발간논문

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Pretransition phenomena observed in four commercial alloy systems of β-C Ti, Al-Mg-Cu, Fe-Cu and Fe-N steel are reported. For the microscopic origin of the modulated structures, it is conjectured that orbital splitting of incomplete 3d electrons in the alloying elements causes electron-phonon coupled lattice displacements through Jahn-Teller`s bain distortions. This metastable phases are to be represented by a periodic regularity of electronic distribution, which could act as a role of an abnormal structural constitution about the modulated strain field and an additional ordered diffraction in TEM observation.

keyword : modulated structure, metastable phase, additional ordered diffraction, bain distortion

In order to study the deformation behavior depending on the initial crystallographic orientations and the morphology of Ni3Al precipitates, the plane strain compression tests were carried out on the single crystals of Ni/Ni3Al (Ni-18at.%Al) two-phase alloys. Flow behaviors were strongly dependent on the initial crystallographic orientations in DS18-3 alloys with rods and plates of Ni3Al precipitates rather than DS18-1 alloys with Ni3Al cuboids. For all orientations of DS18-1 alloys, and (110)[001]- and (110)[112]-oriented specimens of DS18-3 alloys, the flow behavior was thought to have a similar tendency at least up to strain level in this study, whereas the flow stresses were much lower for (100)[011]-, (100)[012]- and (210)[001]-oriented specimens in DS18-3 alloys. Such flow behavior is considered to be closely related to morphology of Ni3Al precipitates and cross-slip within Ni matrix which was related to the operative slip systems.

keyword : Ni/Ni3Al single crystals, plane strain compression, morphology of Ni3Al precipitates, cross-slip

In the present study, a powder injection molding (PIM) process was applied to Cu-based amorphous and Fe-based metamorphic alloys, and microstructure, hardness, and wear resistance of the PIM products were analyzed and compared with those of conventional stainless steel products. Injection-molded Cu-based amorphous powders were not completely sintered even at temperatures just below the melting temperature as all amorphous phases were replaced by crystalline phases. When Fe-based metamorphic powders were injection-molded and then sintered at 1200℃, completely densified products with almost no pores were obtained. They contained 34 vol.% of (Cr,Fe)2B borides in the austenite matrix without amorphous phases. Since these (Cr,Fe)2B borides were very hard and thermally stable, hardness, high-temperature hardness, and wear resistance of the Fe-based metamorphic alloy products were two or three times as high as those of conventional stainless steel products. This property improvement presented new possibilities for their application to structural materials or parts requiring excellent mechanical properties.

keyword : powder injection molding, amorphous alloy, metamorphic alloy, sintering

The FCP behavior of FSWed 5083-H32 and 6061-T651 specimens were examined with the fatigue crack growing either parallel to the DXZ at variable ΔK values and an R ratio of 0.1 and 0.8, respectively, or perpendicular or 45° to the DXZ at various constant ΔK values and an R ratio of 0.1. Residual stress was measured on the top surface of FSWed plate either perpendicular or parallel to the welding direction and the residual stress-corrected ΔK, ΔK(corr), is calculated based on the K(res), the stress intensity factor at residual stress, for the DXZ specimens to quantify the compressive residual stress contribution to the FCP rates. The present study suggests that the FCP behavior of FSWed 5083-H32 and 6061-T651 specimens in the DXZ is mainly determined by the beneficial compressive residual stress reducing effective ΔK and the detrimental grain refinement causing intergranular fatigue failure. The FCP behavior of FSWed 5083-H32 and 6061-T651 specimens is discussed based on residual stress measurement and fractographic observation.

keyword : 5083-H32, 6061-T651, friction stir welding, fatigue crack propagation

The effect of Co additions on tensile properties in the secondary hardening Co-Ni alloy steels has been studied in as-quenched and aged conditions. Three kinds of (5-13)Co-11Ni steels, containing Mo and Cr, were severely multipass-rolled in unrecrystallized region and followed by accelerated cooling. In as-quenched condition, an increase in yield strength and a decrease in uniform elongation with an increase in Co content are closely related with a decrease in the amount of retained austenite. In peak-aged condition, yield and tensile strengths and peak hardness increased with the increase of Co content, but peak time remained around 5 hours. Thus, Co additions would enhance the nucleation of fine M2C carbides rather than their growth.

keyword : ultrahigh strength steel, Co-Ni steel, secondary hardening, tensile properties, severe deformation

Ag-base brazing alloys, i.e. BAg-series according to AWS specification, are the major group of brazing materials for general use. The present study aimed to examine the suitability of the continuous strip casting process as a new fabrication method for the Ag-32%Cu brazing alloy. The effect of aging heat treatment on the microstructure and mechanical properties of an Ag-32%Cu strip was also investigated through tensile test, hardness test, X-ray diffraction (XRD), scanning electron microscopic (SEM) observations, and electron probe microanalysis (EPMA). Three continuous strip casting processes such as planar flow casting, melt dragging, and twin roll strip casting were applied to the manufacture of the Ag-32%Cu strip. Among those processes, twin roll strip casting was found to represent the best strip shaping ability and microstructural homogeneity. 1 mm-thick Ag-32%Cu strip was successfully produced as a continuous strip of 1 charge of melts. The heat-treated strip exhibited the eutectic microstructure feature of an Ag-rich matrix and a Cu-rich phase, with the microstructure clearly evolving with longer aging time. Tensile results showed gradually decreasing strengths and simultaneously increasing elongation with longer aging time. The aging condition of 670℃/2hrs also realized appropriate T.S. strength of 283 MPa and significantly enhanced plasticity of 4l%, due mainly to the homogeneous distribution of fine Cu-rich phases less than 0.8μm. Finally, twin-rolled strips could be cold-rolled smoothly after aging heat treatment for 2hrs at 670℃ to a very thin strip (50~l00μm thickness).

keyword : Ag-32%Cu alloy, brazing strip, continuous strip casting, twin roll strip casting, aging, Cu-rich phase, strength, ductility

The dry sliding wear tests were performed on as-cast samples of A356/20vol.% SiCp composites. The samples were tested by pin-on-disc wear test equipment with different applied load. The wear results show that as-cast samples exhibited better resistance to wear than the T6 treated samples. In addition, as-cast samples exhibited stable friction coefficient values in the range of 0.45-0.5. Wear behavior of as-cast A356/20vol.% SiCp is examined through the observations and analysis of the worn surface, worn subsurface and wear debris. In the low applied load region, wear is occurred mainly by oxidation. With increasing the applied load, mechanical mixed layer (MML) is formed on the worn subsurface and crack is formed on MML. In the higher applied load region, wear is occurred by adhesive and delamination.

keyword : aluminum, silicon carbide, composites, dry sliding wear

Recently, requirement for the ultra thin copper foil increase with smaller and miniaturized electronic components. A study has been made to determine the effect of an electrochemically produced titanium-oxide film on the nucleation and growth of copper. The oxide films were made current step and potential step at 30℃ in 1.5M sulfuric acid on pure titanium metal. The formation of a microporous oxide layer on the titanium electrode greatly enhances the surface density of copper crystals. For a 50seconds of deposition, the surface roughness of the copper deposit is lower than that of the untreated surface.

keyword : electrodeposition, copper foil, crystal growth, anodizing

Electrochemical properties of surface modified stainless steel sheets were evaluated as a separator for polymer electrolyte membrane fuel cell. Tungsten carbide powders and a mixture of tungsten carbide and stainless steel powders were coated on AISI316L stainless steel sheets by cold spray or rolling process. Tungsten carbide coated layers formed by cold spray process showed a sound coating layer and lower interfacial contact resistance than other coated layers. Because of the defects present in coated layers, corrosion current densities for all type of coated specimens did not satisfy the corrosion standard for metal separator.

keyword : coating, stainless steel, cold spray, rolling, fuel cell, separator, interfacial contact resistance, potentiodynamic polarization