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The binding energy for a Sc atom-vacancy couple was determined in an Al-0.0087 at%Sc alloy by the resistivity measurements. For pure Al, the formation energy of a vacancy was determined to be 0.79 eV, being in good agreement with those in the literature. The apparent vacancy formation energy in a dilute Al-0.0087 at%Sc alloy was 0.65 eV. Clearly it is somewhat lower than that for a pure Al which suggests the presence of a vacancy as a vacancy-solute couple in the Al-Sc alloy. On the basis of semi-emprical calculation using the vacancy formation energies in pure Al and in the dilute Al-Sc alloy and using the relationship proposed by Lomer(IOM series, no.23(1958)p.85), the vacancy-Sc binding energy for Sc was found to be 0.35 eV. The value, 0.35 eV, obtained in this study is larger than those observed for other transition elements in Al. This high vacancy-Sc binding energy is considered to affect the precipitation behaviors in Al-Sc alloys.

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The corrosion behavior and mechanical properties were investigated for Zr-1.0Nb alloys which were prepared by vacuum arc remelting and received different final heat-treatments. Corrosion tests were carried out in a static autoclave containing aqueous solution of 360℃ water and 360℃ LiOH (70 ppm). Microstructures of the tested alloys were analyzed using optical microscopy and TEM, and the cross-sectional surface of oxide layer was analyzed using SEM. All of the tested alloys showed higher corrosion rates in the 360℃ aqueous LiOH solution than in 360℃ water. As for the corrosion behavior in the 360℃ aqueous LiOH solution, the alloys exhibited a transition in corrosion rate as early as 30-45 days exposure. With respect to final annealing temperature, Zr-1.0Nb alloy having partialy recrystallized structure in this study showed higher corrosion resistance compared with the alloys having stress relieved, or fully recrystallized structure. It was concluded from TEM study that corrosion behavior would be related to the distribution and size of second phase particles. The yield strength, ultimate tensile strength and hardness decreased as the final annealing temperature increased.

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Residual stress in thin film was evaluated by a nanoindentation technique combined with a new stressrelaxation model. The dominant shape change in the nanoindentation curve during the stress relief in thin film was identified as the increase or decrease of applied load for a given maximum indentation depth. The applied indentation load of film under tensile stress was higher than that of stress-free film. Also, a stress-sensitive deformation morphology around the indenter-thin film contact was proposed. Finally, the in-plane residual stress in thin film was directly calculated from the change in the normal applied load arising from stress relaxation. The stresses for diamond-like carbon (DLC) and Au films evaluated under the proposed analytical model agreed well with the residual stresses measured by the conventional curvature method.

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Superplastic deformation and high temperature load relaxation behavior of a coarse-grained iron aluminide have been investigated in this study. Iran aluminide with the composition of Fe-28 at.% Al was prepared and thermomechanically treated to obtain an average grain size of about 500 ㎛. A series of load relaxation tests was conducted at temperatures ranging from 600 to 850℃. The flow curves obtained from load relaxation tests were found to have a sigmoidal shape and to exhibit stress vs. strain rate data in a very wide strain rate range from 10^(-7)/s to 10^(-2)/s. Tensile tests have been conducted at 850℃ and at the various initial strain rates ranging from 3×10^(-5)/s to 1×10^(-2)/s to investigate the superplastic deformation behavior. The maximum elongation of ∼500% was obtained at the initial strain rate of 3×10^(-5)/s and the maximum strain rate sensitivity was Found to be 0.68. Microstructure observation through the optical microscopy and EBSD (electron back-scattered diffractometry) technique has been carried out on the deformed specimens and revealed the evidences for grain boundary migration and grain refinement to occur during deformation, suggesting the dynamic recrystallization mechanism. The activation energy was evaluated as 387 kJ/㏖ in the strain rate range from 10^(-4)/s∼10^(-2)/s, which is very close to that for creep deformation in Fe₃Al alloys.

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The effects of microstructural factors on high-cycle fatigue properties and fatigue crack propagation behavior of welded regions of a cast Ti-6Al-4V alloy were investigated in this study. High-cycle fatigue test and fatigue crack propagation test were conducted for the welded regions, which were processed by two different welding methods, i.e., tungsten inert gas (TIG) welding and electron beam (EB) welding, and then the test data were analyzed in relation to microstructures, tensile properties, and fatigue fracture mode. The base metal was composed of Widmansta¨tten structure, while the EB and TIG welds consisted of basket-weave type structure having thin α platelets. The high-cycle fatigue results indicated that fatigue strength of the EB weld was lower than that of the base metal or the TIG weld because of the existence of coarse pores formed during welding, although it had highest yield strength. In the case of the fatigue crack propagation, the EB weld composed of very thin α platelets had the faster crack propagation rate than the base metal or the TIG weld. The effective microstructural factor determining the fatigue crack propagation rate was found to be the width of α platelets because it was well matched with the reversed plastic zone size calculated at near-threshold ΔK.

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The effect of niobium(Nb) contents on the mechanical properties of the CrMoV forgings was investigated as a preliminary study to manufacture large forgings having a low fracture appearance transition temperature(FATT) and high tensile strength. The experimental forgings were varied in the Nb contents as 0%(A), 0.03%(B) and 0.05%(C). The tensile strength at room temperature and 600℃ were almost equal regardless of Nb contents, while the FATT lowered in order of alloy C(32℃), B(35℃), and A(62℃). These were resulted from the grain refining. The grain size of alloy A was ASTM No. 5.0 and those of B and C were ASTM No. 6.5 respectively. Based on the preliminary experimental results, large forgings containing 0.03%Nb(D) and 0.05%Nb(E) were manufactured from 30 ton ingot and then those mechanical properties were tested. The higher tensile strength was obtained in both D and E forgings, whereas, the FATT of D and E forgings were 33℃ and 67℃, respectively. The ductile fractured surfaces were developed in the forgings D, while serrated fractographes were alternately occurred with the eutectic niobium carbides and ductile dimples in the forgings E. The eutectic NbC and normal matrix were alternately developed in a severely Nb segregated region of the forgings E. The alternated structure might develop a serrate fracture surfaces, which caused forgings E to increase its FATT. Therefore, in order to manufacture large CrMoV forgings by adding Nb, Nb content to be added should be decided by considering segregation of Nb during solidification. The coarse eutectic niobium carbides were formed in the severely Nb segregated region. The serrated fracture might be caused to increase the FATT of large CrMoV forgings containing Nb.

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Creep-fatigue tests including hold time of 10 minute at the maximum tensile strain were conducted for type 316L(N) stainless steels containing different nitrogen content of 0.04 and 0.10 wt% at 600℃ and 2x 10^(-3)/sec in air atmosphere. Creep-fatigue life was decreased with the hold time but was increased with the addition of nitrogen. Fracture modes were intergranular regardless of nitrogen content. Dislocation structures were changed from cell to planar with the addition of nitrogen. Carbides (M_(23)C_6) were precipitated at grain boundaries and acted as the site for cavitation. Carbides were continuous in the low nitrogen content steel but discontinuous in high nitrogen content steel. The increase of creep-fatigue life with the addition of nitrogen is attributed to the distribution of carbides at grain boundaries rather than the change of dislocation structures.

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The effects of electromagnetic vibration on solidification structures were investigated in Al-Si alloys. In the hypo-eutectic alloy, the grain size decreased and dendrites were fragmented into globoid with increasing frequency(Hz) and amplitude(A) of the electromagnetic vibration. This would be resulted from the disappearance of the constitutional supercooling and remelting of dendrite stems by the vibration of liquid phase. In the hyper-eutectic alloy, it is found that the variation of the vibration frequency did not change the particle size of Si primary crystals significantly. As the vibration intensity increased, the segregation of Si primary near the side wall increased and the Si crystals in eutectic structure have changed from flake to fibrous morphology.

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The effects of alloying elements Cr, Mo and N on the stability of passive film were investigated by measuring pit propagation rate (PPR) and critical pitting temperature(CPT) in aqueous 0.1N H₂SO₄+ 0.1N NaCl solution. CPT tests were carried out by measuring the temperature representing the critical current density obtained by increasing temperature at the specific applied potential within passive potential range while PPR tests were conducted by measuring the pit propagation rate obtained by the current density produced due to the pit growth divided by the pit area. The effects of alloying elements Cr, Mo, N on the stability of passive film were investigated by the comparison between CPT and PPR test results in terms of pitting resistance equivalent. PPR test results showed that pit propagation rate was decreased with the increase in Cr, Mo, and N contents, indicting the synergistic effect of Mo and N. CPT test results exhibited that the higher contents of Cr, Mo, and N, the higher critical pitting temperatures. The synergistic effects of Mo and N on the resistance to pitting corrosion were also observed. As the pitting resistance equivalent increases the passive potential range is extended due to the increased pitting potential. The increase in pitting resistance equivalent also results in the reduction of pit propagation rates, showing the increased critical pitting temperatures. Pitting resistance equivalent may be the good parameter representing the extent of the resistance to pitting corrosion regardless of the kind of stainless steels.

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It was known that the color of Hematite(Fe₂O₃) is red, while Magnetite(Fe₃O₄) is dark blue. To clarify the origin of red color on the steel plate, the relationship between the volume fraction of oxide phases and the color of steel plate was investigated in this study. The volume fraction of Wu¨stite, Magnetite, and Hematite was quantitatively evaluated by a direct comparison X-ray method using crystal structure informations. Furthermore, the effect of volume fraction and the manufacturing process on the surface color of steel plate was investigated. It was found that FRT(final rolling temperature) played an important role on the plate color.

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