발간논문

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The BaTiO₃ powders were prepared by coprecipitation method. BaCl₂ and TiCl₄ were used as starting materials and H₂C₂O₄ as a precipitation agent. Effects of synthesizing conditions on the properties and the crystal structure of powder were investigated by X-ray, SEM, TEM and chemical analysis. The best results were obtained as following conditions : mole ratio(BaCl₂ : TiCl₄ : H₂C₂O₄ = 1.03 : 1 : 2. 2), reaction temperature 80℃, The average particle sizes and specific surface areas of the powders calcined at 700℃ were 0.16㎛ and 6.091㎡/g, respectively.

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Four kinds of Cu-Ni alloys(90Ni-Cu, 78Ni-Cu, 10Ni-Cu) were crept to investigate the effect of solute atom on the creep behavior of these alloys over the temperature range of 558-676K(0.4T_m where T_m is the absolute melting temperature) and at the stress levels from 127 to 256 ㎫. Results show Chat the static and cyclic creep deformation behaviors are affected by the thermodynamic factor, 1-(d ln γ_i/d lnX_i), Where γ_i and X_i are activation coefficient and compositional concentration, respectively. Cyclic creep acceleration is observed for 40Ni alloy under the stress of 197 ㎫, and it can be explained that the excess vacancies generated mechanically under cyclic creep assist the recovery process with dislocation climb. To illustrate the above results, a proper creep equation is proposed as, ε˚=K(1/b²)(γ/Eb)³(D₁.+f_PD_P)(σ/E)³(T.F.), where E is Young`s modulus, b is Burgers vector, r is stacking fault energy, D_L is lattice diffusion coefficient, D_p is pipe diffusion coefficient and T.F. is thermodynamic factor.

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High pure nickel powder was produced from nickel sulfate which was the by-produced in a domestic copper smelter by hydrogen reduction. The purification of nickel sulfate solution was carried out to remove impurites such as Zn, Pb, Fe, Cu and Co by solvent extraction using D2EHPA as an extractant. The purified nickel sulfate solution was reduced in a autoclave by hydrogen pressure and the effects of various additives such as (NH₄)₂CO₃, Tamol 731, and anthraquinone on the hydrogen reduction and the particle size, morphology, and purity of the nickel powder were investigated. Shpheroidal nickel powder of 99.9% purity was obtained through a series of purification, nucleation with anthraquinone, 10 densifications, and heat treatment in flowing hydrogen.

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The mechanical propreties of a squeeze cast AC8A alloy reinforced with alumina short fibers were investigated. The Al₂O₃,AC8A composite was fabricated by the squeeze infiltration method in various casting conditions. Particular attention was given to the condition of preform temperatures. The preheating of the preforms was necessary for an easy infiltration of melt attributable to the improvement of wettability and the desorption of gas from the fiber surface. However, a too high preform temperature produced a severe interfacial reaction layer between the matrix alloy and the fiber, causing deterioration of the mechanical properties of the composite. The tensile strength of the composite decreased significantly as the preform temperature approached the melting temperature of the matrix alloy. In this work, the relationship of mechanical properties of the composite and the structure of interfacial layer has been studied using various analytical electron microscopy techniques. AES and SEM fractographic analyses were also preformed, they showed change of fracture behavior of the composite from a dominant fiber pull-out to a fiber fracture depending on preheating condition of the preform.

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Density distributions of sintered porous metal with various initial densities during hydrostatic compaction have been calculated using a proposed yield function. The calculation was performed using the iteration method. The caculated results were in very good agreement with experimental ones. The geometric hardening and material strain hardening of the metal were found to have an important effect on the porosity-pressure relation.

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This paper addresses the fundamental understanding of sensitization phenomenon in stainless steel and a method of desensitization utilizing a high energy density beam of LASER. Degree of sensitization was evaluated with the state of the art technique of EPR and was correlated with microstructural changes characterized by TEM(STEM), SAD and SEM. Laser desensitization could be obtained with a proper choice of beam power and scanning rate under a constant beam size. As long as the scan rate was lower than a critical rate under a constant beam power, carbides and Cr depleted zone were completely disappeared. Further, as a cooling rate after laser treatment was significantly greater than a critical cooling rate for reprecipitation of carbides, re-sensitization would not be occurred during cooling. The critical scan rate increased with an increase in a beam power, but was constant regardless of the original degree of sensitization.

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To improve the cold workability of non-oriented electrical steel containing 3.1wt% of Si, the effects of Si content and slab reheating temperature were investigated. By lowering the slab reheating temperature from 1250℃ to 1150℃, better magnetic properties have been obtained, regardless of the Si content, due to the increased grain size and the decreased unfavorable (111) texture component in the final annealed sheets. The rate of increase in core loss due to the decrease in Si content was 0.76w/㎏/wt%Si for the slab reheating temperature of 1150℃, which was found to be much higher than 0.24w/㎏/wt%Si obtained for the reheating temperature of 1250℃ It is concluded that both the reduction in grain size and specific resistivity account for the increase in the core loss of the specimens reheated to 1150℃, whereas the specific resistivity alone affects the core loss of the specimens reheated to 1250℃.

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After cold rolling of AA 8091 alloy by 5%∼30%, the static recrystallization kinetics were studied in the range of temperature from 350℃ to 500℃ with varying isothermal holding time from 1 to 5000 sec. It was observed that all recrystallized grains were nucleated at the grain boundaries which were formed during cold rolling and grown. The kinetics of recrystallization could be well described by Avrami equation, Y=1-exp(-ktⁿ) and the values of n were found to be in the range from 0.35 to 0.48 and those of k were known to be in the range from 8 × 10^(-2) to 3 × 10^(-1). It was found that the values of n were lower than those of any other Al alloys which have the value of 0.6, and therefore the recrystallization of AA 8091 alloy could be restricted by the precipitates of δ` and Al₃Zr. It was observed that the fast recrystallization rate was assisted by dislocation elimination, precitates coarsening and large amount of δ` resolution in Al matrix at higher temperatures than 450℃ and the effect of temperature on the recrystallization rate was more pronounced than that of prestrain on the rate at higher prestrains than 10 percents.

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The effect of thermal cycling on the characteristics of martensitic transformation was studied in Fe-17%Mn alloy. The thermal cyclic treatment was carried out with three types; room temperature ↔ 255℃, 300℃and 345℃ As the number of the thermal cycle was increased, the hardness was rapidly increased up to 5 cycles, and then was gradually increased with further cycle. The volume fraction of the εmartensite was also increased with increasing the thermal cycle. The dislocation density in the austenite was increased with increasing the thermal cycle, which resulted in a decrese in the Ms temperature by the strengthening of the austenite, In the zero-cycled specimen, the martensite volume fraction was burstly increased at and below Ms temperature, and was gradually increased with further cooling below about 100℃. On the other hand, in the 15-cycled specimen, the martensite volume fraction was slowly increased in the early stage, but rapidly increased in the range of 100℃ to 60℃. On further cooling, the volume fraction was gradually increased.

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ZrN_x, NbN_x, and MoN_x coatings were deposited onto high speed steel substrates by reactive dc magnetron sputtering in an argon-nitrogen atmosphere. The target and substrate surface reactions were described in order to explain experimental results on the deposition rate and coating composition as a function of the nitrogen partial pressure. As a result of the reactions the target contamination, the sputter yield of the contaminated target surface, and the formal reaction coefficients of gaseous nitrogen with the deposited metal atoms were obtained. Special emphasis was laid on the effect of the position of the metals in the periodic system of elements. The reactivity was strongly governed by the thermochemistry of the respective metal-nitrogen systems.

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