발간논문

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To obtain fundamental informations for ultrafine molybdenum powders, the preparatio of molybdenum powders by vapour phase reaction procedures of MoCl_5 with H₂ has been carried out in the temperature range of 500-900℃. The effects of reaction conditions on the reaction rate, specific surface area, particle size and morphology of ultrafine molybdenum powders were investigated by utilizing conductivity meter, BET method, X-ray diffraction, SEM and TEM techniques, respectively. It is apparent that the reaction rate and specific surface area were increased with increasing reaction temperature, H₂ and MoCl_5 gas partial pressure. The size and shape of the reaction products were nearly about 20-30㎚ and spherical, respectively. Especially, the first generated particles were agglomerated each other in chain type.

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Two types of twins with different atomic structure may exist in GaAs, rotation type and reflection type. An extensive computer simulation was performed of high resolution transmission electron microscopy (HRTEM) images of the two types of twins in GaAs. At the same time, a twin in a practical GaAs crystal was observed with a high-performance HRTEM. Results of simulation study showed that HRTEM images of the two different twins have noticeable differences at certain optimum conditions. However, experimental HRTEM images recorded at the corresponding conditions did not reveal the fine details to show the type of the twin.

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Hg_(0.7)Cd_(0.3)Te epilayers were grown by a silder-type LPE(liquid phase epitaxy) technique using Te-rich growth solution. CdTe substrates of (111)Cd orientation with various degrees of misorientation were prepared by vertical Bridgman crystal growth and subsequent chemo-mechanical polishing processes. Epi-layers, which were grown using a substrate with 1^0 misoriented toward four different directions, showed a typical terrace morphology with terrace fronts perpendicular to the misorientation directions. As the misoriented angle increases, the terrace width decreases, while the terrace height increases. With the misorientation larger than 2^0, the terrace structure begins to disappear transforming into a wave-like surface. The initial stage of the epi-layer growth is thought to be governed by the so-called step bunching process. Dislocations begin to play a major role in the growth process when the terrace becomes as wide as 10 to 30㎛. Epi-layers were examined by Nomarski differential interference contrast optical microscope and stylus

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An attempt has been made to investigate the effects of alloying elements on the mechanical property of structural steels by optimizing the thermodynamic parameters, calculating the chemical driving forces and calculating the nucleation and growth rates in a wide temperature and composition ranges for a carbide, which is thought to give a great effect on the mechanical behavior of steels. For a structural steel including 3 Ni, 1.5 Cr, 0.4 Mo, 0.2 Mn, 0.02 V and 0.17 C by weight percent, calculation of nucleation rates for various carbides predicted that cementite, M_(23)C_6 and M_7C₃ can precipitate in an early stage of tempering at. 923K. Calculation shows that the addition of V up to about 0.1 wt% abruptly decreases the growth rate of M_7C₃ while addition of V more than about 0.1 wt% increases the growth rate of M_7C₃, again. Thus, to get fine distributions of M_7C₃ in this structural steel, it is recommanded from this calculation, that V should be included not more than 0.1 wt%.

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The effects of Mg addition on the aging characteristics of Al-2.5wt%Li alloys have been studied using transmission electron microscopy and differential scanning calorimetry. Results of lattice parameter measurement suggested that the Mg atoms tend to dissolve in the δ` particles. The coarsening of δ` particles generally obeys the LSW kinetics and its rate constant appears not to be much affected by the Mg addition. The kinetics of the precipitation of b phase could be best described by a kinetic law of Avrami type with n=1.5. The addition of 1wt% Mg results in a decrease of the activation energy for the formation of the 8 phase from 141KJ/mole to 137KJ/mole.

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Dispersion morphology of particulate Al/Al₂O₃ composite materials prepared by compocasting has been studied by using optical microscope, SEM and EPMA. An attempt has been made to understand the dispersion behavior and interfacial reaction of Al₂O₃ particles in both Al melt and solid-melt dual phase after homogenization and solution treatment. Cu and Mg form metastable compounds preferably around Al₂O₃ particles and tend to be distributed uniformly in matrix with the content of Al₂O₃ dispersoids. Microstructural analysis of Al/Al₂O₃ composites as prepared by gravitational casting shows that most of Al₂O₃ dispersoids are observed at grain boundary while part of them are distributed inside the grain. The metastable compounds around Al₂O₃ are dissolved when the Al/Al₂O₃ composites are homogenized and solutionized. Particularly, Cu compounds are dissolved remarkably into the matrix.

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Variation of elastic constants and magnetic susceptibility with temperature variation of austenitic Fe-_(13)C_r-_5N₁-_(12)M_6 stainless steel alloy were measured in the temperature range between 77K and 295K. The alloy showed the elastic costant anomalies at the same temperature as that for magnetic transition. Through the analysis of the sharp cusp on the susceptibility curve, the magnetic thermohysteresis, the spin order parameter, etc., it was confirmed that the magnetic transition occured in this alloy is the Ising mean field paramagnetic-spin glass transition of de Alemeida-Thouless type.

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The low temperature phase transition of high-Tc superconducting phase has been investigated in Fe-doped YBaCuO systems by X-ray diffraction analysis, optical microscopy, transmission electron microscopy and electrical resistivity measurement. The phase transition from orthorhombic to tetragonal in YBa₂(Cu_(1-x) Fe_x)₃O_(7-y) superconductor is induced by replacing Cu with Fe. A mixture of the orthorhombic and tetragonal phases exists in specimens containing Fe of about x=0.03. In orthorhombic platelike grains are observed striations which are related to the formation of twins in the grains. The phase transition can be easily identified by the disappearance of the striations in platelike grains under polarized light. As Fe content increases, the twin spacing in orthorhombic phase becomes narrow. In the tetragonal phase at x$gt;0.03, the fine tweed structure is observed instead of the twin structure of the orthorhombic phase. The c parameter does not change even though the phase change occurs, implying that the tetragonal phase in Fe-doped specimen contains more oxygen than that in undoped specimen. The zero resistance temperature(Tc) and oxygen content decrease with Fe content. Nevertheless, the specimen containing Fe up to 2 at% shows high Tc of 84K, suggesting that the super conductivity in YBa₂(Cu_(1-x), Fe_x)₃O_(7-y) is more related to the oxygen content than the crystal structure.

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A study was made of the effects of alloying elements on the structure and mechanical properties of high Li($gt;3wt%) containing RSP Al-Li-X(X=Zr, Cu, Mg) alloys. Particular emphasis was placed on the role of Mg and Cu, which are the major alloying elements in ingot metallurgy (IM) Al-Li alloys. RSP Al-Li-X alloys with several alloy compositions were subjected to various thermal treatments without any pre-aging cold working such as stretching. It has been found that, in the case of plain Al-Li-Zr alloy, the deformation behavior and the resultant mechanical properties are mainly controlled by the presence of uniformly distributed composite Al₃Li/Al₃Zr precipitates. Composite precipitates are also the major strengthening precipitates which control the deformation behavior of the alloys containing Cu and Mg. Addition of Cu and Mg to Al-Li-Zr alloys, when subjected to thermal treatment without any mechanical working, do not promote the precipitation of other strengthening phases such as T₁ and S` as expected ; however, they have a very important effect on increasing the ultimate tensile strength of Al-Li-Zr alloys due to their solid solution strengthening effects. Comparison of the present results with those of IM Al-Li alloys indicates that optimum mechanical properties can be achieved in high Li and Zr containing Al-Li alloys by simple thermal treatment without stretching, and this is due to the presence of uniformly distributed composite precipitates.

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This paper presents the formation of adiabatic shear band during the dynamic torsional test in an AISI 1018 cold rolled steel. A torsional Kolsky bar was used to impose a dynamic deformation rate, viz., γ ◎ 1500/sec, in a short thin-walled tubular specimen. The plastic deformation process in the cold rolled steel could be divided into the following three separate stages ; (1) the homogeneous plastic deformation, (2) the inhomogeneous strain distribution in the axial direction, and (3) the narrowing of the localization into an adiabatic shear band, resulting in a loss in the load-carrying capacity of the steel. It was also found from microscopic observations of the shear band that the fracture occurred by a process of nucleation and coalescence of microcracks which initiated by the the shear cracking of pearlite grains. Elongated dimples were observed on fracture surfaces, indicating ductile fracture due to the local temperature rise during the dynamic deformation.

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