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The effect of electron-beam irradiation temperature on irradiation damage of 12%Cr-15%Mn austenitic steel for structural material of nuclear and/or fusion reactors from the point of view of the reduced activation was investigated by using the 1,250 keV HVEM and an energy dispersed X-ray analyzer(EDX) in a 200 keV FE-TEM with beam diameter of about 0.5 nm. Void formation was not observed in irradiated specimen. The dislocation loop growth was observed and the density and size of dislocation loop were increased with irradiation dose. Irradiation-induced segregations of Cr and Mn at grain boundary were also observed by electron-beam irradiation condition. The amount of Mn segregation was increased with irradiation temperature, however, segregation phenomenon was disappeared in the case of Cr.

keyword : Electron-beam irradiation, Irradiation damage, HVEM, High manganese austenitic steel, Irradiation-induced segregation

This study is concerned with the effects of microstructure on fracture properties of a high-toughness API X70 pipeline steel. Twelve kinds of steel specimens were fabricated by vacuum-induction melting and hot-rolling, and their microstructures were varied by the rolling conditions. Charpy V-notch (CVN) impact test and drop-weight tear test (DWTT) were conducted on the rolled steel specimens in order to analyze low-temperature properties. Charpy impact test results indicated that the energy transition temperature (ETT) was below -100℃ when the finish cooling temperature range was 350~500℃, implying excellent low-temperature toughness. The ETT increased because of the formation of bainitic ferrite and martensite at low finish cooling temperatures and because of the increase in effective grain size due to the formation of coarse ferrites at high finish cooling temperatures. Most of the specimens also showed excellent DWTT properties as the percent shear area well exceeded 85%, irrespective of finish rolling temperatures or finish cooling temperatures, although a large amount of inverse fracture area occurred at some finish cooling temperatures.

keyword : API X70 pipeline steel, Rolling condition, Charpy V-notch, CVN, impact test, Drop-weight tear test, API RP 5L3, Separation

This study is concerned with the effects of microstructure on fracture properties of high-toughness API X70 pipeline steels. Three kinds of steels were fabricated by varying alloying elements such as C, Cu, and Mo, and their microstructures were varied by the rolling conditions such as finish rolling temperature and finish cooling temperature. Charpy V-notch (CVN) impact test and press notch (PN) drop-weight tear test (DWTT) were conducted on the rolled steel specimens in order to analyze low-temperature fracture properties. Charpy impact test results indicated that the specimens rolled at the γ single phase region of the steel having the reduced C and Mo content had high upper shelf energy (USE) and low energy transition temperature (ETT) because of the appropriate formation of quasi-polygonal or polygonal ferrite and the decreased fraction of martensite-austenite constituent. Most of the specimens rolled at γ region also showed excellent DWTT properties as the percent shear area well exceeded 85%, irrespective of finish cooling temperatures, while their USE was higher than that of the specimens rolled at the (α+γ) two phase region. Thus, overall fracture properties of the specimens rolled at rolled at γ region were better than those of the specimens rolled at (α+γ) region, considering both USE and DWTT percent shear area.

keyword : API X70 pipeline steel, Alloying element, Rolling condition, Charpy V-notch, CVN, impact test, Drop-weight tear test

The microstructure and mechanical properties of directional solidified Ti-47Al-3Nb, Ti-47Al-3Nb-0.2Si- 0.1C, Ti-47Al-3Nb-0.3Si-0.2C alloys were investigated. It was found that the lamellar thickness of Si and C doped alloys was fine and uniform, and little or no precipitates were found. Room temperature yield stress increased rapidly with increasing Si and C contents due to fine lamellar distribution and solid solution hardening. However, fracture toughness of type I specimens of the alloys decreased slightly with increasing Si and C contents. It was considered that α2 volume fraction decreased with increasing Si and C contents so that the delamination-type separation was decreased which contributes the high resistance of crack propagation for type Ⅰ specimens. Furthermore, fracture toughness of Ti-47Al-3Nb-0.2Si-0.1C alloy specimens with type Ⅰ orientation, polycrystalline form, and with type Ⅲ orientation increased in order of type Ⅲ, polycrystal and type Ⅰ. These results are discussed and the ability to obtain a respectable combination of strength and toughness through Si and C addition to a TiAl-Nb alloy produced by directional solidification is demonstrated.

keyword : TiA1, Directional solidification, Lamellar microstructure, Fracture toughness, Lamellar orientation

A computational model, which can describe the mechanically-induced martensite transformation of metastable austenite, is suggested. The strain originated from the transformation of metastable austenite into martensite is evaluated by assessing the difference of the nucleation rate of variants, which was determined from the interaction energy between externally applied stress state and lattice deformation based on the K-S relationship. A self-consistent model with the Eshelby`s solution is employed to predict the deformation behavior. A simple iterative calculation was carried out to predict the stress-strain behaviors under the uniaxial stress state. The calculated results are compared with the experimental data for various strain rates. Good agreement is found between the calculated and experimental values of transformation kinetics and stress-strain behavior.

keyword : Mechanically-induced martensite transformation, Metastable austenite, Self-consistent model, K-S relationship

In order to fabricate aluminum matrix composites reinforced by SiC particulates, atmospheric plasma spray forming method was adopted. Aluminum and SiC powders were mechanically mixed for plasma spraying feedstock. The feedstock was deposited into freestanding bulk composite on a graphite substrate. As-sprayed composites had a great amount of porosity, which reduced thermal properties, especially in thermal conductivity. The spray-formed Al-SiCp composites were rolled at the room and elevated temperature and hot isostatic pressed (HIPped) at 500℃ under 100 MPa. By rolling, the amount of porosity decreased and consequently thermal conductivity increased. Although, the amount of porosity hardly decreased by HIP, thermal conductivity increased due to the precipitation of solid solution elements such as Fe and Si. Coefficients of thermal expansion (CTEs) of the composites were affected less significantly by post treatment.

keyword : Composite, Aluminum, SiC, Plasma spray forming, Rolling, Hot isostatic press, CTE, Thermal conductivity

In order to improve the low hardness and low wear resistance of Ti-6Al-4V alloy, plasma carburization and CrN coating were carried out. After composite surface treatment, the carburized layer was about 150 ㎛ in depth and CrN coated layer was about 7.5 ㎛ in thickness, respectively. Based on XRD diffraction patterns, preferential growth along (111) direction was observed. The hardness(Hv) was significantly improved up to about 1,960 after plasma carburized/CrN coating while the hardness value of original alloy was 402 before treatment. This result might be associated with TiC carburized layer and CrN coating formation on the surface of Ti-6Al-4V alloy and such high value of hardness dramatically improve the wear resistance. Through the adhesion test of coating, critical load was as high as 32N as compared with the simply processed CrN coating of a 18N.

keyword : Ti-6Al-4V alloy, Plasma-carburizing, CrN thin film, Hardness, Wear, Adhesion test

The dramatic variations in structure and internal stress in an amorphous silicon carbide film (a-SiC) induced by forming process have been reported extensively. Vapor-depositions of a-SiC film were simulated by molecular dynamics simulation employing the Tersoff potential. To understand effect of substrate temperature that is one of the most important factors involved in the changes in structure and intrinsic stress, the vapor-depositions at various substrate temperatures (500-2500 K) were performed. Then, the MD simulations of cooling to 298 K were carried out. For both cooled and as-deposited a-SiC films, the analyses of structure and internal stress with deposition process parameters entailed the calculation of density, chemical order, and in-plane stress [(σ_(xx)+σ_(yy))/2].

keyword : Amorphous silicon carbide, Molecular dynamics simulation, Substrate temperature, Density, Chemical order, Internal stress

Self-assembled InAs/GaAs quantum dots (QDs) were grown by molecular beam epitaxy and the growth characteristics of QDs were studied using field emission gun-electron transmission microscope. The shapes and optical properties of QDs changed according to spacer layers thickness due to the strain between InAs and GaAs layers. The QDs with 50 nm thick GaAs spacer layers were distributed randomly along the growth direction and changed from dome to flat-pyramidal shape after capping with GaAs. However, QDs with 10 nm thick spacer layers were vertically aligned up to the fifth period and the dome-shape was maintained after capping. The density, distribution and crystalline defects depending on growth conditions were also investigated.

keyword : Quantum dot, InAs/GaAs, Self-assembly, Transmission Electron Microscopy

Solvent extraction experiments with LIX84 have been performed to separate copper from synthetic hydrochloric acid solutions containing copper, nickel and cobalt. Experimental conditions such as solution pH, the concentration of chloride ion and LIX84 and manganese ion, the volume ratio of organic to aqueous phase and pH of stripping solution have been investigated. The optimum conditions for the separation of copper from nickel and cobalt were obtained from the experimental results.

keyword : Copper, Nickel, Cobalt, LIX84, Solvent extraction, Stripping