발간논문

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Effect of aluminum, manganese, and zinc on the iron solubility in molten magnesium alloys has been studied in the temperature range of 650℃ and 745℃ using a sedimentation method. The iron solubility in molten magnesium decreased very rapidly with increasing aluminum or manganese content by 4.0% or 1.0% in weight, respectively, while addition of zinc up to 1.0% in weight did not give any effect on the iron solubility in magnesium. The rapid decrease in the iron solubility with addition of aluminum was due to the corresponding rapid decrease of the iron activity in iron-bearing precipitates in equilibrium with Mg-Al melts. This strong interaction of aluminum with iron was qualitatively analyzed in this paper based on thermodynamics. Using X-ray diffraction method, the phases of iron-bearing precipitates in equilibrium with Mg-Al or Mg-Mn melts were also determined as a function of the temperature and the composition of Al or Mn.

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Amorphous Ni-Zr alloys were produced by mechanical alloying of elemental powders and the hydrogen storage site in these alloys was predicted based on the experimental results of maximum hydrogen content per metal atom and those of atomic distribution functions obtained respectively from hydrogen absorption and X-ray diffraction experiments. Both results were applied to Harris model which is that hydrogen atoms occupy the interstitial sites in five types of A_(4-n)B_n(n=4, 3, 2, 1, 0) [where A(B) is a late(early) transition metal] tetrahedrons in the order of increasing site energies and explained well by this model.

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Strain controlled low cycle fatigue (LCF) tests were conducted in the temperature range between R.T. and 600℃ in type 316L stainless steels with different nitrogen contents. Fatigue life decreased and cyclic stress increased with increasing temperature due to dynamic strain ageing (DSA) at the temperature range of 300∼600℃. The dislocation structures were changed from cell to planar by increasing effect of DSA and short range order(SRO) with increasing temperature and nitrogen addition. Since nitrogen reduced the chromium diffusion coefficients, nitrogen retarded DSA, which resulted in decrease in LCF life at high temperature. So, fatigue life increased with nitrogen addition due to the retardation of DSA and planar slip by SRO.

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In order to solve problems involved with the curved type continuous caster and high speed casting by controlling the steel flow in the mold, the electromagnetic mold brake ruler (EMBR) using a static magnetic field was installed at No. 2 continuous casting plant of POSCO Kwangyang Works. Numerical modelling and plant tests were carried out to optimize the core type and operation condition of the EMBR. The core "Type A" located near the SEN outlets and shaped simple linear, had a detrimental effect on the slab quality. However, the core "Type B" located below the SEN with a magnetic insulator at the core center, reduced the defects of slab, especially at high throughput casting conditions. The optimum range of the surface velocity was found to be in 12∼20 ㎝/sec for reducing both the sub-surface and internal defects of slabs by plant tests with EMBR. The EMBR has been operated with the optimum magnetic density to preserve this surface velocity for each casting width and casting speed.

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Microstructural evolutions of both Si particles and matrix grains of hypereutectic Al-Si alloy were investigated by calculating temperature profiles within the preform during spray forming. Based on calculations, the cooling rates were observed to be relatively higher both at the bottom and the top than the inner regions of the preform along the rotation axis of rod. Such trends in temperatures resulted in size variations of Si particles within the preform. The size of Si particles varies from 3㎛ to 13.5㎛ with distance from the surface to the inner of the preform along the rotation axis of rod. Larger Si particles observed from the inner regions of the preform were found to be poly-crystals having irregular surface boundaries. However, such Si particles were fractured after extrusion, indicating a possibile means for fabricating hypereutectic Al-Si alloys with fine Si particles. In addition, the methodology for predicting the matrix grain size in the presence of small Si particles was also demonstrated.

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