cell responsible for neuron production and the mechanisms that regulate its activity. Here we show, using flow cytometry, that a small population of cells (0.3%) within the stem cell niche of the rat subventricular zone (SVZ) expresses the p75 neurotrophin receptor (p75(NTR)) and that these cells are responsible for neuron production in both newborn and adult animals. In the adult, the p75(NTR)-positive population contains all of the neurosphere-producing precursor cells, whereas in the newborn many of the precursor cells are p75(NTR) negative. However, at both ages, only the neurospheres derived from p75(NTR)-positive cells are neurogenic. We also show that neuron production from p75(NTR)-positive but not p75(NTR)-negative precursors is greatly enhanced after treatment with brain-derived neurotrophic factor (BDNF) or nerve growth factor. This effect appears to be mediated specifically by p75(NTR), because precursor cells from p75(NTR)-deficient mice show a 70% reduction in their neurogenic potential in vitro and fail to respond to BDNF treatment. Furthermore, adult p75(NTR)-deficient mice have significantly reduced numbers of PSA-NCAM ( polysialylated neural cell adhesion molecule)-positive SVZ neuroblasts in vivo and a lower olfactory bulb weight. Thus, p75(NTR) defines a discrete population of highly proliferative SVZ precursor cells that are able to respond to neurotrophin activation by increasing neuroblast generation, making this pathway the most likely mechanism for the increased neurogenesis that accompanies raised BDNF levels in a variety of disease and behavioral situations