발간논문

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The solidification sequence and the formation of the stable quasicrystalline icosahedral phase were investigated in a conventionally cast Al_(65)Cu_(20)Fe_(15) ingot by the combined analysis of SEM-EDX, TEM, XRD, and DTA. An Al_(65)Cu_(20)Fe_(15) alloy was melted in an inductively heated crucible, and then cast into a liquid nitrogencooled wedge-type steel mold under a dynamic argon atmosphere. Rapidly solidified ribbons were also prepared by single roller melt spinning. The microstructural evolution and the formation of icosahedral phase were examined as a function of heat treatment at the solid-liquid mixed region. Icosahedral phase was observed to form at the boundary of λ-Al_(13)Fe₄ phase via a peritectic reaction, and τ-AlCu(Fe) phase was formed in the remaining liquid. The composition corresponding to ω-Al_7Cu₂Fe was also detected in the area of icosahedral phase. It is suggested that the detailed solidification sequence and the resultant morphology of an Al-Cu-Fe alloy are subject to change as a function of cooling rate during solidification and heat treatment condition.

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Microstructure and fracture behavior of Ti alloy reinforced AM60 Mg composite have been investigated in the present study. To clarify the fracture behavior, in-situ fracture test was conducted. The manufactured composite consists of 45.1 vol.% reinforcement and does not show the pore presence. No reaction layers or reaction products are observed at the Ti/Mg interface. However, there is a presence of fine Ti particles in the matrix. It has been suggested that the Ti particles form by diffusion during squeeze casting. It also has been shown that there is a diffusion of Mg into Ti. Microcracks are nucleated mainly along the reinforcement/matrix interface, but they are less sharp than those found in the ceramic reinforced composites. Considering the high volume fraction and large size of the reinforcement used in the present study, the manufactured composite shows the strong potential for further improvement in mechanical properties.

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A comprehensive methodology to consolidate the hypereutectic Al-25Si-X alloy via spray forming was investigated in an attempt to judge the feasibility of this alloy for the application to wear resistant components. Billets having desired shape and microstructures were fabricated using forming parameters obtained from numerical calculations. Prior to tube extrusion of the spray formed billets, effects of various extrusion conditions, such as extrusion ratio, die temperature, and die configuration, on microstructures of the billet were studied. Based on results obtained from the preliminary extrusion tests, the formed billets were then hot extruded into a tubular shape. Various material properties of the extruded billet were measured and compared with the other candidate materials for anti-wear applications.

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The present study has been performed to investigate the effect of variation of binder content, WC grain size, carbon content and TaC addition on the amount of leached binder in WC-Co alloy by electrolytic surface treatment. As the results, the amount of leached binder increases with increasing particle size of WC raw material, binder phase content and carbon content because those factors increase leaching area by increasing the mean free path(distance from a carbide to another carbide) of the alloy. In case that TaC component is added upto 2 wt% in WC-Co alloys, the amount of leached binder decreases with increasing TaC content. The reason is that small addition of TaC to WC-Co alloy inhibits the grain growth of WC and therefore the alloy containing TaC has the finer grain size than that of TaC free alloy, which results in the decrease of mean free path of the alloy.

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The temperature dependences of the electrical and thermoelectric properties of the SbI₃-doped 25% Bi₂Te₃-75% Sb₂Te₃ and 15% Bi₂Te₃-85% Sb₂Te₃ single crystals, grown by the Bridgman method, were measured at temperatures ranging from 77 K to 600 K. For the Sb₂Te₃-rich single crystals, the temperature dependence of the Hall mobility was T^(-1.0) regardless the Sb₂Te₃ contents and added amount of SbI₃. The temperature dependences of (m*/m_o)^(3/2)·μ_c and effective mass m*/m_o were T^(-1.5) and T^(-1/3), respectively. The decrease of the saturated hole concentration and the change of the slope of the Seebeck coefficient with temperature were considered in the view point of the two sub-bands in the valence band. The maximum figure-of-merit at 300 K of 0.2 wt% SbI₃-doped 25% Bi₂Te₃-75% Sb₂Te₃ and 0.4 wt% SbI₃-doped 15% Bi₂Te₃-85% Sb₂Te₃ single crystals were 2.25×10^(-3)/K and 1.95×10^(-3)/K, respectively.

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The structural characteristics of NiFe/Ag multilayers prepared by magnetron sputtering were studied. For the quantitative structural analysis of the multilayers, we used XRD simulation based on a superlattice structural model considering gaussian fluctuations in the thickness of each layer and interface. Because of large lattice mismatch and large surface energy difference in the NiFe/Ag multilayer system, it is difficult to fabricate good uniform multilayer. Among the deposition parameters, the sputtering gas(Ar/H₂) was the most effective to enhance the uniformity of the multilayers. As the period of the multilayer increased, the uniformity of the multiayer was improved.

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Al 4343(filler)/3003(core)/4343(filler) type of brazing sheet(thickness of 100 ㎛ with each side clad ratio of 10%), which is being widely used in clad fin material for parallel flow type condenser of automobile, was fabricated by casting → hot rolling → cold rolling → intermediate annealing(IA) → final cold rolling(FR), and the relationship between IA/FR condition, microstructure and NOCOLOK brazing characteristics was investigated. When specimen was intermediate annealed at temperatures higher than 330℃, followed by cold rolling with the reduction rate lower than 10%, the core was partially recrystallized during brazing process, including unrecrystallized structure such as sub-grain. In this case, the melted filler infiltrated into the core during brazing through the sub-grain boundary which acts as a preferential penetration path, so the eroded area increased and brazeability decreased. When IA/FR was performed in the condition of 330℃/over 37% or 270℃/over 10%, fine recrystallized grains were formed in the core during the brazing process, which promotes filler penetration through the grain boundary. In this case, brazeability was deteriorated with the increment in eroded area. If IA/FR was carried out in the condition of 420℃/20-45% or 330℃/20-30%, the specimen exhibited superior brazeability with suppressed erosion owing to the formation of coarse recrystallized grains during brazing process.

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The microstructure and mechanical properties in the intercritically-reheated heat-affected zone (IRHAZ) of an SA 508 Cl.3 Mn-Mo-Ni reactor pressure vessel (RPV) steel have been investigated systematically using the welding thermal cycle simulated specimens. The double thermal cycles with varying peak temperatures(T_(p1), T_(p2)) in the range 700-800℃ were applied to simulate the various IRHAZs using a Gleeble 1500 thermal simulator. The effect of post-weld heat treatment (PWHT) was also studied. The microstructures of the IRHAZs were mainly composed of bainite, martensite and tempered bainite. But the specimens subjected to T_(p1)=T_(p2)=750℃ (before PWHT) and subjected to T_(p1)=T_(p2)=700℃ (after PWHT) showed lowest toughness. The microstructural factors affecting toughness were discussed with reference to the fraction of high carbon martensite for the specimen before PWHT, and to they coarsened carbide and softened matrix for the specimen after PWHT.

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The distribution ratios of As and Sb between molten silver alloys and CaO-FeO-CaF₂ slags at 1723K was measured to investigate the thermodynamic behaviors of these elements in steelmaking conditions. The oxygen partial pressure was controlled by the activity of FeO in slags, which was estimated from the Fe/FeO equilibrium. Since the distribution ratio of As decreased with increasing oxygen partial pressure, As would be refined by the reducing refining mechanism. However, the distribution ratio of Sb has a minimum value with variation of the oxygen partial pressure, indicating that Sb can be refined by the reducing refining at lower than Po₂=3.02×10^(-12) atm and by the oxidizing refining at higher than this critical oxygen partial pressure. The capacities of each element, obtained from the distribution ratio and the oxygen partial pressure, were independent of the composition of slags, i.e., X_(FeO)/X_(CaF₂), along the CaO saturated slag. It may be due to the constancy of the activity of CaO, which makes the activity coefficient of each reaction product in stags be constant. The effect of carbon content in molten iron on the distribution ratios of As and Sb was also discussed.

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Thin reflective Ag films are synthesized by using sputter methods. For the purposes of improving the reflectance and adhesion of such films, particularly, the substrate bias voltages were applied during sputtering. We succeeded in fabricating a quality silver film which possesses an adhesion of 50 Kgf/㎠ and a high reflectivity of more than 96%. Both of reflectivity and adhesion are better in case of bias sputtering as controlled than nonbias sputtering, specifically the bias of 50-100 V has been shown most effective. The microstructures of sample films were examined by using various techniques and the XRD spectrum in particular showed that $lt;111$gt; direction was the preferred growth orientation.

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