발간논문

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The precipitation of carbide in tempered high tensile strength steel has been characterized by using high resolution electron microscopy and the combined use of a computer-aided image processing system. The lath-like η-Fe_2C carbide precipitated along <211>_M directions with 8-11 ㎚ in width are observed in the martensite plate. The orientation relationship between the η-Fe_2C carbide and the martensite matrix was determined to be (010)_η (110)_M, [101]_η [111]_M. The twin structure having {211} habit plane is occasionally observed in the martensite matrix, and the carbides were not preferentially precipitated at twin boundaries, in contrast with the observation of other investigators.

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In order to clarify the effect of precipitates on creep strength at high temperature in 25Cr-20Ni stainless steels, threshold stress and void-hardening stress have been measured and compared each others. The value of threshold stress for high temperature, measured by stress abruptly loading test, is about 130 MPa. Threshold stress must be measured by the function of stress loading time in the combination hardening steel of solution hardening and precipitate hardening. Average diameter and inter-partial distance of precipitates from TEM microstructure can determine void-hardening stress. The value of void-hardening stress, evaluated by using Scattergood and Bacon`s equation, was from 101 to 130 MPa. The ratio of average void hardening to threshold stress is 1.13 and both values are not equal. The result from analyzing the electron diffraction pattern shows that the dispersed precipitates in SUS310J1TB is NbCrN nitrides. Mechanism of the interaction between dislocations and precipitate particles in SUS310J1TB is Srolovitz mechanism, which is a gravitation-type interaction.

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The purpose of the present research is to develop a model predicting the high cycle fatigue life in relation to the microstructural variable, especially, grain size. In order to achieve this goal, the concept of small crack propagation has been adopted since it is thought that the fatigue crack initiation is similar to the propagation of small crack, which is greatly influenced by the microstructural variables. It is assumed that the whole fatigue crack initiation process can be represented by the microcrack propagation through one grain and one barrier, i.e., consisting of two steps, the propagation through one grain and the arrest at a barrier. For the first step, a modified small crack growth law has been used to consider the influence of grain size on the resolved shear stress, which is the major driving force for the small crack propagation. For the second one, the concept of strain accumulation criterion has been used to calculate the cycles required in overcoming a harrier. Reasonable agreement is found between the experimental data and the predicted curve.

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Strip casting process has been developed to manufacture thin steel strip, 2 to 4 ㎜ thick, directly from molten metal fed in between two water-cooled casting rolls, and is being applied to the industrial steel making plants. The cast strip contains typically 2 - 6% of delta ferrite in the microstructure at room temperature. This work studied the dissolution kinetics of delta ferrite in strip cast specimen for 1050℃ - 1200℃ temperature range by varying the annealing time. The morphology of delta ferrite in the cast strip was in vermicular type with a three-dimensional honeycomb network structure. At early stage of dissolution of delta ferrite, more than 50% of delta ferrite dissolved very rapidly regardless of annealing temperature. The reason for the rapid dissolution was suggested that Cr depletion in the delta ferrite excessively occurred due to (Fe, Cr) carbide precipitation at interface boundary between delta ferrite and austenite phases. Subsequently, the dissolution was controlled by volume diffusion of Cr and Ni in austenite phase. The analytical solution assuming the austenite phase as semi-infinite can be applied to the analysis of experimental data at 1150℃. The activation energy for the dissolution was measured as 251.4 kJ/㏖e, which is coincident with activation energy of Cr volume diffusion constant.

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This study was to analyse the relationship between properties of ion nitrided Fe sintered material and parameters of plasma nitriding process. Pure Fe sintered materials (relative density : 92%) were fabricated by hotpressing and nitrided under pulsed DC plasma for 4 hours. The large number of parameters in plasma diffusion treatment allows close control of the process so that surface layers with defined microstructures and properties can be obtained. The number of micropores in nitriding layer can be reduced in pulsed DC plasma by reducing the plasma power and by varying the gas mixture. The phase constitution of compound layer of nitrided Fe sintered material were influenced by gas mixture (N_2: H_2) and pulsed ratio (pulse on/off time ratio).

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A foam-filled channel having constant wall temperature and allowing convective cooling was numerically analysed to obtain the overall heat transfer coefficient and the characteristic length as the function of foam density, cell spacing, foam thickness, fluid velocity and physical properties of foam and fluid. It is found that the numerically calculated overall heat transfer coefficient shows negative deviation from one calculated by analytical model assuming constant fluid temperature along direction of foam thickness. The deviation was strong at small foam density, small cell spacing, high fluid velocity and thick foam, where heat conduction capacity through solid ligaments was low compared with solid ligament-fluid heat transfer capacity and the fluid temperature variation along direction of foam thickness was remarkable. A general expression for the characteristic length, which was proportional to mass flow rate per unit foam width, and inversely proportional to the overall heat transfer coefficient could be obtained.

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A numerical model representing heat transfer in open-cell metal foam is presented. By comparing the calculated results with experimental ones, the validity of the numerical model is confirmed. Influences of air flow rate, cell spacing as well as cell aspect ratio (ratio of ligand diameter to cell spacing) on heat transfer in open-cell metal foam are examined. Heat flux increases with increase of air flow rate and cell aspect ratio, but it decreases with increase of cell spacing. Solid temperature in foam increases with cell spacing and cell aspect ratio, but it decreases with air flow rate. Gas temperature in foam increases with cell aspect ratio, but it decrease with increase of airflow rate and cell spacing.

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In microelectronics packaging, reliability of the metal/polymer interfaces is an important issue because adhesion strength between dissimilar materials are often inherently poor, and residual stresses arising from thermal mismatches or pressures exerted by vaporized moisture often lead to delaminations of interfaces. Modification of polymer surfaces by the rf plasma and oxidation treatments of metal surfaces before joining are commonly used methods to enhance the adhesion strength of the interface, and therefore it is important to measure and analyze the adhesion strength. In this study: 1) The practical adhesion strength of Cu/polyimide and Cu-based leadframe/EMC interfaces were measured using the peel, pull-out, and fracture mechanics tests, and the effect of rf plasma pretreatment and oxidation pretreatment of leadframe were investigated. 2) The correlations among peel strength, pull out strength, and interfacial fracture toughness were made, and the interfacial fracture energies were derived from the practical adhesion strengths using X-ray measurements and theoretical analyses. 3) The thermodynamic analysis of interface fracture, proposed by Rice^1), was applied to the metal/polymer interfaces.

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We report on fabrication of all solid-state thin film microbattery(TFB) that has a structure of Li/LiPON/ LiNi_(1-x)Co_xO_2/Pt/MgO/Si multilayer. The LiNi_(1-x)Co_xO_2 films were grown by radio-frequency(rf) magnetron reactive sputtering system using synthesized LiNi_(1-x)Co_xO_2 target in a 9:1 Ar/O_2 mixture gas. X-ray diffraction(XRD), transmission electron microscopy(TEM), and selective area diffraction(SAD) analysis show that rapid thermal annealed LiNi_(1-x)Co_xO_2 film has (101) and (104) preferred orientation with increasing film thickness. In addition, scanning electron microscopy(SEM), atomic force microscope(AFM), and Auger electron spectroscopy(AES) examination exhibit that the surface layer on as-deposited LiNi_(1-x)Co_xO_2 film was completely removed by rapid thermal annealing process. All solid state TFB, fabricated by using the annealed LiNi_(1-x)Co_xO_2 cathode film, shows stable cyclibility and high specific discharge capacity of 60.2 ㎂h/㎠-㎛. It was thought that the combined feature of a LiCoO_2 and a LiNiO_2 cathode caused fairly high capacity and stable cyclibility of the LiNi_(1-x)Co_xO_2 based TFB.

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In this study, SU-8 micro-molds and nickel microstructure were fabricated by UV-LIGA process. The optimum conditions for the fabrication of SU-8 micro-mold and the nickel electroforming were investigated. The 150 ㎛-thick SU-8 micro-mold was successfully fabricated through the process conditions. It was observed during electroforming that with increasing of concentration of wetting agent, the current efficiency was increased and declination of nickel microstructure thickness with change of aspect ratio was decreased. This is attributed that when adding wetting agent in electrolyte, contact angle between SU-8 micro-mold and electrolyte is decreased and penetration of electrolyte into microchannel is increased. The thickness uniformity of nickel microstructure was improved by adding wetting agent because of the improvement of secondary current density distribution.

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