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The effects of oxygen content on the microstructure and the phase transformation temperature of modified Zr alloys for nuclear fuel cladding were studied. The changes in microstructure with varying cooling rates were also investigated. The cooling rates of 0.7 and 30℃/s resulted in Widmansta¨tten structure and the cooling rate of 1000℃/s resulted in martensitic structure. As oxygen content increased, Widmansta¨tten structure was changed from basketweave to parallel plate structure and the plate width was increased. The presence of large amount of oxygen causes parallel plate structure by raising phase transformation temperature and widening (α+β) two phase region. However, martensitic structure was not affected by oxygen contents and showed fine acicular α structure in all specimens. Hardness increased with increasing oxygen content due to solid solution hardening and also increased with increasing cooling rate due to α plate refinement in Widmansta¨tten structure and due to the formation of ω phase in martensitic structure.

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In the 4Mo, 6W and 2Mo-3W steels the secondary hardening and fracture behaviors were studied. The alloying elements in these steels contributed to the secondary hardening by forming the fine carbides of the M₂C type. Mo had a strong effect on secondary hardening while W had a very weak effect on it but delayed the overaging. The MoW steel exhibited both the moderately strong hardening by the Mo-addition and considerable resistance to overaging by the W-addition. However, severe intergranular embrittlement for the Mo and MoW steels and the reduction in upper shelf energy for the W steel were observed in spite of thee decrease in hardness during overaging.

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The objective of this study is to investigate dynamic torsional characteristics of Al-Li alloys using torsional Kolsky bar. The materials used were a 2090 and an 8090 Al-Li alloy. Dynamic (γ∼1200/sec) torsional tests were conducted at room temperature and low temperature (-196℃), and the deformed regions and the fracture surfaces of the tested specimens were examined. A number of planar slip bands were formed predominantly in the heavily deformed region in the case of the room temperature dynamic testing, but the amount of planar slip bands tended to decrease with decreasing test temperature. This indicated that the overall deformation mode of both Al-Li alloys changed from the planar slip to the homogeneous deformation with decreasing temperature, resulting in the improvement of the cryogenic properties under dynamic torsional loading.

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An investigation was performed to examine the deformation behavior and the fracture mechanisms of Bi-S free machining steels at elevated temperatures. Especially, focus was paid on the effects of hot rolling and Bi content on the hot ductility and the fracture behavior of the present free machining steels. In order to find the effect of a hot rolling. experiments were conducted on the as-casted state and the as-rolled state, and their high temperature deformation behavior was compared Under the present experimental conditions, the as-rolled state exhibits better hot ductility than the as-casted state due to its homogeneous matrix microstructure. However, the hot ductility in both the as-rolled steels and the as-casted steels decreased with decreasing temperature. This ductility loss would be attributed to the embrittlement which is induced by the existence of a liquid metal, Bi, at testing temperatures. It was found that the hot ductility in the as-rolled steels at the temperature range from 1100℃ to 1050℃ primarily depended on the ductility of matrix, at temperature range from 1050℃ to 950℃ primarily depended on the absolute Bi content, and below 900℃ primarily depended on the liquid Bi film on the grain boundary rather than the absolute Bi content. The as-rolled state exhibited the transition of fracture mode from ductile dimple fracture to brittle fracture at a reduction of area of about 60%. The above results were discussed in the light of the mechanisms of a liquid metal induced embrittlement as well as the fracture mechanisms of low carbon steels.

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Tensile deformation and fracture behavior of ultrafine grain sized, dispersion-strengthened 8009 (Al-8.5% Fe-1.3%V-1.7%Si) plate were examined, within the temperature range between 25 and 300℃. Both tensile strength and ductility gradually decrease with increasing temperature and/or decreasing strain rate. SEM fractographs indicate that 8009 fails by dimpled rupture, regardless of test conditions. The size and distribution of dimples, however, differ significantly with different testing temperatures. At 25℃, bimodal distribution of dimples are evidenced, while equal size of shallow dimples are observed at elevated temperatures. TEM micrographs indicate that at 25℃, both silicide and oxide particles are highly decorated with dislocations. At elevated temperatures, on the other hand, particles are free of dislocations and overall dislocation density is extremely lower than that at 25℃. Most of remaining dislocations are neatly arranged into arrays after high temperature deformation. The enhanced dynamic recovery appears to reduce work hardening and enhance plastic instability and flow localization, leading to reduced tensile ductility at elevated temperature.

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In this study, a two dimensional model for prediction of microsegregation in solidification was suggested on the basis of the previous one dimensional model. Especially, for the growth of a secondary arm, a simple and effective mathematical function was established to show the growing pattern. The solute diffusion in the solid phase was calculated by the finite difference method(FDM). The solid-liquid interface movement was considered to be in local equilibrium state indicated by the phase diagram. The experiments for an unidirectional solidification of Al-4.9wt.%Cu binary alloy were carried out in order to examine the reasonability and feasibility of this model. The concentration profile of solute and eutectic fraction predicted by the simulation agreed well with those found from experimental works. According to the results, it was believed that the pattern function introduced in this study was reasonable for the prediction of a microsegregation in dendritic growth.

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In order to determine the life span of refractory of the blast furnace main runner, six thermocouples were installed in the main runner 4.5 meters distant from a tapping hole and connected to a personal computer via a data acquisition system. The cross-sectional area of a main runner was uniformly divided into many elements, and each element contacting both the pig iron and refractory was defined as boundary elements for the calculation of the temperature distribution as well as for the prediction of refractory erosion. Temperatures obtained at near slag line were wavy, which was closely related not to the tapping condition, but to the erosion state of main runner. A diagnostic method for determining the refractory profile was proposed.

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A 3-dimensional mathematical model to simulate spray forming of billet has been developed. The effect of processing parameters such as spray density constants a and b, substrate angular velocity ω, substrate withdrawal velocity V, eccentric distance l_e and spray angle Φ on the spray forming billet has been studied. The asymmetrical shape was obtained at the low value of substrate angular velocity. The parameters V, l_e, a, b and Φ were found to have no effect on the symmetry of billet. And the necessary condition to reach the steady state of billet growth is as follows : V < a*·(d_0/(d_0-l_e/sinΦ))·cosΦ where d_0 is initial spray distance and a* is spray constant a at spray distance d_0.

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A new method for prediction of intermetallic compounds at metal/ceramics interface has been suggested based on thermodynamic calculation and diffusion kinetics consideration. The present method was applied to the prediction of interface reaction between pure titanium and alumina at 1100℃. A local equilibrium was as sumed at the interface before the formation of intermetallic compounds. By calculating metastable equilibria between the β-Ti phase and Al₂O₃ and by considering the diffusivity of oxygen and aluminum in the β-Ti phase, the formation of Ti-alumides rather than Ti-oxides could be predicted in agreement with experimentally known fact. The effect of initial Ti thickness on the formation of the TiAl phase could also be clarified using the present method.

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Mechanical alloying(MA) powders were prepared by single shaft attritor mill under Ar gas atmosphere. Powders milled for more than 35hours showed Ni solid solution and unimodal particle size distribution having the mean powder size of 12 ㎛ in diameter. These powders milled for 35hours were used for flame spraying. The optimum condition for the flame spraying of Ni-based self-fluxing powders manufactured by mechanical alloying for 35hours was studied and the coating produced under the condition of a spraying distance of 15 ㎝ gave rise to the best results. Sliding wear behaviors of the coating produced by fame spraying on cast iron substrate were investigated by ball-ondisc type wear test using an AISI 52100 steel ball as a counter material. Volumetric wear rates of coatings and substrate were calibrated using the Habig`s equation by measuring a wear volume of ball and disc. Compared with cast iron substrate, relative volumetric wear rate of coatings showed significant improvement on the wear resistance.

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