Search

In the present study, the microstructure and corrosion behaviour of laser beam welded 2A97 alloy are investigated. Welding process results in segregation of alloying elements within the fusion zone and, consequently, formation of T1 (Al2CuLi), TB (Al7Cu4Li) and R (Al5CuLi3) phase interdendritic intermetallics. The distribution of the interdendritic intermetallics is non-uniform across the weld. In NaCl solution, corrosion initiates at interdendritic intermetallic with selective dissolution of Al and Li, resulting in a copper-rich remnant, which subsequently acts as effective cathode supporting the dissolution of alloy matrix. The equiaxed dendrite zone at the centre of the weld is most susceptible to corrosion.

Fluoride ions are commonly present in solutions used in pre-treatments and conversion treatments of aluminium alloys. Because of the intrinsically reactive nature of aluminium metal, alumina layers are either already present on an aluminium surface or form within a matter of seconds on pre-etched aluminium. Hence fluoride ion transport through an alumina layer is an integral part of conversion coating formation. In order to understand the behaviour of fluoride ions in alumina films, the present study investigated the behaviour of fluoride ions during the growth of barrier-type anodic films formed on aluminium at 5 mA cm-2 in 0.1 M ammonium pentaborate solution. The films were examined by analytical transmission electron microscopy, scanning electron microscopy and glow discharge optical emission spectroscopy in order to determine their composition and morphology. Fluoride ions were incorporated into the films by adding sodium fluoride to the electrolyte. Additions of up to 3.5 x 10-3 M sodium fluoride had a negligible influence on the film growth, which occurred at a high efficiency. In contrast, additions of 3.5 x 10-2 M sodium fluoride reduced the efficiency to about 60%. Dissolution studies demonstrated that fluoride ions had promoted the field-assisted ejection of Al3+ ions from the film surface. Using sequential anodizing, the fluoride ions were shown to migrate inwards in the film at a rate about twice that of O2- ions. Thus, a thin fluoride-rich layer of the film was formed next to the aluminium/film interface. Increasing the sodium fluoride content to 3.5 x 10-1 M led to a porous film formed at a low voltage.

High-resolution three-dimensional structure of chromosomes in a human prophase nucleus from electron microscopy., The human genetic material is packaged into 46 chromosomes. The structure of chromosomes is known at the lowest level, where the DNA chain is wrapped around a core of eight histone proteins to form nucleosomes. Around a million of these nucleosomes, each about 11 nm in diameter and 6 nm in thickness, are wrapped up into the complex organelle of the chromosome, whose structure is mostly known at the level of visible light microscopy to form a characteristic cross shape in metaphase. However, the higher-order structure of human chromosomes, between a few tens and hundreds of nanometers, has not been well understood. We show a three-dimensional (3D) image of a human prophase nucleus obtained by serial block-face scanning electron microscopy, with 36 of the complete set of 46 chromosomes captured within it. The acquired image allows us to extract quantitative 3D structural information about the nucleus and the preserved, intact individual chromosomes within it, including their positioning and full spatial morphology at a resolution of around 50 nm in three dimensions. The chromosome positions were found, at least partially, to follow the pattern of chromosome territories previously observed only in interphase. The 3D conformation shows parallel, planar alignment of the chromatids, whose occupied volumes are almost fully accounted for by the DNA and known chromosomal proteins. We also propose a potential new method of identifying human chromosomes in three dimensions, on the basis of the measurements of their 3D morphology.

Scanning Kelvin probe force microscopy (SKPFM) measured local Volta potentials in microstructure of 22Cr-5Ni duplex stainless steel have been correlated to microstructure development with aging treatments at 475◦C. Magnetic force microscopy (MFM) was employed to differentiate crystallographic phases to provide complementary information. The absolute Volta potentials of both ferrite and austenite increased after 5 hours of aging, indicating electrochemical ennoblement of the entire microstructure. Longer aging resulted in a gradual decrease of measured Volta potentials in both phases. The microstructure showed after 255 hours aging up to 2.5-times larger potential differences than in the as-received condition, indicating impaired electrochemical nobility. In the as-received microstructure, the ferrite phase was less noble than the austenite, whereas after 5 hours aging both phases had similar, balanced Volta potentials which indicated a balanced nobility of ferrite and austenite. Longer aging treatment caused severe loss of nobility for the entire microstructure, with ferrite showing larger changes inVolta potential than the austenite. Spinodal microstructure decomposition and associated phase reactions of the ferrite, with elemental redistribution in the austenite, are the reason for the observed changes in microstructure nobility.

The present work investigates the corrosion behaviour of machined AA7150-T651 aluminium alloy. It is found that machining operation introduces a near-surface deformed layer, characterized by ultrafine grains and segregation of Mg and Zn at grain boundaries, to the alloy. The near-surface deformed layer is electrochemically more active compared with the bulk alloy, therefore, is preferentially corroded during immersion in an 3.5 wt.% sodium chloride solution acidified to pH 3.2. The open circuit potential of the machined alloy shows a transient at the beginning of immersion in the solution, due to the preferential dissolution of the near-surface deformed layer.

In the present study, the effects of mechanical polishing on the microstructure and corrosion behaviour of AA7075 aluminium alloy are investigated. It was found that a nano-grained, near-surface deformed layer, up to 400 nm thickness, is developed due to significant surface shear stress during mechanically polishing. Within the near-surface deformed layer, the alloying elements have been redistributed and the microstructure of the alloy is modified; in particular, the normal MgZn2 particles for T6 are absent. However, segregation bands, approximately 10-nm thick, containing mainly zinc, are found at the grain boundaries within the near-surface deformed layer. The presence of such segregation bands promoted localised corrosion along the grain boundaries within the near-surface deformed layer due to microgalvanic action. During anodic polarisation of mechanically polished alloy in sodium chloride solution, two breakdown potentials were observed at −750 mV and −700 mV, respectively. The first breakdown potential is associated with an increased electrochemical activity of the near-surface deformed layer, and the second breakdown potential is associated with typical pitting of the bulk alloy.

第27回宇宙利用シンポジウム (2011年1月24日-25日, 宇宙航空研究開発機構宇宙科学研究所相模原キャンパス), 相模原市, 神奈川県, The Twenty-seventh Space Utilization Symposium (January 24-25, 2011. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)(ISAS)), Sagamihara, Kanagawa Japan, Our communication wilth others depends critically on our ability to read facial expressions. On the ground, such ability is optimized for the face in the upright direction: it becomes hard to read facial expressions when the face inverted. Event related potentials in response to face stimuli change with the direction of the face. In the space, on the other hand, astronauts have to communicate with other members even when their faces are inverted. We hypothesize that adaptive changes would occur in the brain during a long-term stay in space so that astronauts become able to read facial expressions irrespective of the direction of the face. We propose a research plan to test this hypothesis., 形態: カラー図版あり, Physical characteristics: Original contains color illustrations, 資料番号: AA0065129052