발간논문

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A series of isothermal compression tests was carried out to investigate the homogeneous deformation behaviors of the bulk metallic glass (BMG) Zr62Cu17Ni13Al8. The tests were conducted in the temperature range of 390℃-430℃ with the initial strain rate range of 0.0005 s^(-1)~0.005 S^(-l). The Zr62Cu17Ni13Al8 BMG showed the superplastic-like viscous deformation behaviors up to the axial true strain of 1.0 without catastrophic failure. The peak stress and the steady-state stress were increased with increasing temperature and decreasing strain rate. The strain rate sensitivities were about 0.7~1.0 in the steady state deformation stage, and about 0.55~0.65 in the early stage of deformation where peak point was appeared in stress-strain curve. Higher value of the strain rate sensitivity in the steady state deformation stage than that in the early deformation stage was explained by the deformation-induced crystallization model. Viscosity of Zr62Cu17N13Al8 BMG was measured in the range of 10^8~10^11 Pa.s.

keyword : Bulk metallic glass, Newtonian flow, Viscosity, Overshoot stress, Deformation-induced crystallization

New Mg alloys containing Ce-rich misch metal have been developed. The second phase of Mg12(rare-earth) is mostly distributed along the α-Mg phase boundaries as a hard network. The thermally stable network restricts grain boundary sliding during deformation, providing high strength at elevated temperatures. Modification of microstructure can be achieved by an addition of zinc or the variation of solidification process. The refined alloy exhibits higher strength 150 MPa and large elongation to failure around 16% at 150℃, indicating that the effects of stress concentration developed during deformation and residual stress induced during solidification are negligible in the failure behavior of such a refined alloy.

keyword : Magnesium alloy, Misch metal, Mechanical properties, Network structure

The effect of heat treatment condition on microstructure and mechanical properties were investigated in extruded AZ91D alloy. The precipitation of β phase (Mg17Al12 phase) in AZ91D alloy showed different behaviors with cooling method from solution treatment temperature, and also with various annealing treatment temperatures. The microstructure revealed that the tensile properties depend on continuous precipitates within grains and discontinuous precipitates along grain boundaries. The Hall-Petch relation between grain size and yield strength was well fitted in this alloy, and the values of δ0 and Kin Hall-Petch equation are 68 MPa and 31 MPa mm^(1/2) respectively.

keyword : AZ91D alloy, Mg17Al12 phase, Grain size, Tensile properties, Cooling rate

An integrated optical isolator based on multimode interference (MMI) has been studied. Wafer direct bonding technology was used in order to make an integrated optical isolator. Prior to making a direct bonding between Ce : YIG(CeY2Fe5O12) thin film cladding layer and InGaAsP guiding layer, wafer direct bonding between (100)-oriented InP and (111)-oriented GGG(Gd3Ga5O12) was investigated. We found that wafer direct bonding between (100)-oriented InP and (111)-oriented GGG was successfully achieved and effective for the integration of a waveguide optical isolator. In order to accomplish direct bonding between Ce : YIG thin film cladding layer and InGaAsP guiding layer for fabrication of an integrated optical waveguide isolator, the same process has been adopted. The isolation ratio in our device was found to be 2.9 dB at the wavelength of 1.55 μm.

keyword : Optical isolator, Wafer direct bonding, Magneto-optic, Mutimode interference, Garnet

Ti-Cr alloys consists of BCC solid solution, C36, C14, C15 Laves phase at high temperature. Among them, BCC solid solution phase was reported that have a high hydrogen storage capacity. However, activation, wide range of hysteresis at hydrogenation/dehydrogenation and degradation of hydrogen capacity due to hydriding/dehydriding cycles have to be improved for its application. In this study, for improving such a problems, we added on Nb. For getting a target materials, Ti-1Nb-10Cr, Ti-3Nb-10Cr and Ti-5Nb-10Cr specimens were prepared by planetary ball mill. Milling process carried out under a high pressure nitrogen atmosphere. Specimens synthesized were characterized by using x-ray diffractometer (XRD), scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM) and thermo gravimetric analysis/differential scanning calorimetry(TG/DSC). In order to examine hydrogenation behavior, a pressure-composition-isotherm (PCI) was performed at 323, 373, 423, 473 and 523K. In the PCI analysis, The reversible capacity in this system indicated to be about 0.81wt.%, 0.72wt.% and 1.2wt.%, respectively. The Ti-5Nb-10Cr alloy was showed the fully hydrogen capacity about 2.34wt.%. As a result, the Ti-xNb-10Cr(x = 1, 3, 5) alloys of addition to Nb considered that the excellent material of hydrogenation properties.

keyword : Hydrogen storage alloy, Planetary ball miffing, Ti-Cr alloy, Laves phase

Microstructural characteristics of Al sheets joined by a butt welding process named as surface friction weldding (SFW) were analyzed. The SFWed Al alloys were examined by an electron backscattering diffraction (EBSD) installed in field emission-scanning electron microscopy (FE-SEM). The microstructure of welded area consists of dynamically recrystallized zone (DXZ), thermomechanically affected zone (TMAZ), and base metal (BM). A fine-grained microstructure was observed in DXZ. Especially, large number of low angle grain boundaries were observed in TMAZ. Based on the low angle boundary distributions, the shape of DXZ and TMAZ in weld zone could be obviously distinguished. The crystallographic texture observed at weld zone in a SFWed Al alloy sheet resembles that of ideal simple shear texture in FCC metals. This suggests that the predominant deformation in SFW is simple shear.

keyword : Surface Fraiction Welding, SFW, Al sheet, Microtexture, EBSD

Surface friction welding (SFW) is a newly developed solid state joining technology using friction for thin metal sheets. In the present study, the 5052-H32 Al sheets were joined by SFW and investigated. Depth of plastic zone of the SFWed sheets was changed by process conditions such as welding speed, tool rotating speed, sheet thickness, and tool diameter. Tensile properties of the SFWed sheets were similar to that of parent material. Formability of the locally surface-modified sheets using SFW technology was superior to that of the as-received material.

keyword : Surface friction welding, 5052 Al, Mechanical properties, Formability

Unique mechanical properties of bulk metallic glasses have been reported like a viscous flow in an undercooled liquid state as well as high strength and large elastic limit at room temperature. However, most metallic glasses have also been reported to have a tendency to crystallize in an undercooled liquid region even below the crystallization temperature [1,2]. It is therefore believed that understanding the effect of nanocrystallization on the mechanical properties of multiphase alloys is very important in both theoretical and industrial aspects. Crystallization and flow characteristics of a Zr-based bulk metallic glass after isothermal annealing with various preannealing times have been systematically investigated in this study. The high temperature deformation behavior of this BMG has been revealed by conducting a series of compression tests with various initial strain rates and preannealing times at 430℃. The pre~annealing effect on stress overshoot and flow behavior can also be discussed.

keyword : Bulk metallic glass, Pre-annealing, Crystallization, Deformation behavior

A study on the correlation between fatigue properties and defects in electric resistance welded (ERW) seams was investigated. The ERW seams were produced by varying power input in the range of 230-270 kW and seam annealing temperatures of 850-970 in API 5L X70 line pipe steel. ERW defects were mainly in circular shape less than 10 ㎛ in length, while the elongated-shaped ones less than 30 ㎛ in length were occasionally observed in the case of low power input condition, with their occurrence of 1.2% less. They were mainly identified as Si, Mn, Al, (Ca)-rich oxides inclusions. Repeated loading was applied to notched three point bend specimens in order to investigate the susceptibility to pressure reversals of ERW seams. There was no occurrence of pressure reversals (sudden load drop) in all of the conditions investigated. Furthermore, the bond line ductility was found to be satisfactory. These results can be explained by the fact a bond line free from defects was successfully obtained from a wide range of ERW conditions, and also that the fusion zone was tough enough to inhibit initiation of microcracks.

keyword : APT 5L X70, Electric resistomce welding, Fatigue, Pressune reversal, Defect, Three point bending, Bond Line

Low and high cycle fatigue characteristics were investigated for bucket materials of USC fossil power plant, DS2B2 steel (11Cr1Mo1WCo) and commercial material, COST B2 steel at room and service temperature of 593℃. Both DS2B2 and COST B2 materials were refined by vacuum induction melting(VIM) and electro slag remelting(ESR) process. DS2B2 material was superior to COST B2 for LCF and RCF life. TEM analysis showed that the Wadded DS2B2 steel had finer M23C6 precipitates than COST B2 steel. The LCF characteristics of DS2B2 material manufactured by VIM and ESR processes was improved as compared with that by VIM process. It can be seen that the harmful elements such as sulfur and non-metallic inclusions were reduced and the grain size was finer through the ESR refining process.

keyword : Low cycle fatigue, High cycle fatigue, Bucket, 9-12% Cr steel, Ultrasupercritical power plant