발간논문

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To know the effect of the electro-galvanized layer for corrosion protection on the strain distribution in sheet metal forming, we studied and analyzed the variational behavior of strain distribution with the computer aided in bare and electro-galvanized sheet steels which were deformed under the various forming variables. Experimental results, the punch load and concentration of strain distribution of electro-galvanized sheet steel cups which are deformed showed lower values than those of bare steels in low punch speed and are reversed in high punch speed. We could analyze these phenomena in relation with the change of friction coefficient under the various forming variables.

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An experimental study was carried out to investigate the reaction behavior of (sinter ore) and gas phase oxygens in hot metal during dephosphorization treatment using a mixture of CaO and CaF₂ as a flux. In general, the degree of dephosphorization and the partition ratio of phosphorous into the slag increased with an increase in oxygen partial pressure and markedly increased beyond Po₂=10^(-14.5). In most case, the oxygen efficiency for dephosphorization increased with an increase in basisity under the existence of oxygen. Without blowing gaseous oxygen, dephosphorization hardly occured when only the flux was added, although very high degrees of desulphurization as much as 93% was obtained. By the addition of the sinter ore together with flux, dephosphorization was much improved but little desulphurization was observed. When gaseous oxygen was injected without any addition, very low dephosphorization and desulphurization were obtained. With the addition of the flux only dephosphorization was improved and with the addition of the sinter ore together with the flux both dephosphorization and desulphurization were much improved. The temperature of the system was decreased with the addition of solid oxygen and flux, but this temperature drop could by fully compensated by keeping the gaseous oxygen rate above 50% with the injection of gaseous oxygen.

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The interaction phenomena between dislocations and a grain boundary have been examined by conducting in-situ straining experiments with 316 stainless steel in a TEM. An emphasis was put on especially for the identification of the critical factors affecting slip transfer across a grain boundary. Various types of dislocation interactions with grain boundary were observed during in-situ deformation experiments, such as dislocation absorption into grain boundary, reflection from grain boundary, transmission across grain boundary, and microcrack initiation at grain boundary. Each phenomenon is considered as an effective relaxation mechanism for the accumulated internal strain energy induced by piled-up dislocations against grain boundary. The slip transfer across grain boundray was found to be affected by the geometric relationship between the two adjacent grains and also by the internal stress generated by a dislocation pile-up. The activated slip system could, therefore, be predicted by such factors as 1) the minimum intersection angle between the two slip planes in the two adjacent grains, 2) the largest resolved shear stress acting on the neighboring grain under the stress field caused by a dislocation pile-up, and 3) the smallest Burgers vector of residual dislocations remaining in the grain boundary after a slip transfer. These results appear to be in good agreement with the present direct observations of slip transfer behavior.

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The composition and the annealing effect on the giant magnetoresistance of Co-Cu granular films fabricated by RF sputtering were investigated by TEM and NMR analysis. The maximum MR of 17% at 4.2K could be obtained at cobalt composition of 20 at%. From the TEM and NMR analysis, it has been found that the size of cobalt particles and the composition profile near the interfaces of cobalt particles are almost invariable with increase of cobalt concentration, while the interparticle distance decreases. Therefore, it could be concluded that the interparticle distance was mainly responsible for the existence of the maximum MR with respect to the cobalt composition in this system. According to the analysis of NMR spectrum, annealing of the films sharpened the composition profile of the interface between the cobalt particles and the copper matrix, which resulted in enhancement of the GMR values.

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When 200 Å-thick Ni films deposited on Si(111) at room temperature were in situ annealed at 650℃ for 20 minutes, epitaxial NiSi₂ layers were formed. Interfaces between NiSi₂ and Si(111) were observed using a high-resolution transmission electron microscopy. It has been known that the (111￣) sl facet formed on the A-NiSi₂/Si(111) interface. In addition to this, the (001)_(SI, A-NISI)₂, facet was found in the present study. On the B-NiSi₂/Si(111) interface, the formation of (111￣)SI//((11)￣5)B-NISI₂ and ((11)￣5)SI//(111￣)B-NISI₂ facets was found. In sample deposited 10 Å, Ni films and annealed at 500℃ for 3 minutes, nuclei of NiSi₂ were observed. Each nucleus had a hexagonal shape. The NiSi₂ nucleus grew with various facets in order to minimize the total interfacial energy of each nucleus.

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The crack closure phenomenon is important in the aspect of effecitve driving force for crack growth. Therefore, the precise measurement of crack closure load is an essential prerequisite for the determination of the effective driving force. In this study, the acoustic emission(AE) technique was employed as one of crack closure measurement methods and the results were compared with those of crack opening displacement method(CD), back face strain gage method(BFS) and surface strain gage method. Results have shown that the intrinsic fatigue Life curve(da/dN vs. ΔK_(eff)) obtained by use of the AE technique vvas best fit with the curve of high stress ratio (R=0.8) test, which implied that the AE technique could be considered as a reliable crack closure measurement method.

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metallugical studies on the bonded interlayer of directionally solidified Ni base superalloy MM-007 joints were carried out during transient liquid phase diffusion bonding(TLP bonding). The formation mechanism cf solid during solidification process in TLP bonding was also investigated. Microstructures of the bonded interlayer of joints made at 1423K for 3.84ks were charaterized by insert metals. In the case of Ni-Si-B alloy, MBF-30, coarse γ′phases were produced in the bonded interlayer. In the case of Ni-Cr-B alloy, MBF-80, coarse γ′ pases were not produced and fine γ′ phases were produced in the bonded interlayer. In the isothermal solidification process, solids grew epitaxially from mating base metal inward the liquid insert metal ;and grain boundaries were formed at intersections of growing solid. It is clarified that directionally solidified alloy can be bonded ideally without. detriment to hardening mecahnism of base metal. Formation of fine grains in the bonded interlayer is due to epitaxial growth from recrystallized grains which formed by machining. In the result of tensile test at 1033K, all joints were fractured at base metal. TLP bonding process can obtain exellent joints.

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High temperature deformation behavior was studied in a superplastic 7475 Al alloy having a grain size of 14㎛ at temperatures in the range of 723-789K. The objective of this work was to examine the possible rate controlling mechanisms that govern superplastic deformation. The values of the stress exponent and the activation energy, as well as microstructural observation, indicated that grain boundary sliding was the dominant mechanism for superplastic deformation but dislocation creep was rate-controlling. The specimens tested at a superplastic region of 2×10^(-4)∼1×10^(-2)s^(-1) and 753K ∼ 789K revealed dispersoid-free zones(DFZs) near grain boundaries. It was suggested that DFZs were due to the solute diffusion along dislocations piled-up at grain boundaries.

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A study has been made of the effects of HCl concentration, current density and frequency on the etch pit size and distribution in 99.98% aluminum foil etched by alternating current electrochemical etching techinique and the capacitance of the foil after anodization. The highest capacitance was obtained under the etching condition of 1.5 M HCl and 10 ㎐ irrespective of current density. The higher chloride ion concentration and the current density and the lower frequency gave rise to the deeper etched layer and the smaller etch pit size. The individual etch pits were aggregated when the chloride ion concentration and current density decreased and the frequency increased.

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The concentration and temperature dependence of the mixing enthalpies of liquid Cu-Sb, Sb-Sn, Cu-Sn and Cu-Sb-Sn alloys could be described by the regular associate solution model. A new numerical method which uses the least square method and the Newton-Raphson method is proposed for the calculation of the model parameters. The calculation shows that the free energy of mixing of liquid Cu-Sb alloy can be much better described with CuSb and Cu₃Sb associates than with Cu₂Sb or Cu₃Sb associate alone. The mixing enthalpies of Cu-Sn-Sb alloy calculated with the assumption of four kinds of associates are in good agreement with the experimental data. By the calculation the model parameters are fixed using only the data of the three binary bordering systems.

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