발간논문

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In ZMR of SOI structured polycrystalline silicon thin films using tungsten halogen lamps, a heat equation with moving heat source was numerically solved. A new physical model which can describe the solidification behavior of silicon thin film was proposed. From the simulation results, the supercooling in liquid phase due to the difference of reflectivity and emissivity of silicon is expected and the interface is expected to be unstable. As the beam intensity and the scanning velocity increases, the interface morphology is expected to change from nonplanar to planar and the resultant defect density is expected to increase. Experimental results agreed well with those theoretical model. By introducing the interface instability analysis, we proposed the process diagram for the first time as a function of beam intensity and scanning velocity.

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A solidification of hypereutectic Al-30%Si alloy was studied under the condition of the forced fluid flow. The melt was stirred by a rapidly rotating graphite bar on whose surface the primary silicon was solidified. The average silicon content in the solid was more than 80% and the purity increased with the convection increased. The perfect Mixing Parameter(PMP) was introduced and used to rationalize the level of the purity achieved. Both the solute distributions in liquid and solid were influenced by fluid flows and affected the solidification behaviors. The temperature distributions in melts were also analogized by paraffin tests. The microstructural changes of the solid were showed along the position of the specimens and agreed well with its compositional change.

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The oxide reduced iron powders have been applied up to 60wt.% in the mix with the carbonyl iron powders to the injection molding process, and experimental measurements are presented to provide the basis for the practical usefulness of this novel approach. The hydrogen reduction of iron oxide powders is employed to produce fine, rounded powders of agglomerates with a great number of microscopic pores. The apparent and tap density of such an agglomerate as a reduced iron powder are determined to be 1.435g/㎤ and 2.573g/㎤, respectively. Both the spiral length (moldability) and the density of an injected green body decrease with increasing the relative fraction of oxide reduced iron powders in the mix, while the shrinkage ratio and sintered density increase. It is also found that tensile properties and hardness values reach the maximum with the 50wt.% addition of oxide reduced iron powders.

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The effects of niobium and titanium on the weldability of type 409L ferritic stainless steel were investigated. The stabilization ratio for obtaining a fully ferritic structure in the welds was changed to the variation of stabilized alloying elements(Ti, Ti+Nb) and weld thermal cycles. In order to get a fully ferritic structure at heat affected zone of dual(Ti and Nb) stabilized stainless steel after TIG welding, the stabilization (Ti+0.5Nb)/(C+N) ratio was 5.6 minimum, while the ratio of Ti stabilized stainless steel was 8.3 minimum. The difference of the ratio for both stabilized stainless steels might be due to different stability of carbides(TiC, NbC) at high temperature, The dual(Ti+Nb) stabilized stainless steel had a good intergranular corrosion resistance of the weld at the ratio of 6.5. The larger and rectangular shaped particles formed in the melt were identified as TiN and their size was about 3 to 5㎛. Most fine particles with the size of 0.05∼0.1㎛ were identified as (Nb, Ti)(C, N). The higher Nb content in (Nb, Ti)(C, N) was detected in the composition of the finer particles.

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In this study, the crack closure phenomenon was investigated for conservative prediction of fatigue life time. Based on previous experimental results and theoretical analysis, it was expected that the variation of normalized crack closure stress with ΔK could be classified into three regimes. This classification concept was reflected in considering a fatigue crack growth rate equation. In addition to crack opening displacement method and back face strain gage method, acoustic emission technique was introduced to measure local microbehavior of crack closure at the crack tip of CT specimens of 2024-T8 Al alloy. Acoustic emission technique was found to yield better closure results successfully. Finally, FASTRAN program was adapted and executed to predict crack growth rates in the cyclic stress state. The predicted values were relatively consistent with experimental ones except near ΔK_(th) in both CCT and one through crack at hole specimens.

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Evaporated Al/Cr thin films were irradiated with various doses to produce intermetallic compounds. In order to study the first phase formation and phase sequence, RBS, XRD and TEM studies have been used. RBS analysis shows that intermixing has occurred across the Al/Cr interface and mixing variance increases with increasing ion dose. The Al_(13)Cr₂ phase is formed as an initial phase by ion beam mixing and then transforms into the Al_(11)Cr₂ or Al₄Cr phases at subsequent ion bomdardment. This phenomenon is discussed in terms of the enhanced atomic mobility and the thermodynamical driving force by introducing the effective heat of formation(ΔH) model. The experimental results agree with that predicted by effective heat of formation model. It has been studied the first phase formation and phase sequence by Ion Beam Mixing in metal/Si systems as well as metal/metal systems.

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The repressing treatment of the debound part was introduced in MIM of sieve size Fe powders in order to improve the densification and shape stability. The sintered density of the MIM part increased with increasing the applied pressure in repressing step, and mechanical properties of MIM were improved by additional pressing of brown part. Such a post debinding treatment was also proven to influence positively the shape stability of MIM part. The effect of the additional pressing on other MIM processes was further investigated and discussed.

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The structure of Pd/Co multilayer thin films prepared by thermal evaporation was analyzed, compared with the theoretical calculation based on the ideal superlattice structure. The FWHM(Full Width at Half Maximum) of the diffracted peaks for the multilayer films was larger than that of the ideal superlattice, and it was largely affected by the fluctuation of the period of the multilayer film. The fluctuation increased with the period of the film and the lattice mismatch between two sublayers. As increasing the period, the coherent length of superlattice structure was decreased. The observation of the satellite peaks was difficult when the films had small period(Δ$lt;15Å) because the intensity of the structure factor at the satellite peak position is much smaller than that at the main peak position. The Pd underlayer reduced the disorders in the film and therefore the coherence length of the superstructure was increased.

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The mechanical properties of sintered WC-Ni alloy are inferior to those of WC-Co alloy because of its poor sinterability. To improve the sintering response we prepared a WC-Ni alloys from WC powders coated with SiC, which were obtained by coating W powder surface with SiO₂ followed by carburizing. The particle size of carburized powders and microsturcture, density, hardness and fracture toughness of sintered alloy were examined in terms of the amount of SiC addition. The particle size of SiC-coated WC coarsened at a lower rate during carburizing than that of uncoated WC powder. While the SiC coated WC-10%Ni cemented carbide with refined particle size showed an abnormal grain growth. With SiC coating by 0.6∼0.8wt.%, the sintered WC alloy showed restrained grain growth and θ phase(Ni₃W₃C) formation. As the amount of added SiC increased, the density, hardness and K_(IC) of the alloy increased. For those with the amount of SiC above 1.0wt.%, however, the hardness increased whereas the density and K_(IC) decreased.

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In the previous investigations, it has been proposed that the cavities are nucleated at grain boundary due to the aggregation of the mechanically generated vacancies. For that model, the cavity nucleation factor was introduced to correlate the number of cavities and the plastic strain range from which athermal vacancies are generated. In this investigation, a discussion is proposed to clarify the characteristic property of the cavity nucleation factor for the dependence of the plastic strain range using the results of creep-fatigue test of solutionized AISI 304 and aged AISI 304 stainless steel at 873K. From the creep-fatigue test result, the effect of thermal aging is found to be beneficial relative to solution treatment, and it can be considered that the great amount of small carbides are precipitated at the grain boundaries during creep-faitgue tests in the solutionzed material, and they increase the cavity nucleation rate because they can be sites for the cavity nucleation. And, based on this result, it is found that the cavity nucleation factor is a function of the plastic strain range and the dependency is varying with the materials. Using this relation, the modified model for life prediction is suggested.

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