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This study was conducted to investigate the effect of aging treatment from 300 ℃ to 420 ℃ on the mechanical properties of the pressure tube material at 310 ℃, which is outlet temperature pressure tube in CANDU reactor. The short range ordering (SRO) phenomenon during aging process is confirmed by differential scanning calorimetry (DSC), TEM, and slow strain rate tensile test. As-received and aged pressure tube materials exhibit a sudden stress drop in stress at 50 MPa or less when subjected to tensile strain at a strain rate as low as 1 × 10^-6/s at 310 ℃. This is an evidence of destruction of SRO formed in the initial and the aged state. The yield and tensile strengths decreased by 4-11% by aging treatment at 300-420 ℃ for 3,000 hours. Through the DSC analysis, the quadruple-melted pressure tube materials showed an exothermic reaction at around 410 and 590 ℃ in the heating process. This is due to SRO reaction, and the activation energy for the exothermic reaction at around 590 ℃ is obtained as Q = 90 kJ / mole. This reaction seems to be related to the formation of SRO between Zr and O (oxygen) which is dominated by grain boundary diffusion of oxygen in α-Zr. The effects of strain rate and short range ordering (SRO) were discussed in detail. (Received May 16, 2018; Accepted June 8, 2018)

keyword : Zr-2.5%Nb, differential scanning calorimetry (DSC), short range ordering (SRO), aging, tensile property

Austenitic stainless steels are widely used in a variety of industries, such as power plants, aerospace, automotive, construction and electronics, due to their excellent mechanical properties including ductility, toughness and corrosion resistance. For application in various industries, many parts and products require plastic processes such as cold rolling, cold forging and bending. These processes cause a phenomenon in which the strength is increased but the toughness is lowered by the plastic deformation. However, heat treatment of the plastic-deformed material can improve mechanical properties by grain refinement due to recrystallization and phase transformation. In this study, the effect of grain refinement and phase transformation by heat treatment of cold-worked austenitic stainless steels were evaluated in order to improve mechanical properties such as toughness and strength. After the cold working process, strength and hardness were increased by the phase transformation of martensite from austenite, and an increase in dislocations by plastic deformation. After heat treatment, the ductility and toughness of the austenitic stainless steels strengthened by cold working process were increased due to grain refinement caused by recrystallization of the remaining austenite, and the reverse transformation from martensite to austenite induced by heat treatment. (Received May 9, 2018; Accepted June 4, 2018)

keyword : austenitic stainless steel, recrystallization, grain refinement, phase transformation, cold working

This study investigated the effect of Si on the characteristics of FCA multi-layer weld metal with 460 MPa yield strength grade steels for ships and marine structures. The effect on microstructure was evaluated by tensile, charpy impact and CTOD testing of 4 FCAW welds containing various C and Si contents. C content affected weld metal properties depending on Si content, microstructure, strength and impact value. With lower C content, strength and impact value increased when Si content was reduced, because of higher AF volume fraction. With higher C content, those properties were little affected by Si content. These phenomena indicate that C affects phase transformation more than Si. The CTOD value was higher with lower Si and higher C content. This was interpreted to mean that a lower volume fraction of not only PF, FS but also the M-A phase, can affect crack initiation sites, due to higher C and lower Si contents. (Received April 5, 2018; Accepted May 16, 2018)

keyword : impact toughness, strength, CTOD, microstructure. FCAW, Si

Graphite fiber attracted much attention in the area of aluminum based composite due to its low density and high strength. However, one of the major drawbacks in fabricating graphite reinforced metal matrix composites (MMCs) is that aluminum generally does not wet graphite. Therefore, it is difficult to impregnate the graphite fiber in molten aluminum. The application of external electric power has been recently reported as a promising way to improve the wettability. The effect of an electric current on the wettability of molten Al on graphite has not been investigated. The effects of electric current, density of sample and heating time on wettability, thickness of product, and microstructure were investigated. The wettability remarkably improved utilizing electric current due to a larger formation of Al₄C₃ per unit time from electromigration. The wettability of molten Al on graphite substrate is better with an applied negative current and with lower relative density of sample. (Received May 25, 2018; Accepted June 8, 2018)

keyword : wettability, Al₄C₃, electric current, aluminum, graphite

The leaching behavior of silver from electronic scrap using acidic thiourea solution was studied. The results showed that the silver leaching percentage was affected by the concentration of thiourea and hydrochloric acid, the oxidant such as ferric ions, solid-liquid ratio, temperature, rpm, etc. The leaching percentage of silver decreased with an increase in the solid-liquid ratio, and the leaching rate of silver was enhanced by increasing the thiourea and hydrochloric acid concentration. However, when the concentration of hydrochloric acid and oxidant were increased above 1.0 M and 30 mM, respectively, the leaching rate of silver decreased. When the reaction temperature was above 50 ℃, the leaching rate of silver decreased because of the decomposition of thiourea. More than 99 % leaching of silver was achieved under the following conditions: thiourea conc. 0.8 M, HCl conc. : 0.5 M, Temp. : 35 ℃, leaching time : 4 hours, solid-liquid ratio : 25 g/L. (Received May 11, 2018; Accepted June 6, 2018)

keyword : silver, thiourea, leaching, electronic scraps

This study investigated the crystal structure and mechanical properties of copper foil electroplated by adjusting gelatin and chloride ions from 0 to 100 ppm. There was increased formation of spherical crystals and surface roughness on the surface of the electroplated layer when the gelatin was added, and the direction of crystal growth was parallel to the (111) plane. The grain size of the copper foil electroplated in the electrolyte containing 30~60 ppm of gelatin was smaller than that of groups with no additive. As a result, hardness increased, but elongation decreased. In contrast, the elongation and the crystal size of copper foil formed in the electrolyte containing chlorine ions increased more than the group with no additive. The clearly observed tendency was that crystals of the group with no additive as well as the group with added chloride ions grew along the (220) plane. However, the use of the electrolyte with chlorine ions and gelatin additives was not enough to obtain a uniform surface and desirable mechanical properties in the electroplated layer. The addition of a leveler and brightener could be used to fabricate useful copper foil. The tensile strength was similar for the no additive group and mixed additive group. However, it was observed that the grain size of the mixed additive group elongated 34.1% more than the no additive group. (Received April 5, 2018; Accepted May 29, 2018)

keyword : electroplating, organic additive, mechanical properties, microstructural evolution

To improve the hydrogen uptake and release properties of magnesium (Mg), five weight percent of graphene was added to pre-milled Mg by milling in hydrogen (reaction-involving milling). The hydrogen uptake and release properties of the graphene-added Mg (named Mg-5graphene) were investigated. Premilling of Mg (for 24 h) and then adding graphene by milling in hydrogen (for 30 min) significantly increased the hydrogen uptake and release rates and the quantities of hydrogen absorbed and released for 60 min of the Mg and graphene composite. The activation of Mg-5graphene was completed after the second hydrogen uptake-release cycle (n=2). Mg-5graphene had a high effective hydrogen-storage capacity (quantity of hydrogen absorbed for 60 min) of 5.47 wt% at 593 K in 12 bar H₂ at n=3. Mg-5graphene absorbed 0.92 wt% H for 2.5 min, 2.67 wt% H for 10 min, and 5.62 wt% H for 60 min in 12 bar H₂ and released 0.25 wt% H for 2.5 min and 4.99 wt% H for 60 min in 1.0 bar H₂ at 623 K at n=1. The increase in ID/IG, the ratio of intensities of D and G peaks in Raman spectra, after reaction-involving milling of the pre-milled Mg with graphene, compared with that of the used graphene, suggests that defects and disordering in the graphene were increased. We believe that the generation of stacking fault disorder and formation of turbostratic graphite occurred after milling with the pre-milled Mg, rather than the formation of defects, in graphene. (Received April 30, 2018; Accepted June 8, 2018)

keyword : hydrogen absorbing materials, mechanical milling, hydrogen, microstructure, graphene-added Mg alloy

Cu₂Se exhibits very low thermal conductivity and the origin is known to be the "liquid-like" behavior of Cu ions around the sub-lattice of Se. Due to its very high ZT, research interest in the thermoelectric properties of this material has been increasing rapidly. However, most studies have used a conventional melting process, and, to the best of our knowledge, the melt-spinning process has not yet been employed to fabricate the Cu₂Se compound. Herein, we report the thermoelectric properties of Cu_1.98Se compounds prepared by melt-spinning. Depending on the cooling rate, we could control the microstructure of the melt-spun Cu_1.98Se ribbons successfully. The melt-spun ribbons were consolidated using a hot press, and the thermoelectric properties of the compounds were characterized. Although the electrical conductivity of the compound decreased as compared with the Cu_1.98Se compound prepared by the conventional method, an increase in Seebeck coefficient led to an improved power factor in the compound prepared by melt-spinning. Furthermore, a reduction in thermal conductivity due to the reduced contribution of electronic to total conductivity resulted in the enhancement of ZT values in the Cu_1.98Se compound. (Received May 3, 3018; Accepted May 23, 2018)

keyword : thermoelectric, Cu2Se, melt-spinning, hot pressing

We report the effect of interface control using multiwalled carbon nanotubes (MWCNT) on thermoelectric properties of TiO₂. By consolidating TiO₂ nanoparticles with MWCNT (0.5, 1, 2, 4, and 8 wt%) using spark plasma sintering, we prepared interface-controlled TiO₂-MWCNT nanocomposites, where TiO₂ grains were surrounded with a MWCNT network. Simultaneous control of charge and thermal transport was successfully achieved by interface control using MWCNTs. The electrical conductivity increased monotonically as the MWCNT content was increased. As determined in our previous report, the charge transport mechanism in the nanocomposites is based on percolation and hopping models. The formation of new interfaces at the grain boundaries using the MWCNT network led to additional phonon scattering in the nanocomposites, and the thermal conductivities decreased monotonically with increasing MWCNT content. However, the incorporation of the MWCNT did not lead to a significant increase in power factor due to a reduction in the Seebeck coefficient. Consequently, the highest ZT value of 4.6 × 10^-3 was obtained from the TiO2-0.5 wt% MWCNT nanocomposite at 1073 K. Our results introduce a strategy for the independent control of electron and phonon transport based on interface control using carbon nanomaterials in hybrid thermoelectric materials. (Received May 8, 2018; Accepted May 23, 2018)

keyword : thermoelectric, TiO₂, carbon nanotube, interface control

Iodine-doped n-type Bi_2-xSb_xTe_3-ySey:I_m (x = 0.1-0.2, y = 0.15-0.6, and m = 0.0025-0.005) quaternary solid solutions were prepared by encapsulated melting (EM) and hot pressing (HP). The lattice constants of the resulting materials decreased with increasing Sb and Se contents, whereas the amount of I dopant was too small to significantly affect the lattice constants. An average relative density of 97% was obtained for the hot-pressed specimens. All specimens indicated n-type conductions in the temperature range of 323 to 523 K, and the electrical conductivity decreased with increasing temperature, exhibiting the typical characteristics of degenerate semiconductors. The electrical conductivity and the thermal conductivity decreased with increasing Se content, whereas they increased with I doping. This was a result of changes in the carrier concentrations due to Se substitution and I doping, and from the decreased lattice thermal conductivity due to increased alloy scattering. However, the changes in electrical and thermal conductivity caused by Sb substitution were not significant. The maximum figure of merit, ZT_max, of 0.84, was achieved at 473 K for Bi_1.9Sb_0.1Te_2.85Se_0.15:I_0.0025. (Received May 9, 2018; Accepted June 1, 2018)

keyword : thermoelectric, bismuth telluride, charge transport, mechanical alloying, hot pressing