발간논문

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Solidification microstructures and phases in rapidly solidified Al-5, 10wt%Fe alloys have been investigated by TEM bright field and dark field imaging techniques and electron and X-ray diffraction techniques. Rapid solidification of Al-5, 10wt%Fe alloys produces various metastable and stable phases, such as Al_mFe, Al_6Fe and Al_(13)Fe₄. In addition to these phases, clusters of randomly oriented few ㎚ scale particles exist in the form of fine cellular network with α-Al or primary spherical particles. Solidification microstructures of the rapidly solidified Al-5, 10wt%Fe alloys consist of various combination of primary phases such as Al_(13)Fe₄, Al_mFe and cluster of ㎚ scale particles, and cellular/dendritic structures such as fine cellular network structure of ㎚ scale particle clusters and α-Al and cellular structure of Al_mFe and α-Al, depending upon alloy compositions and local cooling rates.

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An air curtain was designed and fabricated in order to maintain an oxygen free environment. With a properly designed air curtain, it was possible to maintain the oxygen concentration less than 0.01 under the conditions of 10∼20 ℓ/min air flow. (fluid velocity:6.17∼12.34 ㎝/sec)When the chamber was purged with nitrogen, the lower equilibrium oxygen concentration($lt;0.001% ) than without purging could be obtained. The oxygen concentrations decreases linearly with time at the beginning of air flow in the air curtain. When the oxygen concentration is decreased to less than 1%, the variation of concentration is proportional to (min.)ⁿ. It was found that n is -2 to -3 without Nitrogen purging and is less than -6 with purging.

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The shape memory characteristics in TiNi alloys are greatly effected by the alloy composition and heat treatment condition. The present work was aimed to investigate the effect of Pd_x(X=5, 10, 15, 20) addition on the shape memory characteristics of TiNi alloys by means of eletrical resistance measurement, X-ray diffraction, differential scanning calorimetry and electron dispersive analysis X-ray measurement. The results obtained from this study are as follows ; 1. The martensitic transformation start temperature, Ms of Ti-Ni_(50-x)-Pd_x alloys decreased considerably with the increase of Pd content up to 10at%, whereas increased largely with the increase of Pd content in the alloys with Pd content more than 15at%. 2. The Ms temperature of Ti-Ni_(50-x)-Pd_x alloys with cold working was significantly lower than that, of the fully annealed alloys because high density dislocation has been introduced by the cold working which suppressed the martensitic transformation.

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In Fe-Ni-Cr-Mn-Si shape memory alloy, the effect of cooling methods and strain on the martensitic transformation was investigated. After the solution treatment at 900℃ for 30 minutes, the specimens were air cooled, water cooled and quenched in liquid nitrogen. For air cooled specimens only austenite phase was detected, whereas austenite and ε-martensite phases were detected for specimens water cooled or quenched in liquid nitrogen. The amount of ε-martensite was increased with the cooling rate and strain. But the increasing rate of the amount of ε-martensite was decreased at 5% strain in air cooling and at 3% strain in water cooling, respectively. The occurrence of α`-martensite was found at about 5% strain in air cooled specimens. For water cooled specimens it was found at about 3% strain. These strains almost coinceded with the strains at which the increasing rate of the amount of ε-martensite was changed. The occurrence of α`-martensite in specimens quenched in liquid nitrogen was found less than 0.5% strain.

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The effects of W and B on the stress-rupture and impact properties of modified 9∼12 Cr-ferritic steel for high temperature use were studied. Stress-Rupture tests were carried out using a lever arm creep tester at 550℃, 600℃ and 650℃ in air, and impact tests were performed at -196℃, 0℃ and 25℃. The stress-rupture strength of 0.1C-9.8Cr-0.6Mo-2W-0.01B steel was lower than that of 0.18C-10.5Cr-1.0Mo-0.2V-0.07Nb steel. The high strength in the latter steel was due to the high C content, larger grain size of prior austenite and carbide precipitation. The deterioration of stress-rupture strength with time of 0.1C-9.8Cr-0.6Mo-2W-0.01B steel was, however, less than that of 0.18C-10.5Cr-1.0Mo-0. 2V-0.07Nb steel. It is, thus, expected that the former steel has better stress-rupture properties in long term tests. In either steel, by improving the cleaness, impart energy was increased and ductile to brittle fracture transition temperature(DBTT) was lowered below 0℃.

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The effects of microstructural change on the superplastic deformation of Ti-6Al-4V alloy were investigated. Three kinds of the specimens($quot;A$quot;, $quot;B$quot;, and $quot;C$quot;) were preparated and used for the present study : specimen $quot;A$quot; was heavily hot-rolled with 20% per pass draft percent at 800℃, specimen $quot;B$quot; was lightly hot-rolled with 3.5% per pass draft percent at 800℃, and specimen $quot;C$quot; was the annealed specimen $quot;A$quot; at 800℃ for 5hours. The maximum elongations were found in the persent work were 2470%, 1100%, and 970% for the specimen $quot;A$quot;, $quot;B$quot;, and $quot;C$quot; respectively. And so, we postulated that an unexampled elongation is, at 880℃ and 4 × 10^(-3)/s, due to very fine grain size that was recrystallized from the substructures created during hot-rolling process. The characteristics of superplastic deformation for Ti-6Al-4V alloy were also found to be strongly influenced by the testing condition. For the specimen tested in the open air, they were thermally oxidized and theirafter the elongation was decreased to the level of about 50% of that tested in the pure Ar gas atmosphere.

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In the present study, the growth behavior of Fe-Zn intermetallic compounds during hot dipping Zn coating (440℃∼650℃) was investgated by means of optical microscope and SEM. The results show that the growth rate of total intermetallic compounds within coating layer is controlled by diffusion rate process with the reaction order of 0.5 in the range of 440℃∼500℃. The growth rate of ζ phase in the range of dipping temperature 440℃∼470℃ and that of δ₁, phase in the range of 440℃ 500℃ is also controlled by a diffusion rate process with the reaction order 0.5. The thickness of coating layer increased with the formation of intermetaltic compounds in the dipping temperature from 440℃ to 470℃ The thickness of coating layer decreased by disolution of coated layer over 500℃ and becomes constant from about 590℃.

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The effect of whisker reinforcement on the precipitation behavior of SiC whisker reinforced Al-Cu-Mg composites has been investigated by the differential scanning calorimetry, transmission electron microscopy, and macrohardness measurement. Through the thermal analysis on the composite and matrix was not changed by whisker reinforcement. However, the result of macro-hardness measurement showed that the age-hardening of whisker reinforced composites was generally accelerated. Based on a simplified model of whisker reinforced composite and by using the finite element method, a umnerical calculation for the misfit strain generated by the solid solution treatment was conducted. Through in situ observation of the precipitation process in AI-Cu-Mg matrix by a transmission electron microscope, the role of increased dislocation density by a large misfit strain in whisker reinforced composite has been investigated. A possible mechanism of accelerated agehardening in whisker reinforced Al-Cu-Mg composites has been discussed in view of misfit strain and increased dislocation density

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Magnetic and magneto-optic properties and microstructure of coevaporated Tb/Fe multilayer thin films as functions of the Tb layer thickness, the ratio of Tb and Fe layer thickness and the period were studied by using a Torque magnetometer, a polar Kerr hysteresisgraphy, a Hall effect measurement apparatus and a TEM. The multilayered films showed that the larger perpendicular anisotropy energy than that of the amorphous TbFe alloy thin films. The multilayered films showed superior properties when they were not piled-up with monatomic layer but with two atomic layers of Tb. Therefore, it was speculated that the directional Tb-Tb and Fe-Fe pairs contribute to the the large perpendicular anisotropy as well as the Tb-Fe pairs to do. A dead zone which has no magnetization or weak magnetization compared to strongly exchange-coupled region was formed at above 3 atomic layer of Tb. Accordingly, the saturation magnetization of the 4 atomic layered film was smaller and decreased more rapidly than that of the 3 atomic layered film in the Tb-rich region, and oppposite phenomenon was appeared in the Fe-rich region.

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A study has been made on the microstructure and mechanical properties of rapidly solidified Al-Fe-(Mo,Si) alloys. Examination of the extruded bar using TEM reveals the microstructure to be inhomogeneous along with the bimodal distribution of precipitates. It is believed that. such inhomogeneous microstructure is caused by the structural difference between zone A and zone B in the starting powder. The tensile testing shows an increase in tensile strength but a decrease in elogation as the content of iron in the alloy increases. It has been found that decrease in elongation can be prevented by the addition of Si to Al-8Fe-2Mo alloy. For the elevated temperature mechanical properties, all extruded specimens with 8% iron show tensile strength of 180-190㎫ at 573K, but their elongation values were decresed with incresing the test temperatures, reaching a minimum at around 423 K.

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