발간논문

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Effects of dopants on the decomposition of synthetic wu¨stite and it`s pickling efficiency were throughly investigated. Chemicals, such as Li₂O, CaO and Al₂O₃ at a concentration of 0.8 wt.% were used as dopants. The isothermal decomposition temperature adopted in this study ranged from 380℃ to 530℃. The decomposition rate of wu¨stite at 460℃ was found to be significantly greaser than that at 380℃ and 530℃. The decomposition rate was increased with the addition of A1₂O₃ due to an increase in the vacancy concentration of wustite, whereas the rate was, decreased with the addition of Li₂O or CaO. Scales doped at two levels 3.23×10^(-4)g/㎠ and 5.38×10^(-4)g/㎠ were used for the pickling tests. The pickling rate of the scale was increased with the addition of either Li₂O or CaO to wu¨stite substrate, whereas decreased with the A1₂O₃ addition. It was also observed that those effects were enlarged with increasing the dopant concentration.

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The free energy of liquid phase of Fe-Si system using sub-regular Model was calculated. It was shown that the free enegy of mixture of pure elements was greater than that of an amorphous phase and the intermetallic FeSi compound. Thus it was estimated that an amorphous phase and the intermetallic compound could be produced by mechanical alloying. Mechanical alloying was carried out on the composition of Fe_(50)Si_(50). The alloying process was investigated by X-ray diffraction and differential scanning calorimetry(DSC). During mechanical alloying, the existence of an amorphous phase was not observed. However, the intermetallic FeSi compound phase was clearly observed.

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The hot-working behavior of rapidly solidified Fe-Nd-B powders was investigated to utilize the hot consolidation techniques in direct fabricating a promising permanent magnet. The examination of stress-strain distribution of both the mandrel extruded ring magnet and the pack rolled magnet enlightens the source of prominent magnetic anisotropy of those hot consolidated magnet bars. It has been confirmed that the maximum normal strain during hot deformation causes the Fe_(14)Nd₂B grains with their c-axes to be aligned parallel to the strain direction resulting in the enhanced remanent magnetization. The highest remanence of Br=8.6 kG was observed along the through-thickness direction of the ring magnet, and of Br=6.9 kG for the pack rolled magnet along the thickness direction. A(001)[1￣10] texture was found to form in the plane normal to the thickness direction of pack rolled magnet bar. The maximum energy product about 10∼11 MGOe was obtained from the hot consolidated Fe-Nd-B magnet deformed in true strain ε_u = 1.0 range.

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Wear resistances of plasma-spray coated alumina, alumina-titania of 3, 15 and 40 wt.%, and pure titania were tested by measuring the wear rates of samples, which were pressed against the rotating cast iron plate with an oil-alumina abrasive. Wear rates of the coatings were compared between samples to determine the optimum composition of the powder in the slurry wear application. Wear rates of the coatings made from the composite powders were nearly the same, with the alumina-15 wt.% titania coating showing the lowest rate of wear. It is shown that alumina-titania composite ceramic can yield coatings wish superior microstructure and wear resistance.

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Aluminum matrix composites reinforced with stainless steel and high carbon steel fibers have been fabricated by liquid casting method, and the effect of surface coating of fibers on the fatigue behavior has been investigated. Interfacial reaction zone is formed for all specimen, and various intermetallic compounds were observed at interfaces ; A1₂Cu, Al₄Cu_9, AlNi, AlFe and Al_5Fe₂ were identified by electron diffraction. For the composites reinforced wish uncoated fibers, 10＾6 cycle fatigue strength is close to the calculated value based on the rule of mixture. The composites reinforced with Ni or Cu-coated fibers show slightly higher fatigue strength than those calculated by the rule of mixture. The composites reinforced with Cu-coated fibers shows the best fatigue characteristics among studied samples. Initial cracks are formed around the metal-fiber interface, and propagate toward the matrix.

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The preferential crystallization, microstructure and growth procedure of euctectic crystal in Fe_(40)Ni_(40)P_(14)B_6 metallic glass were investigated in the temperature range of 621∼671 K. When Fe_(40)Ni_(40)P_(14)B_6 metallic glass is annealed, the region that turbulent melt flow has occurred during melt spinning and the void surface crystallize preferentially. This phenomenon is due to the surface oxidation. The eutectic crystals change their morphology from the normal to the degenerate structure with the annealing temperature. This microstructural change of the eutectic can be explained by the difference of the temperature dependence of the interface kinetics between BCT(Fe, Ni)₃(P,B) and γ(Fe, Ni). Assuming that interface kinetics control mechanism dominates the growth of eutectics, the diffusion coefficient estimated from the measured eutectic growth rate agrees relatively well with the directly measured coefficient.

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Titanium Aluminum Nitride films were deposited on the high speed steel by Biased Activated Reactive Evaportation. Effect of the substrate temperature and nitrogen pressure on the formation, structure, and properties of the films was investigated. Phase identification, preferred orientation, lattice parameter, and effective crystallite size were studied with X-ray diffraction line broading, while the surface and interface of films were observed with a scanning electron microscope. Microhardness and impact adhesion were also measured. Only Ti₂AIN single phase existed below the nitrogen pressure of 4 × 10^(-4) Torr, while Ti₂AIN and Ti_(0.5)Al_(0.5)N coexisted above that pressure. Tendency of having(111) preferred orientation decreased with increasing nitrogen pressure. Effective crysallite size increased with increasing nitrogen pressure and substate temperature. The lower the nitrogen pressure, the finer the film surface morphology. Films have their maximum microhardness of 2500 KHN at the nitrogen pressure of 1×10^(-3) Torr. Impact adhesion property was enhanced with increasing nitrogen pressure.

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The effect of alloying elements on the microstructure was studied in the rapidly solidified Al-Fe based alloys by single roll method. Volume fraction of precipitates in the rapidly solidified Al-Fe binary alloys increased with increasing iron contents. The fine precipitates in the rapidly solidified Al-Fe alloy ribbons were identified as A1_6Fe compound, although precipitates formed in the east ingots were Al₃Fe compound. The Al-Fe based alloy ribbon consisted of a supersaturated solid solution and a microcellular structure. Microscopic study revealed that the Al-Fe-Mo and A1-Fe-Si alloy ribbons had finer cell structure than the Al-Fe binary alloy ribbons. The Al-Fe-Mo-Si quaternary alloy ribbon had subgrain-like structure at zone A, which was different from the binary or ternary alloy ribbons.

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The effect of doping and rolling conditions on the secondary grain growth behavior of the pure and the doped molybdenum were investigated by means of various methods. The results obtained were summarized as follows. 1) In the rolling texture, the intensity of (200), (211) and (222) peaks are increased with rolling reductions but that of (110), (220) and (310) peaks are decreased. 2) The nucleation of secondary recrystallization initiated at the specimen center and the large scale single crystals were obtained in the Cao, Mgo and Na₂O doped sheets. 3) The secondary grain growth is controlled by annealing temperature, doping elements and rolling conditions. In the present work, the single crystal could be obtained under proper doping and rolling conditions above 2100℃.

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In order to establish bainitic transformation mechanism of steel, low carbon-Ni-Cr-Mo steel was used as at. experimental alloy. Transformation-time curve in upper bainite of the steel showed S-shaped curve indicating diffusional transformation of nucleation and growth, while transformation time curve in lower bainite showed C-shaped curve often appearing in isothermal martensitic transformation. Furthermore, the bainitic trans formation rate showed negative dependence on temperature and the degree of the negative dependence was greater in the lower bainite than in the upper bainite. It was concluded by these results that the lower bainite was produced by shear transformation and that the upper bainite was formed by shear and diffusional transformations. A new mechanism to explain how the lower bainite is formed martensitically at temperatures above Ms temperature has been proposed on the basis of carbon concentration fluctuation model. The width of ferrite lath in the lower bainite was decreased and the bainte finishing time was shorter with increasing deformation degree of metastable austenite in bainitic bay. These experimental results well supported the proposed bainite transformation mechanism.

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