발간논문

Home > KJMM 논문 > 발간논문

A stripping solution with thiourea, iron(III) sulfate, and sodium dodecyl sulfate(SDS) was employed to strip Ni-based 14 karat white gold alloys, and the formation of the NiS byproduct and elimination of passivation were investigated in the presence of 0.0-0.2 g/L SDS. White gold alloy samples with a flat shape were cast by gypsum investment and were stripped using the prepared stripping solution. Subsequently, the surface morphology, elimination of the passivation layer, weight loss, microstructure, elemental composition, and electrochemical properties of the samples were analyzed by optical microscopy, Raman spectroscopy, precision scale, scanning electron microscopy, energy dispersive X-ray spectroscopy, and linear sweep voltammetry, respectively. It was found that passivation layers of the as-cast samples were removed by the suggested stripping solution. Upon the addition of SDS, the stripped sample showed a bright silver color without NiS, while the sample showed a dark tarnished appearance due to NiS formation without SDS. The weight loss ratio decreased with increasing SDS content and stabilized at 0.2 % for SDS concentrations exceeding 0.15 g/L, and the sample showed a uniformly etched microstructure. EDS results showed that NiS was formed without SDS addition, while linear sweep voltammetry results indicated that NiS formation was restrained upon SDS addition because SDS suppresses the formation of formamidine disulfide from thiourea. Thus, the suggested thiourea stripping with SDS addition was successfully applied to Ni-based 14 karat white gold alloys. (Received March 3, 2020; Accepted May 23, 2020)

keyword : 14 karat white gold alloy, sodium dodecyl sulfate, thiourea stripping, passivation

Tungsten carbides are quite attractive for their superior properties, e.g., high melting point, high hardness, high thermal and electrical conductivities, and relatively high chemical stability. Tungsten carbides with a binder metal, for example Co or Ni, are mainly used to produce cutting tools, nozzles and molds in the composite form. But these binder materials show inferior chemical characteristics compared to the tungsten carbide phase. There has been enormous interest recently in finding alternative binder phases because of the low corrosion resistance and the high cost of Ni or Co. Al has been reported as an alternative binder for WC and TiC, since Al is less expensive and shows a higher oxidation resistance than Ni or Co. Nanostructured WC-BN-Al composites were rapidly sintered using high-frequency induction heated sintering (HFIHS). The microstructure and mechanical properties (fracture toughness and hardness) were investigated by Vickers hardness tester and FE-SEM. The HFIHS method induced very fast densification, nearly at the level of theoretical density, and successfully prohibited grain growth, resulting in nano-sized grains. The fracture toughness was improved by consolidation facilitated by adding Al to the WC-BN matrix. The 5vol % Al added WC-BN composites showed higher mechanical properties (hardness and fracture toughness than the WC-BN composite. (Received December 19, 2019; Accepted May 21, 2020)

keyword : composite, sintering, mechanical properties, nanomaterial

The microstructure and mechanical properties of a newly developed, β-phase containing TiAl alloy have been studied through hot working and post heat treatment to enhance room temperature ductility and strength. The controlled microstructures hadthree types of structure, fully lamellar, nearly lamellar and duplex, and were produced by cyclic heat-treatment in a single α region and (α+γ) region after a hot-forging process in high temperature (α+β) region. As a result of the room temperature tensile test, the fully lamellar structure exhibited a tensile strength of 622 MPa and ductility of 0.62%. The duplex structure had a tensile strength of 787 MPa and ductility of 1.22%, while the nearly lamellar structure showed a tensile strength of 880 MPa and ductility of 1.76%. In the room temperature tensile test, the nearly lamellar structure exhibited excellent tensile strength and ductility. The strength and ductility were increased by decreasing grain size and β / B2 phase fraction. The newly developed TiAl alloy showed higher tensile values compared with the previous TiAl alloys. The relationship between microstructure and room temperature tensile properties of the newly developed β-phase containing TiAl alloy was examined, and the best approach for hot working and post heat-treatment to obtain the most balanced mechanical properties was proposed. (Received March 26, 2020; Accepted May 21, 2020)

keyword : γ-TiAl, heat treatment, hot working, microstructure, ductility

We report on the phase transition behavior and thermoelectric transport properties of Zn-doped Cu₂Se. Cu₂Se is comprised of nontoxic, low-cost and abundant elements, and has been attracting attention because of its very high thermoelectric performance, ZT, at high temperatures. Many studies have reported enhanced ZT in impurity doped Cu2Se, however, little is known about the effect of dopants on the phase transition behavior of Cu₂Se. In this study, we prepared (Cu_1-xZn)₂Se (x = 0 ~ 0.03) compounds by spark plasma sintering, and investigated the phase transition behavior in terms of the temperature-dependent thermoelectric transport properties. Undoped Cu₂Se consists of monoclinic α-Cu₂Se and berzelianite at room temperature. However, the crystal structure of (Cu_1-xZnx)₂Se compounds is changed into a single phase of α- Cu2_Se with increasing Zn content. As the Zn content increased, electrical conductivity decreased and Seebeck coefficient increased due to the donor role of Zn. The phase transition temperatures of the compounds were characterized as a function of Zn content, based on the temperature-dependent electrical conductivities and Seebeck coefficients. The phase transition temperature was increased by increasing Zn content, and it was attributed to the disappearance of the berzelianite phase whose phase transition temperature is lower than that of α-Cu₂Se. Detailed effects of Zn-doping on both the phase transition behavior and thermoelectric transport properties were discussed. (Received March 19, 2020; Accepted May 14, 2020)

keyword : thermoelectric, Cu2Se, phase transition, spark plasma sintering

The subject of this study was a bronze sword excavated from the early Iron Age area of Sinpung site in Wanju. SEM-EDS, FE-EPMA and Raman Micro-Spectroscopy analysis methods were applied to investigate the manufacturing techniques and corrosion characteristics. The results of the analysis indicated a casting structure composed of both α and α+δ phases. The α phase developed in the form of dendrite, and it seems that no heat treatment was performed. Through the EDS analysis results, the main components of the bronze sword were identified as Cu, Sn, Pb, etc., and it seems that the cooling rate was different depending on the shape of the dendrite, which is the α phase, and the size and distribution of the Pb particles. Regarding corrosion characteristics, the corrosion progressed considerably to the surface, and a high tin corrosion layer was formed, and lead oxide and copper oxide particles were observed between the corroded microstructures. These materials were identified by Raman spectroscopic analysis as Cuprite (Cu₂O), Malachite (Cu₂CO₃(OH)₂), Cassiterite (SnO₂), Cerussite (PbCO₃), Anglesite (PbSO₄), and Litharge (PbO). Among them, the presence of nanocrystalline SnO₂ confirmed the characteristics of long-term corrosion of bronze artifacts in the burial environment. And, materials such as Cerussite (PbCO₃), Anglesite (PbSO₄), and Litharge (PbO) were found only in weapons and tools such as bronze sword and spear when compared with previous studies. (Received March 24, 2020; Accepted May 29, 2020)

keyword : early iron age, bronze sword, microstructure, production technology, corrosion

Polycrystalline diamond compact (PDC) has excellent wear resistance, high impact resistance, superior fatigue properties, and has been used in the oil and gas drilling industries. This study investigated the effect of Co leaching on the microstructure, vertical turning lathe (VTL) wear properties of PDCs manufactured by high-temperature and high-pressure (HTHP) sintering. The VTL wear test has the advantage of simulating the actual oil drilling environment by using granite as the workpiece. PDC sintered material that did not receive Co leaching was named HTHP sintered PDC-A, and the material subjected to Co leaching was called Co-leached PDC-B in this study. As a result of XRD analysis of both PDCs, diamond and WC peaks were detected, and only the HTHP sintered PDC-A exhibited some Co peaks. In the HTHP sintered PDC-A, the binder WC and Co were evenly distributed at the diamond interface. However, in the Co-leached PDC-B, some empty spaces were observed at the diamond interface. The HTHP sintered PDCA exhibited a similar or slightly higher VTL wear resistance than the Co-leached PDC-B, but only in the short sliding distance. In the long sliding distance after 9 km, Co-leached PDC-B showed significantly superior wear resistance compared to the HTHP sintered PDC-A. The HTHP sintered PDC-A exhibited both abrasive and adhesive wear behaviors, while the Co-leached PDC-B showed only abrasive wear. Based on the above results, the VTL wear mechanism of PDCs, and ways of improving wear resistance were also discussed. (Received March 23, 2020; Accepted June 8, 2020)

keyword : polycrystalline diamond compact, PDC, high temperature, high pressure, HTHP, sintering, Co leaching, vertical turning lath, VTL, wear, wear mechanism

Cu-Ni-Si alloys were strengthened by Ni2Si intermetallic compound precipitation in a Cu matrix during aging. The Cu-6 wt% Ni-1.4wt%Si alloy was prepared by water quenching (solution treatment) or air cooling (homogenization treatment) after heating 980 ℃ for 2 hours and aging at 500 ℃ for 6 hours. After maintaining a high temperature single phase region, the structure of the precipitate in the matrix was changed significantly by varying the cooling rate. The solution treated aged alloy had discontinuous precipitation with fiber shaped precipitates throughout the specimen, however, the alloy with homogenization had normal spherical shaped particles. The aged alloy after homogenization treatment had higher strength, ductility and electrical conductivity, of 628 MPa, 18%, 48% IACS, respectively, than aged alloy after solution treatment, which had 582 MPa, 15.5% and 50 % IACS, respectively. In contrast to the tendency of values after aging, after rolling and a 75% area reduction at room temperature after homogenization treatment the alloy had lower strength, ductility and electrical conductivity, of 774 MPa, 10.5%, 48% IACS, than the alloy after solution treatment, with 885 MPa, 6.5%, and 48% IACS, respectively. The Cu-6 wt% Ni-1.4wt%Si alloy with fully discontinuous precipitation had lower strength and higher electrical conductivity than the counterpart alloy with normal precipitation. However, inversely, the strength increased compared to the alloy with normal precipitation after rolling with 75% area reduction, without sacrificing electrical conductivity. (Received April 23, 2020; Accepted May 25, 2020)

keyword : Cu-Ni-Si alloy, aging, Ni2Si intermetallic compound, precipitation hardening, electrical conductivity, normal precipitation, discontinuous precipitation

This study examined the effect of alkalis on lyocell-based activated carbon fibres (ACFs) with high Brunauer-Emmett-Teller (BET) surface areas, using X-ray diffraction (XRD). Here, alkali treatments were conducted using 10 ~ 25 % NaOH and KOH on lyocell samples for 3 h at ambient room temperature. A secondary treatment with 4 % each of KOH and H₃PO₄ for an additional 3 h followed. An activated form of the samples was prepared by oxidation (stabilisation), carbonisation, and activation. The final ACF form of the samples showed a porous structure with high BET surface areas (> 1,000 m² g^-1). A 25 % dosage of NaOH produced the highest BET surface area compared to other samples. In terms of crystallinity, a 15 % dosage of KOH was found to be the optimum dosage to secure the highest degree of crystallinity among all samples. Meanwhile, the NaOH was successfully shifted the samples into the most distinct form of cellulose II. 15 % NaOH was found to secure the most stable thermal characteristics, as determined via thermo-gravimetric analysis. The present work demonstrates the various physio-chemistries of ACFs prepared with different proportions and types of alkalis, leading to intriguing potential applications. (Received April 28, 2020; Accepted May 26, 2020)

keyword : crystallisation, x-ray diffraction, thermodynamic properties, fibre, chemical treatment

The properties and H₂O adsorption characteristics of two types of zeolites (3A and 13X) with the same shape but different element composition were identified. These zeolites are used in gas purification processes to manufacture 9N ultra-pure inert gases for semiconductor process applications. To analyze the shape and microstructure of the zeolites, an optical microscope and FE-SEM were used. EDS, micro-Raman, and XRD analysis were conducted to analyze their compositions and properties. BET analysis was performed to identify their specific surface areas. Finally, a breakthrough analysis was conducted at room temperature. Based on the results of the optical and microstructure analyses, the cylindrical shaped 3A and 13X were determined to be composed of 2.5-um polyhedrons and 1.4-um massive shapes, respectively. The results of the compositional analysis showed that the main components of both zeolites were Si and Al, whereas K and Na existed in 3A and 13X, respectively. The results of the specific surface area analysis demonstrated that the surface area of 13X was 32 times larger than that of 3A. Specifically, the total specific surface areas were 22.42 and 720.33 m²/g for 3A and 13X, respectively. The results of the H₂O adsorption characteristic analysis showed that the H₂O concentration of 13X was 1.33 times higher than that of 3A. Therefore, with respect to using zeolite for gas purification process applications, 13X might be more suitable to achieve excellent specific surface area and remarkable H₂O adsorption. (Received January 9, 2020; Accepted June 3, 2020)

keyword : zeolite, BET, micropore, H, sub, 2, sub, O adsorption