Kiyoshi Yokogawa

Surface oxidation of a Nb(100) single crystal by scanning tunneling microscopy

Abstract The surface structures of a Nb(100) single crystal in the initial stages of oxidation have been investigated by ultrahigh vacuum (UHV) scanning tunneling microscopy (STM). A ladder-like (3×10) structure was observed on the surface after numerous cycles of Ar-ion sputtering and flash annealing at 1973 K. The ladder-like structure could be identified as a NbO 2 overlayer on a Nb(100) surface due to the segregation of oxygen on the surface. The ladder-like structure gradually changed to a disordered structure upon exposure to oxygen at room temperature and was restored by annealing at above 853 K in UHV. The oxidation process on the Nb(100) surface is discussed.

Atomistic simulations of effect of hydrogen on kink-pair energetics of screw dislocations in bcc iron

Atomistic simulations are conducted on the hydrogen-affected kinking process of a screw dislocation in iron by the nudged elastic band method. We find that when a kink pair nucleates at hydrogen, the activation energy is decreased by the transition of hydrogen to a stronger binding site, while it is increased by the transition to a weaker binding site. When a kink pair meets hydrogen during expanding, the sideward motion of the kink pair is impeded by hydrogen. The simulation provides an insight into the complex atomistic process of hydrogen-induced softening and hardening.

Pore structure analysis of activated carbon fiber by microdomain-based model

The pore structures of commercial pitch and PAN-based activated carbon fibers (ACFs) were investigated. The pore size and pore size distribution of pitch-based ACFs were measured by nitrogen adsorption isotherms and 129Xe NMR spectroscopy and compared with each other. Scanning tunneling microscopy showed that the ACFs were composed of spherical microdomain units the size of a few nanometers. The activation mechanism of ACFs was considered and explained by novel hypothesis; the concept of microdomain structure of ACFs was considered and explained to overcome limitation of the conventional fractal hypothesis. Whereas micropores were generated on each microdomain, the origin of mesopores was interdomain pores, resulting from the microdomain hypothesis.

Embedded-atom-method functions for the body-centered-cubic iron and hydrogen

A new reliable embedded atom method potential for hydrogen in body-centered-cubic (bcc) iron is developed by fitting not only to the properties of hydrogen in a perfect bcc iron lattice but also to the properties of hydrogen binding to vacancies. The validity of the potential is examined by calculating the properties of hydrogen trap binding to surfaces and dislocations that are in good accordance with the experiments. A brief application of the potential by molecular dynamic simulation reveals that hydrogen accumulated ahead of the crack tip induces serious hydrogen embrittlement.

Hydrogen embrittlement of a single crystal of iron on a nanometre scale at a crack tip by molecular dynamics

A molecular dynamics simulation was conducted on hydrogen embrittlement at a crack tip of a single crystal of -iron composed of {100} planes under uniaxial tensile load along the 100 direction on a nanometre scale at 293 K. The modified Morse pair-potential function of Fe-Fe and the Morse potential function of Fe-H were used to calculate the interatomic action force. A three-dimensional model with 2618 iron atoms and from one to 260 hydrogen atoms segregated at the notched area was designed for the simulation. The general conclusion on hydrogen embrittlement of a single crystal of -iron could be described qualitatively as the processes of cavity nucleation, cavity linkage and, finally, fracture. Cavity nucleation occurs on the (100) plane in the notched area of the specimen with more than three hydrogen atoms at the early stage of the deformation and does not depend on the hydrogen content. The cavities are linked to each other and fracture occurs on the plane with progressing deformation. The deformation step of cavity linkage and fracture decrease logarithmically with increasing hydrogen content, while neither the cavity nor the fracture occur, even at the maximum total deformation of 50%, due to blunting of the crack tip in the specimen without hydrogen.

Tensile behaviour of type 304 austenitic stainless steels in hydrogen atmosphere at low temperatures

Abstract The tensile behaviour of solution annealed type 304L, solution annealed type 304, and solution annealed and sensitised type 304 stainless steels was investigated in hydrogen and helium under a pressure of 1·1 MPa over the temperature range 300–80 K at strain rates ranging from 4·2×10-5 to 4·2×10-2 s-1. For 304L steel, hydrogen environment embrittlement (HEE) increased with decreasing strain rate. For 304L and 304 steels, HEE increased with decreasing temperature, reached a maximum, and then decreased with further decrease in temperature: the decrease was particularly rapid near the minimum temperature for HEE. Sensitisation enhanced the HEE of 304 steel. Above the maximum HEE temperature, the HEE behaviour was similar to the hydrogen embrittlement behaviour of materials in previous studies, but near the minimum temperature for HEE it was different. Three types of hydrogen induced brittle fracture were observed as a result of HEE: transgranular fracture along strain induced martensite laths and twin boundary fracture on the fracture surfaces of solution annealed 304L and 304 steels, and grain boundary fracture on the sensitised 304 steel. It was found that from room temperature to the maximum HEE temperature, the HEE of the materials depended on the transformation of strain induced martensite and below the maximum HEE temperature it depended on the diffusion of hydrogen.

Surface Superstructure of Carbon Nanotubes on Highly Oriented Pyrolytic Graphite Annealed at Elevated Temperatures

Carbon nanotubes deposited on highly oriented pyrolytic graphite (HOPG) are annealed in ultra high vacuum. The effect of annealing temperature on the surface morphology of the carbon nanotubes on HOPG is examined by scanning tunneling microscopy. The ring-like surface superstructure of (√ 3×√ 3)R30° of graphite is found on the carbon nanotubes annealed above 1593 K. The tips of the carbon nanotubes are destroyed and the stacking misarrangement between the upper and the lower walls of the tube join with HOPG resulting in the superstructure.

Structural changes of fullerene by heat-treatment up to graphitization temperature

Abstract Structural changes of the C60 and C70 mixture during heat-treatment up to 2400 °C were studied by observing the carbonized disk of the fullerene with Raman spectroscopy, X-ray diffraction, FE-SEM, TEM, and AFM/STM. The fullerene lost its five-membered ring and its fcc crystal structure by heat-treatment at 800 °C, as revealed by the Raman spectra and X-ray diffraction, forming hexagonal planes which were randomly arranged by 1300 °C. Further heat-treatment allowed some stacking of layers which grow to dominate, reducing the randomly oriented planes. The graphitized temperature up to 2400 °C provided a very sharp peak at 26 °, suggesting formation of stable turbostratic layers. The TEM characterized turbostratic stacking of 3 to 4 layers. A series of observation under AFM/STM and TEM indicate the crystal of the fullerene, amorphous grain of the hexagonal planes, and the hollow sphere are all in the same range of size around 10–20 nm. Such microdomains induced micro-roughness as observed by FE-SEM on the surface of the carbon disk. Superstructure of hexagonal plane was observed on the surface. A kind of solid state carbonization of the fullerene is suggested to maintain the dimensions of its crystal into the spherical microdomain, even if its marked structural changes take place within the unit.

Characterization of hydrogen-induced crack initiation in metastable austenitic stainless steels during deformation

Hydrogen-induced crack initiation in hydrogen-charged metastable austenitic stainless steels during deformation at 295 K is characterized by performing a combined tensile and hydrogen release experiment and scanning probe microscopy. Strain-induced martensite (α′) not only provides a path for rapid hydrogen diffusion in austenite (γ) but also promotes crack initiation. Hydrogen rapidly diffuses from α′ and accumulates at the boundary between the α′-rich and γ-rich zones during deformation due to the high hydrogen diffusivity and low hydrogen solubility in α′, resulting in crack initiation at the boundary between the α′-rich and γ-rich zones. The hydrogen-induced crack initially grows along the boundary between the α′-rich and γ-rich zones and then propagates in the α′-rich zone.

Evolution of Ar+-damaged graphite surface during annealing as investigated by scanning probe microscopy

The surface evolution of highly oriented pyrolytic graphite irradiated with Ar+ ions of 1.0 keV at doses between 5×1011 and 1×1013 ions/cm2 during annealing was investigated by scanning tunneling microscopy (STM) and atomic force microscopy (AFM) in the tapping mode. Hillocks were observed by both STM and AFM after ion irradiation, where the height of a hillock measured by STM was larger than that measured by AFM. The ion-irradiated surface was recovered in three stages during annealing: the first stage at 473–873 K, the second stage at 873–1473 K, and the third stage at 1473–1873 K. In the first stage, many of the ion-induced hillocks recovered rapidly and irregular domelike protrusions were formed due to both the recombination of the mobile interstitial clusters with the immobile vacancies and the aggregation of interstitial clusters. In the second stage, the hillocks recovered slightly and domelike protrusions aggregated to larger domelike protrusions. In the third stage, the hillocks recovered complet...