발간논문

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The directional solidification and S/L interface stability condition were studied in the horizontal continuous casting of 8mm diameter aluminum rod adopting the heating mold. The directional solidification could be observed at the growth rate R of 10 to 240(㎜/min) and G₁/R of 36.6 to 2.1(℃· sec/㎟). The S/L interface transition from planar to cellular, and, finally, to fine cellular interface could be observed as G₁/R decreased. However, no interface breakdown to dendrite or equiaxed structure could be observed. The striations parallel to the growth direction occurred throughout the given range of growth rate by increasing in amount as the growth rate increased. These striations were considered to be a kind of twin introduced during growth from melt. As the amount of striations increased, the tendency for recrystallization on annealing increased. This fact indicates that these striations are acting as the source of nucleation sites.

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The effects of powder process on combustion characteristics, microstructure and mechanical properties of the Ni₃Al compounds were studied. Ni₃Al-B allays fabricated by the combustion synthesis with the reactive sintering(RS) and self-propagating high-temperature synthesis(SHS) showed a relative density of 97% and 96%, respectively. The microstructure of the alloys by both processes consisted of equiaxed grains of 30㎛ in size and uniformly distributed pores. Proper consolidation of the alloys required an optimum range of the green density, 65∼70% of theoretical. Excessive addition of ZrO₂also impaired the sintered density and hindered complete alloying. While the particle size ratio(d_(Al)/d(Ni)) was important in combustion synthesis, the heating rate was particularly critical in reactive sintering. Ni₃Al-B-ZrO₂alloy processed by RS showed a combination of tensile yield strength, 500MPa and tensile ductility, 10%, whereas that by SHS had inferior mechanical properties due to segregation of ZrO₂particles.

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Surface hardening of carbon steel by electron beam in the atmosphere was carried out using an 1.0∼2.5MeV electron accelerator. The electron beam energy used was 1.4MeV, and the power density of the beam was about 10⁴kW/㎠. The sample piece was rotated and translated during irradiation, so that the point of irridation on the steel surface changed continuously. When the electron beam impinged on the surface, the irradiated area melted very quickly and cooled rapidly owing to the large heat sink provided by the bulk of the steel material. The cooling rate was so fast that the melted area was mostly transformed into martensite phase(or a bainite phase in a limited part) during the self-cooling. The surface hardness of the carbon steel changed from approximately 250Hv to 700Hv, on the Vickers scale. The maximum hardened depth was l.6mm. The melting and cooling times were so fast that oxidation of the steel surface was almost negligible, even though the sample was always in the atmosphere. This method can be applied to heat treatment of steel surfaces for many industrial purposes, owing to its high treatment rate and simplicity.

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In RS aluminum alloy powders, the surface oxide formed on powders usually contains significant amounts of physically adsorbed water and oxygen. Hence, it is necessary to remove both the adsorbed gas and water from the surface of the powders before consolidation, in order to achieve full density. This removal is usually accomplished by a degassing step. A study has been perfomed on the effectiveness of degassing processes of AC8A alloy powders. Mathematical analysis on the degassing time in the degassing process was done and the factors determining the degassing time were investigated. Computer simulation for degassing process in the powder compact was done considering evaporation, diffusion and reaction. According to the results of simulation, degassing time decreases with decreasing size of powder compact and increasing pore radius. Based on the above results, two method of degassing was performed and compared: static degassing process and dynamic degassing process. The mechanical properties of the specimen made from the dynamic degassing method was superior and its degassing time was much shorter than the other method.

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To clarify the precipitation behaviours of secondary phases in Al-Li-Cu-Mg-Zr alloy during ageing, the detailed measurement of hardness, specific heat and the observation of transmission electron micrography have been carried out. In the early stage of ageing, lath shape S´phase formed the corrugated composite sheets on the dislocation and T₁phase precipitated at the grain boundary and sub-boundary heterogeneously were observed, but δ´phase precipitated gomogeneously during quenching were grew uniformly with increasing ageing time. In both stage of peak and over ageing, the concomitance state of δ´, T₁, δ´ and α´composite phase were conformed and especially, fine T₁ and S`phases among those secondary phases played the key role on developement of secondary increasing in hardness. And the most of S´phase spaped fine rod-like type precipitated homogeneously in matrix and another type of S´phase precipitated on dislocation line is observed as lath-like corrugated composite sheet in matrix. It would be exactly confirmed that the precursor phase is formed at the range about 80∼110℃ and resolved in matrix at the range of about 110∼145℃ by thermal analysis of specific heat versus temperature curves.

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In the present study, we made an attempt to fabricate a nitride-reinforced Al alloy composite by blowing N₂gas into Al alloy melts. The addition of reactive element Mg to Al alloy melt facilitated the nitriding reaction, leading to the formation of TiN in Al-5wt.%Ti-10wt.%Mg alloy, but rarely formed in Al-5wt.%Ti alloy due to the absence of Mg. The former alloy showed higher room temperature tensile strength as compared with the latter alloy, which is mainly due to the presence of TiN and partly due to solid solution strengthening effects of Mg. In addition, it is considered that the presence of nitride may also be responsible for high temperature strengthening, judging from the lower decreasing rate of tensile strength with temperature in Al-5wt.%Ti-10wt.%Mg alloy.

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