발간논문

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Formation kinetics of TiNi intermetallics at the interface between Ti and Ni plate has been studied. Two types of the interfaces have been studied: Ni/Ti single-interface type; and Ti/Ni/Ti/Ni multi-interface type. After pressurization at 800℃ Ti₂ Ni and TiNi₃ form at the interface. Higher pressure promotes a more uniform and thicker reaction layer. TiNi intermetallic layer forms between Ti₂Ni and TiNi₃, growing steadily with time. Growth of TiNi satisfies the `parabolic law`, while thickness of Ti₂Ni and TiNi₃, remains nearly constant. In the case of Ti/Ni/Ti/Ni multi-layer interface, TiNi forms more effective y than that in Ti/Ni single-layer interface due to the cooperative growth of TiNi from neighbouring Ti₂Ni and TiNi₃. Thicknes measurement shows that TiNi forms by a volume diffusion mechanism at the interface. Calculated chemical diffusion coefficient, D^v is D^v=1.0014exp(-1148062/RT), and activation energy for the diffusion process is 1148062 J/㏖.

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Effect of Si and Ca additions on the microstructural refinement of AM60 based Mg alloys were investigated. The microstructural characterization was performed by utilizing optical microscope, energy-dispersive spectrometer(EDS), scanning electron microscope(SEM), transmission electron microscope(TEM), differential scanning calorymeter(DSC) and Image analyser. As-cast microstructure of the alloys investigated consisted of Mg_(17)Al_(12). Mg₂Si and α-Mg matrix. Area fraction of Mg_(17)Al_(12) precipitate increased with Al content, the total amount being kept in the range of 4-7%. Continuous or discontinuous precipitation of Mg_(17)Al_(12), was dependant on the amount of Al; i.e., no continuous-type precipitation in alloys containing<2wt.% Al and no discontinuoustype precipitation in alloys containing <4wt.% Al. Two types of Mg₂Si were observed; i.e., chinese script type in alloys containing < 2wt.% Si and polygon type in alloys containing >2wt.% Si. The morphology of Mg₂Si was also depentent on the Al amount; e.g. only chinese script type was observed in alloys containing less than 2wt.% of A1 even though enough Si amount was present for polygon type Mg₂Si. The total amount of Mg₂Si in as-cast alloys was approximately 7%, and kept constant even after heat treatment. The grain size of Si containing alloys was refined by the presence of two types of Mg₂Si: e.g. 237㎛ in AM60 and 125㎛ in AM60-1Si-0.2Ca after aging. Addition of Ca refined the distribution of Mg₂Si by providing heterogeneous nucleation sites for Mg₂Si.

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Zine alloys are widely used for materials of prototypical mold for small-quantity production and sample product since they have Low-melting temperature and good machinability for sample production of automobiles. In this study, Be was added as an alloying element to improve the tensile strength of Zn-Al-Cu-Mg alloys. The variations of tensile properties and microstructures according to Be additions were investigated. To investigate the effect of Be content on the tensile properties of these alloys depending on the cooling rates, 3-step terraced type metal mold and ceramic mold were used. As a result, Be type Cu-rich precipitates were formed by adding Be in Zn-Al-Cu-Mg alloys, and as the increase of Be contents, the precipitates were coarsened. The increase of the cooling rate made Zn-rich primary phase fine and the lamella arm space of eutectic phases decreased. In case of 0.006wt% Be content, tensile stregth was improved by about 10% and by increasing Be addition, tensile strength has been decreased.

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The steel for use as a Reactor Pressure Vessel(RPV) should have the high resistance to thermal fatigue, because it is run in high pressure water vapor of about 288℃ and exposed, in addition, to the irradiation of neutron constantly. In this work, low cycle fatigue characteristics of the three kinds of steels has been investigated, which are produced by different steel making processes, i.e. the conventional Vacuum Carbon Deoxidation(VCD) process, the method of adding Al to the former for the purpose of grain refining, and the method to use Si-killing process in place of VCD and also to add Al. The material made by VCD+Al and Sikilled+A1 process exhibited longer fatigue life than the specimen produced by VCD. This result is mainly caused by the difference in grain size, and the morphological difference at each specimen. The distribution of carbides in bainite structure is also thought to have an effect on their mechanical properties to some extent.

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A two layer composite model was developed using a beam vibration theory and the model was applied for measuring the Young`s modulus of thin film. The Ti coated Si wafer composites were produced by RF magnetron sputtering and used to test the developed model. The measured film moduli using the combination of the developed two layer composite model and the sonic resonance method were checked with those using the static method utilizing the bending of cantilever composite beam. The film modulus values measured in both methods were in good agreements. The film modulus values for the specimens with different film thicknesses were also in good agreements when they were measured in the same method.

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To obtain the fundamental data for the development of ferritic mechanically alloyed ODS steels, the present authors have investigated the effect of three kinds of oxide such as Y₂O₃, Al₂O₃, and TiO₂ on the strain rate, high temperature tensile properties and charpy impact properties of these materials. Due to the most uniform distribution of the finest and spherical particle, ODS steel containing Y₂O₃ raised high temperature tensile and yield strength considerably. Elongation of these materials showed the highest values in containing TiO₂ ODS steel and impact value of these materials showed the highest values in containing Al₂O₃ ODS steel. With increase of strain rate, tensile strengths increased, while elongation increased in the low and high temperature, but decreased at 600℃ showing the maximum elongation.

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The high cycle fatigue properties of a high strength spring steel have been investigated in relation to the microstructural variation through austenzing followed by tempering at different temperatures. The rotating bending fatigue and fatigue crack growth (FCG) tests were carried out and a transmission electron microscope equipped with an energy dispersive X-ray unit was used to characterize the precipitates behavior. The results indicate that the fatigue endurance (σ_f) increases with increasing tempering temperature, reaching a maximum at 450℃, then decreases at 500℃. The increase of σ_f is mainly attributed to the refined distribution of precipitation, together with the structural uniformity of tempered martensite. The inhomogeneity of tempered martensite due to excessive precipitation acounts for the decrease of σ_f, at 500℃. By contrast, the FCG results show an insensitivity of the stage-II growth behavior to the microstructural changes for the whole range of tempering temperature tested. The insensitivity is interpreted in terms of the counterbalancing microstructure-dependent contributions to the FCG, behavior.

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The effects of small addition of titanium and boron on the hot ductility of Type 304 stainless steel were investigated. High temperature tensile tests were performed in the temperature range 900-1200℃ with the specimens prepared from either the hot rolled plates or the continuously cast and reheated slabs. The microstructural characteristics of reheated slabs were analyzed, and the recrystallized structure after hot compression test was observed. In order to examine the effect of boron addition on the grain boundary segregation of sulphur, the fracture surface formed at 1000℃ were analyzed by Auger electron microscopy. The addition of a small amount of titanium was found to significantly improve the hot ductility of 304 stainless steel, particularly in the temperature range 1000-1100℃. It was observed that the titanium addition enhanced the dynamic recrystallization during hot deformation and retarded the grain growth during rehearing process due to the distribution of fine titanium-containing inclusion particles. Hot ductility was further improved by addition of both titanium and boron, which could be attributed to decrease in grain boundary segregation of sulphur due to addition of boron.

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The effect of Ti, Zr and Mo addition on a Ni aluminide intermetallic, Ni-20Al-27.5Fe was studied to improve the mechanical properties, especially the room temperature ductility and the high temperature strength. It was found that the Fe addition in the two phase alloy of NiAl and Ni₃Al gives significant improvement in the room temperature ductility. The hot forming and the microalloying of Ti, Zr and Mo were tried to get optimum condition of the mechanical properties of strength and ductility. The maximum strength at high temperature range was obtained from a hot extruded alloy. It is believed that the tensile properties of Ni-Al-Fe-Ti-Zr should be improved by the controlled microstructure formed by the alloying of Ti and Zr, and the hot forming. The microstructure is consisted of fine grains and precipitates of NiAl and Ni₃Al, which seems to be given by the effect of the hot extrusion related to the Zr compounds in the boundaries of NiAl and Ni₃Al. The phase of boundary precipitates formed by the accumulation of Zr were analysed by XRD and electron microscopes.

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The Argon and Oxygen Decarburization (A.O.D) process is known as the most effective process for the stainless steel making. The aim of this study is to establish the reduction mechanism of Cr₂O₃, at the A.O.D reduction period. Synthetic solid slags diskes of CaO-Cr₂O₃ binary and also ternary systems containing Al₂O₃, or CaF₂ as additives were respectively prepared by compacting and sintering. AISI 304 stainless steel was melted in MgO crucible in semi-opened Ar-atmosphere at 1600-1700℃. The reduction was carried out by adding Al into molten alloy steel and then by placing the disk on it. The reduction rate was measured by analysing steel samples and also reacted slags. The reduction rate was controlled by the surface reaction and the first order with respect to the residual Al concentration. The amount of reduced Cr could be expressed by an exponential form; Δ[Cr]=2[Al]_0(1-e^(-k′(Cr₂O₃)⁴-(time))) The rate constant, k`, is a function of temperature, the viscosity of slag, surface area, and surface roughness factor The activation energy obtained from these was found to be 150㎉/㏖. It is interesting to note that the reaction rate increased significantly as decreasing the viscosity of interfacial liquid slag by adding Al₂O₃, or CaF₂ to the slags.

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