발간논문

Home > KJMM 논문 > 발간논문

Occurrence of center burst during wire drawing or extrusion is related to the tensile stresses developed in central plastic region. The stresses in the plastic zone were calculated using the slip-line field theory incorporated with matrix operators. The combinations of process variables such as reduction, die angle, friction, and front or back tension which incur tensile stress in center were obtained and discussed

keyword :

The formation of quasicrsatals in rapidly solidified Al-Li alloys have been investigated by detailed transmission electron microscopy, including a combination of bright and dark field imaging, selected area diffraction pattern analysis and energy dispersive X-ray microanalysis. Al-Cu-Li, Al-Cu-Li-Mg and 2024-2Li alloys rapidly solidified by melt spinning consist of various types of solidification microstructure, ranging form a fine columnar grain structure near the chilled ,surface to a coarse cellular structure near the unchilled surface, depending upon the wheel rotation speed during melt spinning and the local cooling rate through the thickness of the ribbon. Fine scale δ` precipitates during melt spinning in Al-Cu-Li, Al-Cu-Li-Mg and 2024-2Li alloys. The intercellular particles in as-melt spun Al-Cu-Li alloys are the T_8, Θ or T_2 icosahedral phases, depending upon the alloy composition, but in as-melt spun Al-Cu-Li-Mg and 2024-2Li alloys are always the icosahedral phase. Addition of Mg is effective in stabilizing the icosahedral phase in Al- Cu-Li alloys. The icosahedral phase forms as a second solidification product and is located at primary α-Al cell or dendrite boundaries, with strong dependence of icosahedral particle size upon the melt spinning wheel speed and local cooling rate. The composition of the icosahedral phase in rapidly solidified Al-Li alloys is not stoichiometric. Thus, the composition of the icosahedral phase in as-rapidly solidified Al-Cu-Li-Mg and 2024-2Li alloys can be expressed as (Al, Si)_6 (Cu, Fe, Mn)(Li, Mg)₃, assuming Si, Mg and (Mn, Fe) replacing Al, Li and Cu respectively in the Al_6CuLi₃ icosahedral structure.

keyword :

Superconduction YBa₂Cu₃0_(7-x)./Ag composite thick films on a YSZ substrate were prepared by the screen printing method. Microstructure and supercduction properties such as zero-resistance Tc(Tc^(Zero)), Jc at 77k, and the interface reactions betwean the superconduction thick film and YSZ substrate were investigated. For the YBa₂Cu₃O_(7-x)/Ag composite thick films annealed at 980℃ and 1000℃ in flowing oxygen, Tc^(Zero) and Jc at 77k increased, and the normal state resistivity and the thickness of interface reaction layer decreased strongly with increasing Ag content. The drastic decrease in reaction layer thickness with Ag addition seemed to result from the prevention of interaction between the film and the substrate by liquid Ag which formed during high temperature annealing. SEM analysis showed that the grain size increased as Ag content increased from 7wt% to 24wt%. It seems that the liquid Ag in YBa₂Cu₃O_(7-x)/Ag composite thick films enhances the intergranular coupling of YBa₂Cu_O_(7-x)/Ag grains, which results in the favored sintering process at 980℃.

keyword :

Spray Forming technology has been developed at the Department of Materials Science and Engineering, Univ. of Ulsan, for the fabrication of tube, billet and plate of Al-12%Pb alloys. Spray Forming Process involves atomizing a stream of molten metals and collecting the droplets on a substrate before they have solidified, The effects of geometry(orifice diameter and tip design) and position of the metal delivery orifice in atomization device on the pressure characteristics at the tip of orifice have been studied. Mathematical model has been developed to predict the velocity and temperature profiles of the droplets as a function of their flight distances. Microstructural characteristics of Al-12%Pb hyper-monotectic alloys by spray forming process were examined. Dense (~99. 4%) material with fine-grained equiaxed microstructure was spray-formed using nitrogen as the atomizing gas. It was found that lead(Pb) particles existing in the microstructure were homogeneously distributed in the aluminium matrix and along the grain boundaries. The shape of lead particle were approximately spherical and the mean particle size was about 3-4㎛.

keyword :

The mechanical properties and alloying behavior of Fe-base compacts were investigated. Fe-P and Cu-P master alloys were used for preparing Fe-3Cu-3Ni-P compacts varying the composition of phosphorus from 0.2 wt% to 0.6 wt%. The compacts were sintered at 1120℃ for various time up to 180 minutes under dried hydrogen atmosphere. Remarkable increase in hardness of sintered compacts was observed by adding the phosphorus, however the tendency and the amount of increase in hardness were different with the type of phosphorus compound used. The compacts which prepared from powder mixture containing Cu-P compound powder showed higher values of sintered hardness than the specimens containing Fe-P compound powder. The dimensional expansion took place at early stage of sintering ; during heating the specimen to sintering temperature. Most of the dimensional expansion was attributed differently by the type of used P-compound ; the penetration of liquid phase between Fe particles for the compacts containint Fe-P compound powder, and the alloying phenomena for the compacts containing Cu-P compound powder during heating and sintering. The added P compound powder formed eutectic alloy during heating and melted at lower temperature than the melting point of P-compound. The liquid phase smeared out to Fe matrix and formed solid solution leaving pore at the site of liquid phase located.

keyword :

The precipitation behavior of rapidly solidified Al-Li-Zr alloys has been investigated after melt quenching and after subsequent aging at temperatures ranging from 160 to 240℃, by using a transmission electron microscopy and microhardness measurements. Metastable δ` and β` phases which are nearly spherical in shape are present in the as quenched condition. Composite precipitates are controlled by diffusion of Li atoms onto the initial morphology of β` phase. Heterogeneous precipitations are interpreted in terms of a combination of excess vacancies and capillarity effects on the particle stability at elevated temperatures. Precipitate free zone(PFZ) occurred by heterogeneous precipitation of δ phase formed perferentially at grain boundaries. The width of PFZ increased with (aging time)^½. From the analysis of the growth kinetics of PFZ, diffusivity was obtained to be 1.028×10^(-15)㎝/s at 240℃.

keyword :

TiN has been applied to wear resistant coatings in cutting tools and bearings by PECVD, Sputtering and Ion Plating. Most of the TiN depositing processes take advantage of plasma assistance in lowering required substrate temperature for commercial tool materials like toos steels and high speed steels. BARE(Biased Activated Reactive Evaporation) has wide applications in compound film deposition at low substrate temperature. The plasma density is the main factor to activate the compound formation reaction and to enhance the adhesion between TiN film and the substrate material. In this study, the direct comparison of coatings` performance by screech machining test was carried out for various discharge conditions including the substrate bias, the anode current, and the partial flow rate of less active gas, N₂. The machining life was more dependent on the adhesion than films` microhardness. The maximum increase of machining life was 15 times as the uncoated drills. On the average, 5 times increase was achived with gold color TiN coated high speed steel drills.

keyword :

The Mn-Zn ferrite powder was prepared by using the spray method. The effect of reaction conditions on the size distribution and magnetic properties of Mn-Zn ferrite powder was investigated. The characteristics of the prepared powder were compared with those by the conventional coprecipitation method. The optimum conditions for the powder preparation were as follows : equivalent ratio of 1.2, spray rate of 10 ㎖/min, reaction temperature of 90℃ and the reaction time of 4 h. The powder was calcined, compacted and sintered in a tube furnace by varying the sintering temperature and atmosphere. The initial permeability increased to the maximum value at 1300℃. The initial permeability and Q-factor showed the maximum values at 10% O₂ sintering atmosphere.

keyword :

Numerical analysis was performed to get the information about the shape of the solidification front in growing PbTe single crystals by Bridgman method. Calculations were done by FEM for the mathematical model which was based on the real experimental conditions. For PbTe, the thermal conductivity of the liquid phase is higher than that of the solid. Calculated results showed that the solidification front had an undesirable concave shape respect to solid. Because this type of the solidification front can be formed when the heat flux from the liquid through the ampoule is much larger than that through the solid, this trend may be restrained or reduced by promoting the heat flux to the solid. In this study, copper rod was placed at the bottom of the ampoule to act as a. heat sink. The analytic results showed promoted front shape(flat or less concave shape).

keyword :

Continuous low cycle fatigue properties of 1%Cr-Mo-V steel and 12%Cr-Mo-V steel used in turbine rotors were investigated at 298, 773 and 873K in air atmosphere. The microstructure of 1%Cr-Mo-V steel and 12%Cr-Mo-V steel consisted of tempered upper bainite and tempered martensite, respectively. The experimental results show that the effect of the directionality of rotor on the fatigue life was rarely observed. The fatigue life of 1%Cr-Mo-V steel is significantly reduced at high temperature with decreasing plastic strain range, and the reason is considered to be the earlier crack initiation by oxidation damage. However, the effect of temperature on the fatigue life of 12%Cr-Mo-V steel is known to be not existing throughout all the plastic strain range. This is suggested to be concerned with the increasing plastic ductility in tensile test with increasing test temperatures.

keyword :