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High-entropy alloys (HEAs), which are defined as a single phase crystalline solid solution with the composition of each constituent element in the range of 5-35 at%, have been the subject of intensive study in the last decade. Most of the HEA research is focused on alloy design, microstructural characterization, and mechanical testing, not manufacturing, processing, and industrialization, although the properties, microstructures, and performance are strongly dependent on processing. In this review article, we aim to describe the manufacturing processes applied to HEAs. The manufacturing process of HEAs can be classified into three main routes. First, liquid processing that includes arc melting, Bridgman solidification, atomization, and laser cladding is the most commonly used methods because the processing facilities are already widespread in many laboratory and the liquid processing can be easily scaled-up for commercialization. Second, mechanical alloying. i.e. powder metallurgy, is a process starting from a solid particle state and employing sintering procedures for bulk manufacturing. In particular, the mechanical alloying has been used for achieving super saturated solid solution which cannot be obtaiend using the conventional liquid prcesses. Third, the HEAs can be manufactured by mixing elements of the vapor state, which includes sputter deposition, atomic layer deposition, and vapor phase deposition. Explanation and comparison between various manufacturing methods of the HEAs are systematically described and demonstrated in this review. †(Received July 24, 2017; Accepted August 16, 2017)

keyword : high-entropy alloys, fabrication method, microstructure, properties, characterization

Age hardening and re-crystallization softening of a type of high entropy alloy, CoCrFeMnNi, was performed to investigate the relationship between microstructure and the property of mechanical hardness. Equiatomic CoCrFeMnNi was produced by casting followed by homogenizing, cold rolling and post heat treatments. TEM analysis revealed that the microstructure of the as-rolled alloy was composed of elongated grains with twin laths and a shear band with fine grains, which exhibited a hardness of 371 HV. Following a 500 ℃ heat treatment the microstructure was composed of nano-twins and nano-grains, which represented age hardening, with 454 HV hardness. The microstructure of 700 ℃ heat treated samples was composed of re-crystallized grains and annealing twins, which represented annealed softening, with 244 HV hardness. The 700 ℃ heat treatment also produced spherical particles of 0.1 m maximum grain size, identified as Cr₂₃C₆ type carbide. †(Received March 13, 2017; Accepted May 22, 2017)

keyword : high entropy alloy, age hardening, annealed softening, TEM microstructure, nano-twin, nano-grain, Cr₂₃C₆ carbide

Brake judder is a kind of low-frequency brake vibration and noise phenomenon generated during braking. Various types of judder have been reported, depending on the particular generation mechanism. However, the correlation between the microstructure of gray iron brake discs and judder has seldom been discussed in the literature. In this research, the effect of the microstructure of a gray iron brake disc on judder was investigated using a single end full dynamometer. Judder propensity was evaluated by measuring BTV (brake torque variation), and the effect of cell counts of eutectic gray iron on the friction surface of the brake disc was also measured in the circumferential direction. The eutectic cell counts were controlled by changing the machining thickness of the friction surface. Results showed that the microstructure of the gray iron disc played a crucial role in controlling brake vibration during braking. In this work, reduction in the eutectic cell counts resulted in less brake torque variation, suggesting more stable brake performance. (Received April 10, 2017; Accepted July 7, 2017)

keyword : brake judder, brake disc, eutectic cell, gray iron, brake torque variation (BTV)

In this study the hydrogen embrittlement characteristics of three high-manganese steels were tested using different hydrogen charging methods, and the results were compared with various Mn-containing steels. The results showed that the hydrogen embrittlement susceptibility of the high-manganese steels increased with increasing inherent strength because deformation mechanisms, such as deformation twinning, ε-martensite transformation, and shear- or micro-band formation, enhanced their sensitivity to hydrogen-induced cracking. The different hydrogen charging methods also affected their ability to achieve the critical hydrogen concentration needed for hydrogen-induced cracking under the stress fields of each microstructure. The relative reduction in ductility for different charging methods usually increased in the order of ex-situ electrochemically-charged, ex-situ high-pressure thermally-charged, and in-situ environment tensile testing, although it was somewhat dependent on the charging, testing and specimen conditions. Based on the results of the three high-manganese steels, it was found that the high-pressure thermally-charged steel specimens had higher relative reductions in ductility because a larger amount of hydrogen was uniformly injected into the steel specimens, which promoted hydrogen-induced cracking under smaller strain than that of the electrochemically-charged steel specimens. (Received May 18, 2017; Accepted June 20, 2017)

keyword : high-manganese steel, hydrogen embrittlement, hydrogen charging method, relative ductility reduction

In this study, Al alloy targets were fabricated using the powder metallurgy (pulsed current activated sintering, PCAS) process for metal PVD coating target applications. Powders were prepared from Al, Si and Cu ingots for sintering Al alloy compacts using the gas atomizing process. To fabricate the gas atomized Al alloy powders, processing conditions, such as melting temperature, processing time and gas pressure were optimized and controlled during the gas atomizing process. Al alloy compacts with 200 mm diameters and 1/4 inch thickness were fabricated using a 30,000 A pulsed current activated sintering machine. During the Al alloy compact sintering process, sintering conditions such as temperature, pulse ratio, pressure, and heating rate were controlled and optimized. The Al alloy compacts were fabricated under a uniaxial pressure of 60 MPa at a sintering temperature of 400 ℃ without any significant change in grain size. The grain size and relative density of the Al alloy compacts were 7.3 ㎛ and 100%, respectively. The properties of thin films deposited on a Si substrate using the PCASed target materials were compared with those from a commercial target material prepared using the casting melting process. The thicknesses of the thin films deposited on the Si substrate using the PCASed target material and a commercial target material were about 494 nm and 450 nm, respectively. The specific resistance and surface roughness of the PCASed thin film and commercial thin film were 4.012 × 10^-6 and 4.012 × 10^-6, 6.105 nm and 6.928 nm, respectively. (Received June 16, 2017; Accepted July 4, 2017)

keyword : gas atomization process, Al alloy powder, pulsed current activated sintering method, thin film, property evaluation

Interstitial free (IF) steel disks were subjected to various degrees of revolution during application of the high-pressure torsion (HPT) process, and the resulting distributions of hardness and microstructure during the early torsion stage of high-pressure torsion (HPT) were investigated using experimental and simulation approaches. The results indicated that the deformation in the HPT-processed IF steel disk was inhomogeneous, producing low hardness in the center and high hardness in the edge region. The experimental results, including the hardness and microstructure distributions, indicated that the severe deformation zone proceeds gradually from the center to the edge of the HPT disks in the early torsion stage, and also confirmed verify that the deformation on the upper surface of the disks lags behind that on the bottom surface. Simulation results from a finite element method analysis strongly supported the experimental conclusions. (Received December 30, 2016; Accepted July 2, 2017)

keyword : high pressure torsion, early torsion stage, IF steel, deformation, finite element analysis

A new aluminum welding wire with 0.8 wt% higher in Mg content was developed and compared with commercial welding wire of the 5000 series. The weldability of the Al5083 base material was evaluated using high current metal inert gas (MIG) welding, using the developed Al5183 and commercial Al5183 welding wire. For the wire with 5.1 wt% Mg content, the hardness of the weld metal decreased with increasing heat input to the center of the weld under low and high heat input conditions. For the wire with 5.9 wt% Mg content, the hardness of the weld metal did not decrease under low and high heat input conditions. The wire with 5.1 wt% Mg content exhibited a tensile strength of about 300 MPa under the low/medium heat input conditions. The tensile strength decreased to 284 MPa under high heat input conditions. However, the welding wire with 5.9 wt% Mg content exhibited a tensile strength of 300 MPa regardless of the heat input condition. Dendritic microstructures were observed in the center of the welds in both wires, with Mg contents of 5.1 wt% and 5.9 wt%, and the second dendrite arm spacing (SDAS) increased with increasing heat input. (Received May 8, 2017; Accepted July 10, 2017)

keyword : aluminum 5083 alloy, magnesium, welding wire, metal inert gas welding, tensile strength, hardness

The separation of Re(VII) and V(V) by solvent extraction with TBP was investigated by varying the HCl, metal and chloride ion concentrations. Re(VII) was selectively extracted over V(V) from the HCl solution. The highest separation between Re(VII) and V(V) occurred at a concentration of 2.0 M HCl. The extraction reaction of Re(VII) by TBP was verified using the slope analysis method. Changing the concentration ratio of the two metals and the chloride ion concentration had little effect on the extraction of V(V). Re(VII) could be completely stripped from the loaded TBP when the pH of the stripping solution was higher than 3.0. The obtained results can be utilized to develop a process for separating Re(VII) and V(V) from HCl solutions. (Received February 1, 2017; Accepted July 2, 2017)

keyword : separation, rhenium, vanadium, tributyl phospate, solvent extraction

In this study, we made large-scale ingots of CoCrFeMnNi high entropy alloy by vacuum induction melting. The as-cast CoCrFeMnNi high entropy alloy contained a high proportion of columnar structures with a few equiaxed grains, and showed single phase fcc solid solutions without macro segregation and low interstitial levels. The tensile properties along the three different loading directions were investigated at low-temperature and room temperature. A decrease in temperature led to an increase in yield and tensile strengths. The increase in anisotropy due to the columnar structure was negligible. Unlike wrought CoCrFeMnNi high entropy alloys, twins were actively formed in the specimen deformed at room temperature. Mechanical twinning was found to be a result of the coarse grain size, over 500 μm, of the as-cast CoCrFeMnNi high entropy alloy, which effectively reduced the critical stress for twinning. Charpy impact tests were also conducted, and the absorbed energy of the CoCrFeMnNi high entropy alloy showed no dependence on the temperature or loading directions. The results of this study provide an understanding of pilot-scale high entropy alloy castings, and are expected to be utilized as basic properties for manufacturing large-scale high entropy alloy castings, which are effective in severe environments. (Received May 23, 2017; Accepted June 4, 2017)

keyword : high entropy alloy, mechanical property, microstructure, twinning

Nanocomposite films, which are comprised of metal nanoparticles dispersed in a matrix layer, have attracted great interest due to their unique electrical and optical properties, which are mainly due to the excitation of localized surface plasmons. In this study, we employed an alternating sputtering method to fabricate such nanocomposite films. We then systematically investigated the effect of SiO₂, (Ba,Sr)TiO₂, and Si matrix materials on the growth behavior of Au nanoparticles and their geometric distribution, which strongly affects their plasmonic properties, using both electrical and optical means. From the results, we suggest an effective way of monitoring the growth stages of Au nanoparticles and adjusting their geometric distribution to a desired form, by properly selecting the combination of materials and process controls. (Received May 16, 2017; Accepted June 17, 2017)

keyword : metal-dielectric nanocomposite film, alternating sputtering, localized surface plasmon resonance, growth of Au nanoparticles