Jongryoul Kim

Grain refinement mechanism during equal-channel angular pressing of a low-carbon steel

Abstract The grain refinement mechanism during equal-channel angular pressing of a plain low-carbon steel was explored by a careful analysis of the slip systems operating at each pass of repetitive pressing. The steel was subjected to one to eight passes of pressing, in which a single passage yielded an effective strain of ∼1, at 623 K. At the initial stage of pressing, submicrometer-order ferrite grains enclosed by serrated and low-angled boundaries were formed. Transmission electron microscopy examination revealed that these boundaries resulted from interaction between the slip systems that are typical in body-centered cubic structures. Further pressings mainly resulted in rotation of ultrafine subgrains rather than grain refinement, providing the formation of high-angle grain boundaries. Since the serrated boundaries restrict dislocation movement, the rotation of subgrains with the serrated boundaries is more favorable for accommodating further deformation than intragranular strain, and therefore boundaries become high-angled.

Formation of nanocrystalline Fe–Co powders produced by mechanical alloying

Abstract The structural and magnetic properties of Fe–Co alloy powders produced by mechanical alloying of elemental powders were investigated. In this research, two kinds of milling methods (cyclic and conventional operation) were adopted to optimize the milling process. Consequently, the alloy powder with average grain size 10–15 nm was obtained. The cyclic operation had an advantage over conventional milling in that a smaller-grained structure could be obtained with less milling time. Maximum M s was obtained at the composition of 30 at.% Co. Due to the high amount of internal strain introduced during milling, relatively high coercivities were obtained. The coercivity was decreased fairly through the relief of the internal strain by heat treatment. Over 95% of the theoretical density was attained after the spark plasma sintering at 900°C for 5 min with applying pressure of 60 MPa. The sintered specimen remains nanostructured with a grain size of about 30 nm.

Microstructures and mechanical properties of equal-channel angular pressed low carbon steel

At the advent of developing ultrahigh strength steels by refining their microstructures without the major compositional modification, the authors reported for the first time the successful grain refinement of commercial low carbon (0.15 wt.% C) steel by utilizing the equal-channel angular (ECA) pressing technique. In that investigation, only 2 passes of ECA pressing in which a strain of a single pass was {approximately}1 resulted in grain refinement of the steel from 30 {micro}m to {approximately}1 {micro}m and the increase of ultimate tensile strength (UTS) from 450 MPa to over 800 MPa. In this paper, the microstructural evolution and the corresponding mechanical properties of the same steel subjected to further strain through ECA pressing of four passes are described.

Shear strain accommodation during severe plastic deformation of titanium using equal channel angular pressing

In the present study, microstructural changes in commercially pure titanium during equal channel angular (ECA) pressing were examined. Elongated grains inclined by 25° to the longitudinal direction were developed and these grains contained high-density twins. TEM analysis of the twins revealed that their twin plane is {1011} and the twins are accompanied with dislocations on non-basal planes. These results suggest that the severe plastic deformation imposed on titanium via ECA pressing is accommodated mainly by the {1011} twinning, rather than dislocation slips commonly observed in pressing of other metals such as aluminum and steel. The twinning modes that might accommodate the severe strain were proposed based on the dislocation slip systems observed during the pressing.

Deformation structures of pure Ti produced by equal channel angular pressing

Abstract A microstructural examination was conducted on a pure titanium deformed by a single pass of equal channel angular pressing. The microstructure was manifested by elongated grains composed of multiple {1 0 1 1} twins. This indicates that the severe plastic deformation imposed on titanium is accommodated by {1 0 1 1} twinning rather than dislocation slip.

Development of FeCo-based thin films for gigahertz applications

For improving high-frequency characteristics and soft magnetic properties of (Fe/sub 7/Co/sub 3/)B-based thin films, the effects of Ni addition and an oblique deposition method on the microstructures have been examined. The Ni addition to (Fe/sub 7/Co/sub 3/)B films was found to enhance B segregation on grain boundaries, which resulted in the formation of a well-defined columnar structure and the increase of film resistivity. Utilizing the columnar structure, large in-plane anisotropy was induced by an oblique deposition method which enabled the control of the inclination angle of the columns. As a result, the ferromagnetic resonance of (Fe/sub 7/Co/sub 3/)/sub 71/Ni/sub 7/B/sub 22/ thin films reached 6.2GHz.

Coupled finite-element-analytic technique for prediction of temperature rise in power apparatus

In order to design the power apparatus such as bus bar or power cable, the current carrying capacity (or ampacity) should be determined exactly since it is limited by maximum operating temperature. The temperature rise in the power apparatus is due to Joules loss in the current carrying conductor and due to the induced eddy current in the tank. This paper presents a new technique that can be used to estimate the temperature rise in the extra high voltage bus bar. In this paper, the power losses are calculated from the magnetic field analysis for various materials when ac current flows into the single- and three-phase bus bar and are used as the input data to predict temperature rise for the thermal analysis. However, it is not easy to apply the heat transfer coefficient on the boundaries correctly, because the coefficient is not a constant, but depends on temperature as well as model geometry, etc. The heat transfer coefficient is calculated according to the model geometry and varying temperature and is coupled with the finite element method. The temperature distribution in the bus bar by proposed method shows good agreement with the experimental data, compared to that of the analytic method using fixed coefficient.

Effects of boron contents on magnetic properties of Fe-Co-B thin films

Effects of B on the magnetic properties and high frequency characteristics of the Fe/sub 7/Co/sub 3/ based thin films were investigated through the microstructural analysis. It was found that B addition was very effective in increasing anisotropy field and resistivity and in decreasing coercivity without much loss of saturation magnetization. These enhancements were shown to be strongly related to grain size, grain morphology and preferred orientations. As a result, a Fe/sub 55/Co/sub 28/B/sub 17/ film with small dispersion angle of /spl sim/0.5/spl deg/ showed the ferromagnetic resonance frequency of 3.3 GHz and a linear effective permeability of /spl sim/350 up to 2 GHz.

Development of deformation structures in low carbon steel by equal channel angular pressing

Abstract Experiments were conducted on a low carbon steel to analyze the structural evolution during repetitive ECAP. The analysis shows that several slip systems that can follow the shear strain of the pressing operate. This is considered to result from the limited slip systems of the steel and the obstruction of dislocation movements by the existing bands.

An Integrated LTCC Inductor Embedding NiZn Ferrite

An integrated inductor using the low-temperature cofired ceramics (LTCC) technology for low-power electronics was fabricated. In the inductor, NiZn ferrite sheet (muR=230) was embedded to increase inductance. The inductor has Ag spiral coil with 14 turns (seven turns times two layers), a dimension of 0.6 mm in width, 10 mum in thickness, and 0.15 mm in pitch. To evaluate the inductance, including the parasitic resistance, the fabricated inductor was simulated and measured. It was confirmed that simulated values were very close to the measured values. Finally, the maximum quality factor of 20.53 was obtained at 5 MHz for the inductor