발간논문

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The inhomogeneous texture through the thickness direction can be developed in automotive hot-rolled steel sheets by rolling condition such as hot-rolling ratio, rolling pass and rolling temperature. The inhomogeneous texture results in inhomogeneity of texture in the cold-rolled and annealed-steel sheets. In this study, the inhomogeneous transformation texture through the thickness direction of hot-rolled steel sheets was theoretically predicted using Kurdjumov-Sachs (K-S) relationship. The plane strain and the shear strain textures typical in FCC austenite phase were considered to calculate the inhomogeneous transformation textures in BCC ferrite phase, respectively. A rate sensitive polycrystal model was used to simulate the texture evolution of the inhomogeneous deformation texture through the thickness direction during the cold rolling deformation. The change of volume fraction for the major texture components was also analyzed in a cold-rolled state at different strain levels.

keyword : automotive steel sheets, K-S relationship, inhomogeneous texture, rate sensitive polycrystal model

In the present study, Zr-based amorphous alloy matrix composites reinforced with tungsten continuous fibers were fabricated without pores or defects by liquid pressing process, and their microstructures and mechanical properties were investigated. About 60 vol.% of fibers were homogeneously distributed inside the amorphous matrix, although the matrix of the composite contained a small amount of crystalline phases. The compressive test results indicated that the tungsten-fiber-reinforced composite was not fractured at one time after reaching the maximum compressive strength, but showed some plastic strain as the compressive load was sustained by fibers. Tungsten fibers favorably affected the strength and ductility of the composite by interrupting the propagation of shear bands or shear cracks formed in the amorphous matrix, by dispersing the stress applied to the matrix, and by promoting fiber buckling. These findings suggested that the liquid pressing process was useful for the development of amorphous matrix composites with improved strength and ductility.

keyword : composite, amorphous alloy, Liquid pressing process

This study demonstrated that elastostatic stress imposed on amorphous alloys induces the structural change, of which rate largely depends on their atomic packing density characterized by the fractions of, various short-range ordered (SRO) clusters. This structural change was accompanied by the formation of excess free volume, which in turn affected the mechanical properties. These findings were analyzed by using a combined technique of experiments and molecular dynamics simulations in view of the atomistic-scale structural changes.

keyword : amorphous alloy, structural change, atomic packing density

A low temperature plasma carburizing technique offers the enhancement of surface hardness of austenitic stainless steels without sacrificing their corrosion resistance. However, it is hard to obtain the reliable thickness of hardened layer due to the sufficiently low process temperature. In the present paper, attempts have been made to investigate the influence of the addition of Ar gas (0~30%) in the carburizing atmosphere during plasma carburizing. The expanded austenite (γc) and precipitates of chromium carbides were detected in the hardened layer (C-enriched layer). The precipitation of chromium carbides formed at the grain boundaries led to the degradation in the corrosion resistance. The introduction of the reasonable amount of Ar into the atmosphere can contribute to the increase of the thickness of the hardened layer. The surface hardness was determined to be in a range between 250 and 750 Hv0.l. The thickness of the hardened layer was increased up to about 30 μm in 30% Ar atmosphere.

keyword : austenitic stainless steel, Low temperature plasma carburzing, expanded austenite

The nanocrystalline FeCuNbSiB alloy flakes with the thickness of skin depth embedded in a polymer sheet are expected to be an excellent electromagnetic noise suppressor available for the quasi-microwave band. The effects of the milling conditions such as the size of milling ball, the rotation speed of impeller, the weight ratio of ball to powder, and the milling time on the formation of flakes in the amorphous FeCuNbSiB powders which is the precursor of nanocrystalline flakes have been investigated with an attrition mill. The effective thinning of the amorphous powders without severe crushing through the repetitive cold welding and fracture was obtained under the conditions of relatively large ball size, low rotation speed, and medium ball to powder weight ratio, where the shear action between milling balls could be activated preferentially.

keyword : electromagnetic noise suppressor, amorphous & nanocrystalline FeCuNbSiB alloy, flakes, attrition mill

Molecular dynamics simulations have been performed under three different temperature conditions to investigate the velocity and kinetic energy distributions of atoms during natural cooling, and the crystal growth and nucleation processes of liquid Si have been investigated. The interatomic forces acting on the particles were calculated using the Tersoff potential. The newly developed natural cooling system simulated by determining the atomic movements with a combination of Langevin and Newton equations was employed for cooling the overall system. The influence of the effect of temperature conditions on the crystal growth and nucleation processes was also investigated. We found that the velocity of crystal growth and nucleation of Si could be separated into the general epitixial growth, and crystal growth and concomitant nucleation processes according to the process temperatures. It was found that the nucleus was nucleated in the temperature range of 1,850 K ~ 1,950 K under the temperature gradient of about 11.1Å.

keyword : molecular dynamics, velocity distribution, nucleation, crystal growth

The wet sliding wear tests were performed on A356/20vol.% SiCp composites against semi-metallic pad materials. The samples were tested by pin-on-disc wear test equipment. The wet wear properties of A356/20vol.% SiCp against semi-metallic pad have exhibited good results with low wear rate up to 700N applied load and 4.0m/s sliding velocity and low friction coefficient in the range of 0.17~0.23. In addition, the wear properties under wet sliding conditions have been compared with those under dry sliding conditions. Wear behavior has been examined through the observations and analysis of the worn surface, worn subsurface and wear debris. It is revealed that thin lubricant films are formed on the worn surface to reduce the wear rate in the low applied load and/or sliding velocity conditions. In addition, platelike worn debris has been formed on the worn surface with increasing the applied load and/or sliding velocity due to the plastic flow and increase of temperature on top of the worn surface.

keyword : aluminum, silicon carbide, composites, wet sliding wear, semi-metallic pad

Fe-based amorphous matrix composites reinforced by tungsten powders were fabricated by warm consolidation process using atomized Fe-based bulk metallic glass powders. Mixture of atomized Fe-based amorphous powders and tungsten powders were milled to form composite powders comprising fine tungsten particle layers within amorphous matrix. Small amounts of unknown phases were also formed in the composite powders. Through warm consolidation process within the supercooled liquid region, a bulk composite (Φ7 × L7) with a density of up to 95% of the theoretical density could be produced using the layered composite powders.

keyword : bulk metallic glass, composite, warm consolidated processing