Effect of heat treatment on microstructure and mechanical properties of an Fe-6.5Mn-0.08C medium-manganese steel is investigated in this study. Three kinds of medium-manganese steel specimens are fabricated by varying heat treatments of intermediate quenching (IQ), step quenching (SQ), and intercritical annealing (IA). Hardness and tensile tests are performed to examine the correlation of microstructure and mechanical properties for the Fe-6.5Mn-0.08C medium-manganese steel specimens. The IQ and SQ specimens have microstructures of martensite matrix with ferrite, whereas IA specimen exhibits microstructure of acicular ferrite matrix with martensite. The tensile test results show that the SQ specimen with martensite matrix has the highest yield strength and the lowest elongation. On the other hand, the SQ specimen has the highest hardness due to the relatively lower reduction of carbon content in martensite during intercritical annealing. According to the fractography of tensile tested specimens, the SQ specimen exhibits a dimple and quasi-cleavage fracture appearance while the IQ and IA specimens have fully ductile fracture appearance with fine-sized dimples caused by microvoid coalescence at ferrite and martensite interface.

이소시아네이트와 아민의 빠른 반응성으로 인해 고가의 전용 도포 설비가 있어야만 시공이 가능하였던 우레아 수지를, 일반적인 도포 설비로도 시공이 가능하도록 수분산 상태의 우레아 수지를 합성하기 위한 조건을 설정하기 위하여, 1차적으로 분자량과 관능기의 수가 다른 폴리이서 아민과 다이이 소시아네이트를 사용하여 기계적 물성이 우수한 수분산 우레아 수지 합성 조건을 찾고, 2차적으로 DMPA [2,2-Bis(hydroxymethyl)propanoic acid]의 함량이 수분산 우레아 수지의 수분산 안정성에 미치는 영향을 연구하였다. 우레아기의 형성은 FT-IR ATR 분광법으로 확인하였다. 그 결과 수분산 우레아 수지의 기계적 물성은 분자량과 관능기수가 다른 폴리이서 디아민과 폴리이서 트리아민을 함께 투입한 PU-4와 PU-6 배합의 경우, 각각 인장강도가 10.5 N/㎟, 12.7 N/㎟ 및 신장률이 1165 %, 969 %로 우수하였다. 또한, 인장강도가 가장 높은 PU-6 배합에 DMPA을 0.5 몰 첨가하여 합성한 후 트리에틸 아민으로 중화시킨 수분산 우레아 수지가 14.2 N/㎟의 인장강도와 993 %의 신율로 가장 우수하였으며, 8 주 동안의 저장안전성 평가에서도 수분산 상태가 가장 안정하게 유지되었다.

In the development of advanced ceramic tools, material improvements and design freedom are critical in improving tool performance. However, in the die press molding method, many factors limit tool design and make it difficult to develop innovative advanced tools. Ceramic 3D printing facilitates the production of prototype samples for advanced tool development and the creation of complex tooling products. Furthermore, it is possible to respond to mass production requirements by reflecting the needs of the tool industry, which can be characterized by small quantities of various products. However, many problems remain in ensuring the reliability of ceramic tools for industrial use. In this study, alumina inserts, a representative ceramic tool, was manufactured using the digital light process (DLP), a 3D printing method. Alumina inserts prepared by 3D printing are pressurelessly sintered under the same conditions as coupon-type specimens prepared by press molding. After sintering, a hot isostatic pressing (HIP) treatment is performed to investigate the effects of relative density and microstructure changes on hardness and fracture toughness. Alumina inserts prepared by 3D printing show lower relative densities than coupon specimens prepared by powder molding but indicate similar hardness and higher fracture toughness values.

Commercial ultra-high-strength PAN-based carbon fibers (T1000G) were heat-treated at the temperature range of 2300– 2600 °C under a constant stretching of 600 cN. After continuous high-temperature graphitization treatment, microstructures, mechanical properties and thermal stability of the carbon fibers were investigated. The results show that the T1000G carbon fibers present the similar round shape with a smooth surface before and after graphitization, indicating the carbon fibers are fabricated by dry–wet spinning. In comparison, the commercial high-strength and high-modulus PAN-based carbon fibers (M40JB and M55JB) present elliptical shapes with ridges and grooves on the surface, indicating the carbon fibers are fabricated by wet spinning. After graphitization treatment from 2300 to 2600 °C under a constant stretching of 600 cN, the Young’s modulus of the T1000G carbon fibers increases from about 436 to 484 GPa, and their tensile strength decreases from about 5.26 to 4.45 GPa. The increase in Young’s modulus of the graphitized T1000G carbon fibers is attributed to the increase in the crystallite sizes and the preferred orientation of graphite crystallites along the fiber longitudinal direction under a constant stretching condition. In comparison with the M40JB and the M55JB carbon fibers, the graphitized T1000G carbon fibers are easier to be oxidized, which can be contributed to the formation of more micropores and defects during the graphitization process, thus leading to the decrease in the tensile strength.

Aluminum nitride (AlN) has excellent electrical insulation property, high thermal conductivity, and a low thermal expansion coefficient; therefore, it is widely used as a heat sink, heat-conductive filler, and heat dissipation substrate. However, it is well known that the AlN-based materials have disadvantages such as low sinterability and poor mechanical properties. In this study, the effects of addition of various amounts (1-6 wt.%) of sintering additives Y2O3 and Sm2O3 on the thermal and mechanical properties of AlN samples pressureless sintered at 1850oC in an N2 atmosphere for a holding time of 2 h are examined. All AlN samples exhibit relative densities of more than 97%. It showed that the higher thermal conductivity as the Y2O3 content increased than the Sm2O3 additive, whereas all AlN samples exhibited higher mechanical properties as Sm2O3 content increased. The formation of secondary phases by reaction of Y2O3, Sm2O3 with oxygen from AlN lattice influenced the thermal and mechanical properties of AlN samples due to the reaction of the oxygen contents in AlN lattice.

Injection molding is extensively used for mass production of plastic products. Over the years, the plastic products have been manufactured in a variety of colors, materials and mechanical properties to fulfill the market demand. The purpose of this study is to identify the relation between the color of resin and the product quality. To proceed this study, different colored PBT specimens have injection molded, and mechanical properties were compared. Tensile tests and bending tests have carried out to study mechanical properties of the specimens, and differentials have occurred in tensile strength, bending strength and tensile elongation by their respective color. And the red specimens were broken during the bending test. The experimental results reveal that the color of the resin influences the mechanical strength of the injection molded product. As a result, the color of resin should be considered when setting parameters for injection molding in order to improve the product quality.

The objective of this study is to investigate the influence of powder shape and densification mechanism on the microstructure and mechanical properties of Ti-6Al-4V components. BE powders are uniaxially and isostatically pressed, and PA ones are injection molded because of their high strengths. The isostatically compacted samples exhibit a density of 80%, which is higher than those of other samples, because hydrostatic compression can lead to higher strain hardening. Owing to the higher green density, the density of BE-CS (97%) is found to be as high as that of other samples (BE-DS (95%) and P-S (94%)). Furthermore, we have found that BE powders can be consolidated by sintering densification and chemical homogenization, whereas PA ones can be consolidated only by simple densification. After sintering, BE-CS and P-S are hot isostatically pressed and BE-DS is hot forged to remove residual pores in the sintered samples. Apparent microstructural evolution is not observed in BE-CSH and P-SH. Moreover, BE-DSF exhibits significantly fine grains and high density of low-angle grain boundaries. Thus, these microstructures provide Ti-6Al-4V components with enhanced mechanical properties (tensile strength of 1179 MPa).

TiO2-particles containing Co grains are fabricated via thermal hydrogenation and selective oxidation of Ti- Co alloy. For comparison, TiO2-Co composite powders are prepared by two kinds of methods which were the mechanical carbonization and oxidation process, and the conventional mixing process. The microstructural characteristics of the prepared composites are analyzed by X-ray diffraction, field-emission scattering electron microscopy, and transmission electron microscopy. In addition, the composite powders are sintered at 800℃ by spark plasma sintering. The flexural strength and fracture toughness of the sintered samples prepared by thermal hydrogenation and mechanical carbonization are found to be higher than those of the samples prepared by the conventional mixing process. Moreover, the microstructures of sintered samples prepared by thermal hydrogenation and mechanical carbonization processes are found to be similar. The difference in the mechanical properties of sintered samples prepared by thermal hydrogenation and mechanical carbonization processes is attributed to the different sizes of metallic Co particles in the samples.

Whisker-type magnesium hydroxide sulfate hydrate (5Mg(OH)2·MgSO4·3H2O, abbreviated 513 MHSH), is used in filler and flame-retardant composites based on its hydrate phase and its ability to undergo endothermic dehydration in fire conditions, respectively. In general, the length of whiskers is determined according to various synthetic conditions in a hydrothermal reaction with high temperature (~180oC). In this work, high-quality 513 MHSH whiskers are synthesized by controlling the concentration of the raw material in ambient conditions without high pressure. Particularly, the concentration of the starting material is closely related to the length, width, and purity of MHSH. In addition, a ceramic-coating system is adopted to enhance the mechanical properties and thermal stability of the MHSH whiskers. The physical properties of the silica-coated MHSH are characterized by an abrasion test, thermogravimetric analysis, and transmission electron microscopy.

Bamboo forests are fast-growing, renewable resources, and their carbon sequestration potential has attracted increasing attention. Although bamboo can be used for many purposes, bamboo forests in Korea represent a generally underutilized resource. The main objective here was to perform an assessment of the physical and mechanical characteristics of different species of bamboo found in Korea. The main species of domestic bamboo are Phyllostachys bambusoides, P. pubescens, and P. nigra; we measured the air-dried density for each of the species, with obtained values of 0.89 g/cm2, 0.79 g/cm2, and 0.83 g/cm2, respectively, giving the density order of P. bambusoides > P. pubescens > P. nigra, with P. bambusoides having the highest density. We then measured the compressive strength of each species, which were 802.84 kgf/cm2, 624.69 kgf/cm2, and 743.77 kgf/cm2, respectively, in the order of P. bambusoides > P. pubescens > P. nigra, with P. bambusoides having the highest compressive strength. Volume and maximum load decreased with increasing node height in the three bamboo species, whereas air-dried density and compressive strength increased. Our results thus add to the pool of essential knowledge about Korean bamboo species, and consequently to the development of a potentially valuable domestic resource in Korea.

A cold roll-bonding process is applied to fabricate an AA6061/AA5052/AA6061 three-layer clad sheet. Two AA6061 and one AA5052 sheets of 2 mm thickness, 40 mm width, and 300 mm length are stacked, with the AA5052 sheet located in the center. After surface treatment such as degreasing and wire brushing, sample is reduced to a thickness of 1.5 mm by multi-pass cold rolling. The rolling is performed at ambient temperature without lubricant using a 2-high mill with a roll diameter of 400 mm at rolling speed of 6.0 m/sec. The roll bonded AA6061/AA5052/AA6061 complex sheet is then hardened by natural aging(T4) and artificial aging(T6) treatments. The microstructures of the as-roll bonded and age-hardened Al complex sheets are revealed by optical microscopy; the mechanical properties are investigated by tensile testing and hardness testing. After rolling, the roll-bonded AA6061/AA5052/AA6061 sheets show a typical deformation structure in which grains are elongated in the rolling direction. However, after T4 and T6 aging treatment, there is a recrystallization structure consisting of coarse equiaxed grains in both AA5052 and AA6061 sheets. The as roll-bonded specimen shows a sandwich structure in which an AA5052 sheet is inserted into two AA6061 sheets with higher hardness. However, after T4 and T6 aging treatment, there is a different sandwich structure in which the hardness of the upper and lower layers of the AA6061 sheets is higher than that of the center of the AA5052 sheet. The strength values of the T4 and T6 age-treated specimens are found to increase by 1.3 and 1.4 times, respectively, compared to that value of the starting material.

This study deals with the microstructure and tensile properties of 600 MPa-grade seismic reinforced steel bars fabricated by a pilot plant. The steel bar specimens are composed of a fully ferrite-pearlite structure because they were air-cooled after hot-rolling. The volume fraction and interlamellar spacing of the pearlite and the ferrite grain size decrease from the center region to the surface region because the surface region is more rapidly cooled than the center region. The A steel bar specimenwith a relatively high carbon content generally has a higher pearlite volume fraction and interlamellar spacing of pearlite and a finer ferrite grain size because increasing the carbon content promotes the formation of pearlite. As a result, the A steel bar specimen has a higher hardness than the B steel bar in all the regions. The hardness shows a tendency to decrease from the center region to the surface region due to the decreased pearlite volume fraction. On the other hand, the tensile-to-yield strength ratio and the tensile strength of the A steel bar specimen are higher than those of the B steel bar with a relatively low carbon content because a higher pearlite volume fraction enhances work hardening. In addition, the B steel bar specimen has higher uniform and total elongations because a lower pearlite volume fraction facilitates plastic deformation caused by dislocation slip.

Pitch precursors affording excellent spinnability, high-level oxidation-resistance, and good carbonization yields were prepared by bromination–dehydrobromination of various ratios of pyrolyzed fuel oil and coal tar pitch. The pitches exhibited spinnabilities that were much better than those of pitches prepared via simple distillation. A pitch prepared using a 1:2 ratio of fuel oil and coal tar pitch exhibited the best tensile strength. Pitch fibers of diameter 8.9 ± 0.1 μm were stabilized at 270 °C without soaking time after heating at a rate of 0.5 °C/min and carbonized at 1100 °C for 1 h after heating at 5 °C/min. The resulting carbon fibers exhibited a tensile strength, elongation, Young’s modulus, and average diameter of 1700 ± 170 MPa, 1.6 ± 0.1%, 106 ± 37 GPa, and 7.1 ± 0.2 μm, respectively.

한국원자력연구원 부지 내에 위치한 지하처분연구시설(KAERI Underground Research Tunnel, KURT) 에서는 선진핵주기 고준위폐기물처분시스템(A-KRS)을 기반으로 고준위방사성폐기물을 처분하였을때, 예상되는 공학적방벽(Engineered Barrier System, EBS)과 자연방벽(Natural Barrier System, NBS)에서의 열-수리-역학적 복합거동(Thermo-Hydro-Mechanical coupled behavior)의 특성을 규명하고자 현장시험(In-situ Demonstration of Engineered Barrier System, In-DEBS)을 2012년부터 계획 및 설계를 시작하여, 2016년 5월부터 지하처분연구시설 3번 연구 갤러리(Research gallery 3)에서 진행하고 있다. 현장시험의 데이터를 분석하고 열-수리-역학적 복합거동 특성을 명확히 규명하기 위해서는 경주 벤토나이트와 KURT 암석 및 암반의 열적, 수리적, 그리고 역학적 물성 특성을 반드시 파악하고 있어야만 한다. 이에 본 연구에서는 지금까지 수행된 KURT 부지 특성과 KURT 화강암 및 경주 벤토나이트의 열적, 수리적, 그리고 역학적 특성을 정리하고, 열적, 수리적, 그리고 역학적 모델을 제시하였다. 특히, 온도에 따른 암석의 열팽창계수 변화, 응력에 따른 암석의 투수계수 변화, 포화도에 따른 벤토나이트 및 암석의 열전도도 변화, 포화도에 따른 벤토나이트의 비열 및 흡입력 변화와 같은 열-수리-역학적 복합물성에 대한 다양한 모델을 도출함으로써, In-DEBS 현장시험 결과 분석과 열-수리-역학적 복합거동 특성 평가를 위해 수행 될 수치시험에 필요한 벤토나이트와 암석 및 암반의 입력자료를 제시하고자 하였다.

Oxide-dispersion-strengthened (ODS) alloy has been developed to increase the mechanical strength of metallic materials; such an improvement can be realized by distributing fine oxide particles within the material matrix. In this study, the ODS layer was formed in the surface region of Zr-based alloy tubes by laser beam treatment. Two kinds of Zr-based alloys with different alloying elements and microstructures were used: KNF-M (recrystallized) and HANA-6 (partial recrystallized). To form the ODS layer, Y2O3-coated tubes were scanned by a laser beam, which induced penetration of Y2O3 particles into the substrates. The thickness of the ODS layer varied from 20 to 55 μm depending on the laser beam conditions. A heat affected zone developed below the ODS layer; its thickness was larger in the KNF-M alloy than in the HANA-6 alloy. The ring tensile strengths of the KNF-M and HANA-6 alloy samples increased more than two times and 20–50%, respectively. This procedure was effective to increase the strength while maintaining the ductility in the case of the HANA-6 alloy samples; however, an abrupt brittle facture was observed in the KNF-M alloy samples. It is considered that the initial microstructure of the materials affects the formation of ODS and the mechanical behavior.

This study examines the role of the nano- and micro-particle ratio in dispersion stability and mechanical properties of composite resins for SLA(stereolithography) 3D printing technology. VTES(vinyltriethoxysilane)-coated ZrO2 ceramic particles with different nano- and micro-particle ratios are prepared by a hydrolysis and condensation reaction and then dispersed in commercial photopolymer (High-temp) based on interpenetrating networks(IPNs). The coating characteristics of VTES-coated ZrO2 particles are observed by FE-TEM and FT-IR. The rheological properties of VTEScoated ZrO2/High-temp composite solution with different particle ratios are investigated by rheometer, and the dispersion properties of the composite solution are confirmed by relaxation NMR and Turbiscan. The mechanical properties of 3Dprinted objects are measured by a tensile test and nanoindenter. To investigate the aggregation and dispersion properties of VTES-coated ZrO2 ceramic particles with different particle ratios, we observe the cross-sectional images of 3D printed objects using FE-SEM. The 3D printed objects of the composite solution with nano-particles of 80 % demonstrate improved mechanical characteristics.

In this study, three kinds of bainitic steel plates are manufactured by varying the chemical compositions and their microstructures are analyzed. Tensile and Charpy impact tests are performed at room and low temperature to investigate the correlation between microstructure and mechanical properties. In addition, heat affected zone (HAZ) specimens are fabricated by a simulation of welding processes, and the HAZ microstructure is analyzed. The base steel that has the lowest carbon equivalent has the highest volume fraction of acicular ferrite and the lowest volume fraction of secondary phases, so the strength is the lowest and the elongation is the highest. The Mo steel has a higher volume fraction of granular bainite and more secondary phases than the base steel, so the strength is high and the elongation is low. The CrNi steel has the highest volume fraction of the secondary phases, so the strength is the highest and elongation is the lowest. The tensile properties of the steels, namely, strength and elongation, have a linear correlation with the volume fraction of secondary phases. The Mo steel has the lowest Charpy impact energy at -80 oC because of coarse granular bainite. In the Base-HAZ and Mo-HAZ specimens, the hardness increases as the volume fraction of martensite-austenite constituents increases. In the CrNi-HAZ specimen, however, hardness increases as the volume fraction of martensite and bainitic ferrite increases.

The microstructure, phase, and mechanical properties of three aged porcelain insulators which were manufactured in different years (1973, 1995 and 2008) and which were used in the field for different amounts of time, were investigated. With X-ray 3D computed tomography (CT), defects with ~mm size can be detected without destroying the aged insulators. Defects of small specimens, which are cut from the aged insulators and polished, are analyzed with optical and scanning electron microscopy (OM and SEM), and defects of um size are detected by OM and SEM. The number and size of defects in all the aged insulators are similar. Porcelain insulators manufactured in 1973 contain more SiO2 (quartz and cristobalite) than those manufactured in 2008. Those manufactured in 2008 contain more Al2O3 than those manufactured earlier. The Vickers hardness of the insulator manufactured in 1973 has the lowest value. The formation of the cristobalite (SiO2) in the insulator manufactured in 1973 which can come from the phase transformation of quartz can cause stress in the insulator by formation of microcracks, which can lead to the low hardness of the insulator.

In the current steel structures of high-rise buildings, high heat input welding techniques are used to improve productivity in the construction industry. Under the high heat input welding, however, the microstructures of the weld metal (WM) and heat-affected zone (HAZ) coarsen, resulting in the deterioration of impact toughness. This study focuses mainly on the effects of fine TiN precipitates dispersed in steel plates and B addition in welding materials on grain refinement of the HAZ microstructure under submerged arc welding (SAW) with a high heat input of 200 kJ/cm. The study reveals that, different from that in conventional steel, the γ grain coarsening is notably retarded in the coarse grain HAZ (CGHAZ) of a newly developed steel with TiN precipitates below 70 nm in size even under the high heat input welding, and the refinement of HAZ microstructure is confirmed to have improved impact toughness. Furthermore, energy dispersive spectroscopy (EDS) and secondary-ion mass spectrometry (SIMS) analyses demonstrate that B is was identified at the interface of TiN in CGHAZ. It is likely that B atoms in the WM are diffused to CGHAZ and are segregated at the outer part of undissolved TiN, which contributes partly to a further grain refinement, and consequently, improved mechanical properties are achieved.