발간논문

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A model expression on the diffusion coefficients of multicomponent metallic alloys has been derived as a review of previous works by many-other researchers. A systematic method for evaluation of model parameters of multicomponent diffusion coefficients from experimental data reported in various form was suggested. From a thermodynamic consideration on the diffusion coefficient model, it was found that the off-diagonal diffusion coefficient of an interstitial element can be evaluated from phase diagram information on isopotential curve of that element resulting in the following relation. D_(CSi)/D_(cc)=-(dy_c/dy_(Si))_(μc) This relation has been applied on the Fe-Si-C ternary fcc alloys to evaluse D_(CSi) from known values of D_(cc) and experimentally reported isoactivity data of carbon. A computer simulation of well known Darken`s uphill diffusion in the fcc Fe-Si-C alloys has been carried out and it was concluded that the above relation can be used as an alternative way to evaluate the off-diagonal diffusion coefficients of interstitial elements when critically assessed thermodynamic data is not available.

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Silicon Nitride(Si₃N₄) and Ni-Cr steel were bonded by using Ti/Cu laminate layer and the interfacial structure along with mechanical properties were investigated. It could be shown that the reaction products formed at Si₃N₄/Ti interface were TiN and Ti-silicide. The thickness of reaction product layer formed at Si₃N₄/Ti interface was almost constant, independent on bonding condition. However room temperature bond strength was affected strongly by the thickness of Ti/Cu reaction layer and decrease of copper layer due to press in high bonding temperature. Bond strength according to testing temperature was similiar to room temperature value (162MPa) to 400℃, while above 400℃ it was decreased abrubtly due to softening of copper layer. In this study, dominant factors controlling mechanical properties of the joint were the extent of the reaction between Ti and Cu and the change of copper layer thickness.

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The effect of heating rate and initial porosity on the ignition temperature, combustion temperature and microstructure of products formed by combustion synthesis was investigated. Ignition temperature increased with increasing heating rate and initial porosity. Temperature rise(ΔT) caused by exothermic reaction decreased with increasing heating rate and decreasing initial porosity. Microstructure of the product formed by 2℃/min. heating rate consisted of Al₃Ni, Al₃Ni₂ precombustion crystal and martensite matrix. Typical polygonal grain structure was observed in specimen produced with initial porosity of 20% and heating rate of 20℃/min. Whereas irregular grain boundary was observed in specimen produced with initial porosity of 53% and heating rate of 600℃/min.

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The stress-induced martensitic transformation was studied by tensile deformation in Fe-17% Mn alloy quenched to room temperature, as a function of strain, strain rate, and deformation temperature. The structure of the alloy consisted of ε, α´ and γ. When the alloy was deformed at room temperature, the sustenite stabilization due to deformation began at 4%, while the epsilon stabilization due to deformation started at 14%. During the deformation, a large number of ε plates coalesced to some big blocks parallel to the tensile axis, and α´ martensites were formed at the angle of 45˚ to the tensile direction within them. When the deformation was carried out with variation of strain rate between 1×10^(-3)/ sec and 2×10²/sec, the contents of the three phases such as ε, α´ and γ were kept constant values in the range of strain rate below 1/sec, however the γ content was decreased in accordance with the increase in α´ content on the further strain rate.

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The P/M consolidation of the rapidly solidified powders is an effective way to produce very fine microstructure in the hyper-peritectic Al-Ti alloys, where the grain size is less than a few microns in diameter with fine and uniform dispersion of the Al₃Ti particles. The RSP Al-Ti alloys show good high temperature strength above 300℃ mainly due to excellant thermal stability of the aluminide. In order to investigate the deformation behavior of the Al-Ti alloy at high temperatures, fine-grained Al-1.2wt%Ti, Al-3.6wt%Ti and Al-7wt%Ti specimens were produced through hot extrusion of the rapidly solidified powders made by gas atomization. The strain-rate(ε˚) vs. flow stress(σ) relations were obtained by the strain-rate change tests in tension at 300-500℃. It has been analysed that the deformation behavior could be represented by the (ε˚/D)=A·exp[-Q_a/(RT)]·(σ/E)^n where D and E are diffusivity and elastic modulus of pure Al, respectively. Stress exponent, n, was 8.5, and the activation energy for deformation was 163∼183kJ/㏖. this observation indicates that tensile deformation of the fine-grained RSP Al-Ti alloys is governed by dislocation slip activated by lattice diffusion of aluminum. Even though the grain size is very fine, the fine dispersion of Al₃Ti particles prevents grain boundaries from being activated at the high temperature range investigated.

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To clarify the influence of Be addition on the coarsening behavior and the early stage of decomposition process of δ´phase in binary Al-Li alloys, the detailed measurement of micro vickers hardness and specific heat and the observation of transmission electron micrographs have been carried out. Al-Li-Be alloys were found to have higher hardening rate than the binary Al-Li alloy when aged at 190℃ and Be addition on binary Al-Li alloys promoted the early stage of decomposition process of metastable phases such as formation of ordered domains (δ´precursory structures). δ´precursory structures in the binary and ternary alloys were stable with aging time at 190℃. The split of endothermic peak R on the S-T their incompletely orderd structure and size effect, respectively. It was deduced that the coarsening rate constants of δ´phase in Al-Li-Be alloys were higher than that of the binary Al-Li alloy because of the change of interfacial characteristics of δ´phase which resulted from microsegregation of Be at interface between δ´phase and the matrix or Be incorporation into δ´phase.

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