발간논문

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Effects of Sc on the coarsening process of δ`-Al₃Li precipitate in Al-Li allays, which have been regarded as a main hardener, were investigated by means of DSC, TEM and hardness tester. From the result of DSC analysis, both exothermic and endothermic peaks of the Sc-containing alloy, which correspond to the precipitation and dissolution of δ` particles, were observed to shift to the higher temperature sides, compared to those of the Sc free alloys. This suggests that Sc solutes could act as a prohibiter of precipitation and growth of the δ` particles. The estimated activation energy for δ` precipitation and growth in the Sc-containing alloy is 60 kJ/㏖ larger than Sc-free alloy 47 kJ/㏖. From the result of microstructure observation by means of TEM, both alloys revealed that the growth of mean diameter of δ` particles is proportional to the cube root of time and, especially in the Sc containing alloy, a much decreased coarsening rate of the δ` particles is observed. According to the result of EDX analysis, no peak of Sc solute was detected in the δ` particle of the Sc containing Al-Li alloy, which indicates that there is no Sc enrichment in the δ` particle. Therefore, based on the present experimental results, the reason for the retardation of δ` precipitate by the Sc addition is closely related to the strong interaction between Sc solutes and vacancies in the matrix.

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The binary system Ti-Al intermetallic compound thin films were deposited on Corning glass and silicon wafer substrate by a two-facing-targets sputtering system using two binary Ti-Al alloy targets with stoichiometric composition. The crystallization process of the Ti-Al thin film has been studied by X-ray diffraction, field-emission scanning electron microscope and transmission electron microscope. The as-deposited films are amorphous phase when deposited at ambient temperature. The Ti-Al films deposited were annealed at 773-973 K. When annealed at 778 and 873 K, the films deposited on the corning glass substrate were crystallized into Ti₃Al and Ti₂Al_5 phases. Ti₃Al, Ti₂Al_5 and TiAl phases were obtained after annealed at 973 K. Crystallization behavior of Ti-Al films deposited on silicon wafer substrate, when annealed at 773 K, was the same as the case of corning glass substrate. On the other hand, when annealed at 823 K, the films were crystallized into Ti₃Al, Ti₂Al_5, α-Ti(Al) and Ti_7Al_5Si_(12) phases. When annealed above 873 K, α-Ti(Al) and Ti_7Al_5Si_(12) phases still remained. The results of the selected area electron diffraction patterns analyses are consistent with XRD results.

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A new conceptual metal forming process termed as the continuous confined strip shearing(C2S2) based on equal channel angular pressing (ECAP), which enables sheet farming in a continuous mode, is introduced. Likewise rolling, the strength of the material could also be controlled through the multi-pass operation using the continuous ECAP. Strengthening mechanisms were investigated for the processes of cold rolling and ECAP, respectively, by observing microstructural evolutions of the Al specimens using TEM. Experimental observations indicated that the hardness was not increased in the cold rolled and the ECAPed 1050 Al sheets, when the shear strain was increased larger than the critical value, i.e., γ$gt;4. When γ$gt;4, subgrains formed as a result of the dynamic recovery were observed in the cold rolled specimens, while ultra fine grains with an average size of ∼1 ㎛ were formed in the Al specimens processed by continuous ECAP as a result of the dynamic recrystallization.

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The hydride formation characteristics and its effect on the failure mechanism of nuclear fuel claddings were investigated. The effect of cooling rate on the size, morphology and distribution of hydrides was also examined. Hydrides were more homogeneously distributed with high cooling rate (i.e, air-cooled) and clustered with low cooling rate (i.e, furnace-cooled). The room temperature strength increased slightly and the ductility decreased with hydrides formation. The size and distribution of hydrides were found to have insignificant effect on the mechanical properties. Secondary cracks along the platelets of hydrides were observed on the fracture surfaces of claddings fractured at low temperature. The loss of ductility at low temperature is thought to be associated with the cracking along the hydride platelets. Above 200℃, hydrides were dissolved during deformation and no secondary cracks were observed, suggesting that hydrides may not be detrimental to the high temperature ductility.

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Nowadays, the aluminum films have wide application in micromechanical devices such as micro sensors and actuators, so that their mechanical properties come to be very important for reliability evaluation. However, there is no standardized method to evaluate the mechanical properties of material used in MEMS (micraelectromechanical system) device since the measured mechanical properties are influenced by many factors such as surface condition, friction force, etc. Hence, we intended to evaluate the mechanical properties of thin film in our study, which is important in mechanical operation. Because MEMS devices are usually operating in elastic range, the Young`s modules and yield strength were evaluated by using microcantilever beam technique. First, Al cantilever beams were fabricated using the silicon bulk micromachining technology to have various film thickness. And then, the load-displacement curve was obtained by nanoindentation method. The linear relation in elastic range was utilized in deriving the Young`s modulus of Al film, which gives us reproducible result regardless of film thickness. In high load range, the deviation from the linear relation was detected, so that the yield strength of Al film could be evaluated. It is found that the yield strength increases with decreasing film thickness. By applying the misfit dislocation theory and the Hall-Petch relation, the theoretical estimation could predict well the trend of yield strength.

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Yield point elongation(YP-EL) should be suppressed in shadow mask after decarburization annealing due to higher yield strength and non-uniform deformation. The effect of decarburization annealing atmosphere was examined to optimize the processing conditions. Laboratory experimental results revealed that YP-EL was decreased with higher dew point and annealing temperature. However, hydrogen partial pressure and gas flow rate did not show significant effect on YP-EL. Calculated diffusion rate of carbon showed that the decarburization was controlled by surface reaction. The YP-EL of shadow mask was varied according to the stacked position in the production line. This phenomenon was analyzed by thermodynamic calculation of equilibrium carbon content with CO+CO₂ gas, which could be formed by decarburization reaction.

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Deformation behavior of a Fe-27.6 at.% Al alloy was investigated at various temperatures-ranging from 300℃ to 800℃, corresponding to ordered DO₃ and imperfectly ordered B2 phase fields. A series of load relaxation tests has been conducted to obtain the flow stress vs. inelastic strain rate curves and the results were analyzed to elucidate the effect of strain rate on the anomalous temperature dependence of strength. While the strain rate sensitivity was negligible at the temperatures corresponding to the DO₃ phase field and appeared to increase near the DO₃-B2 transition temperature, flow curves in the B2 phase field showed much higher values of strain rate sensitivities compared to that in DO₃ structure. Anomalous temperature dependence of flow stress became prominent with increasing strain rate higher than 10^(-5)/s. The peak strength was observed to shift to the lower temperatures with decreasing strain rate. It is expected that the different deformation mechanism and/or associated dislocations may be operating at the different levels of strain rate in the temperature range corresponding to the B2 phase field.

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A new concept metal forming process termed as the continuous confined strip shearing based on equal channel angular pressing (ECAP), which enables sheet forming in a continuous mode and repeatable manner, is introduced. Both the numerical and experimental studies were carried out to observe the shear deformation patterns during continuous confined strip shearing. An experimental methodology to observe the shear flow patterns formed as a result of the continuous confined strip shearing is demonstrated. Finite element method was used to predict the shear deformation as well as the effective strain induced during the strip shearing process. The effect of the oblique angle, i.e. the shear strain, on the variations in the hardness was studied based on the microstructural changes observed using TEM.

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In this study, we performed experiments on the coercive force and the Barkhausen noise(BN) for the various microstructure of ultra-fine grained steels and SA508 cl.3 steels according to the pre-processing condition. We tried to quantify the BN from the relationship between the velocity of magnetic domain walls and the retarding force replaced with the coercive force in the domain wall movement. In order to quantify the BN, we observed the variation of microstructure of ultra-fine grained steels and SA508 cl.3 steels and analyze the relationship between the microstructural variation and BN. We then compared the results by the standardized fracture toughness test with the fracture toughness by the quantified Barkhausen noise from the two intermediary parameters. These two parameters are considered as the grain size and the distribution of non-magnetic particles on each specimen having the different BN. From these results, we can testify the quantification of the BN and the possibility of nondestructive fracture toughness test by BN method.

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The fracture mechanisms operative in the hypereutectic Al-Si-X composites fabricated via spray forming and extrusion were investigated in an attempt to improve the mechanical properties of the composites. Two different types of Al-25Si-X composites were prepared by varying matrix composition and heat treatment conditions; the hard/brittle sample revealed 0.8% elongation with 420 ㎫ U.T.S and the soft/ductile one revealed 16% elongation with only 165 ㎫ U.T.S. The effect of the crystallographic factor of Si particles on the crack initiation in Al-25Si-X composite was regarded not to be important. In the case of the hard/brittle sample, the cracks were observed to be initiated from Si particles at a relatively high tensile stress of about 420 ㎫ followed by sudden crack propagation through the composite. On the other hand, in the soft/ductile sample, the stress was also concentrated at the Si particles. However, cracks were not developed until the sample was deformed in about 7-9% elongation.

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