발간논문

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The local structural states of amorphous alloys have been depicted previously via short-range orders (SROs). However, the concept of SROs alone is inadequate and sometimes insufficient to explain the structure-property relation of the amorphous alloys. In this study, we propose new types of medium-range building structures that affect the mechanical properties, plasticity in particular. Using a combination of molecular dynamics simulations and the Voronoi tessellation method, we demonstrate a three-dimensional configuration of icosahedral medium-range orders (I-MROs) and elucidate how these icosahedral orders evolve by the application of shear deformation. It was observed that the structural stability of the icosahedral orders relies largely on how they are linked via percolation and this linking is explained in detail.

keyword : amorphous materials, deformation, computer simulation, short(medium)-range order

In the present study, a Zr-based amorphous alloy matrix composite reinforced with tungsten porous foam was fabricated without pores or defects by liquid pressing process, and its microstructures and mechanical properties were investigated. About 69 vol.% of tungsten foam was homogeneously distributed inside the amorphous matrix, although the matrix of the composite contained a small amount of crystalline phases. The compressive test results indicate that the composite was not fractured at one time after reaching the maximum compressive strength, but showed considerable plastic strain as the compressive load was sustained by tungsten foam. The tungsten foam greatly improved the strength (2764 MPa) and ductility (39.4%) of the composite by homogeneously dispersing the stress applied to the matrix. This was because the tungsten foam and matrix were simultaneously deformed without showing anisotropic deformation due to the excellent bonding of tungsten/matrix interfaces. These findings suggest that the liquid pressing process is useful for the development of amorphous matrix composites with improved strength and ductility.

keyword : composite, infiltration, mechanical properties, compression test, Liquid pressing process

The Mg-Zn-RE alloy cladded with the thin Al1050 sheet was fabricated by means of a roll bonding process at 280℃. Microstructures and mechanical properties of the clad sheets were investigated. After heat treatment at 230℃ for 30 min, an Mg-rich diffusion layer with about 2 μm in thickness was developed at the Mg and Al interface. Tensile tests were carried out in a temperature range up to 300℃. The clad sheet exhibits superior elongation to failure not only at room temperature but also at elevated temperatures compared with those of the Mg alloy sheet. For the deformed specimens, interface debonding does not occur and the diffusion layer shows only a few cracks.

keyword : clad sheet, alloys, diffusion bonding, mechanical properties, tensile test

Higher strength and fracture toughness of reactor pressure vessel steels can be obtained by changing the material specification from that of Mn-Mo-Ni low alloy steel (SA508 Gr.3) to Ni-Mo-Cr low alloy steel (SA508 Gr.4N). However, the operation temperature of the reactor pressure vessel is more than 300℃ and the reactor operates for over 40 years. Therefore, we need to have phase stability in the high temperature range in order to apply the SA508 Gr.4N low alloy steel for a reactor pressure vessel. It is very important to evaluate the temper embrittlement phenomena of SA508 Gr.4N for an RPV application. In this study, we have performed a Charpy impact test and tensile test of SA508 Gr.4N low alloy steel with changing impurity element contents such as Mn and P. And also, the mechanical properties of these low alloy steels after longterm heat treatment (450℃, 2000hr) are evaluated. Further, evaluation of the temper embrittlement by fracture analysis was carried out. Temper embrittlement occurs in KL4-Ref and KL4-P, which show a decrease of the elongation and a shifting of the transition curve toward high temperature. The reason for the temper embrittlement is the grain boundary segregation of the impurity element P and the alloying element Ni. However, KL4-Ref shows temper embrittlement phenomena despite the same contents of P and Ni compared with SC-KL4. This result may be caused by the Mn contents. In addition, the behavior of embrittlement is not largely affected by the formation of M3P phosphide or the coarsening of Cr carbides.

keyword : metals, ageing, embrittlement, impact test, RPV steel

10 nm thick Ni layers were deposited on 200 nm SiO2/Si substrates using an e-beam evaporator. Then, 60 nm or 20 nm thick α-Si:H layers were grown at low temperature (<200℃) by a Catalytic-CVD. NiSi layers were already formed instantaneously during Cat-CVD process regardless of the thickness of the α-Si. The resulting changes in sheet resistance, microstructure, phase, chemical composition, and surface roughness with the additional rapid thermal annealing up to 500℃ were examined using a four point probe, HRXRD, FE-SEM, TEM, AES, and SPM, respectively. The sheet resistance of the NiSi layer was 12Ω/□ regardless of the thickness of the α-Si and kept stable even after the additional annealing process. The thickness of the NiSi layer was 30 nm with excellent uniformity and the surface roughness was maintained under 2 nm after the annealing. Accordingly, our result implies that the low temperature Cat-CVD process with proposed films stack sequence may have more advantages than the conventional CVD process for nano scale NiSi applications.

keyword : thin film, annealing, crystallization, Auger electron spectroscopy, hydrogenated amorphous silicon

An Ni-Al intermetallic coating has been produced by induction heating on mild steel. The effect of the induction heating conditions on the microstructure of the coating has been investigated. The reaction synthesis of the intermetallic compounds was promoted while increasing the heating rate and the holding time at reaction temperature. Especially, an NiAl phase corresponding to the initial composition of mixed powder was predominantly formed. However, the synthesis at low reaction temperatures occurred by solid state diffusion during the holding time and an Fe-Al reaction layer was formed at the interface with the substrate, regardless of the heating rate. The combustion synthesis of the intermetallic compound occurred at a temperature higher than 1023 K and resulted in an almost single phase NiAl structure.

keyword : intermetallics, coating, microstructure, scanning electron microscopy/X-ray diffraction, induction heating

The effects of Cu(II) and Al(III) on the extraction and separation of Pt(IV) and Pd(II) have been investigated in a mixed chloride solution using Alamine336 as the extractant. In the HCl concentration range of 1 to 5 M, more than 99% of Pt and Pd could be extracted by Alamine336 from all of the mixed chloride solutions investigated in this study. Lower HCl concentration led to a higher separation factor between Pd and Pt when Alamine336 concentration was constant. Extraction percentage of Cu increased with the increase of HCl concentration, while that of Al was nearly constant at 33% in our experimental range. The optimum conditions to extract Pt and Pd from Cu or Al and the separation factor under these conditions were obtained.

keyword : metals, recycling, recovery, spectroscopy, separation

A fundamental study of the distillation behavior of ferromanganese alloy melts was carried out at 1773 K and 0.1333 kPa (=1 Torr). During the distillation of ferromanganese alloy melts under reduced pressure, manganese vaporizes preferentially to phosphorus and other solute elements. High purity manganese metal with a very low content of solute elements can be obtained by distillation of ferromanganese alloy melts. The evaporation of manganese is suppressed as the carbon content of ferromanganese alloy melt increases due to the decrease of activity and vapor pressure of the manganese. When the carbon content of ferromanganese alloy melt is high, melt droplets are ejected from the bath, especially in the early stages of the distillation, and the solute elements in the splashed droplets contaminate the condensed material. The ejection of melt droplets is presumed to be caused by the increase of melting temperature and viscosity of the surface layer of melt due to the enrichment of solute elements such as carbon and iron.

keyword : metals, purification, melting, SEM, evaporation of manganese

We here describe the laser-direct patterning of nanostructured metal thin films. This method involves light-matter interaction in which a pulsed laser beam impinging on the film generates a thermoelastic force that plays a role to detach the film from the substrate or underlying layers. A moderate cohesion of the nanostructured film enables localized desorption of the material upon irradiation by a spatially-modulated laser beam, giving good fidelity with the transfered pattern. This photoresist-free process provides a simple high-resolution scheme for patterning metal thin films.

keyword : thin films, metals, nanostructured materials, scanning electron microscopy, microstructurds

A micro gas sensor for formaldehyde (HCHO) gas was fabricated by using MEMS (Micro Electro Mechanical System) technology and the sol-gel process. The sensing materials of the SnO2-ZnO system were synthesized by the sol-gel method. The crystal structure and thermal analysis of the SnO2-ZnO were characterized by XRD and DSC-TGA. The fabricated gas sensors were tested at various gas concentrations (0.5~5.0 ppm) and different operation temperatures (350~550℃). The SnO2-10 mol%ZnO sensor showed the highest sensitivity (Rs=0.24) for 1.0 ppm-formaldehyde at 500℃ and response time (90% saturation time) was within 20 seconds.

keyword : formaldehyde sensor, nanostructured materials, sol-gel, electrical properties, electrical conductivity/resistivity