간행물
한국재료학회지 KCI 등재 SCOPUS Korean Journal of Materials Research
- 발행기관 한국재료학회
- 자료유형 학술지
- 간기 월간
- ISSN 2799-8525 (Print)2799-8681 (Online)
- 수록기간 1991 ~ 2024
- 주제분류 공학 > 재료공학 공학 분류의 다른 간행물
- 십진분류KDC 530DDC 620
권호리스트/논문검색
제21권 제11호 (2011년 11월) 10건
1.
2011.11
구독 인증기관 무료, 개인회원 유료
This study describes a hydrogen embrittlement evaluation of the subsurface zone in 590DP steel by micro-Vickers hardness measurement. The 590DP steel was designed to use in high-strength thin steel sheets as automotive materials. The test specimens were fabricated to 5 series varying the chemical composition through the process of casting and rolling. Electrochemical hydrogen charging was conducted on each specimen with varying current densities and charging times. The relationship between the embrittlement and hydrogen charging conditions was established by investigating the metallography. The micro-Vickers hardness was measured to evaluate the hydrogen embrittlement of the subsurface zone in addition to the microscopic investigation. The micro-Vickers hardness increased with the charging time at the surface. However, the changing ratio and maximum variation of hardness with depth were nearly the same value for each test specimen under the current density of 150 mA/cm2 and charging time of 50 hours. Consequently, it appears that hydrogen embrittlement in 590DP steel can be evaluated by micro-Vickers hardness measurement.
4,000원
2.
2011.11
구독 인증기관 무료, 개인회원 유료
The effect of the precipitator (NaOH, NH4OH) and the amount of the precipitator (150, 200, 250, 300 ml) on the formation of Fe3(PO4)2, which is the precursor used for cathode material LiFePO4 in Li-ion rechargeable batteries was investigated by the co-precipitation method. A pure precursor of olivine LiFePO4 was successfully prepared with coprecipitation from an aqueous solution containing trivalent iron ions. The acid solution was prepared by mixing 150 ml FeSO4(1M) and 100 ml H3PO4(1M). The concentration of the NaOH and NH4OH solution was 1 M. The reaction temperature (25˚C) and reaction time (30 min) were fixed. Nitrogen gas (500 ml/min) was flowed during the reaction to prevent oxidation of Fe2+. Single phase Fe3(PO4)2 was formed when 150, 200, 250 and 300 ml NaOH solutions were added and 150, 200 ml NH4OH solutions were added. However, Fe3(PO4)2 and NH4FePO4 were formed when 250 and 300 ml NH4OH was added. The morphology of the Fe3(PO4)2 changed according to the pH. Plate-like lenticular shaped Fe3(PO4)2 formed in the acidic solution below pH 5 and plate-like rhombus shaped Fe3(PO4)2 formed around pH 9. For the NH4OH, the pH value after 30 min reaction was higher with the same amount of additions of NaOH and NH4OH. It is believed that the formation mechanism of Fe3(PO4)2 is quite different between NaOH and NH4OH. Further investigation on this mechanism is needed. The prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and the pH value was measured by pH-Meter.
4,000원
3.
2011.11
구독 인증기관 무료, 개인회원 유료
It is well known that modified 9Cr-1Mo steel has a low thermal expansion and high thermal conductivity with excellent high temperature properties compared to austenitic stainless steel. For these advantages, the steel is very popular for the boiler tube of thermal power plants. Normalizing is commonly utilized to obtain martensite in this steel, which shows an unusual toughness for martensite. However, some accidents related to this steel have been reported recently, opening the necessity for further study. As a particular behavior of the steel, an abrupt drop of the impact value has been identified upon tempering at 750˚C for about 1 hour. It is well known that Fe3C forms during autotempering and turns to Cr2C at an early stage and then transforms to Cr23C6. In this study, the cause of the abrupt drop of the impact value was investigated with an impact test, microstructural observation, nanodiffraction and phase analyses using instruments such as optical and transmission electron microscopes (TEM) with an extraction carbon replica of the carbides. The analyses revealed that the M2C that formed when retained for about 1 hour at 750˚C causes a drastic decrease in the mechanical properties. The sharp drop in mechanical properties, however, disappeared as the M2C transformed into M23C6 with longer retention.
3,000원
4.
2011.11
구독 인증기관 무료, 개인회원 유료
The very high temperature gas reactor (VHTR) is one of the next generation nuclear reactors for its safety, long-term stability, and proliferation-resistance. The high operating temperature of over 800˚C enables various applications with high energy efficiency. Heat is transferred from the primary helium loop to the secondary helium loop through the intermediate heat exchanger (IHX). The IHX material requires creep resistance, oxidation resistance, and corrosion resistance in a helium environment at high operating temperatures. A Ni-based superalloy such as Alloy 617 is considered as a primary candidate material for the intermediate heat exchanger. In this study, the microstructures of Alloy 617 crept in pure helium and air environments at 950˚C were observed. The rupture time in helium was shorter than that in air under small applied stresses. As the exposure time increased, the thickness of outer oxide layer of the specimens clearly increased but delaminated after a long creep time. The depth of the carbide-depleted zone was rather high in the specimens under high applied stress. The reason was elucidated by the comparison between the ruptured region and grip region of the samples. It is considered that decarburization caused by minor gas impurities in a helium environment caused the reduction in creep rupture time.
4,000원
5.
2011.11
구독 인증기관 무료, 개인회원 유료
The effect of BaF2 flux in Y3Al5O12:Ce3+(YAG:Ce) formation was investigated. Phase transformation ofY3Al5O12(YAG) was characterized by using XRD, SEM, and TEM-EDS, and it was revealed that the sequential formation ofthe Y4Al2O9(YAM), YAlO3(YAP) and Y3Al5O12(YAG) in the temperature range of 1000-1500oC. Single phase of YAG wasrevealed from 1300oC. In order to find out the effect of BaF2 flux, three modeling experiments between starting materials(1.5Al2O3−2.5Y2O3, Y2O3−BaF2, and Al2O3−BaF2) were done. These modeling experiments showed that the nucleationprocess occurs via the dissolution-precipitation mechanism, whereas the grain growth process is controlled via the liquid-phasediffusion route. YAG:Ce phosphor particles prepared using a proposed technique exhibit a spherical shape, high crystallinity,and an emission intensity. According to the experimental results conducted in this investigation, 5% of BaF2 was the bestconcentration for physical, chemical and optical properties of Y3Al5O12:Ce3+(YAG:Ce) that is approximately 10-15% greaterthan that of commercial phosphor powder.
4,000원
6.
2011.11
구독 인증기관 무료, 개인회원 유료
Red-orange phosphors Gd1-xPO4:Eux3+ (x=0, 0.05, 0.10, 0.15, 0.20) were synthesized with changing theconcentration of Eu3+ ions using a solid-state reaction method. The crystal structures, surface morphology, and optical propertiesof the ceramic phosphors were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), andphotoluminescence (PL) spectrophotometry. The XRD results were in accordance with JCPDS (32-0386), and the crystalstructures of all the red-orange phosphors were found to be a monoclinic system. The SEM results showed that the size ofgrains increases and then decreases as the concentration of Eu3+ ionincreases. As for the PL properties, all of the ceramicphosphors, irrespective of Eu3+ ion concentration, had orange and red emissions peaks at 594nm and 613nm, respectively. Themaximum excitation and emission spectra were observed at 0.10mol of Eu3+ ion concentration, just like the grain size. Anorange color stronger than the red means that 5D0→7F1 (magnetic dipole transition) is dominant over the 5D0→7F2 (electricdipole transition), and Eu3+ is located at the center of the inversion symmetry. These properties contrasted with those of a redphosphor Y1-xPO4:Eux3+, which has a tetragonal system. Therefore, we confirm that the crystal structure of the host materialhas a major effect on the resulting color.
4,000원
7.
2011.11
구독 인증기관 무료, 개인회원 유료
The effect of oxygen on the shape memory characteristics in Ti-18Nb-6Zr-XO (X = 0-1.5 at%) biomedical alloys was investigated by tensile tests. The alloys were fabricated by an arc melting method at Ar atmosphere. The ingots were cold-rolled to 0.45 mm with a reduction up to 95% in thickness. After severe cold-rolling, the plate was solution-treated at 1173 K for 1.8 ks. The fracture stress of the solution-treated specimens increased from 450 Mpa to 880 MPa with an increasing oxygen content up to 1.5%. The fracture stress increased by 287MPa with 1 at% increase of oxygen content. The critical stress for slip increased from 430 MPa to 695 MPa with an increasing oxygen content up to 1.5 at%. The maximum recovery strain of 4.1% was obtained in the Ti-18Nb-6Zr-0.5O (at%) alloy. The martensitic transformation temperature decreased by 140 K with a 1.0 at% increase in O content, which is lower than that of Ti-22Nb-(0-2.0)O (at%) by 20 K. This may have been caused by the effect of the addition of Zr. This study confirmed that addition of oxygen to the Ti-Nb-Zr alloy increases the critical stress for slip due to solid solution hardening without being detrimental to the maximum recovery strain.
4,000원
8.
2011.11
구독 인증기관 무료, 개인회원 유료
This is a study on the distribution of acoustic emission parameters during a burst test for a type-II CNG vehicle fuel tank. A resonant AE sensor with a central frequency of 150 kHz was attached to the composite materials in the center of the fuel tank. The pressure was increased from 30 to 100% of the expected burst pressure and was maintained for 10 minutes at each level. Damage at 70% of expected burst pressure occurred by various damage mechanisms including fiber breakage and delamination, while that of below 60% only occurred by matrix crack initiation and growth. The count, duration and rise time of the AE signal at 60% of the expected burst pressure are distributed below 500, 5000 μs and 300 μs, respectively. Then, at above 70% they increased with pressure by superimposing of individual AE signal generated at a nearby place. These results confirmed that the analysis of the distribution of AE parameters is an effective tool for estimating damage of a CNG fuel tank.
4,000원
9.
2011.11
구독 인증기관 무료, 개인회원 유료
Studies on lead-free piezoelectrics have been attractive as means of meeting environmental requirements. We synthesized lead-free piezoelectric (Bi1/2Na1/2)TiO3-Ba(Cu1/3Nb2/3)O3 (BNT-BCN) ceramics, and their dielectric, piezoelectric, and strain behavior were characterized. As BCN with a tetragonal phase was incorporated into the rhombohedral BNT lattice, the lattice constant increased. A small amount of BCN increased the density and dielectric constant forming the complete solid solution with BNT. However, BCN above 10 mol% was precipitated into a separate phase, and which was detected with XRD. In addition, EDX measurement revealed that Cu in BCN was not distributed homogeneously but was accumulated in a certain area. A lower density with a large amount of BCN was attributed to the nonsinterable property of BCN with large tetragonaliy. The dielectric constant vs the temperature change and the strain vs the electric field indicated that the ferroelectric property of BNT was diminished and paraelectric behavior was enhanced with the BCN addition. BNT-7.5BCN showed a 0.11% unimorph strain with a 9.0 kV/mm electric field with little hysteresis.
4,000원
10.
2011.11
구독 인증기관 무료, 개인회원 유료
We report growth of epitaxial AlN thin films on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy. To achieve two-dimensional growth the substrates were nitrided by nitrogen plasma prior to the AlN growth, which resulted in the formation of a two-dimensional single crystalline AlN layer. The formation of the two-dimensional AlN layer by the nitridation process was confirmed by the observation of streaky reflection high energy electron diffraction (RHEED) patterns. The growth of AlN thin films was performed on the nitrided AlN layer by changing the Al beam flux with the fixed nitrogen flux at 860˚C. The growth mode of AlN films was also affected by the beam flux. By increasing the Al beam flux, two-dimensional growth of AlN films was favored, and a very flat surface with a root mean square roughness of 0.196 nm (for the 2 μm × 2 μm area) was obtained. Interestingly, additional diffraction lines were observed for the two-dimensionally grown AlN films, which were probably caused by the Al adlayer, which was similar to a report of Ga adlayer in the two-dimensional growth of GaN. Al droplets were observed in the sample grown with a higher Al beam flux after cooling to room temperature, which resulted from the excessive Al flux.
4,000원
