발간논문

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The effects of composition and heal. treatment on the magnetic properties of Nd-Fe-Oxides were investigated. High coercivity of 5.5 kOe was obtained from the oxide which had atomic ratio of Fe₂ O₃/Nd₂=4/1. The optimum heat treatment condition of this sample was 1300℃ For 3 hours. The X-ray and SEM experiments revealed that the sample was composed of NdFeO₃ and α-Fe₂O₃. It seems that the high coercivity was due to NdFeO₃ phase.

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Change of microstructure in the bonded interlayer and tensile strength of joints were carried out during transient liquid phase diffusion bonding(TLP bonding) using SUS304 as base metal and SUS304+B alloy as insert metal. Main experimental results obtained in this study are as follows. 1) The optimum amount of B addition into the insert metal was found about 4wt%. 2) The completion time of isothermal solidification was 6.0ks and 3.8ks at the bonding tempetature of 1523K and 1553K respectively. With this bonding condition, microstructure in the bonded interlayer was same to that in the base metal because of the grain boundary magration in the bonded interlayer macroscopically. However, there exist fine car-borides and borides such as M_(23)(C, B)_6, Cr₂B and CrB at the grain boundary microscopically. These precipitates almost disappeared after homogenizing treatment at 1373K for 86. 4ks. 3) Exellent tensile properties of joints could be obtained by TLP bonding(1523K×6.0ks) using insert metal of base metal+4wt%B. Joint water-quenched at 1273K had higher tensile property than that of furnace-cooled joint. It is considered this is caused by the effect of retarding the precipitates at the grain boundary by water-quenching. Tensile properties of joints bonded under the vacuum and Ar atmosphere respectively were almost same.

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Partial binary phase diagrams of systems, Na₃AlF_6-Co₃O₄ and Na₃AlF_6-FeO mere investigated by means of the Differential Thermal Analysis(DTA) and K-ray diffraction technique. Computer aided optimization method, combining thermodynamic data and experimental phase diagram data was also applied to reinforce phase diagrams. Experimental results show that, the system Na₃AlF_6-Co₃O₄, was a simple eutectic over the range studied(0 to 0.25㏖ Co₃O₄) with the eutectic point at 0.044㏖ Co₃O₄ and 1225.1K, and for the system Na₃AlF_6-FeO, the eutectic point was found to be 0.142 ㏖ FeO and 1258.4 K. X-ray diffraction patterns proved no solid solution present in the cryolite rich area in both cases.

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Microstructure and decomposition behaviour of Mg-BLi-xAl(x=1, 3, 5) alloys have been investigated. Detailed TEM study shows that. the microstructure of Mg-Li-Al alloys consists of two types of matrix phases, the h.c.p. α and b.c.c. β phases. With the presence of Al, additional two phases, MgLi₂ Al and AlLi are present in the form of 0.1-0.3㎛ or 1-3㎛ size particles in the αrt and βphase matrix. With increasing Al content, the amount of the MgLi₂Al and AlLi phase particles increases. Present study indicates that these particles play an important role in improving the strength of the Mg-Li alloys with the addition of Al. These particles are, however, dissolved into the surrounding matrix phase by annealing at the temperature of 350℃ for 1 hour.

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The purpose of this work was to develop an engineering materials having a high volume fraction of carbide. For this purpose, rapidly solidified Fe-3.7wt%C-(1∼4)wt%Cr alloy powder was made using high pressure gas atomizer and followed by hot extrusion. In this study, the microstructures and mechanical properties of extruded bars were discussed. The microstructures of extruded bars were found to be the mixture of carbide and fine pearlite. The average carbide particle size was about 2㎛ and the pearlite lamellar spacing was about 0.1㎛. The tensile strength increased with increasing extrusion ratio and reached 85∼90㎏/㎟ when extrusion rate was 13. Toughness was 4∼5㎏/㎟ when extrusion race was 13. Toughness was 4∼5㎏ m/㎠ and hardness was 55∼60 HRc.

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