발간논문

Home > KJMM 논문 > 발간논문

Decomposition processes of α phase in Cu-9Ni-6Sn alloy have been investigated by means of side band observation around (200)_α peak, hardness and lattice parameter measurements with aging time and microstructural observations. When the specimens are aged below 400℃ after solution treatment, the supersaturated solid solution α is decomposed by three stages. In the first stage, the modulated structure forms along the <100>_α direction, the side band appears around (200)_α peak and the satellite occurs in the selected area diffraction pattern, these results indicating that the supersaturated solid solution α is decomposed by the spinodal decomposition. And then in the second stage, the matastable phase of γ′ with DO_(22) type superlattice nucleates from the modulated structure. At the final stage of transformation, the y phase decomposes into the unequilibrium cellular structure consisting of α phase and DO_(19) type superlattice. When the specimens are aged above about 450℃ after solution treatment, however, the equilibrium cellular structure consisting of α phase and DO₃, type superlattice forms from the initial stage of transformation without any reactions such as spinodal decomposition and transition of metastable phase.

keyword :

High resolution transmission electron microscopy(HRTEM) was used to investigate the atomic structure of the δ-G.P. zone complex precipitate which forms in an Al-Li-Cu-Mg-Zr alloy. In the alloy quenched from 530℃, the L12-type microdomains with 2-3㎚ in sine are homogeneously observed in disordered matrix. The δ-G.P. I zone complex precipitate is formed by nucleation of δ on both faces of G.P. I zone plate during aging at 190℃. The interphase boundary at the plate faces is atomically flat and coherent with the surrounding δ phase. The δ phase on the top face of the G.P. I zone is translated exactly one half of the unit cell dimension with respect to the δ phase on the bottom face of the G.P. I zone. This forms an antiphase boundary(APB) in the δ phase across the G.P. I zone and the δ appears to form such that Li atoms are adjacent to Cu atoms in the G.F. I zone. The preferred formation of this configuration can be explained by considering the bond energies among Al, Li and Cu atoms.

keyword :

50Ti-40Ni-IOCu(at%) and 50Ti-46Ni-4Cu(at.%) shape memory alloy(SMA) ribbons were prepared by melt spinning method. Using differential scanning calorimeter(DSC), X-ray diffractometer, optical microscope, and thermal cycling test under constant load(strain-temp. measurement), the effect of annealing temperature on the phase transformation and the shape memory behavior of melt-spun has been investigated at temperatures between 400℃ and 700℃. From thermal cyclic test in the Ti_So Ni_(40)Cu_(10) ribbon two-step martensitic transformation is clearly observed, while in the Ti_(10)Ni_(46)Cu₄ ribbon one-step martensitic transformation takes place. Transformation hysteresis(ΔT=A-M) of Ti_(50)Ni_(40)Cu_(10) alloys is still more narrow than that of alloys added 4at.%Cu. Rapidly solidified TiMiCu shape memory ribbon has good crystallinity and preferred orientation. And its preferred orientation is associated with the transformation strain. As the annealing temperature increases, the shape of grain changes to lath-shaped grains along the direction of width and transformation strain decreases. It was confirmed that transformation strain clearly increases with increasing the number of fine equiaxed grains and preferred orientation.

keyword :

Thermo-mechanical stress in MCM-D substrate is an important reliability and fabrication issue. The difference in coefficient of thermal expansion(CTE) between substrate, polymer, and metal leads to complicated stress fields in multilevel interconnect structures. This study uses materials set representative of typical MCM-D technology to monitor the stress level in the thin film layers of polymer. A Kapton PI film is bonded to a substrate with heat and pressure using a thin layer of Ultem 1000 thermoplastic or Coverlay thermosetting adhesive. Mechanical test structures consist of single or multilayer polymer films are fabricated on silicon wafers. The substrate deflection caused by composite stresses due to fabrication and thermal cycling is determined by a curvature measurement technique. Initial stress or bending is intrinsic but becomes extrinsic upon thermal cycling. The composite stress or bending in multilayer structures is due to a contribution from each individual layer. In a thermo-mechanical stress viewpoint, Ultem 1000 thermoplastic and the epoxy thermosetting adhesive exhibit quite different behaviors. While the epoxy thermosetting adhesive exhibits an ideal elastic behavior predicted from the elastic analysis, the total thermo-mechanical stress tends to decrease when Ultem thermoplastic is used as an adhesive. It seems that this relaxation effect of Ultem thermoplastic is due to the self-relaxation nature of thermoplastic materials themselves, while the ideal elastic behavior of the epoxy thermosetting is due to the heavily crosslinked structure of thermosetting materials. A simpler model which predicts a stress contribution from each individual layer during MCM-D substrate fabrication is proposed and verified.

keyword :

The variation of tensile properties and hardness as a function of tempering temperature and time has been investigated using a hardfacing 12%Cr stainless steel. The mechanical properties of the hardfacing 12%Cr stainless steel could be generalized by the Larson-Miller parameter, which concurrently considers the effects of tempering temperature and time. Thermal cycling fatigue behavior of a hardfacing 12%Cr stainless steel has been investigated using a special thermal fatigue testing apparatus. The resistance of thermal fatigue was deteriorated mainly by the low ductility and true fracture strength of material. The temperature distribution in the specimen was calculated using finite element program and compared to experimental results. The strain and stress distributions were evaluated taking into account the temperature distribution and the temperature dependence of the material properties. The results showed that maximum values of strain and stress were produced within the induction-heating region. The strain amplitude obtained in this study was much smaller than that of fully constrained case, which corresponds to thermal expansion due to temperature difference. This result arises from the reduction of the temperature gradient due to thermal conduction to the neighboring region. The magnitude of strain raised with the increase in the temperature gradient, which is due to the rapid cooling and heating rates in the induction-heating region.

keyword :

The thermodynamic behaviors of antimony in molten iron-carbon alloys at high temperature was investigated in connection with the possibility of removal of tramp elements by chemical equlibrating technique. The effects of antimony, carbon content and temperature on the activity coefficient of antimony in molten iron was determined by measuring distribution ratio of antimony between Fe-C alloy and silver. Though the effect of Sb content on activity coefficient, ε^(Sb)_(Sb) , was null at the lower than 0.1wt% Sb, the effect of carbon on Sb activity coefficient, ε^(Sb)_(Sb) was estimated as 10.32 at 1673K. The dependence of the acvtivity coefficient of antimony on temperature was determined as follows ; For carbon saturated iron : ln γ ^(Fe-C○)_(Sb)=-14280/T+10.20 For molten iron : ln γ^O_(Sb)=-23000/T+13.19 Combining the effects of temperature and carbon, the activity coefficient in Fe-C alloy could be expressed as follows; ln γ ^(Fe-C_(Sat.)_(Sb)=(-23000/T+13.19)+ε^O_(Sb)·X_o where, ε^O_(Sb)= 64260/T-28.09 and [%C]^(sat)_(Fe)=0.647+2.54X 10^(-3)·T(K) The possibility of the removal of antimony from molten iron by evaporation was discussed.

keyword :

In order to investigate the effect of particle shape on the gaseous reduction rate of iron ore, iron oxide pellets of different shape were reduced with CO-CO₂ gas mixture. A reduction model for a non-spherical particle was proposed. Applicability of the proposed model was examined by comparing the analysis results from this model and one of the existing models for non-spherical particle. According to the result of applicability comparison, the fractional reduction calculated by the former model were on the better agreement of the observed ones than by the latter model. Investigation revealed that the assumption of the proposed model was reasonable and practicable.

keyword :

Thermodynamic and kinetics of pack siliconizing on pure molybdenum were investigated in the range of 1273K-1473K. Siliconizing was carried out in a hydrogen atmosphere using 40wt.%Si-5wt.%NaF-55wt.%Al₂O₃ packs. The growth rate of MoSi₂ coating was observed to be proportional to the square root of time. Apparent activation energy for growth of the MoSi₂ coating was obtained between that for gas diffusion through Si depleted zone and that for solid diffusion of Si in MoSi₂. The rate limiting step for pack siliconizing process was subject to theoretical considerations. Three models such as a gas diffusion model from the pack to substrate surface, a solid diffusion model in MoSi₂, and an equilibrium model of gas diffusion and solid diffusion were evaluated, Theoretically predicted results based on the quasi steady state parabolic kinetic theory induced from an equilibrium model and experimentally obtained results for pack siliconizing on pure molybdenum were found to be good argeement. The Si halide vapors contributed to growth of the MoSi₂ wating were SiF₂, SiF₃, SiF₄, SiH₃F, SiH₂F₂, SiH₃F and the deposition of Si from Si halide vapors on the molybdenum substrate may occur by reactions involving hydrogen reduction, disproportionation, or a Na vapor species.

keyword :

Ti-48Al specimens were coated with Al-21Ti-23Cr film at 200W, 0.8Pa and 573K by RF magnetron sputtering. The oxidation behavior of the coated specimens was investigated through isothermal and cyclic oxidation tests, and the tensile deformation properties of the uncoated and the coated specimens were also investigated before and after oxidation. The isothermal and cyclic oxidation curves showed that the AI-21Ti-23Cr film was very effective in decreasing the oxidation rate of the Ti-48Al at 1273 K. This excellent oxidation resistance is attributable to the formation of a protective Al₂O₃, layer on the surface of the Al-21Ti-23Cr film. From the tensile test, it was found that the protective Al₂O₃, layer on the surface of the Al-21Ti-23Cr film enabled the Ti-48Al to maintain its tensile properties in an oxidizing environment. Moreover, from the microhardness test, it was also found that the Al-21Ti-23Cr film suppressed the embrittlement of the Ti-48A1 during oxidation. Although extensive cracking with a transverse direction was observed on the surface of the film, the delamination of the film from the substrate and the spallation of the Al₂O₃ scale were not observed. Thus, it is suggested that the adhesion of the Al₂O₃ scale on the Al-21Ti-23Cr film and the Al-21Ti-23Cr film on the Ti-48Al is excellent before and after oxidation.

keyword :

19%Cr-Ti ferritic stainless steel has been used for stud pin as a glass sealing material for cathode ray tube. The characteristics of scale formation with various Ti content under the oxidation treatment in wet gas mixtures of 75%H₂+25%N₂ has been studied. The oxide scale is mainly composed of chromia with 2-4㎛ thickness. Mn-oxide is concentrated on the gas/scale interface. Ti-oxide is randomly distributed not only in oxide layer, but also scale/alloy interface. The oxidation rate was linearly increased with the addition of Ti up to 0.6%. This is related with the increase of the number of vacancy in chromic. The acid resistance in 5%HF+5%HNO₃ solution was decreased as the Ti content lowered below 0.4% because of irregular and thin oxide layer formation. Therefore, it is required to have at least 0.4% Ti to achieve adequate anti-acid properties.

keyword :