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The aim of this investigation is to prove the mechanism of hydrogen induced crack (HIC) of 316L stainless steels in hydrogen pressured tensile test. Microstructures like twin, planar slip, and abnormal phase transformation around the HIC were analyzed by transmission electron microscopy. Deformation twin accompanied by planar slip could be related to the main cause of HIC in the hydrogen pressured tensile condition, because intragranular HICs were mainly observed along the boundaries of twins and planar slip lines. An abnormal forbidden diffraction was also accompanied by HIC in the hydrogen attacked area. Examination of the HIC mechanism in austenitic stainless steel can be applied to the fitness of use for alloys with the possibility of various susceptible cracks in a hydrogen and stress atmosphere.

keyword : hydrogen induced crack, stainless steel, deformed twin, planar slip, forbidden diffraction, phase transformation

Berkovich pyramidal indents formed on stressed metallic glass samples were observed with an atomic force microscope. Three-dimensional morphologies of the remnant impressions with pile-up lobes were approximated as six-sided pyramids and their indent strains were estimated according to Milman’s strain definition. Under a residual stress state, it was possible to formulate the indent strain using the sum of the elastic indentation stresses and the residual stress parallel to the sample surface. Finally, the residual stress can be expressed by the multiple of the stress-dependent change in the indent strain and the Young``s modulus of the indented sample. The predicted stresses estimated from the pyramidal indent morphologies were comparable to the applied stresses except in a high stress regime. This deviating phenomenon under high tensile stress is discussed and found to be due to exaggerated plastic deformation and image scatter due to mild pile-ups.

keyword : amorphous materials, deformation, residual stress, indentation, indent strain

In this study, the effect of heat treatment (peak aging, T6) on the porosity and high-cycle fatigue deformation behavior of A356 alloy was investigated. As a result of tensile and high-cycle fatigue tests, A356-T6 exhibited superior tensile strengths (44% increased), fatigue life, and fatigue limit (43% increased) compared to A356-F alloy, due mainly to the evolution of microstructure followed by T6 heat treatment. Globular Si particles, evenly distributed fine Mg2Si particles, and decreased dendrite arm spacing were found to have enhanced tensile and high-cycle fatigue properties effectively. Note, however, that the deteriorating effects of pores were greater on A356-T6 alloy compared to A356-F alloy; those were analyzed using ΔK (stress intensity factor range) parameters.

keyword : alloys, casting, fatigue, scanning electron microscopy, porosity

The microstructure of carbon film, deposited using hot-filament chemical vapor deposition, was investigated in relation to deposition pressure and methane concentration as deposition variables. Methane concentration in hydrogen gas was varied from 0.5 to 5% in volume. Deposition pressures were 5, 15, 45 and 75 Torr. Filament temperature and deposition temperature were fixed at 2100 ℃ and 950 ℃, respectively. With increasing methane concentration, the surface morphology changed its microstructure from microcrystalline diamond, to nanocrystalline diamond, to graphite. Raman spectroscopic analysis and X-ray diffraction analysis confirmed the bonding structures corresponding to each microstructure. At 5 Torr, the surface showed a fine grained morphology, different from the microstructures in the other pressure cases; however, the bonding nature also changed from diamond to graphite with increasing methane concentration.

keyword : carbon and graphite, deposition, microstructure, SEM, diamond

This paper reports the tunable structural design and thermal characteristics of a SiO2 layer on Cu2O pigment via a dual-effect of NH4OH catalyst. We prepared octahedron Cu2O 200~300 nm in size and successfully synthesized a core-shell Cu2O@SiO2 structure using a modified Stober method in which the NH4OH concentration was adjusted. The NH4OH solution dipping time was controlled such that the Cu2O core was generated with a yolk-shell and a hollow-shell. The silica-coated Cu2O raw materials were calcined at various temperatures (150~500 ℃). The thermal and color effects of the silica on the Cu2O were characterized by scanning electron microscopy, CIE L*a*b* color parameter measurements, transmission electron microscopy, and X-ray diffraction.

keyword : Cu2O, nanostructured materials, surface modification, phase transformation, transmission electron microscopy (TEM)

Aluminum titanate (AT) ceramics has high potential as alloy casting refractory materials due to their good thermal shock resistance which results from their low coefficient of thermal expansion. In a previous study, AT doped SiO2 and ZrO2 were developed to achieve stability and mechanical strength at high temperature. In this work, the changes of microstructure of AT doped SiO2 and ZrO2 were investigated in molten steel. A finger rotating test of sintered AT was carried out at 1300 ℃ for 30 min, 1 h, 2 h, and 3 h. Even if there was no reaction between AT and molten steel, a new surface was formed, followed by the decomposition of AT by the heat from molten steel, which was dominantly constituted of Al and Si due to decomposition of AT and Mullite phases. Some of the new surface layer was found to have been eroded by fluctuations of the molten steel.

keyword : aluminum titanate, high-temperature corrosion, low thermal expansion, high-temperature strength

An n-type Bi2Te3 thermoelectric compound was analyzed by using a high resolution transmission electron microscopy, and was further analyzed by using an image processing unit. A sinusoidal strain contrast of the structural modulations appeared in a multi-beam image and the fringes showed a wavelength of 12 nm with a wave vector parallel to the {101.10} planes. A numerous dislocations were found to glide on to the (0001) plane with a Burgers vector of 1/3<21.1.0>-type on the strain field. In order for the elastic strain energy to be relieved, the resultant 1/3<21.1.0>-type dislocation was thought to be dissociated into two partial dislocations gliding on the {1.015} planes. The variation in stacking periodicity of the 5-layers lamellar structure with respect to the basal plane of Bi2Te3 was also observed locally. However, this stacking variation is not responsible for the formation of structural modulations. The displacement of the sinusoidal strain field on the structural modulations seems to be generated by dislocation, with presence of dipoles that are about a few nanometers apart. It is understandable that phonons should be scattered throughout the strain field of the structural modulation. Hence, the lattice thermal conductivity is expected to be decreased due to the phonon scattering on sinusoidal strain field of the structural modulations.

keyword : thermoelectric materials, structural modulations, dislocation, transmission electron microscopy (TEM), extrusion

A sample with a composition of 86 wt% MgH2-10 wt% Ni-2 wt% NaAlH4-2 wt% Ti (designated MgH2-10Ni-2NaAlH4-2Ti) was prepared by reactive mechanical grinding. The sample was subjected to thermal cycling (between room temperature and 623 K) as well as cycling between hydriding (at 593 K under 12 bar H2) and dehydriding (at 623 K in vacuum). At n=1, MgH2-10Ni-2NaAlH4-2Ti absorbed 3.10 wt% H for 5 min, 4.14 wt% H for 10 min, 4.35 wt% H for 15 min, and 4.45 wt% H for 60 min at 593 K under 12 bar H2. From the beginning to about 10 min, the hydriding rate of the sample increased roughly as the number of cycles, n, increased. The quantity of hydrogen absorbed for 60 min, Ha (60 min), decreased on the whole as the number of cycles increased from 1 to 15. At n=15, MgH2-10Ni-2NaAlH4-2Ti absorbed 3.31 wt% H for 5 min, 3.60 wt% H for 10 min, 3.66 wt% H for 15 min, and 3.82 wt% H for 60 min. The quantity of hydrogen desorbed for 30 min, Hd (30 min), increased slowly as the number of cycles increased from n=2 to n=15. After hydriding-dehydriding cycling, β-MgH2, Mg, Mg2Ni, TiH1.924, NiAl, and MgO were observed in the samples. NiAl was formed from the reaction of Ni with Al which was formed by the decomposition of NaAlH4.

keyword : hydrogen absorbing materials, mechanical alloying/milling, microstructure, scanning electron microscopy (SEM), NaAlH4, Ni and Ti addition

ZnO nanorods were hydrothermally grown on R-plane sapphire substrates coated with an as-grown ZnO seed layer and with ZnO seed layers annealed at different temperatures. The effects of the seed-layer annealing temperature on the structural and optical properties of the ZnO nanorods were investigated using scanning electron microscopy, X-ray diffraction, and photoluminescence. ZnO nanorods and nanosheets grew on the as-prepared seed layer. Only ZnO nanorods grew on the ZnO seed layer annealed above 700 ℃. The structural and optical properties of the ZnO nanorods were significantly enhanced when the seed layers were annealed at 700 ℃. A cubic equation was used to establish the non-linear exciton radiative lifetime of the free exciton emission peak. Varshni’s empirical equation fitting parameters were α = 4 × 10.3 eV/K, β = 1 × 104 K, and Eg(0) = 3.335 eV; the activation energy was ~94.6 meV.

keyword : hydrothermal synthesis, optical materials, nanostructured materials, annealing, optical properties, scanning electron microscopy

For implement the white light-emitting diode (LED) package, glass-phosphor composite was produced with different glass composition and glass/phosphor ratio. To maximize the LED light emission characteristic, the glass frit was required a low firing temperature and high refractive index. We selected the bismuth phosphate glass due to their low melting and high refractive index. With further addition of Bi2O3, the value of Tg, Ts was decrease and coefficient of thermal expansion, refractive index were increased, due to the Bi2O3 acts as modifier in the glass increasing the non-bridging oxygens. The refractive index changes of the glass frit affect the optical property of the composite. With higher contents of Bi2O3 glass composition, the transmittance and emission intensity of the composite was increased. These results suggest that the difference in refractive index between the phosphor and glass frit is minimized, the loss of light is minimized.

keyword : composites, sintering, optical properties, thermal analysis, white LED