발간논문

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A thermomechanical process was developed which gives a fine grain size of 8㎛ in 7475 Al alloy. Tensile and superplastic properties of this material were evaluated in the temperature range of 673-818 K over the strain-rate range of 2.8×10^(-4)-2.8×10^(-2) s^(-1). The maximum ductility exhibited by the alloy was approximately 1300%, and optimum su-perplastic properties were achieved at a strain rate which is higher by an order of mag-nitude than other 7475 Al alloys. This result is attributed to the presence of fine diaper-soils which maintain the fine grain size at high homologous temperatures. The superplastic 7475 Al alloy has strain rate sensitivity of about 0.67, and an activation energy which is similar to the one for lattice diffusion of aluminum. The superplastic deformation can be explained by a diffusion accomodated grain boundary sliding model.

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A study on the mineralogical properties and soap flotation characterisics of goethite from Yongwol Okdong mine was carried out. The results are as follows, 1. Ye11ow color sample was found to be a typical goethite fine powder. It has a needle like shapes and its chemical formula is Fe₂O₃·1.36 H₂O. Brown color sample is a mixture of goethite and hematite. It has a irregular shapes and its chemical formular is Fe₂O₃·1.28 H₂O. 2. Oleic acid is an effective collector in flotation of iron oxide. In pH 7.0, the flotation recovery and concentrate grads shows maximum value. 3. In yellow color sample, the grade of goethite rougher concentrate shows 96 % at 70 % flotation recovery. On the other hand, brown color sample represents 96 % grade of rougher concentrate at nearly 100 % flotation recovery. 4. From the experimental results, both samples has a possibility to be used as an iron oxide pigment. Moreover, it is possible to obtain iron oxide for magnetic materials by further treatment such as cleaning flotation method.

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