발간논문

Home > KJMM 논문 > 발간논문

It is proved that there exists an ordering reaction in alloy 600. The differential scanning calorimetric (DSC) tests were carried out for the alloy 600 and a Ni2Cr alloy from room temperature to 800℃ Both an exothermic and an endothermic reaction are interpreted in a aspect of order-disorder reaction in the Ni-Cr and the Ni-Fe binary system. The exothermic reaction was observed only in the water quenched specimens from above 600℃ and the cold worked specimens. However, the endothermic reactions were observed at 490-560℃ in all specimens. Especially, a convex endothermic reaction was observed at about 500-620℃ for specimens aged at 474℃ for 1,100 and 10,000 h. In addition, the activation energy for an exothermic reaction in a quenched specimen after a 1095℃-0.5 hr treatment was Q = 190 kJ/mol (~2 eV/atom), which suggests that this reaction is related to a substitutional diffusion process.

keyword : Differential scanning calorimetry, DSC, Alloy 600, Order-disorder reaction, Short range order, SRO, Ni2Cr

In the study, we investigated a new type of severe plastic deformation method, `Twist Extrusion`, passing a workpiece through a die with a twist channel. Large strains in the workpiece can be generated by repeating the process, because of the characteristics of an equal channel, i.e. all cross-sections of the whole channel (before, middle and after a twisting section) are the same in geometry and size. Plastic deformation behavior of a metallic workpiece during the twist extrusion process was numerically analyzed using the finite element code, ABAQUS. The plastic deformation generated in the workpiece is as inhomogeneous as the case of simple torsion, i.e. increases with a distance from the center. However this feature of inhomogeneous deformation with distance from the center can be utilized as an effective way for surface hardening of pure metallic parts and composite materials due to the enhanced grain refinement in surface regions. The strain and stress distributions are parallel to the extrusion direction, which presents a prospective method to control an alignment of particles in composites, e.g. carbon nanotube reinforced metallic composites.

keyword : Twist extrusion, Severe plastic deformation, Surface hardening, Finite element analysis, Grain refinement, Strain hardening

The material pile-up around indenter/surface contact region can make the conventional Oliver and Pharr`s analysis of the nanoindentation curve inaccurate. We suggests a novel methodology for characterizing pile-up incorporated contact properties by adopting the Matlab(R)visualization tool in this study. Based on our approximation of contact boundary as peak of the pile-up region, radial indentation surface profiles observed with an atomic force microscope are differentiated with the distance from the indent center and a closed contact boundary is formed by connecting special differentiated points having their values as zero. The calculated load-off hardness was nearly consistent with the result of nanoindentation curve analysis for pileup free material such as fused quartz. This experimental result supports validity of the newly proposed method, However, the load-off hardness of metallic solids such as (100) tungsten monocrystal and pure copper showed that the nanoindentation curve analysis could overestimate the hardness more than 50% in the cases of severe pile-up.

keyword : Nanoindentation, Material pile-up, Load-on/Load-off hardness, Matlab(R), visualization tool

Corrosion tests of alloy 625 in subcritical- and supercritical water containing 0.1 M HCI were performed in the range of temperature of 350℃~450℃ at 25 MPa. The corrosion rate of alloy 625 was faster in sub critical water than in supercritical water. 0.1 M of hydrogen peroxide seemed critical concentration of oxidizer that accelerated the corrosion rate of alloy 625. Below 0.1 M of hydrogen peroxide, corrosion rate was not influenced, however, over 0.1 M, corrosion rate was increased rapidly with increasing the hydrogen peroxide. Alloy 625 had two corroded layers. Intermediate layer was the porous structure layer in which Ni and/or Fe were dissolved out selectively. The outer layer was a powder-like porous oxide layer mainly composed with chromium oxide. It was found that the main corrosion mechanisms of alloy 625 were selective dissolution of nickel and iron under the subcritical water and selective dissolution of nickel under the highly oxidized supercritical water condition.

keyword : Supercritical water, Supercritical water oxidation, Corrosion, Alloy 625, High pressure

Bi2Sr2CaCu2Ox(2212) and Bi2Sr2Ca2Cu2Oy(2223) high-Te superconductor (HTS) have been prepared by plasma spraying and PMP(Partial Melt Process). The 2212 HTS coating layer is synthesized through the peritectic reaction between Sr-Ca-Cu oxide coating layer and Bi-Cu oxide coating layer by partial melting process. The superconducting characteristics, it can be predicted from wetting secondary phase and mis-alignment of whisker, depends on the spray distance which was related to the spray particle melt. The 2212 HTS layer consists of the whisker grown in the diffusion direction. 2223 phase and secondary phase in 2212 layer were observed. The secondary phase is distributed uniformly over the whole surface which was caused to the mis-orientation of 2223. But it can be removed by oxygen pressure control. Spray coating layer shows superconducting with an onset Tr of about both 115 K and 77 K (Para-conductivity) in this study. There are two steps. step 1 at 115 K is due to the diamagnetism of the 2223 phase and step 2 at 78 K is due to the diamagnetism of the 2212 phase.

keyword : Plasma Spray, BSCCO, Superconductor, Partial Melt Process, 2212/2223 phase

A class of intermetallics of MNiSn (M=Ti, Zr Hf) half-Heusler compound is currently of interest as a promising thermoelectric material for application to power generation in high temperature range. We have studied the high temperature thermoelectric properties of TiNiSn half-Heusler compound. The present study reports the effect of heat treatment on high temperature thermoelectric properties of TiNiSn half-Heusler compound. It is shown that the transport properties of TiNiSn compound change from metallic to semiconducting by heat treatments. From the evaluation of microstructure, Seebeck coefficient, electrical resistivity and band gap for the heat-treated TiNiSn compounds, it is revealed that the heat treatment at 1073 K for 2 weeks is the optimum condition. This finding facilitates the research aiming the enhancement of high temperature thermoelectric properties of TiNiSn-based half-Heusler compounds.

keyword : TiNiSn half-Heusler compound, Heat treatment, Thermoelectric properties, Band gap

The effect of Hf alloying on Ti site, and Sb doping on Sn site for the optimization of high temperature thermoelectric properties of TiNiSn-based compounds was studied. Also, to achieve a low thermal conductivity, a powder metallurgy technique is used for the fabrication of the compounds. It was found that Hf alloying on Ti site reduced the thermal conductivity significantly to a low value of 3.8 W/mK at room temperature. Also, Sb doping on Sn site led to a remarkable enhancement of power factor of 4.1 mW/mK2. Further reduction of thermal conductivity was achieved in hot pressed samples. In hot pressed Ti(0.95)Hf(0.05)NiSn(0.99)Sb(0.01) sample, the dimensionless figure of merit, ZT reaches 0.78 at 770 K.

keyword : TiNiSn half-Heusler, Powder metallurgy, Thermoelectric properties, Seebeck coefficient, Electrical resistivity, Thermal conductivity, Dimensionless figure of merit

Self-assembled InAs/GaAs quantum dots (QDs) were grown by molecular-beam epitaxy and their structure and strain characteristics were studied by using transmission electron microscopy (TEM). TEM investigations were performed by conventional bright field TEM, high-resolution electron microscopy (HREM), annular dark field (ADF) and high angle annular dark filed (HAADF)-STEM techniques. In addition, strain analysis was performed on an atomic-length scale by measuring the space of lattices in HREM. The QDs were found to form a lens-shaped structure with side facet. Compressive strain was induced to uncapped QDs from GaAs substrate and the strain increased more than 3.5% after GaAs cap layer growth. On the other hand, tensile strain by capped QDs was induced to GaAs up tolO nm over the QDs, i.e. 15 nm over the wetting layer. It was also confirmed that the tensile strain extension resulted in aligned QD growth with 15 nm thick spacers. Vertically aligned QDs could be grown by the tensile strain from predeposited QDs when the thickness of GaAs spacer was less than 15 nm.

keyword : InAs/GaAs Quantum Dot, Microstructure of Quantum Dot, Misfit Strain, Multilayer-Stacked Quantum Dots, Transmission Electron Microscopy

The purpose of this study is to identify the effect of additives amount and composition rate on initial copper nucleation and growth during copper electrodeposition. Additives such as arabic gum, chloride ion were used in this study. In the production of electrodeposited copper foil, the surface roughness and crystal size of copper was able to be controlled by using appropriate combination of additives. Among various electrodeposition conditions, the minimum surface roughness of copper foil was obtained when electrodeposited at the current density of 500 mA/㎠ for 50 seconds with 1.5 ppm of gum arabic and 15 ppm of chloride ion. The minimum value of surface roughness was 0.195 ㎛. The addition of chloride ions (5 ppm~20 ppm) showed more effect on reducing the surface roughness compared with the case adding gum arabic only in the electrolyte.

keyword : Electrodeposition, Arabic gum, Chloride ion, Nucleation, Copper foil