• Transactions of Materials Processing
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• v.32 no.4
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• pp.173-179
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• 2023

In this study, the age-hardening behavior and tensile properties of a cast AZ91-0.3Ca-0.2Y (SEN9) alloy are investigated and compared with those of a commercial AZ91 alloy. Even after homogenization heat treatment, the SEN9 alloy contains numerous undissolved secondary phases, Al₈Mn₄Y, Al₂Y, and Al₂Ca, which results in a higher hardness value than the homogenized AZ91 alloy. Under aging condition at 200 ℃, both the AZ91 and SEN9 alloys exhibit the same peak-aging time of 8 h, but the peak hardness of the latter (86.8 Hv) is higher than that of the former (83.9 Hv). The precipitation behavior of Mg₁₇Al₁₂ phase during aging significantly differs in the two alloys. In the AZ91 alloy, the area fraction of Mg₁₇Al₁₂ discontinuous precipitates (DPs) increases up to ~50% as the aging time increases. In contrast, in the SEN9 alloy, the formation and growth of DPs during aging are substantially suppressed by the Ca- or Y-containing particles, which leads to the formation of only a small amount of DPs with an area fraction of ~4% after peak aging. Moreover, the size and interparticle spacing of Mg₁₇Al₁₂ precipitates of the peak-aged SEN9 alloy are smaller than those of the peak-aged AZ91 alloy. The homogenized AZ91 alloy exhibits a higher tensile strength than the homogenized SEN9 alloy due to the finer grains of the former. However, the peak-aged SEN9 alloy has a higher tensile elongation than the peak-aged AZ91 alloy due to the smaller amount of brittle DPs in the former.

• Korean Journal of Materials Research
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• v.33 no.6
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• pp.242-250
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• 2023

Titanium, which has excellent strength and toughness characteristics, is increasingly used in the aerospace field. Among the titanium alloys used for body parts, more than 80 % are Ti-6Al-4V alloys with a tensile strength of 931 MPa. The spark plasma sintering (SPS) method is used for solidification molding of powder manufactured by the mechanical milling (MM) method, by sintering at low temperature for a short time. This sintering method avoids coarsening of the fine crystal grains or dispersed particles of the MM powder. To improve the mechanical properties of pure titanium without adding alloying elements, stearic acid was added to pure titanium powder as a process control agent (PCA), and MM treatment was performed. The properties of the MM powder and SPS material produced by solidifying the powder were investigated by hardness measurement, X-ray diffraction, density measurement and structure observation. The processing deformation of the pure titanium powder depends on the amount of stearic acid added and the MM treatment time. TiN was also generated in powder treated by MM 8 h with 0.50 g of added stearic acid, and the hardness of the powder was higher than that of Ti-6Al-4V alloy when treated with MM for 8 h. When the MM-treated powder was solidified in the SPS equipment, TiC was formed by the solid phase reaction. The SPS material prepared as a powder treated with MM 8 h by adding 0.50 g of stearic acid also formed TiN and exhibited the highest hardness of Hv1253.

• Korean Journal of Materials Research
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• v.33 no.7
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• pp.315-322
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• 2023

In the present work, we address the new route for the green synthesis of manganese dioxide (MnO₂) by an innovative method named the solution plasma process (SPP). The reaction mechanism of both colloidal and nanostructured MnO₂ was investigated. Firstly, colloidal MnO₂ was synthesized by plasma discharging in KMnO₄ aqueous solution without any additives such as reducing agents, acids, or base chemicals. As a function of the discharge time, the purple color solution of MnO₄^- (oxidation state +7) was changed to the brown color of MnO₂ (oxidation state +4) and then light yellow of Mn²⁺ (oxidation state +2). Based on the UV-vis analysis we found the optimal discharging time for the synthesis of stable colloidal MnO₂ and also reaction mechanism was verified by optical emission spectroscopy (OES) analysis. Secondly, MnO₂ nanoparticles were synthesized by SPP with a small amount of reducing sugar. The precipitation of brown color was observed after 8 min of plasma discharge and then completely separated into colorless solution and precipitation. It was confirmed layered type of nanoporous birnessite-MnO₂ by X-ray powder diffraction (XRD), fourier-transform infrared spectroscopy (FT-IR), and electron microscopes. The most important merits of this approach are environmentally friendly process within a short time compared to the conventional method. Moreover, the morphology and the microstructure could be controllable by discharge conditions for the appropriate potential applications, such as secondary batteries, supercapacitors, adsorbents, and catalysts.

• Korean Journal of Materials Research
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• v.33 no.7
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• pp.285-294
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• 2023

Porous ceramics are used in various industrial applications based on their physical properties, including isolation, storage, and thermal barrier properties. However, traditional manufacturing environments require additional steps to control artificial pores and limit deformities, because they rely on limited molding methods. To overcome this drawback, many studies have recently focused on fabricating porous structures using additive manufacturing techniques. In particular, the binder jet technology enables high porosity and various types of designs, and avoids the limitations of existing manufacturing processes. In this study, we investigated process optimization for manufacturing porous ceramic filters using the binder jet technology. In binder jet technology, the flowability of the powder used as the base material is an important factor, as well as compatibility with the binder in the process and for the final print. Flow agents and secondary binders were used to optimize the flowability and compatibility of the powders. In addition, the effects of the amount of added glass frit, and changes in sintering temperature on the microstructure, porosity and mechanical properties of the final printed product were investigated.

• Geomechanics and Engineering
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• v.31 no.4
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• pp.395-407
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• 2022

In this paper, cracks with different angles are prefabricated in rock specimens to study the fracture characteristics of rock based on CT images. The rock specimens are prepared for compression tests according to the standard recommended by ISRM (International Society for Rock Mechanics). The effects of different angles on rock mechanical properties and crack propagation fracture modes are analyzed. Then, based on the cohesive element method and CT images, the relationship between porosity and Young's modulus as well as the fracture property is explored by the numerical modelling. In the modelling, the distribution of Young's modulus is determined by the CT image through the field variable method. The results show that prefabricated cracks reduce the mechanical properties of rock. The closer the angles of the prefabricated crack is, the greater the Young's modulus of the rock sample is. The failure process of each specimen with prefabricated cracks is formed by the initiation and propagation of crack, and the angle of the prefabricated crack will affect the type of extended crack. As part of the numerical model proposed in this paper, the microstructure of rocks is reflected by CT images. The numerical results verify the effectiveness of the cohesive element method in the study of crack propagation for rock. The rock model in this paper can be used to predict engineering disasters such as collapse and landslide caused by rock fracture, which means that the methodology adopted in this paper is comprehensive and important to solve rock engineering problems.

• Fisheries and Aquatic Sciences
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• v.26 no.9
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• pp.535-547
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• 2023

Sea cucumbers (Holothuria scabra), also known as beche-de-mer, are highly valued as a luxurious food item and have been utilized as a traditional tonic food in various Asian countries for centuries. The body walls of sea cucumbers are the main edible part, which are primarily composed of glycosaminoglycan (GAG). The rehydration of dried sea cucumber is a crucial step prior to further processing. The aim of this study was to assess the impact of ultrasound-assisted rehydration (UAR) on the quality of dried sea cucumbers. The experiment used four different rehydration methods, including conventional methods at 27℃ (KV27℃) and 15℃ (KV15℃), as well as a combination of ultrasound at 27℃ with conventional at 15℃ (UAR27 + KV15℃) and ultrasound at 15℃ with conventional at 15℃ (UAR15 + KV15℃). Results indicated that the rehydration rate (RR) was significantly affected by both the rehydration method and the temperature used (p < 0.05). UAR27 + KV15℃ was identified as the most effective method in terms of rehydration behavior and quality characteristics of dried sea cucumber, with a RR of 0.58 ± 0.53 gH₂O/hour and reduced rehydration time of up to 28 hours. Moreover, the UAR27 + KV15℃ method demonstrated superior rehydration potential, nutritional value (proximate composition and sulfate content), color, lower energy, and microstructure properties compared to the other methods. The sulfate content and yield of sulfated GAGs were determined to be 89.4 mg/g and 52.8 ㎍/g, respectively. Confirmation of the absorption band of the sulfate group showed the presence of 3-N-acetyl galactosamine at a wavelength of 1,269 cm^-1 and C-O-S at 860 cm^-1. The sea cucumbers treated with UAR exhibited a GAG content approximately 2.9 times higher than those rehydrated with the conventional method. Eventually, the combination of UAR at 27℃ with conventional at 15℃ methods can significantly accelerate the rehydration of sea cucumber without negatively affecting its physical quality properties.

• Steel and Composite Structures
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• v.49 no.4
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• pp.361-380
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• 2023

A size dependent bending behavior of piezoelectrical flexoelectric layered perforated functionally graded (FG) composite nanobeam rested on an elastic foundation is investigated analytically. The composite beam is composed of regularly cutout FG core and two piezoelectric face sheets. The material characteristics is graded through the core thickness by power law function. Regular squared cutout perforation pattern is considered and closed forms of the equivalent stiffness parameters are derived. The modified nonlocal strain gradient elasticity theory is employed to incorporate the microstructure as well as nonlocality effects into governing equations. The Winkler as well as the Pasternak elastic foundation models are employed to simulate the substrate medium. The Hamiltonian approach is adopted to derive the governing equilibrium equation including piezoelectric and flexoelectric effects. Analytical solution methodology is developed to derive closed forms for the size dependent electromechanical as well as mechanical bending profiles. The model is verified by comparing the obtained results with the available corresponding results in the literature. To demonstrate the applicability of the developed procedure, parametric studies are performed to explore influences of gradation index, elastic medium parameters, flexoelectric and piezoelectric parameters, geometrical and peroration parameters, and material parameters on the size dependent bending behavior of piezoelectrically layered PFG nanobeams. Results obtained revealed the significant effects both the flexoelectric and piezoelectric parameters on the bending behavior of the piezoelectric composite nanobeams. These parameters could be controlled to improve the size dependent electromechanical as well as mechanical behaviors. The obtained results and the developed procedure are helpful for design and manufacturing of MEMS and NEMS.

• Journal of Powder Materials
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• v.30 no.6
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• pp.463-469
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• 2023

Aluminum alloys, known for their high strength-to-weight ratios and impressive electrical and thermal conductivities, are extensively used in numerous engineering sectors, such as aerospace, automotive, and construction. Recently, significant efforts have been made to develop novel aluminum alloys specifically tailored for additive manufacturing. These new alloys aim to provide an optimal balance between mechanical properties and thermal/electrical conductivities. In this study, nine combinatorial samples with various alloy compositions were fabricated using direct energy deposition (DED) additive manufacturing by adjusting the feeding speeds of Al6061 alloy and Al-12Si alloy powders. The effects of the alloying elements on the microstructure, electrical conductivity, and hardness were investigated. Generally, as the Si and Cu contents decreased, electrical conductivity increased and hardness decreased, exhibiting trade-off characteristics. However, electrical conductivity and hardness showed an optimal combination when the Si content was adjusted to below 4.5 wt%, which can sufficiently suppress the grain boundary segregation of the α-Si precipitates, and the Cu content was controlled to induce the formation of Al₂Cu precipitates.

• Korean Journal of Materials Research
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• v.33 no.12
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• pp.558-564
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• 2023

In this study, we designed and manufactured a large angular contact ball bearing (LACBB) with low deformation using JIS-SUJ2 steel and analyzed changes in its structural characteristics and chemical composition upon heat treatment. The bearing was produced by hot forging and heat treatment including a quenching and tempering (Q/T) process, and its properties were analyzed using 4 mm thick specimens. A difference in the size distribution of the carbide in the outer and inner parts of the bearing was observed and it was confirmed that large and non-uniform carbide was distributed in the inner part of the bearing. After heat treatment, the hardness value of the outer part increased from 13.4 HRC to 61 HRC and the inner part increased from 8.0 HRC to 59.7 HRC. As a result of X-ray diffraction (XRD) measurements, the volume fraction of the retained austenite contained in the outer part was calculated to be 3.5~4.8 % and the inner part was calculated to be 3.6~5.0 %. The surface chemical composition and the content of chemical bonds were quantified through X-ray photoelectron spectroscopy (XPS), and a decrease in C=C bonds and an increase in Fe-C bonds were confirmed.

• Proceedings of the Plant Resources Society of Korea Conference
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• 2023.04a
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• pp.30-30
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• 2023

한국산 체꽃속(Scabiosa L.)의 외부형태형질을 바탕으로 구분한 5가지 변이 유형[솔체꽃(S. comosa) Type 1-2, 체꽃(S. tschiliensis f. pinnata) Type 3-4, 구름체꽃(S. tschiliensis f. alpina) Type 5]에 대해 잎 표피와 화분의 미세구조를 관찰하였다. 잎 표피세포의 크기는 향축면은 평균 12.4 × 6.7 ㎛로, Type 1이 가장 작고 Type 3가 가장 컸으며, 배축면은 평균 9.4 × 4.6 ㎛로 모든 유형이 비슷하게 나타났다. 모용은 단세포성 선상 모용이 일부 유형에서, 다세포성 비선상 모용이 모든 유형에서 관찰되었다. 선상 모용은 향축면에서 Type 1과 5에만 있었으며, 두 유형 중 Type 5의 모용이 길이 72.2 ㎛로 더 길고, 단위면적 (0.01 mm²) 당 개수도 5개로 더 많았다. 배축면에서는 Type 1, 3 및 5에 있고, 단위면적 당 개수가 Type 1과 5는 3~6개로 비슷하였으나, Type 3은 1개 이하로 매우 드물게 나타났다. 기공복합체는 부등형으로 양면에 분포하며, 향축면의 공변세포 크기는 평균 4.4 × 2.7 ㎛로 너비는 유사하나 길이는 Type 1과 5가 3.1 ㎛, 3.9 ㎛로 나머지에 비해 작았다. 단위면적 당 기공의 수는 향축면과 배축면이 평균 40.2개, 149.2개이고, 양면 모두 Type 1이 66.3개와 177.2개로 가장 많았다. 화분은 모두 단립으로 삼구형이고, 극축 길이(P)와 적도면 지름(E)의 비율은 0.83~1.12로 모양은 아단구형~약장구형이다. 표면무늬는 과립상 표면에 자상 돌기가 있고, Type 3과 4에서만 유공상이 확인되었다. 이상의 결과에서, 솔체꽃 유형인 Type 1과 2가 잎 표면의 선상 모용 유무 및 기공의 수에 뚜렷한 차이를 보여 Type 1은 솔체꽃과는 다른 새로운 변종 또는 품종일 가능성이 있다고 생각된다. 또한 체꽃 유형인 Type 3과 4의 경우, Type 3의 잎 배축면에 털이 드물게 있으나, 두 유형의 화분 표면무늬가 동일하고, 다른 형질들도 모두 중복되어 연속적인 변이로 보이며, 구름체꽃 유형인 Type 5는 나머지 유형과 뚜렷이 구분되었다.