발간논문

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Li-ferrite powders were prepared by coprecipitation method. The crystal structure, the microstructure and the magnetic property of synthesised poveders were investigated using X-ray analysis, chemical analysis, SEM, and VSM. The synthesis of Li-ferrite by coprecipitation method was subjected to precipitate with stearic acid forming insoluble salt of Li^+ and then through calcination. The mole ratio of precursor(Li^-/Fe^(++))was a very important factor on the crystal structure and the magnetic property of product and the powders produced were weakly agglomerated. The optimum condition of synthesis were the mole ratio of FeSO₄: Li₂SO₄ : procipitan = 1 : 0.1 : 0.1, pH 95 and reaction temperature 80℃

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Water-based magnetic fluids were successfully prepared using the synthesized ultrafine magnetite coated with fatty acid. The effect of pH change on the stability of dispersion in water-based magnetic fluids was investigated and the results were obtained as follows; The dispersion ratio of magnetic fluids stabilized by decanoic acid was constantly maintained over 95% when the amount of decanoic acid for magnetite of 20g was more than 0.035㏖. It was 2.2 times as much as the amount for monolayer adsorption. The stable pH region for the magnetic fluids prepared with saturated fatty acid was in the range of 6.5 to 11.0. As the length of carbon chin of fatty acid is longer, the stable dispersion pH became higher. In decanoic acid stabilized magnetic fluid, there was a close inter-relationship with the amount of chemisorption. The dispersion mechanism of water-based magnetic fluid is proposed and it is consisted of the magnetite surrounded with double layer surfactants.

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The main objective of the present study is to clarify the microstructural factors such as tempered martensite matrix, coarse primary carbides, eutectic structures, and fine secondary carbides, which influence fracture properties of high speed steel (HSS) rolls. Three HSS rolls with different Cr content were manufactured by a centrifugal casting method, and the microstructures of their shell regions were investigated quantitatively. MC type primary carbides, together with eutectic ledeburite structures, were observed in the intercellular regions. The observation of fracture processes indicated that these primary carbides cleaved first to form microcracks at very low stress levels. These microcracks were easily propagated along the intercellular networks, leading to predominantly cleavage mode of fracture on fracture surfaces. To improve microstructures for use in the HSS rolls, therefore, it is suggested that the cell size must be minimized to achieve the homogeneous distribution of primary carbides

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Mechanical properties of multi-step forged Alloy 718 are affected by the processing parameters, such as upset ratio, temperature and holding times for forging, and post heat-treatment. The aim of this study is to find processing conditions to enhance mechanical properties of Alloy 718 pancakes at elavated temperatures up to 700℃. A double step forging process is employed, i.e., firstly at 980℃ or 1025℃, and then at 980℃. Finally, aging treatments are conducted to enhance the mechanical properties through γ´ or γ˝ precipitation. Since the grain size is controlled by the presence of δ phase, the grain size of the pancakes is affected by forging temperature especially near the δ solvus temprature, 980℃. Dynamic recrystallization becomes prominent with increasing temperature for the first step forging, leading to greater grain size and uniform size distribution with increasing temperature. The microstructure after the second step forging at 980℃ and the aging treatment retains the general feature of the microstructure of cat the first step forging, and tensile properties are similar with increasing first step forging temperature.

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A powder mixture of elemental aluminum(62.8wt.%) and titanium(37.2wt.%) was mechanically alloyed in an argon-filled jar-type ball mill from 100 to 900 hours. The solid state transformation of each mechanically alloyed powder was investigated by means of X-ray Diffraction(XRD), Differential Scanning Calorimetry(DSC), and Electron Probe Micro-Analyzer(EPMA). The X-ray examinations reveal that elemental particulate sizes decrease down to 3.4㎚ with increasing the mechanical alloying (MA) time of up to 500 hours and then drop slowly to a saturation value of 2.7㎚ for MA 900hours powder. X-ray and DSC studies show that there are three exothermic peaks which are due to the formation of Al₃Ti, AlTi and Al_(24),Ti_8

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The high temperature deformation behavior of an lnvar alloy was investigated in the temperature range of 800∼1100℃ at the strain rate range of 4.0×10^(-2)∼2.6×10^0/sec. Flow curves and microstructures exhibited this alloy had restoration by dynamic recrystallization combined with dynamic recovery. The temperature(T) and strain rate(ε˙) dependence of the peak stress (σ) was described by the Zener-Hollomon parameter, Z=ε˙ exp(-335kJ/RT)=A[sinh(0.0103α)]^(4 96). The size of dynamically recrystallized grains decreased as Z increases. The distribution of precipitates (Mn, Cr, etc.) was also investigated. The strain induced precipitation was occurred during the hot deformation.

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Growth rate of MoSi₂ layer formed by the chemical vapor deposition of Si on Mo substrate and oxidation behavior of Mo coated by MoSi₂ were investigated and the experimental data were analyzed by theoretical parabolic rate equations for growth of intermediate phases in a Si-Mo diffusion couple. The equations well described the facts that growth of MoSi₂ layer on Mo and Mo_5Si₃ layer formed at interface in MoSi₂/Mo couple during oxidation test obeyed the parabolic rate law. The theoretical lifetime of MoSi₂ layer against oxidation was proportional to the square of its thickness. However, coating lifetime was substantially less than the predicted one because MoSi₂ layer was not defect-free. Activation energy of growth of MoSi₂ layer on Mo substrate by the CVD method was 11.6㎉ per mole.

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Continuous hot torsion tests of 10vol%SiC_P/Al-14Fe composites were performed in the range of 350∼530℃ and 1.15×10^(-3)∼1.15×10^(-1)/sec. Flow curves of the hot torsion tests were analysed with respect to the characteristics of work hardening rate θ, dynamic recrystallization (DRX) and constitutive equation. The critical strain (ε_c) for DRX initiation was determined from changes in slope of the strain hardening rate -effective stress (θ-σ) curves, and it was decreased with increasing temperature and strain rate. Although the SiC particles helped the initiation of DRX, they also retarded dynamic recrystallization progress. Relationship between critical strain and peak strain (ε_P) was found to be ε_c ＝ 0.51 ε_P. The stress dependence on the strain rate (ε˙) and temperature (T) could be described by hyperbolic sine relationship, ε˙=2.21×[sinh(1.92×10^(-2)·σ)]^(4.7)exp(-193.5 kJ/RT). The microstructural development during hot deformation were interpreted by dynamically recrystallized structures which were composed of equiaxed grains, subgrains and dislocation tangles.

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Heat treatment of high-carbon chromium bearing steel, STB 2 (AISI 52100 or JIS SUJ 2), with excellent resistance to rolling contact fatigue was attempted in this study by spheroidized annealing, partial austenitizing and then isothermal transformation. The rolling contact fatigue test was conducted at a shaft speed of 1700 rpm under axial load, 506 kgf, circulating the lubricant using thrust-type rolling contact fatigue testing machine. It was found that lower bainite was formed after isothermal transformation. Strain hardening of lower bainite occurred due to formation of tangled dislocations, which resulted from accommodation of severe plastic deformation in lower bainite during rolling contact fatigue. It was also observed that propagation of subsurface crack leading to flaking during rolling contact fatigue in the isothermal transformed specimen was retarded by deflection due to tough lower bainitic matrix, just under the raceway surface subjected to rolling contact stressing. Consequently, the rolling contact fatigue life of the isothermal transformed steel was improved in comparison with that of the conventional quench-tempered steel.

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The effects of casting processes and heat treatment on the microstructure and wear behavior of hypereutectic Al-Si alloys have been investigated. Two casting processes, gas-pressurized Hot Top process(alloy A) and spray deposition casting(alloy B), with different solidification rate have been employed. The microstructure of alloy A solidified at the rate of 100℃/sec was composed of coarse faceted primary silicon and fine round eutectic silicon crystals surrounded by aluminum dendrites. There was no primary silicon in the Alloy B which was rapidly solidified. Small and evenly distributed Si crystals were obtained in the alloy B due to the rapid solidification of liquid droplets. The improved dry wear resistance of alloy B compared to that of alloy A was attributed to these fine silicon particles. High temperature formability of alloy B was also evaluated by compression tests.

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