발간논문

Home > KJMM 논문 > 발간논문

Torsion tests were performed on AA6082 samples to investigate their flow softening behavior and to determine optimum process conditions. Based on the experimental data and dynamic materials model, high strain processing maps were established which demonstrate the constitutive relationships among temperature, strain rate, strain and stress. The flow curves exhibited a single and smooth maximum, followed by a softening stage. Power dissipation efficiency increased with increasing deformation temperature, and decreased with increasing strain rate. The main softening mechanism was continuous dynamic recrystallization. The power dissipation efficiency could be divided into three stages. Two flow instability domains occurred. One was found at deformation temperatures ranging from 623 to 673 K. The other flow instability domain was found at deformation temperatures ranging from 673 to 773 K, and with strain rates ranging from 0.5 to 5/s at strains from 1.0 to 3.0, which were mainly located in the upper part of the processing maps. (Received January 6, 2016; Accepted May 10, 2016)

keyword : alloys, hot working, recrystallization, torsion test, processing map

In this paper, we demonstrate that the microstructure and mechanical properties in the Ti₄₀Zr₁₀Cu₃₆Pd₁₄ alloy can be tailored by controlling the cooling rate during solidification. A lower cooling rate increases the volume fraction of crystalline phase such as B2 but decreases the free volume of the glassy matrix. The increase of the B2 volume fraction can dramatically enhance the toughness of the composites, since the B2 phase is relatively ductile compared to the glassy matrix and seems to have good interface stability with the matrix. From the experimental results, it was found that there is a transition point in the plasticity of the composites depending on the cooling rate. Here, we explain how the toughness of the composites varies in accordance with the cooling rate in the Ti₄₀Zr₁₀Cu₃₆Pd₁₄ alloy system. (Received April 22, 2016; Accepted May 25, 2016)

keyword : metallic glass matrix composite, cooling rate, B2 phase, microstructure, mechanical properties

The effect of the strength coefficient (K_B) of bainite on micromechanical deformation and failure behaviors of a hot-rolled 590MPa steel (590FB) during uniaxial tension was simulated using the elasto-plastic finite element method (FEM). The spatial distribution of the constituent phases was obtained using a phase identification technique based on optical microstructure. Empirical equations which depend on chemical composition were used to determine the stress-strain relationship of the constituent phases of the 590FB steel. The stress-strain partitioning and failure behavior were analyzed by increasing the K_B of bainite. The elasto-plastic FEM results revealed that effective strain in the ferrite-bainite boundaries, and maximum principal stress in fibrous bainite, were enhanced as the  increased. The elasto-plastic FEM results also demonstrated that the K_B significantly affects the micromechanical deformation and failure behaviors of the hot-rolled 590FB steel during uniaxial tension. (Received February 2, 2016; Accepted May 19, 2016)

keyword : ferrite-bainite dual phase steel, metals, deformation, fracture, optical microscopy

In many plants, including nuclear power plants, pipelines are composed of numerous fittings such as elbows. When plants use these fittings, welding points need to be increased, and the number of inspections also then increases. As an alternative to welding, the pipe bending process forms bent pipe by applying strain at low or high temperatures. This work investigates how heat treatment affects on the boric acid corrosion of ASME SA335 Gr. P22 caused by the induction heat bending process. Microstructure analysis and immersion corrosion tests were performed. It was shown that every area of the induction heat bent pipe exhibited a high corrosion rate in the boric acid corrosion test. This behavior was due to the enrichment of phosphorous in the ferrite phase, which occurred during the induction heat bending process. This caused the ferrite phase to act as a corrosion initiation site. However, when re-heat treatment was applied after the bending process, it enhanced corrosion resistance. It was proved that this resistance was closely related to the degree of the phosphorus segregation in the ferrite phase. (Received October 19, 2015; Accepted May 19, 2016)

keyword : low alloy steel, induction heat bending, boric acid corrosion, phosphorus

CoTi and ZrO₂ nanopowders were mechanochemically synthesized from CoTiO₃ and Zr powders according to the reaction (2CoTiO₃ + 3Zr → 2CoTi + 3ZrO₂). The milled powders were then consolidated by pulsed current-activated sintering within two minutes under an applied pressure of 80MPa. The average hardness and fracture toughness of the nanostructured 2CoTi-3ZrO₂ composite sintered from high energy ball milled powder were 940 kg/mm² and 6 MPaㆍm^1/2, respectively. The mechanical properties of the composite were higher than those of monolithic CoTi or ZrO₂, respectively. The microstructure and phases of the composite were investigated using FE-SEM and XRD. (Received May 4, 2016; Accepted May 23, 2016)

keyword : sintering, composite materials, nanomaterials, mechanical properties, synthesis

Herein, we report the results of our investigation on the effect of friction and anvil design on the heterogeneous plastic-deformation characteristics of copper during the compressive stage of high-pressure torsion (HPT), using the finite element method. The results indicate that the friction and anvil geometry play important roles in the homogeneity of the deformation. These variables affect the heterogeneous level of strain in the HPT compressed disks, as well as the flash in the disk edge region. The heterogeneous plastic deformation of the disks becomes more severe with the increasing depth of the cavity, as anvil angle and friction coefficient increase. However, the homogeneity increases with increases in the wall angle. The length of flash and the area of the dead metal zone increase with the depth of the cavity, while they decrease at a wall angle of 180°. (Received March 17, 2016; Accepted May 25, 2016)

keyword : high pressure torsion, copper, deformation, finite element analysis, anvil structure

Directional solidification of CM247LC was performed at a constant growth velocity (300 mm/h) using a Bridgman type furnace. Microstructural features, including the number of grains, PDAS, and EBSD IPF map, were investigated with progressive positions from the starter block. As directional solidification proceeded, the number of grains decreased and PDAS increased. These results indicate that the thermal gradient decreases with distance from the starter block. Thermal gradient changes were estimated by dendrite spacing change in the directionally solidified rod. The EBSD IPF map showed that almost all of the grains were close to the < 001 > preferred direction, but with heights of about 300 mm from the starter block, some grains at the center of the specimen exhibited quite different crystallographic directions near the gate instead of the preferred direction. (Received October 5, 2015; Accepted May 24, 2016)

keyword : Ni base superalloy, solidification, electron backscattering diffraction(EBSD)

The world is gradually running short of high quality iron resources. This forces ironmaking processes to focus on the use of low quality iron ores. Carbon composite iron oxide briquettes (CCB) are one of the alternative choices. CCB enables the utilization of low quality powdered iron ores and coal, and also enhances the fast reduction rate of iron oxides by providing direct contact with coal. The iron ore and coal used for making CCB initially contain impurity components such as gangue and ash. Impurity components are generally well known to suppress contact between the iron ore and coal. In this study, the effect of basicity on the reduction behavior of CCB is discussed. Basicity in the range of 0 and 1.632 was considered. CCB mixed with reagent iron oxide and graphite was prepared to have a molar ratio C/O = 1. Reduction experiments were carried out in N2 gas condition by heating from 200 ℃ to 1200 ℃ in an electric furnace. CO and CO₂ gas were measured by a gas analyzer to analyze the reduction mechanism. In the case of CCB with a basicity less than 0.816, the degree of reduction was decreased due to the formation of fayalite phase. CCB whose basicity was higher than 1.224 also showed that the decrease of the reduction degree was due to the formation of high melting point slag. As a result, the highest value of degree of reduction was measured at the basicity in between 0.8 and 1.2. (Received March 30, 2016; Accepted May 14, 2016)

keyword : iron oxides, ironmaking process, reduction, scanning electron microscopy (SEM), CCB

Studies were carried out to apply electroplating technique to the inner surface of a nuclear fuel cladding tube to prevent a fuel-cladding chemical interaction (FCCI) in a sodium-cooled fast reactor (SFR). 20 μm thick Cr was uniformly plated onto the inner surface of a HT9 cladding tube with a 7.4 mm outer diameter and 0.56 mm thickness. Diffusion couple testing of simulated metal fuel (rare earth alloy) as well as tensile and burst tests were carried out, which revealed that the Cr plating exhibited good FCCI resistance as well as mechanical compatibility. In-reactor performance of Cr-plated cladding tube was performed using the HANARO research reactor, where Cr plating showed excellent FCCI resistance in spite of some local cracks. (Received February 2, 2016; Accepted April 5, 2016)

keyword : metals, plating, mechanical properties, tensile test, fuel-cladding chemical interaction (FCCI)