발간논문

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The present article aims at evaluating experimentally impact absorption energies of extruded aluminum tube for space frame and at investigating the effect of cross-sectional shape on the compressive deformation behavior under dynamic loading. The dynamic compressive test was conducted on extruded aluminum tube specimens, whose shape and thickness to width (or diameter) ratio are different. using a high performance servo-hydraulic machine. Zig-zag type plastic hinges were formed mainly in the rectangular shaped specimens, whereas ring type hinges were formed in the circular shaped specimens. However, the hinge shape tended to become ring type as the thickness to width (or diameter) ratio increased, and the calculated values of impact absorption energy increased linearly. To improve the impact absorption energy and the maximum repulsive force against the compressive load of extruded tube for apace frame. it is recommended to make the thickness/width ratio of extruded tube as large as possible and to design the circular cross-sectional shape in a way to promote the formation of ring type plastic hinges instead of zig-zag type hinges.

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Formation of AlN by means of a melt-gas in-situ reaction in Al-10wt.%Mg and Al-20wt.%Mg alloys has teen investigated. N₂+ 5% H₂ gas was used for AlV formation. Melt-gas in-situ reaction was carried out at 900℃ for 16 and 32hours. Formation of AlN has been identified by a variety of analytic methods : optical, scanning electron, transmission electron microscopies, X-ray diffraction, energy dispersive X-ray spectroscopy and image analyzing technique. AlN particles exist uniformly in the form of clusters mainly at interdendritic area together with the eutectic mixture of α-Al and Al₃Mg₂ The reason for clusters to be formed may be : 1) heterogeneous nucleation on the preexisting AlN particles . 2) reduction of interfacial energy by forming aggregates of particles : or 3) reaction of Al and Mg₃N₂. Individual AlN particle has a shape of a hexagon type thin plate. Amount and size of AlN increased with reaction time. Thickness and width of AlN are approximately 0.3㎛ and 0.9-1.8㎛ respectively after reaction for 16 hours, and approximately 0.3-0.4㎛ and 1.5-2.2㎛ respectively after reaction for 32 hours. AlN/α-Al interface was stable after being exposed at 500℃ Thermodynamic reasoning shows that Mg plays an important role in transferring a nitrogen atom from the gas phase to molten Al by forming intermediate Mg₃N₂ particles which are eventually changed into AlN by reaction with Al melt.

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Compression tests for a single crystalline and polycrystalline Ti₃Al alloys of the hexagonally ordered DO_(19) structure deformed in compression were carried out to determine a temperature range in which deformation twinning is induced. Deformation twinning does not occur in a single crystalline Ti₃Al alloys. Prism plane slips are activated when single crystalline Ti₃Al alloys are compressed in the vicinity of the a axis. On the other hand, pyramidal and basal plane slips are activated when single crystalline Ti₃Al alloys are compressed in the vicinity of the c axis. In a polycrystalline Ti₃Al alloys, deformation twinning does occur in the temperature range of 1073K to 1373K only in Ti-34㏖%Al alloy. Dominant deformation twinning system is identified as [101￣1] (1012￣) in the temperature range of 1073K to 1373K. The volume fraction of deformation twins increases with increasing temperature in the temperature range of 1073K to 1273K. With further increasing temperature, however, the volume fraction of deformation twins decreases. The volume fraction of deformation twins increases with increasing total strain at 1173K and 1273K. Non-a dislocations and APBs in Ti-34㏖%Al alloy are visible in the temperature range induced deformation twinning.

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Experimental approaches to a mechanism of deformation twinning in Ti₃Al alloys of the hexagonally ordered DO_(19) structure deformed in compression in the temperature range of 973K to 1373K and room temperature are examined focusing on relative intensity (long range order degree), non-a dislocation, the size of APD and the width of APB. The relative intensity ratio(I_(○)/I_(○)) measured by X-ray diffraction decreases with increasing temperature in the range of 973K to 1373K, indicating that the incidence of deformation twinning results from the decrease of long-range order degree. Moreover, non-c dislocations are visible in Ti-34㏖% Al alloy deformed at room temperature after water quenched from 1273K. The width of APBs in Ti-34mol% Al alloy compressed at 1273K is wider than that in Ti-34㏖% Al alloy compressed at room temperature. The size of APD in Ti-34㏖% Al alloy after annealed at 1273K is much smaller than those in Ti-30㏖% Al and Ti-25㏖% Al allovs.

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Mechanically alloyed Ni₃Al and ODS Ni₃Al powders were consolidated by hot extrusion. The microstructures of as-extruded and heat-treated Ni₃Al and ODS Ni₃Al were observed by transmission electron microscopy, electron energy-loss spectroscopy, energy-dispersive X-ray spectroscopy and optical microscopy. Fine oxide dispersoids were uniformly distributed in as-extruded both Ni₃Al and ODS Ni₃Al materials. EELS and EDS analysis showed that fine dispersoid in Ni₃Al is α-Al₂O₃. The dispersoid volume fraction was 3.6% in Ni₃Al, and 4.5% in ODS Ni₃Al. The Y₂O₃ added in ODS Ni₃Al reacted with Al₂O₃ and formed YAG, Y₃Al_5O_(12). The size measurement of dispersoid in Ni₃Al and ODS Ni₃Al annealed at 1100℃ for 2 to 30h showed that the dispersoids coarsen by diffusion-controlled mechanism. In addition, the growth kinetic constants of oxide dispersoids in Ni₃Al and ODS Ni₃Al were 1.31×10^(-27)㎥/sec. 1.67×10^(27)㎥/sec, respectively.

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The present study is concerned with the processing and the microstructural analysis of surface alloyed materials with TiB₂ powders using irradiation of high-energy electron beam. The mixtures of TiB₂ powders and flux were deposited on a plain carbon steel plate, and then electron beam was irradiated on these mixtures using an electron beam accelerator. The microstructures of the irradiated surface layers were examined by optical and scanning electron microscopy. A few residual micropores were found in the sample processed without flux, but their number was decreased in the samples processed with a considerable amount of flux. As a result of irradiation, the Ti content was homogeneously maintained throughout the melted region, whose hardness was greatly improved. This was associated mainly with the microstructural modification including the segregation of Ti and B along solidification cell boundaries and the formation of fine Ti(C,M) particles.

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The present article aims at investigating the effects of high-energy electron beam irradiation on surface hardening and improvement of fatigue properties in an AISI 4140 steel currently used for automotive crankshaft. The 4140 steel samples were irradiated at atmosphere by using an electron accelerator and, then the microstructure, hardness, and fatigue property were examined. The beam power was varied in the range of 53-70㎾ by changing the beam current. Upon irradiation, the unirradiated microstructure containing band structure was changed to martensite, bainite in the carbon-rich zone or ferrite, bainite, and martensite in the carbon-depleted zone. This microstructural modification improved greatly the surface hardness and fatigue properties due to transformation of martensite whose amount and type were determined by the heat input during irradiation. Thus, the high-energy electron beam irradiation can be effectively applied to the surface hardening process of automotive parts.

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A coupled analysis of rod growth model using vector calculus and heat transfer model using boundary fitted coordinate has been formulated. Temperature profile in a spray formed rod of 2024 aluminum has been calculated by the proposed model. At initial stage of spray forming. the temperature of substrate and preform increases rapidly. And then temperature of the vertex point is kept as about 525℃ During the cooling stage after the spray farming steps, temperature of all points inside rod decrease and the cooling rate of surface is higher than that inside the rod.

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Analytical study on the thermomechanical deformation behavior of multilayered structures was performed. The solutuions for thermal strain, curvature change and internal stresses in response of temperature variation are derived by recourse of plate theory. The theory accounts for the effects on the thermomechanical characteristics due to the thermal mismatch between the constitute layers. The results are used to develope performance diagrams which enable the identification of possible geometrical combinations and compositional gradients for the optimal themomechanical behavior of multilayers. The resultant analytical solutions could be utilized for the optimal design of multilayered structures including functionally graded materials (FGM`s). Aluminum/Stainless steel multilayer with and without compositional graded layer is used as a model system for the analysis of thermomechanical behavior.

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A previously developed two dimensional model was applied to predict the degree of microsegregation in weld metal. In the model, both primary and secondary arm regions were defined for the analysis region. For the growth of a secondary arm, a simple and effective mathematical function was introduced to show the growing pattern(polynomial interface). The solute diffusion in the solid phase was calculated by 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 the weld metal solidification of 2024 Al alloy were carried out in order to examine the reasonability and feasibility cf this model. The concentration profile of the solute and Eutectic fraction predicted by the simulation agreed well with that found from experimental works. According to the results, it was believed that the pattern function introduced in this study be reasonable for the prediction of any binary systems which may or may not have an eutectic reaction.

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