발간논문

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ZrN and Zr/ZrN layers were deposited on tool steel(SKH9) by a reactive DC magnetron sputtering technique and their microhardness and adhesion strength were measured for the films processed by changing the partial pressur of N₂gas(4∼10×10^(-4)mbar) and the substrate bias voltage(0∼250V). The adhesion strength was evaluated by acoustic signals through the scratch-test with the incremental applied load. As the bias voltage was applied, the adhesion strength of the films was observed five times as strong as that obtained without bias voltage. The adhesion strength of the Zr/ZrN doublelayered structure was measured to 20∼24N and it was deposited two times as strong as that in the monolayered Zr interlayer increased about twice more than that of the load of ZrN monolayered structure ZrN(10N). The microhardness of the ZrN film was shown to be Hv 1800Kg/㎟ when the bias boltage was 200V and the partial pressure was N₂4×10^(-4)mbar. From the failure mode on the film during scratch adhesion test, it was found that the adhesive failure was observed in the ZrN monolayer and the cohesive failure was found in the Zr/ZrN doublelayed structure.

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Sm₂Fe_(17)N_x compounded powders, where the x was found to be 2.9, were prepared from parent alloys of Sm₂Fe_(17) made by both the arc melting and rapid solidification processes. The cast Sm₂Fe_(17) compound contained mixed phases of SmFe₃and α-Fe, and correspondingly formed less amount of Sm₂Fe_(17)N_(x3) , with rather a large amount of residual α-Fe after nitrogenation. The melt-spun Sm₂Fe_(17), however, was single phase and exhibited negligible amount of residual α-Fe after nitrogenation. The residual amount of free iron was found to increase as a function of milling time and impede promising permanent magnetic properties. The melt-spun Sm₂Fe_(17)N_x compound powders exhibited a coercivity value(_iH_c) of 5 kOe, which is double that of the as-cast Sm₂Fe_(17)N_x powders and a high remanence, B_r=60emu/g. The average magnetic moment of Fe atoms in the crystal is estimated to be 2.29μ_B and the Fe atom in the 6c site shows the highest magnetic moment of 2.65μ_B. The expansion in c-axial direction of the nitrogenated crystal was mainly due to extension of Sm(6c)-Fe(6c) and Fe(18f)-Fe(18f) distances.

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The smelting reduction of manganese oxide has been employed to increase the Mn level of steels. In order to understand the behavior of manganese oxide in smelting reduction process, the manganese equilibrium between ironmaking slag containing FeO and pure silver melt was investigated over the temperature range of 1400∼1500℃. The oxygen partial pressure was varied by controlling the mixing ratio of CO and CO₂gases. The Mn contents in silver melts have been thermodynamically converted into those in supercooled and carbon-saturated iron melts. The activity coefficients of MnO in the slags were determined experimentally and showed a tendancy to decrease by increasing the FeO content in the slag. The equilibrium manganese distribution between slag and iron was found to increase with the increase of FeO content and the decrease of stag basicity. The temperature dependence of equilibrium manganese distribution was also determined in the temperature range of 1400∼1500℃ through the regression analysis of experimental data.

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The soldification behaviors of the drop-tube processed droplets were analysed through computer simulations. The model was assumed that an undercooled droplet had a single nucleation event at the surface and solidified with releasing heat of fusion by the conduction. The coordinate system is a bispherical rotational bipolar coordinate. The model also has been tested experimentally for the Cu droplets, and agrees well with the experiments.

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The effect of Fe addition on Ni-Aluminide intermatallics was studied in order to improve the mechanical properties, especially the room temperture ductility and the high temperature strength. It was found that the Fe addition in the two phase alloy of (NiAl＋Ni₃Al) gives significant improvement in the room temperature ductility. The high ductility of Ni-Al-Fe alloy at room temperature seems to be caused by the mobile dislocations formed at NiAl which occurred by the accumulated dislocations in Ni₃ Al. The strength improvement, especially at high temperature, was tried by micro-alloying and hot forming. When the micro-alloying of B, Cr, Ti and Zr was tried the optimum condition in strength and elongation was obtained from the addition of 1 at % Ti in Ni-20Al-27.5Fe. When the alloy is hot-formed, the Zr addition gives significant improvement in the high temperature strength, that is to say, the maximum strength at high temperature range was obtained from the hot extruded bars of Ni-20Al-27.5Fe-1Ti-0.5Zr. It is believed that the microstructure is consisted of fine grains and precipitates by the hot extrusion, which is accentuated by the Zr compounds.

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The LASER weldability response (solidification cracking and cold cracking susceptibility) of austenitic, ferritic and martensitic grades revealed a significant alloy to alloy and heat to heat variation. the ferritic and martensitic alloys appeared to be less sensitive to solidification cracking but sensitive to cold cracking such that when the hydrogen content in shielding gas increased the fracture strength and time decreased, indicating that LASER welding of these grades should be kept from absorption of hydrogen. On the other hand, austenitic alloys showed a high propensity to solidification (hot) cracking in stead of cold cracking. Sensitivity was predominantly dependent upon the primary solidification mode (Cr_(eq)/Ni_(eq)) and impurity (S, P, Si) contents.

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