발간논문

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The solidification behavior in a spray cast high speed steel has been investigated, with particular attention on the formation of eutectic carbides and an equiaxed cell structure. The microstructure of the spray cast material was comprised of two separated regions that were classified by carbide types of cell structure : M₂C and M_6C regions. In the M₂C region, MC carbides were finely dispersed throughout the microstructure and M₂C carbides formed discontinuous cell network. M₂C carbides were metastable which could be formed by a rapid cooling or at a high carbon content. However, in the M_6C region, the carbide network was more continuous than that in the M₂C region. The microstructural feature was mainly determined by the thermal enthalpy depending on atomized droplet sizes depositing on the substrate. The formation of the equiaxed cell structure was associated with a dendrite fragmentation and a rapid solidification of the liquid metal.

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In order to investigate the phase stability of the milled powders with respect to thermal energy, the mixture of elemental powders with the chemical composition of 10, 15 and 48at%Cr in Al was milled for 2000hrs. and subsequently heat treated at various temperatures. The Al-10at%Cr powder consisted of Al solid solution and Cr phase transformed to Al and Al_(13)Cr₂ by heat treatment at 450℃, while the fcc phase in Al-15at%Cr changed into Al_(13)Cr₂ and remaining Al and Cr into amorphase phase by heat treatment at 460℃. The fcc phase in Al-48at%Cr also transformed to intermetallic compounds AlCr₃ and AICr₂after heat treatment. The mechanically alloyed Al-15at%Cr powders were successfully consolidated into amorphous phase by vacuum hot pressing at 460℃.

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Two and three dimensional elasto-plastic finite element calculations have been carried out to analyze the strip buckle in the continuous annealing line. The maximum compressive principal stress in the strip was the most important factor on the strip buckle. The buckle started at the tapered region due to compressive stress. The self centering force due to roll taper propagated the buckle into the strip. The three dimensional analysis indicated that the line tension was the most important factor on the maximum compressive principal stress and the maximum compressive principal stress was much more influenced by the flat width than the taper angle among the geometric variable of roll.

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Dynamic deformation behavior of a 93W-4.9Ni-2.1Fe heavy alloy was investigated using torsional Kolsky bar in order to evaluate its adiabatic shear banding behavior. Dynamic (γ˙∼10³/sec) torsional tests were conducted for four samples fabricated by the different processing conditions, i.e., sintered, heat-treated, swaged, and aged, and then the test data were compared with those of the quasi-static(γ˙10^(-4)/sec) tests. Dynamic test results showed that the maximum shear stress was increased in the order of the sintered, the heat-treated, the swaged, and the aged samples. In the swaged and the aged samples, the shear stress dropped abruptly after it reached to the maximum shear stress point and the shear strain to the maximum shear stress point were considerably small compared to those of the sintered and the heat-treated samples. The phenomena indicated the higher possibility of forming adiabatic shear bands in the swaged and the aged samples than the sintered and the heat-treated samples. The observation of the deformed areas of the fractured torsional specimens indicated that shear deformation was concentrated to the center of the gage section in the case of the swaged and the aged samples, whereas it was homogeneously distributed in the case of the sintered and the heat-treated samples. This deformation behavior correlated well with the shear stress-shear strain curves, suggesting that the torsional Kolsky bar technique is a good tool for evaluating the adiabatic shear banding behavior of the tungsten heavy alloys.

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Al-(20,30wt%)Si-(3,5,8wt%)Fe alloy powders were consolidated by hot extrusion and the change powder microstructure during extrusion was investigated. The effects of Fe addition to Al-Si alloy on the mechanical properties, wear and heat resistance of extruded bars were examined, too. During extrusion of Al-Si-Fe alloy powders, primary Si is broken into fragments from 10∼20 ㎛ size to size less than 10 ㎛ and intermetallic compound from 20∼50 ㎛ size to size less than 5 ㎛. The intermetallic compound in matrix is broken finely, too. Additionally, during extrusion metastable δ phase(Al₄FeSi₂) intermetallic compound disappears and the equilibrium β phase(Al_5FeSi) is formed. Al-Si-Fe alloys showed better mechanical properties, wear and heat resistance than Al-Si binary alloys, resulting from the composite like hardening and matrix hardening by finely distributed primary and eutectic intermetallic compound. Especially, Al-20Si-8Fe alloy showed best physical properties in this work

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In order to maintain elevated temperature strength of mechanically alloyed Al-Ti alloys, the rapid coarsening of Al₃Ti must be prevented. It has been known that the addition of the 3rd elements reduces the lattice misfit between Al and Al₃Ti and produces a smaller Al₃Ti precipitate. Room and high temperature tensile strength of MA Al-Ti-X(X=B, V, Ce, Zr) have been reported to be superior to MA Al-Ti because of the dispersion of fine Al₃(Ti+X) precipitates. The creep-rupture properties of Al-Ti-X alloys were studied at 300℃ and 400℃. The rupture strengths were higher than those of Al-8wt.%Ti alloy, and this improvement of creep-rupture strength was due to the dispersion of fine Al₃(Ti+X) precipitates and the strain induced by altering the dimension of the intermetallic`s unit cell at Al-Al₃(Ti+X) interface. Extraordinarily high creep activation energy could be corrected to the values near for self diffusion of Al by the concept of threshold stress of creep. Then the creep mechanism of MA Al-Ti-X alloys could be described as interface controlled.

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The formation of Lu¨ders lines was characterized in Al-4.5%Mg and Al-5.6%Mg alloys. All of the test specimens of Al-4.5%Mg and Al-5.6%Mg alloys showed type B Lu¨ders line in the strain rate range from 2×10^(-4)/sec to 2×10^(-1)/sec. Type B Lu¨ders line was identified as serration on stress-strain curves. However, type A Lu¨ders line was not observed. The critical strain(ε_c) for the occurrence of the Portevin Le Chatelier effect(type B Lu¨ders line) in Al-4.5%Mg alloy increased with increasing the strain rate. It was considered that dislocation locking was delayed at high strain rates because dislocations moved faster than solute atoms. Specimens were reheat-treated at 400℃, 450℃ and 500℃ for 30 minutes to study the effect of microstructure on the formation of type A Lu¨ders line. It was shown that Al-4.5%Mg alloy showed type A Lu¨ders line but Al-5.6%Mg alloy didn`t. It was considered that the formation of type A Lu¨ders line in Al-4.5%Mg alloy was caused by the grain refinement, increasing the quantity of soluble solute atoms and formation of cell structure after reheat-treatment.

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The overall characteristic of the supported liquid membrane(SLM) containing HEH(EHP) was studied to separate cobalt from nickel. The basic separation properties of the SLM including the effect of operation variables were investigated using a flat type SLM separator. The SLM in a module of hollow fibers considered as a suitable SLM configuration for commercial applications was examined on its efficiencies in cobalt separation and concentration. In addition, the life time of the SLM was tested by a long time continuous operation of the flat SLM. The pressure stability was also tested employing various supportors and diluents.

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A study has been made of the solvent extraction of copper with diluted tributyl phosphate(TBP). Systems involving hydrochloric acid as extracting-promoting agent have been studied in the distribution of copper(II) chloride. The distribution coefficient for copper(II) chloride goes through a maximum at just over 8M HCl and 75% TBP(in toluene), but the extraction of copper was negligible below 3.0M HCl and 75% TBP(in toluene). It was found that the extracting species has the formula 2TBP·H₂CuCl₄·xH₂O Unextracted copper in the aqueous phase is determined directly and copper in the organic phase is stripped with H₂O and determined by Sodium diethyldithiocarbamate method, respectively.

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In order to improve the mechanical properties and the corrosion resistance, paste boronizing was carried out in the temperature range of 700℃∼1050℃. Mechanical properties were measured by the microhardness and abrasion tests, and corrosion resistance was examined by potentio dynamic polarization test. The microhardness and wear resistance were increased with the increase of the boronizing temperature. The microhardness of boronized samples was about 1600Hv, which is approximately ten times higher than those of the low carbon steel. The increase of microhardness can be attributed to the formation of borides such as FeB and Fe₂B within the samples. The wear resistance of the boronized samples also were 5 times higher than that of the low carbon steel. Annealing of the samples enhanced the wear resistance and the corrosion resistance significantly, but did not enhance the microhardness. The activation energy for boronizing on the low carbon steel measured as 227 kJ/㏖ below 874℃ and 117kJ/㏖ above 874℃

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