발간논문

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Aging behavior of Al-Mg-Si alloys with high silicon content was investigated in the present study. Excess silicon leads to unique aging behavior. The Al-Mg-Si alloys show seven exothermic peaks in DSC thermogram. These peaks seem to be induced by the formation of clustering of Si and Mg atoms, G-P zone, unknown small precipitates, β$quot; phase, B` phase, Si particle and β phase. The content of excess Si has no relationship with the precipitation of β$quot; phase and B` phase, but it affected on the formation of Si particles. Natural aging has a distinct influence on the subsequent precipitation process. It is proposed that the formation of stable silicon clusters during natural aging is the major reason for such an effect.

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This study is aimed at investigating the anisotropy of shape memory effect in Fe-based alloy. An Fe-15Mn-3Si-4Co-5Cr alloy was cold rolled by 92%, and annealed at 630℃ for 46 min. The alloy undergoes γ→ε→α’ transformation by cold rolling and γ is restored with a major preferred orientation of (110)γ[001]γ by the annealing. The specimens for shape memory effect and tensile test were taken 0, 45, 70, 80 and 90 degs to the rolling direction. The specimen taken along 70 degs to the rolling direction exhibits the best shape memory effect. An analysis of the (110)γ[001]γ texture reveals that the specimen experiences the deformation along [221]γ which is the most expandable direction in the γ→ε transformation. In tensile test, the best SME specimen shows the lowest yield stress, indicating that the transformation take place most easily since the deformation is applied to the preferential direction to the deformation.

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The objective of this study is to investigate the effect of hydride precipitates on the distributions of the lattice distortion and stress in CANDU Zr-2.5Nb pressure tube. The three sections taken from CANDU Zr-2.5Nb tube, the longitudinal normal section(LS), the radial normal section(RS) and the transverse normal section(TS) were charged with hydrogen at 530℃ for 15 h. X-ray diffraction analysis was carried out on the sections with or without charged hydrogen using a X-ray diffractometer equipped with a position sensitive detector. The effect of hydride precipitates on the line broadening, the interplanar lattice spacing, and the macro-stress was determined from a change in the peak position and the half width of the diffraction lines ｛12.3｝ on the three sections. The hydride precipitates had an effect of increasing the interplanar lattice spacing of ｛12.3｝ only on the LS, causing tensile stress of as high as 400 ㎫ but decreased the interplanar lattice spacing on the TS causing it subjected to compressive stress. Based on a change in the distribution of line broadening with hydride precipitates on each section depending on the pole density of ｛12.3｝, the preferential site for the precipitation of hydrides is discussed.

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An ultrafine grained structure was introduced in a commercial 5083 Al alloy with the initial grain size of ∼200 ㎛ using the equal channel angular pressing technique. Equal channel angular pressing was successfully conducted at 200℃ and the results showed that the microstructure was reasonably homogeneous after single passage and consisted of parallel bands of elongated substructure having the average width of 0.2 ㎛ and the average length of 0.8 ㎛. Repetitive passes were conducted on the same sample up to total 8 passes through the die such that the sample was rotated 180° around its longitudinal axis between passes. An equiaxed ultrafine grained structure of ∼0.3 ㎛ was obtained in the present alloy after 8 passes. In addition, in this study, the feasibility of low temperature superplasticity of a ultrafine grained 5083 Al alloy was examined. It was found that a 5083 Al alloy with the grain size of ∼0.3 ㎛ exhibited a superplastic-like behavior with elongation to failure in excess of 200% below 250℃.

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Small punch(SP) test was performed to evaluate tensile properties and ductile-brittle transition behavior of various reactor pressure vessel(RPV) steels with bainitic microstructure in the temperature range of R.T. to -196℃. The testing specimens were taken from 1/4t location of one weld and six forged RPV steels manufactured by the different processes and machined to a dimension of 10×10×0.5 ㎜. Yield strength and maximum strength from SP test were successfully correlated with yield strength in all temperature ranges and tensile strength in the upper shelf region of SP energy versus temperature curves. Weibull statistical analysis was a useful method to determine the mean value of SP energy in the transition region and the SP transition temperature even for the very limited number of SP specimens. Transition temperature determined at mid-point of upper and lower SP energy(T_(SP)) was successfully correlated to the index transition temperatures from standard Charpy impact test and T_o value from the precracked V-notched three-point bend fracture(PCVN) test. From the observation of the outer surface of the tested SP specimens, it was found that the SP energies were proportional to the lengths of the largest residual crack that did not lead to a fatal main crack. Small punch local fracture stress obtained by considering Griffith theory exhibited some meaningful agreement with local fracture stress obtained from the PCVN test.

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Recently, in the line with the growing interest of sanitary problem, the requirement for the development of anti-bacterial material has been an issue with more weight. In Japan, not only some steel makers already completed the development of the anti-bacterial stainless steel, but other makers are also showing an interest in that subject. The function anti-bacterial power of stainless steel is closely related with the addition of Cu element. In this case, Cu decomposes as Cu^(2+) and reacts with COA-SH enzyme which acts as a catalyst in respiratory process of bacteria, resulting in deactivation of the enzyme. The current study derived an optimum anti-bacterial alloy composition, proper process conditions including heat treatment and the overall comparison with the previous developed POSCO-304J1 stainless steel. With the proper heat treatment the anti-bacterial power of the developed alloy is increased due to the uniform distribution of ε-Cu precipitates (0.5∼1.0 ㎛) restraining the formation of passive film on the steel surface.

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Mode II fatigue crack growth tests under a superimposed static compression stress field were made using a newly developed apparatus, in order to establish a test method to obtain the intrinsic mode II crack growth resistance of steels. The difference between the crack growth in simple mode II loading and the crack growth in rolling contact fatigue is, we suppose, whether or not there is a superimposed compression stress. Some preliminary results on da/dN-ΔK_(II) relations have been obtained by using this apparatus on a steel and aluminium alloys. Subsurface fatigue crack growth in contact rolling fatigue were successfully simulated by this testing system.

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Two main factors of microsegregation in secondary dendrite arms during solidification have been known as the back diffusion of solute atoms in the solid domain and the coarsening of dendrites during and after solidification. The effects of the back diffusion and the coarsening have not been considered simultaneously in most previous analytical solutions reported in the literature. In the present study, an analytical model, where both the back diffusion and the coarsening effects are included, has been developed to predict microsegregation in solidification of alloy. The present model was successfully applied to the prediction of eutectic fractions in directional solidification of an Al-4.9 wt%Cu alloy and concentration profiles of Mn in continuous casting of steels.

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Ti and Ti-based alloys have been widely used for the biomedical applications due to their superiorities of biocompatibilty, mechanical properties and corrosion resistance. However, there has been the limiting factor for these metals to show the low affinity to the living bone. The purpose of this study is to improve the bone-bonding ability between Ti-base alloys and living bone through the chemically activated process and thermally activated one. After those treatments, the effects of the pre-treatments on the bonding property were evaluated by in vitro test. Two kinds of Ti-based alloys, Ti-In-Nb-Ta and Ti-6Al-4V, were used in this study. These specimens were alkali treated in 5.0M NaOH solution and heat treated in vacuum furnace at 600℃ and soaked in SBF(simulated body fluid) including nearly same ion concentration as human blood plasma. The microstructural changes of activated surface were observed by OM, SEM, EDS, XRD, and AES. Two surface modified alloys showed improved bioactive behavior. And Ti-In-Nb-Ta had the better bioactivity than Ti-6Al-4V in SBF.

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Photocatalytic characteristics of nano-sized TiO₂ powder with rutile phase produced using homogeneous precipitation process at low temperatures (HPPLT) were compared with those of commercial P-25 powder by Degussa Co. The TiO₂ powder by HPPLT showed very higher photoactivity in the removal rate, showing lower pH values in the solution, than the P-25 powder when eliminating metal ions such as Pb and Cu from aqueous metal-EDTA solutions. This can be inferred the more rapid photo-oxidation or -reduction of metal ions from the aqueous solution, together with relatively higher efficiencies in the uses of electron and hole formed on the surface of TiO₂ particle, under UV light irradiation. Also, in the view of the TiO₂ particle morphology, compared to the well-dispersed spherical P-25 particle, the agglomerated TiO₂ particle by HPPLT consists of acicular typed primary particle with the thickness ranged of 3∼7 ㎚, which would be more effective to the photocatalytic reactions without electron-hole recombination on the surface of the TiO₂ particle under the UV light irradiation. It is, therefore, thought that the higher photoactivity of the rutile TiO₂ powder by HPPLT in the aqueous solutions resulted from having its higher specific surface area as well as acicular shape primary particle with very thin thickness.

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