• Nuclear Engineering and Technology
• /
• v.3 no.3
• /
• pp.129-140
• /
• 1971

Two laboratory-melt heats of 3.25 silicon-iron were made and processed according to a normal commercial practice. Some of the important processing variables that were studied in relation to secondary recrystallization and texture development were contents of manganese and sulfur, heat-treatments after hot-rolling that were used to achieve different hot-rolled microstructures, and amounts of second cold-reduction. The main effort of the present study was directed toward elucidating the relationships among the amount of second cold-reduction, activation energies associated with secondary recrystallization and texture development. The specimens that had been cold-reduced 10% exhibited only grain growth by strain-induced grain boundary migration and did not exhibit secondary recrystallization. Secondary recrystallization did rot appear to completely occur in the 30% cold-reduced specimens, although the nucleation for secondary recrystallization was observed. The second cold-reduction in an amount of 50% was shown to be the optimun for secondary recrystallization and texture development by subsequent processing.

• Korean Journal of Materials Research
• /
• v.26 no.12
• /
• pp.688-695
• /
• 2016

Recrystallization behavior has been investigated for commercial purity AA1050 (99.5wt%Al) and high purity 3N Al (99.9wt% Al). Samples were cold rolled with 90% of thickness reduction and were annealed isothermally at 290, 315, and 350o C for various times until complete recrystallization was achieved. Hardness measurement and Electron Backscatter Diffraction(EBSD) analyses, combined with Grain Orientation Spread(GOS), were employed to investigate the recrystallization behavior. EBSD analysis combined with GOS were distinctly revealed to be a more useful method to determine the recrystallization fraction and to characterize the recrystallization kinetics. As the annealing temperature increased, recrystallization in AA1050 accelerated more than that process did in Al 3N. Both AA1050 and Al 3N showed the same temperature dependence of the n value of the Johnson-Mehl-Avrami-Kolmogorov equation(JMAK equation), i.e., n values increased as annealing temperature increased. Activation energy of recrystallization in AA1050 is about 176 kJ/mol, which is comparable with the activation energy of grain boundary migration in cold-rolled AA1050. This value is somewhat higher than the activation energy of recrystallization in Al 3N.

• Journal of Welding and Joining
• /
• v.33 no.5
• /
• pp.26-30
• /
• 2015

Hastelloy X in this study was applied in jet engine F-15 air fighter as shroud to isolate the engine from outer skin. After 15 years operation at elevated temperature the mechanical properties decreased gradually due to the precipitation of continues second phases in the grain boundaries and precipitated inside the grain. The crack happened at the edge of the shroud due to the thermal and mechanical stress from jet engine. Selective TEM analysis found that the grain boundaries consist of $M_{23}C_6$ carbide, $M_6$ Ccarbide and small percentage of sigma(${\sigma}$) phase. Furthermore, it was confirmed the nano size of ${\sigma}$ and miu (${\mu}$) phase inside the grain. In this study, it was investigated the microstructure of the degraded shroud component and HAZ of repair welded shroud. In the HAZ, it was observed the dissolution of the $M_{23}C_6$ carbides and smaller precipitates, the migration of the undissolved larger $M_{23}C_6$ carbide and $M_6$ Ccarbide. It is also observed the liquation due to the simply melt of the segregated precipitates in the grain boundaries. Interestingly, the segregated second phases which simply melt in the grain boundaries more easily happened at higher heat input welding condition. High temperature tensile test was done at $300^{\circ}C$, $700^{\circ}C$ and $900^{\circ}C$. It was obtained that the toughness of welded sample is lower compare to the non-welded sample. The solution heat treatment at $1170^{\circ}C$ for 5 minutes was suggested to obtain a better mechanical properties of the shroud. The high cycle fatigue number of the repair welded shroud shows a much lower compare to the shroud. In addition, the high cycle fatigue number at room temperature after solution heat treatment was almost double compare to the before solution heat treatment under 420-500MPa stress amplitude. However, the high cycle fatigue number of repaired welded sample was shown a much lower compare to the non- welded shroud and solution treated shroud. One of the main reasons to decrease the tensile strength and the high cycle fatigue properties of the repair welded shroud is the formation of the liquid phase in HAZ.

• Transactions of the Korean Society of Pressure Vessels and Piping
• /
• v.14 no.1
• /
• pp.15-23
• /
• 2018

A compact heat exchanger is one of critical components in a very high temperature gas-cooled reactor (VHTR). Alloy 617 (Ni-Cr-Co-Mo) is considered as one of leading candidates for this application due to its excellent thermal stability and strengths in anticipated operating conditions. On the basis of current ASME code requirements, sixty sheets of this alloy are prepared for diffusion welding, which is the key technology to have a reliable compact heat exchanger. Optical microscopic analysis show that there are no cracks, incomplete bond, and porosity at/near the interface of diffusion weldment, but Cr-rich carbides and Al-rich oxides are identified through high resolution electron microscopic analysis. In high-temperature tensile testing, superior yield strengths of the diffusion weldment compared to the code requirement are obtained up to 1223 K ($950^{\circ}C$). However, both tensile strength and ductility drop rapidly at higher temperature due to the insufficient grain boundary migration across the interface of diffusion weldment. Best fit curves for minimum yield strength and average tensile strength are drawn from the experimental tensile results of this study.

• Journal of the Korean Society for Heat Treatment
• /
• v.6 no.1
• /
• pp.9-16
• /
• 1993

To investigate the properties of diffusion bonding of 7050 Al alloy, the diffusion bonding joints have been produced in self-made diffusion bonding hot-press which admits a defined application of the bonding pressure during the heating phase and also rapid cooling after the bonding process with various bonding condition. The strength of the bond increases with increasing the bonding time and temperature. Shear test at toom temperature showed that high strength up to 70% that of parent metal (320 MPa), 220 MPa for the specimen bonded 14 hr at $560^{\circ}C$, with 3 MPa. In this case, however, there is large deformation more than 20% reduction in thickness. The results were correlated with joint characteristics found by optical microstructure and by fractography by SEM. When the strengths of the bonds are more than 50% that of parent metal, a great deal of dimples stretched along the direction of shear stress are observed over the fractured surface of the bond. On the microstructure of the bond line, initial mophology of the bond line disapeared for the grain boundary migration with increasing the bonding time.

• Korean Journal of Materials Research
• /
• v.10 no.11
• /
• pp.725-731
• /
• 2000

Recrystallization behaviors of cold-worked and $\beta$-quenched Zr-Zn alloys were investigated by the microhardness tests and microscopic examinations. The recrystallization of the $\beta$-quenched alloys was retarded in comparison with that of the cold-worked alloys, suggesting that the stored energy of the cold-worked alloys is larger than that of the $\beta$-quenched alloys. Although initial hardness for the cold-worked and the $\beta$-quenched specimens had an equal value, the recrystallization behaviors were observed to be quite different. Based on the transmission electron microscope(TEM) studies, it was suggested that the recrystallization of the cold-worked specimen would have occurred by subgrain coalescence while that of $\beta$-quenched specimen by strain-induced grain boundary migration.

• Journal of the Korean Society of Fisheries and Ocean Technology
• /
• v.31 no.3
• /
• pp.279-286
• /
• 1995

As Al-Li based alloys are to find widespread use in aerospace and other structural applications in which their low-density high specific stiffness properties be exploited, their mechanical properties must adequately match those of the which they intended to replace. In order to develop these purposed, the precipitation behaviors of the rapidly solidified Al-Li-Ti alloys aged at various temperature were investigated. ${\delta}'$ phase precipitated homogeneously in the matrix during not only melt quenching but also aging at the aging temperature of $160^{\circ}C\;and\;210^{\circ}C$. The addition of the Ti in Al-Li alloy promoted the formation of $\delta$' phase. The addition of Ti on the ${\delta}'$ solvus line had a little effect over the thermodynamics ${\delta}'$ solvus line. The reason for these behavior was that the ${\delta}'$ phase was suppressed to precipitate as much as supercooling by melt quenching. The discontinuous precipitation reaction occurred by the preferential growth of ${\delta}'$ phase due to the migration of grain boundary provided the driving force dependent of solute concentration fluctuations.

• The Journal of the Petrological Society of Korea
• /
• v.24 no.2
• /
• pp.125-139
• /
• 2015

Rock structural and microstructural analyses on the deformed Cheongsan granite, which is characterized by abundant feldspar megacrystals, have been carried out to understand the microstructural change during the mylonitization by ductile shear deformation. In K-feldspars, the characteristic microstructures are recognized as microkinks, microfractures, myrmekites, flame perthites, and core-and-mantle structures without the development of subgrains in outer core-zone. Microkinks are observed in both the microfractured and unmicrofractured K-feldspars and the directions of their axes are generally extended across the adjacent K-feldspar fragments bounded by microfractures. Myrmekites and flame perthites are found on the strain-localized boundaries of the microfractured K-feldspars. In plagiclases, microfractures, deformation twins and kink bands are predominant. Grain size reduction of plagioclase megacrysts also occurs by microfracturing but the core-and-mantle structures like the case of K-feldspars are uncommon in the microfractured plagioclases. The deformation twins, which overlap the igneous zoning structures, are often found in less deformed rocks. The twin lamellae in more deformed rocks generally bisect the obtuse angles of conjugate kink-band boundaries, and are microfractured or microfaulted and randomly oriented. From such characteristic microstructures, thus, it can be suggested that the micostructures during the mylonitzation of Cheongsan granite was developed as follows: production of microkinks in the K-feldspar megacrysts and of deformation twins and kink bands in the plagioclase megacrysts, and then grain-size reduction of the feldspar megacrysts through microfracturing, and then production of core-and-mantle structures (grain-size reduction of the microfractured K-feldspars through grain boundary migration), myrmekites and flame perthites in the microfractured K-feldspars.

• Journal of the Microelectronics and Packaging Society
• /
• v.27 no.1
• /
• pp.25-29
• /
• 2020

This study demonstrates direct-patternable amorphous TiO_x resistive switching (RS) device and the fabrication method using photochemical metal-organic deposition (PMOD). For making photosensitive stock solutions, Ti(IV) 2-ethylhexanoate was used as starting precursor. Photochemical reaction by UV exposure was observed and analyzed by Fourier transform infrared spectroscopy and the reaction was completed within 10 minutes. Uniformly formed 20 nm thick amorphous TiO_x film was confirmed by atomic force microscopy. Amorphous TiO_x RS device, formed as 6 × 6 ㎛ square on 4 ㎛ width electrode, showed forming-less RS behavior in ±4 V and on/off ratio ≈ 20 at 0.1 V. This result shows PMOD process could be applied for low temperature processed ReRAM device and/or low cost, flexible memory device.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.39 no.12
• /
• pp.1221-1227
• /
• 2015

Diffusion bonding is the key manufacturing process for the micro-channel type heat exchangers. In this study, austenitic alloys such as Alloy 800HT, Alloy 690, and Alloy 600, were diffusion bonded at various temperatures and the tensile properties were measured up to $650^{\circ}C$. Tensile ductility of diffusion bonded Alloy 800HT was significantly lower than that of base metal at all test temperatures. While, for Alloy 690 and Alloy 600, tensile ductility of diffusion bonded specimens was comparable to that of base metals up to $500^{\circ}C$, above which the ductility became lower. The poor ductility of diffusion bonded specimen could have caused by the incomplete grain boundary migration and precipitates along the bond-line. Application of post-bond heat treatment (PBHT) improved the ductility close to that of base metals up to $550^{\circ}C$. Changes in tensile properties were discussed in view of the microstructure in the diffusion-bonded area.