발간논문

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The effect of boron addition on the wear and oxidation resistance of nickel aluminides was investigated. Wear tests of various heat-treated specimens at room temperature and at 500℃ were performed under no lubricant condition in air by using a ball-on-disk type tribotester. Isothermal oxidation tests were made at 1100℃ in air flowing at the rate of 70cc/min. and at 1000℃ in air by using TGA. Experimental results from wear tests showed that boron-added nickel aluminide had remarkably improved wear resistance at both temperatures. Especially, the minimum weight loss and the minimum wear volume in solutionized and aged boron-added specimen were measured from the wear test at 500℃. On the contrary, the addition of boron to the nickel aluminides has affected the oxidation resistance of the aluminides adversely. Ni-32Al-0.5B aluminide showed poorer oxidation resistance than Ni-32Al aluminide during the isothermal oxidation test. Although both types of aluminides revealed a parabolic oxidation behavior and the formation of Al₂O₃ on the surface, the differences in the oxidation resistance between the aluminides with and without boron addition seemed to be attributed to the microstructural difference between the aluminides. An accelerated oxidation along the thin layer of Ni₃Al along the grain boundary observed in the microstructure of the Ni-32Al-0.58 aluminide could be attributed to the poor oxidation resistance compared with the Ni-32Al showing no grain boundary Ni₃Al phase.

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Microstructural evolution of Ti/Al layered structures annealed at various temperature has been investigated by using optical microscopy, scanning electron microscopy, and X-ray diffractometry. Ti/Al layered sheets have been produced by dipping two Ti sheets in Al melt. The Ti/Al interfaces showed good bonding characteristics in as-dipped sheet. Microstructure of the heat-treated Ti/Al sheet varies depending on the subsequent annealing temperature. In the Ti/Al layered composite annealed at temperature below Al melting point, thin TiAl₃ layer was formed by the diffusion process at the Ti/Al interfaces. In the case of composite exposed at temperature above Al melting point, the equiaxed TiAl₃particles protruded into Al matrix from the thin TiAl₃ layer as well as the formation of thin TiAl₃ layer. TiAl₃ particles become homogeneously distributed within the Al matrix away from the Ti/Al interface with increasing annealing time. The formation of TiAl₃ phase in Ti/Al sheet annealed at temperature above Al melting point is believed to be formed by the exothermic chemical reaction.

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Physical properties were investigated in the NiCrAlY/PSZ composites with various NiCrAlY contents(0vol%, 20vol%. 40vol%. 60vol%, 80vol%, 100vol%). Samples of the NiCrAlY/PSZ composites were sintered in the temperature range of 1450 to 1200℃ by a Plasma Activated Sintering(PAS) process. SEM micrographs revealed that grain size of the PSZ, as a matrix in NiCrAlY/PSZ composites, decreased with increasing NiCrAlY content. Hardness values also decreased with increasing NiCrAlY contents. Sintering temperature was found to decrease with increasing NiCrAlY contents in hardness and density measurements. X-ray analysis revealed that the volume fraction of the tetragonal phases in PSZ, as a matrix in NiCrAlY/PSZ composites, increased gradually with increasing NiCrAlY contents and reached the maximum in the NiCrAlY content of 40vol%, and then decreased remarkably. Optical microscopy showed that the continuous phase in the NiCrAlY/PSZ composites changed from PSZ to NiCrAlY in the NiCrAlY content of 80vol%. On the other hand, fracture toughness values linearly increased with increasing NiCrAlY contents, reached the maximum in the NiCrAlY content. of 60vol%, and then decreased remarkably in the NiCrAlY content of 80vol%.

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It is important to predict the corrosion fatigue crack growth rate of Inconel 600 for the structural integrity of nuclear power plants. In order to evaluate the corrosion fatigue crack growth rate of Inconel 600 in various applied load types, 3 different types of cracking loads which simulate corrosion fatigue, SCC and mixed mechanism of corrosion fatigue and SCC were applied to CT type Inconel 600 specimens in high temperature water at 290℃ with 4ppm dissolved oxygen concentration. There was synergistic effect on corrosion fatigue crack growth rate when mixed mode of SCC and corrosion fatigue load is applied to Inconel 600 specimens in the environment. The measured crack growth rate of the specimen to which mixed load were applied wsa faster than the predicted crack growth rate based on the superposition model. The increase in the crack growth rate indicates that there is sysnersistic interaction of fracture process at the growing crack tip which experiences mixed load of SCC and corrosion fatigue. So it is necessary to account for the synergistic acceleration of corrosion fatigue crack growth to predict the life of structural components of Inconel 600 alloys in corrosive environments.

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Effects of sulfur and manganese on the hot workability of STS 316L and 309S steels have been investigated. From the results of hot workability test, the hot ductility was decreased with increasing sulfur content and reheating temperature. This is considered to be caused by sulfur segregations and sulfide precipitates at grain boundaries. Sulfur would be dissolved under the reheating conditions and reprecipitated with decreasing temperatures during hot rolling. The content of reprecipitated sulfur is decreased with increasing manganese content and decreasing reheating temperature. Therefore, the hot ductility is increased with increasing manganese content. It was also found that the hot ductility is increased with decreasing reheating temperature.

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Electromagnetic casting(EMC) technology of a soft contact type with high frequency electromagnetic field has been examined for steel application. A special copper mold of cold crucible type was fabricated and usage of cold insert was established to analyze and predict the effect of the operating parameters. The continuous casting experiments with tin were conducted to quantify the electromagnetic field effect on the surface quality of the billet. The result shows that the surface roughness was improved to the 1/10 of the conventionally cast billet for the optimal electric power. The optimum value of the electric power was heavily dependent on the relative position of the meniscus and the coil top. It was also observed that the electromagnetic casting made the solidified shell thinner near the meniscus and made it thicker just below the mold than the conventional casting. The postulate that EMC is an effective tool on preventing the hook formation at the early solidifying stage was confirmed.

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In this study we have conducted experiments for alloying Tb-Dy-Fe of which the target composition was Tb_(0.27)Dy_(0.73)Fe_(1.95) to achieve energy efficiency and mechanical strength. Because of the high oxidizing tendency of rare earth metals, Tb, Dy and the high vapour pressure of Dy, the charging composition had to be adjusted to Tb_(0.27)Dy_(0.73)Fe_(1.69). After alloying, Terfenol-D rods were grown in directional solidification by Bridgman technique and the solidification characteristics was investigated as function of solidification conditions. It was found that the characteristics primarily depended an the temperature gradient and the cellular-dendrite transition occurred at the growth rate around 25 ㎛/s. The morphology of dendrite was plate type and these dendrite plates displayed a strong preference of <112> alignment along growth direction. The magnetostriction measurements were performed at various pressures and magnetic fields. With 3N purity of elements, the maximum strain(λ_(max)) was increased from 887 to 1000 ppm at 7.35 MPa as the interface morphologies were changed from dendrite to cellular.

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Cracking sensitivity during solidification of the high-alloyed austenitic stainless steel was evaluated in terms of P and S. Gleeble test apparatus was made to measure the reduction area (RA) of austenitic non-magnetic (NM) steel. The solidification mode and micro-segregation were also investigated using the unidirectional solidification equipment. From the muti-regression analysis among the zero ductility temperature (ZDR), P and S, the following equation was obtained, ZDR(℃)=331[P wt.%]+1802[S wt.%)+43. It was found that the effect of S on the ZDR is more significant than P. It was confirmed that the solidification pathway of the NM steel is L→L+γ→γ and most of the elements such as P, S ,Ni, Mn and Si were segregated positively in the interdendritic regions.

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Rapidly solidified Al-misch metal-Ni powders were produced by using gas atomizer. Studies about change of formation phase and solidification mechanism were performed. And optimal composition range for amorphous single phase formation was determined. We obtained amorphous powder which showed glass transition phenomena in the composition range of 15.7 at%≤(Ni+Mm)≤17.4 at%. In the other compositions, mixed phases consisted of α-aluminum and various metallic compound were obtained. It was found that cooling rate of powder which the particle size was below 20 ㎛ was at least two times faster than that of powder ranging from 20 ㎛ to 38 ㎛. Al_(84.2)Mm_6Ni_(9.8) and Al_(84.3)Mm_(6.9)Ni_(8.8) alloys had a good glass forming ability and were prospected as optimal composition for fabrication of bulky form. In addition, we observed the transition phenomena of nonequilibrium phase during crystallization by using XRD and in-situ TEM in these alloys.

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The formation mechanisms of intermetallic compound particles during the tumbler-ball milling of 2Cu+Al powders and the vibratory ball milling of Mo+2Si powders were investigated by X-ray diffraction and microscopic observations, respectively. Any temperature variations at the mill surface during a tumbler-ball milling of 2Cu+Al powders have not occurred, but the thermodynamically stable intermetallic compound particles, Cu₂Al phases in the vicinity of surface of the equiaxed globular powder particles milled have been formed. The other side, a abrupt rise of temperature, namely, exothermic phenomenon have been occurred in 50h at the mill surface during a vibratory-ball milling with 25.4㎜ balls of Mo+2Si powders, as the results the intermetallic compound particles, i.e, tetragonal MoSi₂ phases have been observed. It was considered that the formation of intermetallic compounds occurred by a combustion synthesis reaction due to the exothermic heat on the basis of the impact energy of ball during the milling.

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