• Journal of the Korean Ceramic Society
• /
• v.35 no.12
• /
• pp.1294-1300
• /
• 1998

Yttria-stabilized zirconia ceramics with irregular grain shapes and curved grain boundaries was prepared by ceria doping. The amount of ceria doped into zirconia compacts by a dipping method were at range of 2 to 20 mol% Irregular grain shapes and curved grain boundaries were formed only inspecimens doped with more than 8mol% cerial. Ceria-doped specimens showed large grain size and low sintered density compared with pure yttria-stabilized zirconia which was due to the increase in the contents of stabilizer and cubic phase. The amount of doped ceria was larger on the surface than the inside regions and therefore mi-crostructure and phase on the surface were different from those in the inside regions. Transgranular frac-ture mode was observed ion ceria doped specimens due to irregular large grain sizes.

• Korean Journal of Materials Research
• /
• v.27 no.7
• /
• pp.369-373
• /
• 2017

Unidirectionally solidified TiAl alloys were prepared by optically-heated floating zone method at growth rates of 10 to 70 mm/h in flowing argon. The microstructures and tensile properties of these crystal bars were found to depend strongly on the growth rate and alloy composition. TiAl alloys with composition of 47 and 50 at.%Al grown under the condition of 10 mm/h showed $Ti_3Al({\alpha}_2)/TiAl({\gamma})$ layer structures similar to single crystals. As the growth rate increased, the alloys with 47 and 50 at.%Al compositions showed columnar-grain structures. However, the alloys fabricated under the condition of 10 mm/h had a layered structure, but the alloy with increased growth rate consisted of ${\gamma}$ single phase grains. The alloy with a 53 at.%Al composition showed a ${\gamma}$ single phase regardless of the growth rate. Room-temperature tensile tests of these alloys revealed that the columnar-grained material consisting of the layered structure showed a tensile ductility of larger than 4 % and relatively high strength. The high strength is caused by stress concentration at the grain boundaries; this enhances the secondary slip or deformation twinning across the layered structure in the vicinity of the grain boundaries, resulting in the appreciable ductility.

• The Journal of Korean Institute of Communications and Information Sciences
• /
• v.37A no.9
• /
• pp.738-745
• /
• 2012

This paper proposes an approximated soft decision demapping algorithm with low computational complexity for coded 4+12+16 amplitude phase shift keying (APSK) in an additive white Gaussian noise (AWGN) channel. To derive the proposed algorithm, we approximate the decision boundaries for 4+12+16 APSK symbols, and then decide the log likelihood ratio (LLR) value for each bit from the approximated decision boundaries. Although the proposed algorithm shows about 0.6~1.1dB degradation on the error performance compared with the conventional max-log algorithm, it gives a significant result in terms of the computational complexity.

• Korean Journal of Metals and Materials
• /
• v.50 no.7
• /
• pp.511-516
• /
• 2012

Low temperature ${\delta}$-precipitation phenomenon below 2-step aging ($718^{\circ}C$, $8hr+621^{\circ}C$, 8 hr) temperature of cold drawn Inconel 718 alloy wire was investigated. The investigation was carried out on wires with a cold drawn ratio of 0, 20, 50 and 70% using OM, SEM, XRD, TEM, and DSC. In microstructures of 50 and 70% drawn wire, many precipitates were found along the grain boundaries and the twin boundaries in deformation band. From the results of the XRD and TEM analysis, the precipitates were identified as plate-like ${\delta}$-phase. From the results of the DSC analysis, it was also found that a temperature of ${\delta}$-precipitation decreases with an increase of the cold drawn ratio. We concluded that cold drawing of inconel 718 wire promotes the ${\delta}$-precipitation, and under the condition of a high drawing ratio, the ${\delta}$-phase could be precipitated at a temperature below the aging temperature ($718^{\circ}C$).

• Journal of Welding and Joining
• /
• v.15 no.4
• /
• pp.78-89
• /
• 1997

This study has evaluated the weld metal solidification cracking behavior of several Ni base superalloys (Incoloy 825, Inconel 718 and Inconel 600). Austenitic stainless steels(304, 310S) were also included for comparison. In addition, a possible mechanism of solidification cracking in the fusion zone was suggested based on the extensive microstructural examinations with SEM, EDAX, TEM, SADP and AEM. The solidification cracking resistance of Ni base superalloys was found to be far inferior to that of austenitic stainless steels. The solidification cracking of Incoloy 825 and Inconel 718 was believel to be closely related with the Laves-austenite (Ti rich in 825 and Nb rich in 718) and MC-austenite eutectic phases formed along the grain boundaries during solidification. Cracking in Inconel 600 was always found along the grain boundaries which were enriched with Ti and P. Further, solidifidcation cracking resistance was dependent not only upon the type of love melting phases but also on the amount of the phases along the solidification grain boundaries.

• Laser Solutions
• /
• v.7 no.3
• /
• pp.25-36
• /
• 2004

This study was performed to clarification of the formation of weld discontinuities in the dissimilar laser fusion zone. Welding parameters were beam power of 1300, 1430, 1560, and 1700 W and travel speed of 1, 1.3, and 1.7 m/min. Most cavities in the fusion zone were observed near the tip. Cavities in the fusion zone observed to be formed and grown from pores in the tip. More cavities were formed as the beam position moves to the tip side. Small cavities were decreased but large cavities were increased when the energy input increased. W content in the fusion zone was increased with heat input and as the beam position close to the tip. In the fusion zone, W content in the dendrite boundary was increased with heat input. Considering the propagation path and fracture morphology, cracks were solidification cracking, and were initiated and propagated along the dendrite boundaries. The formation of cracks might be related with the W rich ${\mu}$ phase which was formed in the grain boundaries and dendrite boundaries.

• Journal of the Korean Society for Heat Treatment
• /
• v.5 no.2
• /
• pp.85-94
• /
• 1992

Microstructural evolution of wrought Inconel 718 superalloy with different heat treatment conditions was studied. Heat treatment was performed via conventional(CHT), modified(MHT), Merrick(MeHT) and modified Merrick (MMeHT) methods. The size of ${\gamma}^{\prime}$ and ${\gamma}^{\prime\prime}$ precipitates which are principal strengthening phases in Inconel 718 superalloy increase in order of CHT, MHT, MeHT. For the case of MMeHT, a coexistence of fine ${\gamma}^{\prime\prime}$ precipitate and very coarse particles due to exess growth of ${\gamma}^{\prime\prime}$, which is called bimodal distribution, was observed. CHT gave the finest grain size. (Ti, Nb)C carbide and needle-like ${\delta}$ phase were formed together at grain boundaries for CHT, and were formed both inside and at boundaries of grains for MHT, MeHT and MMeHT. Morphology of partially serrated grain boundaries was developed in all heat treatment conditions except CHT.

• Proceedings of the Korea Association of Crystal Growth Conference
• /
• 2000.06a
• /
• pp.243-248
• /
• 2000

Effect of NiO addition on microstructure, stability of tetragonal phase and mechanical properties was investigated. (Y, Ni)-TZP solid solution was obtained by pressureless sintering. The fracture toughness was increased by solid solution of NiO. Neither reaction phase nor glassy phase was observed at the grain boundaries. From these results, it was determined that solid solution of NiO was destabilized tetragonal phase of Y-TZP. Y-TZP/Ni nanocomposite that contained nano-sized Ni particles was also fabricated by internal reduction method. Some evaluations and discussions were carried out for both (Y, Ni)-TZP solid solution and Y-TZP/Ni nanocomposite.

• Applied Microscopy
• /
• v.43 no.1
• /
• pp.40-45
• /
• 2013

Phase transformation by the Mn-deprivation was observed in the high-Mn twinning-induced plasticity-aided steel. Mn-depletion was induced by the formation of Mn-O oxide during the dew-point control process at temperature above $-20^{\circ}C$, which changed austenitic parent phase to multi-grained ferrite. Mixture of Al-O, Al-Mn-Si-O oxides were observed at the grain boundaries of transformed ferrite.

• Transactions of Materials Processing
• /
• v.16 no.6
• /
• pp.473-478
• /
• 2007

In this study, a deformation mechanism map of metallic nanocrystalline materials(NCMs) using the phase mixture model is proposed. It is based on recent modeling that appears to provide a conclusive description of the phenomenology and the mechanisms underlying the mechanical properties of NCMs. The proposed models adopted the concept of a 'phase mixture' in which the grain interior and the grain boundaries are treated as separate phases. The volume fraction of this grain boundary 'phase' may be quite appreciable in a NCM. Based on the theoretical model that provides an adequate description of the grain size dependence of plasticity covering all grain size range from coarse down to the nanoscale, the tensile deformation response of NCMs, especially focusing on the deformation mechanisms was investigated. The deformation mechanism map is newly proposed with axes of strain rate, grain size and temperature.