발간논문

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The plastic instability in high purity aluminum, commercial purity aluminum and 2024 aluminum alloy sheets was studied using uniaxial tension testing. The emphasis of the study was on the effects of thickness and anisotropy in cold worked specimens and annealed specimens. Localized necking was observed in the specimens whose thickness to width ratio was less than about one fifth regardless of the condition of specimens. The angle between the localized neck trough and the tensile direction is shown to vary with the plastic strain ratio or the R value measured with respect to the tensile direction, whether specimen is planar isotropic or not. The strain at which local necking occurs can also be influenced by anisotropy of the specimen.

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The present study examined influence of oxygen-active elements in the bond coating of thermal barrier coating on a nickel-base superalloy. Utilizing plasma spray techniques, bond coating of Ni-Cr-Al alloys containing various additions of Zr, Y, Hf and Si were coated on a substrate and, then, ceramic layer of ZrO2(Y2O3-stabilized) was coated. After static exposure at 1200℃, Al-depleted zones were formed at the ceramic coating side due to selective oxidation of Al and at the substrate side due to interdiffusion of Al. Cracks appeared to initiate at the Al2O3-bond coating interface, and to propagate into the ceramic coating. Resistance against cracking increased in the order of Y-, Zr- and Hf-addition in the bond layer, while additional Si always improved the cracking resistance.

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The value of atomic size effect for Co-base thin film alloys made by RF sputtering was about 0.06, and this value increased with decrease of cooling rate. With increasing the negative heat of mixing between the constituent elements the value of atomic size effect was linearly decreased, and this trend was interpreted as kinetic effect. Saturation magnetization had a constant value except for the case of high Ar pressure. Coercive force decreased with increasing the input power and time, and rapidly increased with increase of Ar pressure. Change of coercive force was discussed as effects of film thickness, residual stress and columnar structure. To get an amorphous thin film alloys with good soft magnetic properties, it is concluded that 300 W of input power and 10 m torr of Ar pressure were optimal conditions for sputtering, and Cu and cover glass preferred to polyimide as substrate material.

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The effect of Nb element and annealing condition on the magnetic domain properties were investigated in Fe_(80)B_(16)Si₄(at %) amorphous alloy which has high saturation magnetization and low core loss. With increasing Nb contents in (Fe_(1-x)Nb_x)(80)B_(16)Si₄(x=0-0.10) amorphous alloy ribbons, the crystallization temperature increased from 448℃ to 542℃ and soft magnetic properties were improved by decreasing coercive force and increasing maximum permeability. These improvements were attributed to the decreasing of magnetostriction and the reduction of domain width by Nb addition. The annealing treatment also improved the soft magnetic properties of (Fe_(1-x)Nb_x)_(80)B_(16)Si₄ amorphous alloys and the optimum annealing temperature was 425℃-450℃. (Fe_(0.92)Nb_(0.08))_(80)B_(16)Si₄. and (Fe_(0.90)Nb_(0.10))_(80)B_(16)Si₄, amorphous alloys which were annealed at these temperatures had low coercive force and high maximum magnetic permeability. It could be thought that these improvements were attributed to the relaxation of internal stress and were related to the disappearance of maze domain and the for mation of 180˚ domain.

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As a fundamental study on the physico-chemical properties of slags containing fluoride ion, a measurement has been made on the activity at the equilibrium of hydrogenwater vapor gas mixtures with the slags of Fe_tO-CaO-CaF₂ ternary system contained in solid iron crucible at 1400℃. From the results obtained, the activity of iron oxide and the ratio of ferrous to ferric iron (Λ=Fe^(3+)/Fe^(2+)) have been determined as a function of slag composition. The addition of F- anions increased the activity coefficient of iron oxide exponentially, but it seemed to be independent on Λ-values. The obtained results are as follows, lnγ_(Fe_tO)/lnγ^0_(Fe_tO) =1.78 N_F-

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Delamination behavior of Nb-microalloyed HSLA steel, produced by different processes, has been studied by tensile and impact tests. The plate production processes are: quenched from hot-rolling (designate as HQ sample), air-cooled from hot-rolling (HA), and subsequently cold rolled HQ and HA samples (HQC, HAC). In tensile test, only HQC sample, and in impact test, HQC and HAC samples have shown delamination. The occurrence of delamination in tensile tested sample is strongly affected by carbide precipitates on grain boundaries. However, the contributing factor of delamination in the impact tested samples are suggested to be pancaking of grains and texture by cold rolling. Carbide precipitate on grain boundaries may also contribute delaminating the impact tested HQC sample. Variation of the amount of delamination by aging treatment has been also studied in correlation with the microstructure.

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Effects of graphite shape and pearlite ratio on thermal properties of a compacted vermicular graphite cast iron(CVGCI) were studied. Thermal expansion coefficients, thermal conductivities, specific heats and thermal diffusivities of flaky graphite cast iron(FGCI), CVGCI and spheroidal graphite cast iron(SGCI) were correctly measured. Thermal diffusivity decreases as the nodularity of graphite, temperature and the pearlite ratio in matrix increase. FGCI shows higher temperature dependency compared with CVGCI and SGCI. Linear correlation is obtained between thermal diffusivity(α) and the pearlite ratio in matrix(p) as α=α_0(1-4.3×10^(-3). p), where α_0 is thermal diffusivity of the ferrite matrix. (Received January 26, 1987)

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