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The effect of hydrogen on fatigue behaviour of dual phase steels with various martensite morphology and volume fraction was studied. Two different heat treatments, an intermediate quenching and an intermediate air cooling, were employed to change martensite morphology and its volume fraction. Fatigue lifetime in hydrogen was decreased with increase in martensite volume fraction because martensite promoted fatigue crack initiation and propagation in hydrogen. I.Q, heat-treated specimen with fibrous martensite morphology was more sensitive to hydrogen than LA. heat-treated specimen with globular martensite morphology. All of the specimens showed the crack initiation at the martensite-ferrite interfaces and the propagation through both the ferrite and the martensite.

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The microstructural relationship among carbide, ferrite and austenite in a medium carbon bainitic steel containing Si and Mn was studied. Si was added to inhibit the formation of carbide, Mn was added to stabilize the untransformed austenite at room temperature. The steel was annealed both in the fully austenite region and in the ferrite + austenite region. The 1.5%Si-0.5%Mn-0.45 %C steels examined in the present investigation was found to exhibit upper and lower bainites transformed through different transformation mechanism respectively. In the case of the upper bainite, the bainite reaction was thought to be shear transformation in the stage of nucleation, but to be diffusional one in the stage of growth of bainitic ferrite. But in the case of the lower bainite, transformation was thought to be basically shear transformation in all stages of reactions. The variation of the bainite transformation with annealing temperature was found to be that the reaction was slower in the case of intercritical annealing, but no change of transformation mechanism was investigated in both upper and lower bainites in comparison with full austenitizing.

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The sintering behaviour of AISI 4600(Ni-Mo) alloy steel powder for powder forging has been investigated. The powder compacts with the green density of 60-80%TD(theoretical density) were sintered at 1120℃ for 15 min in flowing Ar, N₂+3H₂ or H₂ atmosphere. The volume shrinkage and weight loss of powder compacts during sintering were larger in reducing atmospheres than in Ar atmosphere. In case of an addition of 0.45wt%C, the sintering kinetics was enhanced irrespective of the sintering atmosphere. In the Ar atmosphere, however the enhancement effect was the highest; this was attributed to the reduction of surface oxide and acceleration of Fe-atom diffusion by free carbon. The sintering behaviour of C-added powder compacts was also discussed, based on the chemical analysis of residual C and O content.

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Convection phenomena in the floating zone melting for the purification of the materials and single crystal growth were studied by ring heater method. Two kinds of NaNO₃ rods, 4㎜ and 6㎜ in diameter, were used in order to simulate the convection phenomena under the micro-gravity condition. The experiments on the effect of coupling between natural convection and forced convection, and between Marangoni convection and forced convection were conducted to find the effect of Marangoni convection in the floating zone. The forced convection induced by rotating the rod of 13rpm suppressed the natural convection and the shape of solid/liquid interface was changed from convex to planar when the counter-rotation was applied in the experiment of coupling between natural convection and forced convection. But it was found that the forced convection induced by single rotation of 73 rpm could not suppress the Marangoni convection completely in the experiment of coupling between Marangoni convection and forced convection. It can be suggested that the flow and the temperature fields in the floating zone method by electron beam where large temperature gradient is expected should be controlled by the application of forced convection.

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Marangoni convection in the floating-zone melt under gravity-free condition was studied numerically by a finite difference method. The purpose of the computer simulation was to ana lyze the effect of pure Marangoni convection on the flow and temperature fields in the floating zone. The ranges and values of the parameters investigated were as followings : Ma number from 10¹-10^5, Re_6 number from 1-10⁴, Pr number 10, Aspect ratio 1, Gr number 0. All the solutions showed a single cell stream line pattern and the core of the cell was located at about three quarter radius away from the axis. For Ma number)10⁴, the heat transfer by Marangoni convection relieved the temperature gradient along the centerline(R=1) and the flow and temperature fields were little influenced by temperature gradient from a heat source. The form of thermal boundary layer and the shape of solidifying interface were discussed.

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The development of the {111}//ND and the $lt;110$gt;//RD fiber in the texture of a cold rolled deep drawing steel was studied by the x -ray orientation distribution function measurements and TEM observations. The rolling texture was simulated with the Taylor and its relaxed strain models to explain the variation of preferred orientations under different boundary conditions. The formation of the preferred orientation {001}$lt;110$gt; in the hot coil was caused by the fcc stable orientation {110}$lt;112$gt; formed in austenite phase during the hot rolling. At a low reduction the boundaries of microbands were parallel to the trace of {110} slip planes. Though {11 11 8} $lt;4 4 11$gt; was only stable orientation under the Taylor condition, the orientation near {111}$lt;112$gt; was found in the rolling texture which is rotated 8°from {11 11 8}$lt;4 4 11$gt;. {111}$lt;112$gt; was calculated exclusively under the boundary condtions which involve the relaxation of δ_(ε13). Also, if the boundary conditions which involve the relaxation of δ_ε23, the orientation along the $lt;110$gt;//RD fiber was found. However the orientations along the {111}//ND fiber were formed under the various bounday conditions.

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Metals of low melting points are usually added to Al-Mg alloys in order to increase the machinability. These metals form eutectic compounds together with Mg, crystallized at grain boundaries. Low melting point mother alloys with three different shapes-wire, ribbon, and powder-having various particle sizes and surface areas were made by varying cooling rates and used to make Al-Mg alloys. The smaller the particle size of the powder was, the higher the size distribution of eutectic compounds was. Ribbons with medium size particles had the value of size distribution between the ones of wire and powder. It is thought that using wire-shaped mother alloys results in statisfactory machinability of Al-Mg alloys.

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The main objective aim of this study is to improve mechanical properties of a 7075 aluminum alloy. The alloy composition of a 7075 alloy was slightly modified. To enhance the precipitate hardening effect the content of zinc was increased by 2wt% also, 1wt% cobalt and 1wt% nickel were added to provide the dispersion hardening effect. These alloys were prepared by rapid soidification and hot extrusion. The experimental results suggest that newly developed P/M aluminum alloys exhibit much higher tensile strength than an I/M 7075 alloy. Especially, the alloys produced from fine powders show higher tensile strength and elongation compared to those from coarse powders. Improvements in tensile strength are attributed to suppression or elimination of coarse secondary phases, refinement of grain size and homogeneous distribution of fine precipitate particles, Co₂Al_9, Al₃Ni and oxide particles.

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Al-1%Si thin films with a small amount of copper were deposited on SiO₂ in a single wafer magnetron-sputtering system, followed by BPSG(boro-phospho-silicate glass) deposition. Hillock, grain size variations and etchability of metal films were investigated scanning electron and optical microscopes. Cross-sectional transmission electron microscopy analysis showed that CuAl₂ precipitated along the interface of BPSG and the metal film in as-deposit condition and they were show redistributed homogeneously after annealing treatment. These results were consistent with those obtained from AES and RBS. Electromigration tests were conducted for the metal films with 0.5% and 2% copper contents. The increase in electromigration resistance may be attributed to the homogeneous redistribution of copper in Al-1%Si-X%Cu metal films.

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Preheating processes before the extrusion of rapidly solidified Al-7.9wt%Zn-2.4wt% Mg-1.5wt%Co-0.92wt%Ni alloy powders were studied. Cellular dendrite structures of aluminum alloy powders are found to be decomposed when cold compacted powders are preheated at the temperatures above 490℃. In an aluminum alloy preheated at 490℃ and hot extruded, the density of pores is lower than 0.1% and the maximum diameter of pores is less than 1㎛. The grain size of the aluminum alloy is less than 1㎛ and fine oxide particles are dispersed uniformly within grains and at grain boundaries. After solution (490㎛/1hr) and aging (120㎛/1hr) treatments, the room temperature yield, strength tensile strength and elongation of the aluminum alloy are 71.9㎏/㎟, 76㎏/㎟ and 5.0%, respectively.

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