Papers by Author: Jong Keuk Lee

Abstract: In this study, joining characteristics of dissimilar Ti and Cu metals have been investigated, when using both an Ag-based eutectic alloy as a filler and an Ag layer present on Ti base metal as a diffusion barrier. The observed microstructures were classified into three characteristic types, depending on the presence of a Ag layer at the Ti interface, e.g. first, the sample retaining thick continuous intermetallic layers, e.g. Ti2Cu, TiCu, Ti3Cu4, Ti2Cu3, and TiCu4 by a significant dissolution of the Ti atoms into the molten filler, in the absence of a Ag coating layer onto the Ti base metal, second, the sample with relatively thin Ti-Cu intermetallic layers by the reduced reaction of Ti with Cu due to a prominent decrease in the Ti dissolution, owing to the role of the Ag coating layer as a diffusion barrier, and finally, the sample without any brittle Ti-Cu intermetallics in the joint by a complete suppression of both the dissolution of the Ti atoms and its reaction with the Cu elements in the molten filler due to the presence of a Ag layer.

Abstract: The microstructure and mechanical properties of Ti joints brazed with a Zr41.2Ti13.8Ni10.0Cu12.5Be22.5 (at.%) amorphous filler were investigated. With a Zr-based amorphous filler, in this study, Ti joints with a homogeneous composition could be obtained by heating to well below the α-β transformation temperature for a short time, so that the undesirable effects of the high temperature heating are considerably diminished. The joints brazed at 790 °C for 10 min consisted of the coarse acicular structure rather than the fine Widmanstätten structure which generally deteriorates the ductility of the joints. The joints with the homogenous coarse acicular structure, i.e. without a residual liquid region, show almost the same mechanical properties as those required for base metals without heating. Although the residual liquid region in the joints deteriorates the ductility of the joints, this region could be successfully removed by a diminution in the quantity of the filler.

Abstract: In this study the effects of harder ultrafine Al2O3 particles on the mechanical milling of ductile Cu flakes (200mesh, 70μm) have been investigated. The small hard Al2O3 particle in the mixture acted as an effective milling agent not only by inducing a drastic change of the Cu morphology from flattened flakes to equiaxed crystals but also by reducing the milling time required for a uniform dispersion of Al2O3 in the Cu matrix. This was more pronounced as the Al2O3 concentration increased. A critical concentration of the reinforcing Al2O3 required for a shape change was observed at the range of 8 to 9wt. %. All the hard Al2O3 particles were uniformly embedded in the ductile Cu matrix regardless of the concentration of Al2O3. By increasing the milling time, the weldment and fragmentation of the Cu flakes became significant and a uniformly distributed equiaxed composite structure (13wt.% Al2O3) was obtained at above 70 min..

Abstract: Highly dense Y2O3 ceramics have been fabricated by a magnetic pulsed compaction (MPC) which is capable of reaching a sufficiently high pressure (~1GPa) in a very short duration (a few microseconds), and a subsequent pressureless sintering at 1600°C. The Y2O3 green bodies with a relative density of about 68% were achieved by the application of the MPC process due to the effect of an enhanced rearrangement and a high speed movement of the particles, without the help of ceramic binder. Those compacts showed densities greater than 95%, which is very close to the theoretical density, after the subsequent pressureless sintering process at 1600 oC. The shrinkage rates of the diameter for the samples compacted by the MPC process were markedly reduced, when compared to those for the ones by the conventional compaction (CC) process.

Abstract: The effect of MPC pressure on the density, microstructure, mechanical properties, and electrical property of MPCed and sintered bulk was investigated. A detail characterization of the MPCed and sintered bulk has been performed using XRD, SEM, TEM, Vickers hardness tester, and breakdown voltage tester. The alumina powder used in this research has a size of 50-200 nm, a smooth surface and elliptical shape. The obtained density of MPCed and sintered bulk is increased with increasing MPC pressure from 0.5 to 1.25 GPa. The highest density of 92% in this research is obtained in the MPCed at 1.25 GPa and sintered bulk, while it is 90 % in the MPCed at 0.5 GPa. The different Vickers hardness with MPC pressure is associated with the different density and grain size of bulks. The maximum breakdown voltage of 47 kV/cm is achieved in the bulk MPCed at 1.25 GPa due to the higher density than that of others. In addition, the fracture mechanism of MPCed and sintered bulk is discussed.

Abstract: In this study the nanostructured α-Al2O3 ceramics has been fabricated by the combined application of magnetic pulsed compaction (MPC) and spark plasma sintering (SPS), and their density and hardness properties were investigated. The α-Al2O3 prepared by the combined processes showed an increase of 8.4 % in density, approaching a value close to the theoretical density, a enhancement by 210∼400 Hv in hardness, compared to those for the ones by the MPC or static compaction method followed by a sintering treatment. Its grain size was almost equivalent to or slightly higher than the size of the starting Al2O3 powder, suggesting that the grain growth was remarkably reduced during the MPC and SPS processes.