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Annealing effect on soft magnetic properties of Co_(70.5-x)Fe_(4.5)(Ni, Nb)xSi_(10)B_(15) amorphous alloy has been investigated. Nb addition is effective to improve the permeability and the coercive force while Ni is favorable to increase the remanence. The combined addition of Nb and Ni is effecitive to optimize the permeability and remanence. The change of magnetic properties to annealing temperature is classified by three steps and discussed in terms of the structural relaxation, cluster formation and the crystallization process.

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The effect of cooling rate on the mechanical properties of Zircaloy-4 alloy was studied for the specimens which were heated in the region of βphase and then cooled in various cooling processes, such as, iced brine, water, oil, air and furnace cooling. It was observed that the iced brine and water quenching of the specimens resulted in higher strength and greater strain hardening rate than the oil quenching, air and furnace cooling. It is believed that the increase in the strength and the strain hardening rate is attributed to the increase in stress required to move glide dislocations due to twins and tangled dislocations introduced during the quenching process, i.e., martensitic transformation. It was also observed that the strength and the strain hardening rate were increased gradually as the cooling rate increased from furnace cooling (0.05℃/s) to oil quenching(110℃/s). This means that the two properties are mainly controlled by α-lath size. From the microstructure and hardness, it was shown that the iced brine and water quenched specimens resulted in faster recrystallization than the oil quenched and air cooled specimens. It was suggested that the iced brine quenched specimen was recrystallized through the homogeneous nucleation, while the recrystallization of water quenched specimen seemed to follow the bulge mechanism.

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In this study an attempt has been made to improve mechanical properties of a cast aluminum matrix composite. Detailed microstructural analyses of a cast A356.0 Al-SiC composite indicated that SiC particulates and eutectic Si particles were segregated to the intercellular regions. The presence of these eutectic Si particles resulted in cleavage fracture along the intercellular regions, which deteriorated mechanical properties of the cast composite. In-situ SEM fracture observations clearly showed that eutectic Si particles were cracked first before the matrix-SiC decohesion or the SiC breakage occurred. In order to modify the distribution of eutectic Si particles the cast composite was re-cast with adding small amount of Sr. Tensile properties of the re-cast composite with 0.03wt.% Sr were significantly improved. The improvement of mechanical properties is attributed to a more homogeneous distribution of SiC and eutectic Si particles due to the addition of Sr and recasting.

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An attempt to explain the fracture mechanism of tempered martensite embrittlement (TME) was made by investigating microstructural changes and impurity segregation during tempering of an AISI 4340 steel Particular emphasis was placed on studying microfracture processes at crack tips using an in-situ SEM technique. At the relatively low stress levels, shear bands were formed preferentially along the arrays of interlath cementite particles and microcracking occurred at the prior austenite grain boundaries. The failure eventually took place along localized shear bands in a ductile manner, although the crack path was occasionally deviated from the shear band only when the main crack met the discrete microcracks associated with the prior austenite grain boundaries. Examination of the fracture surfaces clearly revealed a transgranular ductile fracture mode without any evidence of intergranular fracture. Thus, the direct observations of the crack initiation and the fractographic results strongly suggest that the presence of carbide particles is the most important metallurgical factor leading to the tempered martensite embrittlement.

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Forming limit diagrams for various A-K steel sheets and 60-40 brass sheet have been measured. Experimental results. have been obtained by hemispherical - punch stretching and compared with those calculated, based on a shear instability criterion. FLDs have been analyzed by material properties obtained from uniaxial tension testing. Most of measured FLDs for steel sheets were located on the Keeler-Goodwin band. The level of FLD for brass is a little lower than those for A-K sheets. The calculated results are in good agreement with the measured data.

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Effect of the RI-ITMT variations on grain refinement and the superplasticity of 7475 aluminum alloy has been studied. The increased number of warm rolling passes contributed for better grain refinement, that is the average grain diameter of 7.5㎛ was obtained. As the solution treatment time increased, the distribution and the size of secondary particles became more uniform and finer, respectively, and the superplasticity was enhanced. The maximum superplastic elongation of this study was 2000% at 516℃, and at the deformation rate of 1.7 × 10^(-3)/s. The attained superplasticity of this study is much higher than any of the previously published results on 7475 and 7075 alloys. Cold rolling process, instead of the warm rolling of the conventional RI-ITMT, has resulted a little decreased superplasticity. However the superplastic elongation was still quite high.

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The morphological changes in the solid-liquid interface and the microstructural development during unidirectional solidification of Al-Cu and Al-Ni eutectic alloys were investigated by the $quot;Peltier Interface Demarcation$quot;(PID) method in conjunction with theories of interface stability. PID method with the controlled current pulse was verified as a direct and precise technique to obtain fine details of the solid-liquid interface. The well-developed unidirectional microstructure maintaining $quot;planar interface$quot; was obtained at G₁/Y$gt;130℃ hr/㎝ and $gt;40℃ hr/㎝ for Al-Cu and Al-Ni eutectic alloy respectively. The breakdown of planar interface for Al-Ni eutectic alloy was resulted from the gradual amplification of the sinusoidal perturbation(λ=120㎛) as the growth rate increased, and the morphological stability was able to be predicted by Mullins-Sekerka theory. However, the growth rate of cellular structure development in Al-Cu system was observed to be somewhat slower than the value expected by the theory. This indicated that the different type of perturbations could be operative.

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The bath smelting process has been drawing the particular attention that it could solve the problems of coking coal, environmental pollution, and production flexibility. In bath smelting process coal, partially prereduced ore and oxygen are reacted in an iron bath. The CO and H₂ gases are post combusted above the bath and those reactions supply more of the required energy. The resulting gas is highly oxidized with the post combustion ratio up to 50%. In this work, the rate of reduction of industrial pellets in Co-H₂-CO₂-H₂O gas mixtures simulating those from a bath smelting process were measured. The reduction temperatures were 973, 1073, 1173 and 1273K. Both acid and basic pellets were tested. The reduction was uniform throughout the pellets and the time to reduce to FeO was about 100 minutes at 973K, and to 10 minutes at 1273K. The implications present on the results are discussed.

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A study was carried out on the dependence of mechanical alloying on the process variables for the Al-Ti alloy in order to determine the optimal MA process condition. The homogeneity of composite particles was able to be obtained by the following condition; addition of 1.5wt.% stearic acid as PCA, the attrition velocity of 400rpm and MA milling time more than 10hrs. The MA powder prepared by the above condition reveals the achievement of a steady state processing, i. e., equiaxed powder particles, constant powder size distribution and a saturated hardness. Also, the intermetallic compound phase of Al₃Ti in the matrix was identified by X-ray diffraction analysis

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To improve the ductility and toughness of Al-5.2%Si-0.4%Mg casting alloy with a reduced amount of Si, the specimens have been prepared by quenching from 450℃ after solidification and the changes in mechanical properties during solution treatment and aging have been investigated. Needle like eutectic Si crystals have been observed to be small and uniform due to rapid quenching and the spheroidization of Si crystals has therefore rapidly occured. When solution treated and aged, the mechanical properties of rapid quenched specimen have been found to be superior to that of slow cooled specimen. In case of aging at 140℃, both elongation and impact strength have been measured to increase by about 30% and solution treatment time can be reduced by rapid quenching after solidification. The Jominy test has been effectively conducted to determine the quenching sensitivity and the air cooled specimen has been found to be more sensitive than the quenched specimen.

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