발간논문

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Transformation behaviour of a Ti-35Ni-15Cu(at%) shape memory alloy has been investigated by means of electrical resistivity measurement, differential scanning calorimetry, X-ray diffraction and transmission electron microscopy. Transformation occurred in two stages, i.e., B2◎B19◎B19` and the B19→B19` transformation was not complete even at 83K. The B19◎B19` transformation was not clear in DSC curves, while it was clear in the electrical resistivity vs. temperature curve. The B19 martensite did not involve internal defects such as dislocation and twin. This corresponded well to the fact that the lattice distortion associated with the B2→B19 transformation satisfied the invariant plane strain condition

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The oxidation behavior and morphological changes of Ti-6Al-4V alloy have been investigated in air over the temperatures ranging from 973K to 1273K. The phases of oxidation product have teen examined by X-ray diffractometer and scanning electron microscopy. They are composed of films of TiO₂ and Al₂O₃ mixture. The weight gains during the oxidation increase rapidly at temperature above 973K, which obey the parabolic law. The oxidation behavior of Ti-6A1-4V alloy lead to breakaway oxidation at high temperature, which is similar to that, of pure Ti. However, the oxidation rate of Ti-6Al-4V alloy is higher than that of pure Ti, because a porous mixture film of TiO₂ and α-Al₂O₃ formed in Ti-6Al-4V alloy is less protective than TiO₂ film formed in pure Ti. The growth rate of TiO₂ oxides in Ti-6Al-4V alloy is faster than that of α-Al₂O₃ at high temperature.

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The superplastic properties of fine-grained 7475 Al alloy were investigated at 516℃ over the strain-rate range of 2.8 × 10^(-4)s : to 1.7 × 10^(-2)s^(-1). A significant improvement in superplastic elongation was achieved by adopting two-stage strain-rate deformation compared to a usual single strain-rate test. The initially higher strain-rate deformation in the first stage caused rapid microstructural changes including the conversion of subboundaries to high angle grain boundaries, The optimum stain-rate in the first stage was found to be 8.3 × 10^(-3)s^(-1). The specimens tested at lower strain-rates, revealed the dispersoid-free zones(DFZs) near grain boundaries normal to the stress axis. When a higher strain-rate was applied to the specimens with DFZs, the grain refining was not observed. This absence of grain refining was due to the concentration of plastic deformation in the weak DFZs.

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The microstructures of sputtered TbFe thin films were investigated with transmission electron microscopy. In the films sputtered at low Ar pressure of 0.13Pa, the microstructures appeared smooth and featureless, but in those sputtered at higher Ar pressures, columnar structures have been produced. As the Ar pressure increased, the microstructures showed larger columns and clearer columnar boundaries, which are developed due to increase of oblique incidence component and decrease of the average energy of incident atoms in the coating flux. .4n evidence of preferential oxidation of Tb appeared in the films sputtered at. high Ar pressures through the analyses of electron diffraction patterns.

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The fatigue crack initiation behavior of SUTS304 austenitic stainless steel with different strain induced martensite morphologies resulted from 3 different heat treatments was investigated. The speciment, without Cr-carbide precipitate showed that primary crack initiations sites were twin boundaries with preferred strain induced martensitic transformation in this region. On the other hand, the sensitized specimen showing Cr-carbide precipitation and consequent strain induced martensitic transformation along grain boudaries showed the enhanced intergranular crack initiation accompanied by shorter crack initiation life than that with no Cr-carbide. The sensitization in STS304 austenitic stainless steel gives contrasting effects on crack initiation and near-threshold fatigue crack propagation behaviors. The heat treatment which resulted in not only coarsened Cr-carbides on grain boundarlies but also Cr-carbide precipitation on twin boundaries inside the grains showed the shortest crack initiation life and the lowest ΔK_(1h) value among 3 conditions. It was considered that in this condition the crack initiation was due to strain induced martensite transformed at. Cr-carbides and these crack initiation sensitive regions were distributed more continuously along crack path showing little extrinsic toughening, resulting in the same detrimental effects on both crack initiation and near-threshold fatigue crack propagation behavior.

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