발간논문

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An investigation was conducted into the effects of κ-carbides on the cracking phenomenon, which often occurred in cold-rolled light-weight steel plates. Three kinds of steels were fabricated by varying the C content, and their microstructures and tensile properties were investigated. In the two steels that contained a high carbon content, the band structures of ferrites and κ-carbides that were severely elongated along the rolling direction were well developed, whereas continuous arrays of κ-carbides were formed in the steel that contained a low carbon content. Detailed microstructural analyses of the deformed region beneath the tensile fracture surface showed that the cracks initiated at arrays of κ-carbides or κ-carbides formed interfaces between the band structures, which initiated cleavage fractures in the ferrite bands, while the bands populated with a number of κ-carbides did not play an important role in propagating the cracks. Thus, the minimization of interfacial κ-carbides or κ-carbide arrays by increasing the carbon content was essential for preventing cracking from occurring during cold rolling.

keyword : metals, rolling, fracture, scanning electron microscopy, SEM, κ-carbide

High-strength aluminum alloy sheets with high magnesium contents were fabricated by a strip caster equipped with an asymmetric nozzle, which has been proven to be effective for reducing surface defects and internal segregation. 4 mm thick as-cast sheets consisting of fine dendrites and minor Al8Mg5 segregation were hot-rolled successfully to 1 mm sheets and subsequently annealed at various temperatures. The sheet revealed the tensile strength and elongation of 306 MPa and 34%, respectively, when it was rolled at 250℃ and subsequently annealed at 475℃, which exhibits the feasibility of the practical application for autobodies. The observed mechanical properties were explained on the basis of the microstructural characteristics of the alloy sheets.

keyword : Al-Mg alloy, hot rolling, strength, optical microscopy, twin-roll strip casting

The effects of oxidation behavior on the hot ductility of plain carbon steels were investigated at various temperatures in order to simulate the continuous casting process more precisely, in which the process undergoes in air atmosphere rather than Ar atmosphere. The high temperature oxidation behavior and scale morphology of the carbon steels exposed to the air and Ar atmosphere at various temperatures were also investigated in order to assess the mechanism of the RA value decreasing in an air atmosphere. The RA values obtained from the air atmosphere were marked below 45% by the test temperature, except for over 1000℃, with the RA values remaining in low values in both the low and high temperature region, at which the RA values generally recovered in the Ar atmosphere. The surface roughness of the specimen was developed by external and internal oxidation when the specimen was deformed in an air atmosphere at high temperature, with the result being the stress concentrated at the roughness of the specimen surface, resulting in low RA values. The hot ductility in the air atmosphere was found to be likely controlled by the oxidation rate instead of the microstructures corresponding to test temperatures.

keyword : metals, thermomechanical processing, oxidation, tensile test, hot ductility

This study was performed to develop an antibacterial film that can be dried at room temperature. A nanosized TiO2 particle-dispersed solution was prepared by the hydrothermal treatment of peroxo-titanic acid at 160℃ for 4h. The binder was synthesized through the hydrolysis and condensation reactions of TEOS (10cc) and GPTS (3.5cc) in the mixture of H2O (30cc) and EtOH (30cc). The synthesized binder was mixed with 0.1 M of TiO2 solution in a volume ratio of binder/TiO2 solution=0.25~0.5. The glass substrate was coated after using the dip coating method, which was then followed by drying for over 2h at room temperature. Although the TiO2 particles did not chemically-bond to the binder, the coating layer strongly adhered to the substrate and displayed good antibacterial properties.

keyword : thin film, coating, microstructure, scaning electron microscopy, SEM, antibacterial TiO2

Studies were carried out to establish the technology for sodium-clad compatibility and to analyze the compatibility behavior of the Sodium-cooled Fast Reactor (SFR) cladding material under a flowing sodium environment. The natural circulation facility caused by the thermal convection of the liquid sodium was constructed and the 316-series stainless steels were exposed at 650℃ liquid sodium for 1458 hours. The weight change and related microstructural change were analyzed. The results showed that the quasi-dynamic facility represented by the natural convection exhibited similar results compared to the conventional dynamic facility. Selective leaching and local depletion of the chromium, re-distribution of the carbide, and the decarburization process took place in the 316-series stainless steel under a flowing sodium environment. This process decreased as the sodium flowed along the channel, which was caused by the change in the dissolved oxygen and carbon activity in the liquid sodium.

keyword : metals, aging, embrittlement, indentation, sodium

A dense nanostructured MoSi2-SiC composite was synthesized by a pulsed current activated combustion synthesis method within 2 min of one step from mechanically activated powders of Mo2C and Si. Simultaneous combustion synthesis and consolidation were accomplished under the combined effects of a pulsed current and mechanical pressure. Highly dense MoSi2-SiC with a relative density of up to 98% was produced under simultaneous application of an 80 MPa pressure and pulsed current. The average grain size and mechanical properties of the composite were investigated.

keyword : nanostructured materials, intermatallics, sintering, mechanical properties, powder processing

In the RH process, it is possible to obtain quicker processing times by enhancing the decarburization rates at a low carbon range of steel melt through Ar gas injection into the vacuum vessel. The RH decarburization reaction was simulated through a dissolved oxygen removal reaction by injecting nitrogen into a 1/8 scale RH water model system. The gas nozzles for the N2 injection into the vacuum vessel were located at the lowest level of the vessel`s outer wall. The nitrogen bubbling in the vacuum vessel resulted in an increase in the reaction rate constant, which rose in accordance with an increase in the bubbling flow rate and number of nozzles used. However, there was almost no variation in the reaction rate constant, which depended on the horizontal positions of the bubbling nozzles.

keyword : metals, purification, water model

Solvent extraction experiments of Rh(III) and Ir(IV) were performed on the HCl solution by using Alamine336 and TBP. The extraction percentage of Rh and Ir by Alamine336 was much higher than that by TBP. For the solvent extraction with Alamine336, the extraction percentage of Rh and Ir decreased with a HCl concentration. However, the extraction percentage of both metals by TBP was below 12% in our experimental range and increased with an increasing HCl concentration of up to 8 M. From the mixed solution of Ir with an excess SnCl2, most of the tin was extracted by Alamine336 and TBP. However, the extraction percentage of Ir by Alamine336 was reduced and no iridium was extracted by TBP. The extraction behavior of Ir and Sn was investigated by scrubbing experiments on the loaded Ir with a SnCl2 solution.

keyword : purification, recycling, electronic materials, fuel cells, metals

The hot tear susceptibility of Al alloys was investigated by using a constrained-rod mold designed to quantify 8 types of tear tendency. The severity of the crack was scored by 5 grades on a scale of 0 to 4, with 0 being "no crack formed" and 4 being "complete separation by crack". The Hot Tear Susceptibility index (HTS) which consists of crack type scores and position scores, was proposed to compare the hot tear tendency of Al alloys. A356.0 cast alloy and AA6061 wrought Al alloy showed an HTS value of 27.5 and 53 respectively. The effects of Si, Cu, and Mg content on hot tear tendency were also investigated with a constrained-rod mold. The variation of HTS values with alloying elements represents similar behavior in the variation of the solidification range in a pseudo binary phase diagram.

keyword : alloy, casting, solidification, image analysis

For an enhanced thermoelectric performance, one-dimensional heterostructure nanowires were created that consisted of aBi core and Te shell. The structure was fabricated by depositing Te in-situ onto a Bi nanowire grown by our unique OFF-ON (on-film formation of nanowires) method. After examining a cross-sectional TEM image, it was found that diffusive interface was formed between Bi and Te. Selected area electron diffraction revealed that the crystallinity of the Te shell was some what lower compared to the highly single-crystalline Bi core. The Bi-Te core/shell nanowires can be a smart structure that suppresses phonon transport by several scattering mechanisms, making the OFF-ON method the simplest way to realize that structure.

keyword : nanostructured materials, thermoelectic materials, microstructure, thermal conductivity, deposition