발간논문

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In order to improve the wear-resistance and heat-resistance of carbon tool steel, a study of tungsten surface alloying by the irradiation of CO₂laser was carried out. Mixtures of tungsten and cobalt powders were applied directly on specimens by coating a powder slurry suspended in a alchohol to the thickness of 70㎛. Surface alloyed layers were obtained by single and overlapping laser tracks at 900 W. Microhardness test, microstructural observation, X-ray diffraction analysis and EPMA test were carried out on each specimen. Well developed tungsten surface-alloyed layers were produced on tool steel. The thickness of these layers was about 250㎛ and the hardness was 1000 Hv Tungsten was distributed homogeneously throughout the alloyed layer by well-intermixing of components, and its concentration was about 10%. The microstructure of this layer consisted of martensite in equiaxed grains. Mixtures of W +5% Ce increased the depth and width of the lasermelted zone. A futher increase of Co in mixture decreased the depth of allayed layer. When a specimen was annealed at 1100℃ for 5 hours, a large amount of tungsten carbides (WC) precipitated in the alloyed layer When a large surface is to be alloyed by overlapping laser tracks, the degree of overlapping should be at least within a half of that of single laser pass to obtain homogeneous alloyed layer.

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Original austenite, martensite, reversed austenite, ausformed martensite, and two-phase structure of martensite and austenite were obtained by controlled heat treatment in Fe-32% Ni alloy. The mechanical properties of these structures have been investigated with respect to deformation degree. At all deformation degrees, the tensile strength becomes higher in the order, original austenite. reversed austenite, ausformed martensite, martensite. Undeformed states of original austenite, reversed austenite, and martensite show necking phenomena in tension test, and the strain hardening rate from yield point to necking becomes greater in the order, original austenite, reversed austenite, martensite. Although martensite and reversed austenite ale produced by diffusionless shear transformation, the susbstructure of the former mainly consists of internal twins in lenticular martensite crystal, while that of the latter consists of dislocations in lath type austenite crystal. Cells are formed in original austenite by deformation and their sizes are decreased with increasing deformation degree. However, they are not formed in martensite by deformation and transition bands at higher deformation degrees.

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The magnetic properties of amorphous Fe_(80)Si_6B_xP_(14-x) alloys prepared by single roller method were studied using vibrating sample magnetometer (77K-1100k), magnetostriction measurement apparatus and AC B-H loop tracer (60Hz-4OkHz). As the atomic percent of B was increased in the amorphous Fe_(80)Si_6B_xP_(14-x) alloys the soft magnetic properties were imporved with increased Curie temperature and saturation magnetization, and decreased magnetostriction, coercive force and power loss.

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In this work Fe-Al-Mn alloys of different phase structure of γ-single phase and mixed phase of α+ γhave been studied. By decreasing the test temperature, elongation was increased in most of the tested samples. However, elongation of the alloys containing more than 25% α-phase decreased at -196℃. Increase of elongation at low temperature may be interpreted with TRIP phenomena. The samples of y-single phase and she mixed structure of γ-5% αphase have shown very low decrease of impact energy with decreasing test temperature. Their low temperature impact toughness are superior to the conventional 304 stainless steel. The impact energy of the samples containing more than 25% α-phase decreased quite a large amount with decreasing test temperature. Weldability of all the alloys was not so good by arc welding. By decreasing the test temperature the strength of the welded materials increased and elongation has decreased.

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In this study, computer programs applicable to heat-treated components of complicated geometries were developed for prediction of thermal history in various heat treatment processes. These programs made it possible to analyse the austempering and the continuous cooling phase transformation processes of selected ductile irons and steels so that the heat treatments could be better quantified. This study warrants that it is necessary to develop computer simulation methods to analyse the relationships of temperature-phase transformation-stresses as well as the temperature-time relationship to imporve the quality and the productivity of the heat treatment processes.

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Aluminum-base composites reinforced with high carbon steel and stainless steel fibers were prepared by a liquid phase casting method. The fibers were helical type with various helical angles and they were coated with nickel and copper, respectively. The mechanical properties of these composities and interfacial phenomena between fibers and matrix were investigated. A thin film of intermetallic compound produced on the fiber surface improves the wettability between fibers and matrix. The aluminum-helical fiber composite having a helical angle of 65 degrees shows a better combination of strength and ductility. The ultimate tensile strength of composite become greater in the order, uncoated fiber, nickel coated fiber and copper coated fiber.

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Alloying effects on the crystallization temperature and the crystallization process of Fe-Cr-C-B alloys were studied by DSC and X-ray diffractometer. The crystallization temperature linearly increased with increasing the concentration of Cr and metalloids. The activation energy for crystallization was in the range of 83-97 ㎉/㏖, and its dependence to composition was similar to that of the crystallization temperature. Three crystallization processes of primary, eutectic and polymorphic were occurred with alloy composition. The final stable crystalline phases were α-(Fe, Cr) (bcc), (Fe, Cr)₃B(orthorhombic) and (Fe, Cr),B (tetragonal) in Fe_(47)Cr_(35)B_(18) alloy, (Fe, Cr)₃(C, B) (orthorhombic) in Fe_(41)Cr_(35)C_(12)B_(12) alloy, and α-(Fe, Cr)(bcc) and (Fe, Cr)₃ (C, B) (orthorhombic) in all Fe-Cr-C-B alloys except Fe_(41)Cr_(35)C_(12)B_(12) alloy.

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By means of Brillouin scattering spectroscopy, hypersonic velocities in the NaNO₃-KNO₃binary melts have been measured at temperatures of the liquidus temperatures of the melts to 200k above them over the entire range of compositions. The light beam scattered by the melt is analyzed with a pressure scanned Fabry-Perot spectrometer. Hypersonic velocities obtained are about the same as the ultrasonic velocities reported at compositions being rich in NaNO₃, but are markedly larger than those at compositions being rich in KNO₃, indicating the occurrence of structural relaxation. Thermodynamic values such as adiabatic and isothermal compressibilities, constant volume heat capacity and internal pressure calculated from the sound velocity obtained show that the melt can be treated as a typical ionic liquid though the melt contains a certain amount of associated species.

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