발간논문

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A bimetal composed of D₂tool steel and Cu-Sn alloy as base metal and weld metal, respectively, was made by MIG welding process. Tensile test, Charpy V_notch impact test (load-deflection) and elastic-plastic fracture toughness test (J_(IC)) were made on D₂tool steel Copper bimetal to investigate the bonding strength and wear test was accomplished for base metal. Results obtained from the interface are as follows σ_(uts)=21.5(㎏/f㎟) Kid=47.3(J/㎠) σ_(ys)=18(㎏.f/㎟) j_(IC)=1.9(㎏.f/㎜). The fracture in tensile test and elastic-plastic fracture toughness test took place in copper around interface but, in Charpy V-notch impact test, fracture occured partly in D₂tool steel. Wear resistance of the base metal (D₂) was superior to those of AISI 1045 and 304 stainless steel.

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The effects of alloying addition on the wetting of graphite and industrial diamond were, studied using sessile drop tests conducted in Ar-atmosphere at 1,100℃. Alloy systems used in this work were Cu-Mn-Sn. Cu-Mn-V and Cu-Mn-Cr. For all of these alloys, wetting of graphite and industrial diamond were found to be improved by increases in the content of carbide formers such as Mn, V and Cr. Among them, Cr was the most effective one. This result was consistent with earlier observations that the wettability of carbon-based materials was strongly dependent upon Gibb’s free energy of formation for carbides involved. The expected increase in activity of the melts, however, was not found in the Cu-Mn-Sn alloys. In the Cu-Mn-V and the Cu-Mn-Cr alloys, the effect of Mn on wettability was different in that Mn was beneficial in the former, while not in the latter alloys.

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The effect of heat treatment on the oxidation behavior of Zircaloy-4 plates was investigated in 400 to 600℃ steam for alpha-annealed, beta-quenched, and laser surface beta-treated samples. It was observed that the laser treatment resulted in the formation of surface layer of 40∼50㎛ thick beta-treated structure on the mass of alpha-annealed plates, and the microstructure of laser treated layer was much finer than those of alpha-annealed and beta-quenched samples. The corrosion resistance of laser treated Zircaloy was found to be superior to other two samples in a1l the temperatures. At 600 ℃, the corrosion rate was changed from cubic (W=kt＾(1/3)) to linear rate(W=kt) after about 7 hour exposures. The marked improvement in corrosion resistance of laser treated samplers is thought to be due to the change of the second phase particle distribution. The coarse and discrete particles in the alpha-annealed structure changed to the fine and chain-like particles in the laser heat treated samples. In this study, the structure of oxide layer was observed to be only one type of monoclinic ZrO₂.

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CdTe single crystals were grown by a vertical Bridgman method. The growth rate was varied between 1.1 and 5.4 ㎜/hr with maintaining the temperature gradient of 16℃/㎝ at the solid-liquid interface. Etch-pit structures of CdTe single crystals were investigated. The dislocation density was larger and the subgrain size was smaller at the surface and bottom of the crystal. With the increasing growth rate, the dislocation density gets larger and the subgrain size becomes smaller The thermal stress, which is supposed to build up during the solidification process, can explain our results.

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Solidification analysis of shaped castings and prediction of shrinkage cavity formation by the finite difference method were demonstrated with several examples. Several actual castings of aluminum alloy in sand mold wore experimented and the results were compared with those obtained by computer simulation. The two prediction parameters, temperature gradient and critical fraction solid for fluid flow, and an auxiliary parameter shrinkage potential, were adopted for predicting the formation of shrinkage cavity or porosity in aluminum alloy castings. It was found that a quantitative design of shaped castings can be made by the computer aided solidification analysis.

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The effect of external stress on the phase transformation behavior of annealed and aged Ni-Ti alloys has been studied by the tensile test and the measurement of electrical resistance. Martensitic (M_s) and austenitic transformation starting temperatures (A_s) increased gradually with increasing stess up to 129 MPa and increased rapidly at a rate of 0.2℃/MPa above this stress level. However, incommensurate (T`_R) and commensurate phase transformation starting temperatures (T_R) increased very slowly at a rate of 0.06℃/MPa with the increasing stress. Martensitic transformation temperature range, ΔT(M_s∼M_f), and the stress at which ΔT was kept constant were larger for the aged specimen than for the annealed specimen. The stress at which the strain accompanied by incommensurate and commensurate phase transformation began to decrease (ε-1c) was higher for the annealed specimen than for the aged specimen. these facts imply that the incommensurate and commensurate phases are more stable in the annealed specimen than in the aged specimen. While the residual stress (ε_p) in the annealed specimen remained low regardless of the stress level, ε_p in the aged specimen began to increase rapidly at 240 MPa. This means that slip deformation is more difficult for the annealed specimen than for the aged specimen.

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The partial binary phase diagram of the system Na₃AIF_6-PbO was investigated by means of the Differential Thermal Analysis(DTA) using platinum cells with caps to minimize vaporization effects. Peak points on the DTA curve were taken as transition temperatures due to the difficulty in interpreting overlapping eutectic-and liquids peaks. The system was found to be a simple eutectic over the range studied(0 to 45wt.% PbO), with the eutectic point at 28.02wt.% PbO and 897.6℃, X-ray powder diffraction patterns showed no solid solution.

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Ribbon type Pb-Sn eutectic alloy of 0.2∼0.8 ㎜ thickness fabricated continuously by using an unidirectional solidification apparatus. When the controlled growth velocity was in the range of 0.011∼0.038 ㎝/min. at the constant temperature gradient in solid, G= 33.79 ℃/㎝, well aligned eutectic specimen of lamellar spacing, λ= 2∼4 ㎛ could be prepared into ribbon shape longer than 100 ㎜ in length. A micro-morphological stability of solid-liquid interface was analysed by means of three theoretical stability models.

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The crystallographic texture of cold worked and heat treated Zircaloy-4 sheet was analyzed quantitatively and its relationship with the mechanical anisotropy was investigated. The anisotropy parameter, R, was determined by Knoop micro-hardness test while the texture parameters, f_n and F_n, were obtained from pole figures. The texture parameters and the anisotropy parameter were influenced by cold work or heat treatment temperature. They increased with α-phase recrystallization heat treatment, but decreased with 950℃ heat treatment in the β-phase region. The theoretical relationship between the mechanical anisotropy parameter and the crystallographic texture parameter was in fair agreement with the single crystal slip model of Van Swam et al.. For less severely textured materials, however, measurements of R-values tended higher than the theoretical prediction.

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A two dimensional mathematical model was developed to analyze the heat flow involving phase change during welding of thin plates. A modified finite difference method was adopted for computer simulation. Latent heat of fusion or freezing during welding was treated by the temperature recovery method. Radiative and convective heat losses from the surface and the dependence of the relevant physical properties on temperature were also taken into account. The effects of welding speed, heat input, preheating temperature, and plate thickness on the thermal history and isotherms of the welding zone were quantitatively demonstrated by computer simulation based on 7020 aluminum alloy and l.5% Mn steel.

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