발간논문

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In the present study, the effects of P and/or Misch metal (Ms) on the microstructure and high temperature tensile behavior of Bi-added, Pb-free brass were examined. It was found that the tensile ductility of Bi-added. Pb-free brass decreased significantly at 300℃. The addition of P and/or Ms, on the other hand, greatly improved elevated temperature tensile ductility of the present alloy. The microstructural observation demonstrated that the formation of Bi film along the α-β, phase boundary. which is the main cause for the low tensile ductility at elevated temperature, was substantially reduced with the addition of P and/or Ms. The retarded Bi film formation was believed to be due to the thermally stable Bi-related compounds formed in Bi-added, Pb-free brass. (Received April 8, 2004)

keyword : Bi-added, Pb-free brass, P, Ms, Elevated temperature tensile ductility

Effects of ferrite grain size and martensite volume fraction on quasi-static and dynamic deformation behavior of dual phase steels were investigated in this study. Dynamic torsional tests were conducted on six steel specimens, which had different ferrite grain size and martensite volume fraction, using a torsional Kolsky bar. Under a dynamic loading condition, maximum shear stress and fracture shear strain increased with decreasing the ferrite grain size and increasing the martensite volume fraction. Observation of the deformed area beneath fracture surface after the dynamic torsional test indicated that adiabatic shear bands of 5-15 μm in width were formed along the shear stress direction; voids or microcracks initiated at martensite/ferrite interfaces below the shear band. The width of the shear band decreased with increasing ferrite grain size and with decreasing martensite volume fraction. These phenomena were then analyzed by introducing concepts of critical shear strain. (Received April 7, 2004)

keyword : Dual phase steel, Dynamic deformation, Martensite volume fraction, Ferrite grain size, Step quenching

The prediction of bending fatigue life was carried out for Type 304 stainless steel sheets containing various nickel contents from 8.3 up to 12 wt.%. After the slopes of the elastic strain (Δε_(e)/2) and the plastic strain (Δε_(p)/2) lines were obtained by the tensile test and bending fatigue test data at room and cryogenic temperatures, the prediction equations for the bending fatigue life were made by using the method of universal slope. At the ranges above 10³ Cycles, the predicted equations showed higher bending fatigue life same as measured one with lowering the nickel contents in the range of 8.3-12 wt. %. But, at the ranges below 10³ Cycles, the predictions of low-cycle bending fatigue life varying the nickel contents were different from the measured ones. A limitation was encountered for the prediction of the bending fatigue life with using the tensile data. In order to increase the reliability of the prediction equations at the ranges below 10³ Cycles, it is necessary to take into account the behavior of the α`-martensite transformation during fatigue affecting the generation and propagation of fatigue cracks. (Received March 8, 2004)

keyword : Type 304 austenitic stainless steel, Nickel contents, Bending fatigue life, Prediction of bending fatigue life, Cryogenic temperature

In this study, a microstructural investigation was conducted on the side cracking phenomenon occurring in low-carbon steel plates during hot rolling. Particular emphasis was placed on the role of iron oxides and pearlite band structure in the side cracking. Detailed microstructural analyses of the hot-rolled plates showed that the pearlite band structure in the side region was considerably slanted to the surface, while that in the central region was parallel to the surface. Small cracks were often observed when the iron oxide layer formed by inhomogeneous oxidization along the slanted pearlite band structure in the side region was intruded into the plate interior during hot rolling. In addition to these small cracks, a few large cracks were also found when some protrusions formed on the plate surface were decarburized, folded, and then intruded into the plate interior. Thus, in order to prevent the side cracking, 1) the minimization of the segregation of C and Mn in slabs by the sufficient homogenization, 2) the parallel arrangement of the pearlite band structure in the side region by the appropriate control of the hot rolling process, 3) the minimization of the iron oxide formation by the shortest time exposed to high temperatures, and 4) the careful control of the spacing between steel plates and side guide rolls were suggested. (Received January 17, 2004)

keyword : Side cracking, Low-carbon steel plate, Hot rolling, Iron oxide layer, Pearlite band structure

Mechanical tests, such as tensile, fatigue and creep, of type 316L(N) stainless steel containing 0.04 and 0.10 wt% nitrogen content were conducted at 600℃ to investigate the grain size effect on the behaviors of mechanical properties since grain size was refined with the addition of nitrogen. Yield strength and fatigue life increased with the addition of nitrogen and the decrease of grain size. The creep rupture time was increased with the addition of nitrogen but decreased with increasing grain size. The change in minimum creep rate with grain size was consistent with the result predicted from Garofalo`s equation. (Received November 17, 2004)

keyword : 316LN, Stainless steel, Grain size, Nitrogen, Fatigue

WC-Co coatings have been deposited by detonation gun thermal spraying of conventional (WC-12 wt.%Co) and nanocomposite MC-8 wt%Co) powders and wear properties were compared each other. Various stand-off distances and fuel amounts were used in order to control phase composition and microstructure of nano WC-Co coatings. Phase decomposition of WC in nano coatings into W₂C and free W was observed due to hot detonation wave during spraying process and amounts of W₂C phase increased in longer stand-off distance. The microstructure of nano-coatings, such as porosity and hardness, was widely varied according to fuel amounts. Most dense nano-coatings exhibited higher hardness and wear resistance than those of conventional coatings. Nano-coatings showed different wear behavior from conventional coatings due to extremely small carbide size. (Received March 5, 2004)

keyword : WC-Co, Thermal spraying, Nano-Particle, Wear resistance

The microstructural changes under varying temperature conditions of Cu-15 wt%Nb filamentary nanocomposite fabricated by the bundling and drawing process were characterized using transmission electron microscopy(TEIVI). In the sample heat treated at 400℃, twin bands were observed in a subelemental wire with the Nb filaments acting as barriers to twin propagation, and the multi-twins having {111} twin planes were observed in Cu sheath. The spheroidzation of Nb filaments appears in specimen heat treated at 500℃. Nb filaments completely transformed into spherical particles in the sample heat treated 700℃, and the amorphous layers with 4 nm in width were observed at the interface of Nb particles and the Cu matrix. The texture of the Cu matrix(<100> and <111>) and Nb particles(<110>) remains unchanged following annealing at 700℃. (Received March 12, 2004)

keyword : Cu-Nb filamentary nanocomposite, Transmission electron microscopy, Multi-twin structure, Spherical Nb particles, Amorphous layers

In this study, equal channel angular pressing (ECAP) was carried out on Cu-Fe-Cr microcomposites at room temperature. The microstructure and mechanical properties of the specimens pressed at different ECAP routes were investigated. All specimens pressed at different routes showed ultrafine-grained microstructures with the shape and distribution of Fe-Cr phase dependent on the processing routes. As the number of pressing increased by route A, the intial dendrite of Fe-Cr phases were elongated along the shear direction and developed into filaments. On the other hand, as the number of pressing increased by route Sc and route C, the initial dendrites became finer by fragmentation with no pronounced change of the shape. The comparison of the hardness between wire drawn Cu-Fe-Cr and ECAP-ed Cu-Fe-Cr showed that wire drawing was more effective in increasing the hardness than ECAP. More effective microstructural refinement due to the decrease of the interfilammentary spacing with increasing drawing strain is thought to be responsible for much faster increase of hardness during wire drawing. (Received October 6, 2004)

keyword : Equal channel angular pressing, Micorcomposite, Twin, Shear Direction

In the present investigation, we studied effects of composition of organic additioves and electrolysis conditions for surface appearance of electrodeposited zinc. When organic additive PEG (Ployethylene Glycol) increased in electrolyte, the current efficiency decreased and grains of plated film changed with fine structures of staking typed form. The current density increased with decreasing limit current density and current efficiency, but an increase of the flow rate allowed electroplating at high current density. When PEG(Ployethylene Glycol) additives increase, high angle pyramid plane is tilted to 65?and preferred crystallographic orientated with (101) and then (112) peak increase. Whiting degree of plating film surface is decided by crystallographic orientation and grain size. In the case of the matrix with paralleled basic plane(002) which is preferentially oriented, higher degree of whiteness is shown because incident light on sheet surface is regularly reflected. (Received November 17, 2004)

keyword : Electrodeposited zinc, Current efficiency, Flow rate, Crystallographic orientated, Grain size, Whiting degree

Results of stress analysis on Metal 1/Via 1/Metal 2 AI-0.5%Cu interconnect are reported as it cools from ILD (Inter-Level Dielectric) deposition temperature to room temperature and subsequent heating to 200℃ (electromigration test temperature) for three values of Metal 1 reservoir length for a fixed length of Metal 2 reservoir length. Computed stress fields are connected to the vacancy concentration distribution in line and via by assuming both steady state for the vacancy concentration and no current flow in the reservoir region during electromigration test. Finally, the reservoir effect on the electromigration lifetime is analyzed using the normalized vacancy concentration distribution in the reservoir region of the multilevel Al-Cu interconnect structure, which were used to estimate the probability of the initial void formation in the reservoir region. (Received May 10, 2004)

keyword : Stress, Electromigration, Via, Microelectronic devices, Reservoir