Papers by Author: Kap Ho Lee

Abstract: Stress-strain responses and microstructure of multi-phase CoCrCuMnNi and CoCrMnFeCu alloys in which Fe or Ni was replaced by Cu from Cantor alloy were studied. The deformation mechanisms of CoCrCuMnNi and CoCrMnFeCu were observed to be influenced by the presence of brittle sigma phase and the separated Cu-rich and the matrix phase. CoCrCuMnNi exhibited the relatively lower strength and excellent deformability, while CrMnFeCoCu alloy exhibited higher strength and lower ductility. The higher strength and the lower ductility of CoCrCuMnNi is associated with the presence more frequent and coarser sigma phase than those in CoCrCuMnNi.

Abstract: In this study, creep properties and fracture behavior of CrMnFeCoNi high entropy alloy (HEA) were studied at intermediate temperatures. The invert-type transient primary creep behaviors were observed in CrMnFeCoNi high entropy alloy. Creep behaviors of HEA are similar to those of class I solid solution alloys. The transient creep curves upon increase of stress by 5MPa in the steady state creep region did not change much except the sudden strain increase. And, no decrease of creep rate was observed upon increase of stress. Instead, the slightly invert transient creep or almost straight creep curves were observed, supporting the high friction stress. CrMnFeCoNi high entropy alloy has a stress exponent of 3.75 and the creep activation energy was calculated to be 278KJ/mole. The fracture strain increased from 1.3 to 1.6 with the decrease of stress from 96 MPa to 48MPa. The lower stress exponent along with the invert type primary creep curves strongly suggest that the creep of CrMnFeCoNi high entropy alloy at 600°C~650°C occurs by a glide controlled process.

Abstract: Cu-Cr/Al-Fe-Cr/Cu-Cr clad-composite was processed by cold roll-bonding and the effect of interface bonding on the mechanical properties was examined. The clad plates were peeled-off and their separated fracture surface was analyzed. The interface of as-roll-bonded Cu-Cr/Al-Fe-Cr/Cu-Cr hybrid composite was observed to be intact and the interface intermetallics were visible in Cu-Cr/Al-Fe-Cr/Cu-Cr heat-treated at 450°C. The effective area fraction of region with strong mechanical metallic bonding for the as-roll-bonded Cu-Cr/Al-Fe-Cr/Cu-Cr hybrid composite was observed to be lower than that observed in Cu/Al/Cu. The less effective bonding in Cu-Cr/Al-Fe-Cr/Cu-Cr in roll-bonding is thought to be due to the higher strengths of Cu-Cr and Al-Fe-Cr than Cu and Al of commercial purity. The decrease of strength of Cu-Cr/Al-Fe-Cr/Cu-Cr hybrid composite after heat treatment is associated with the softening of Al-Fe-Cr matrix at 450°C, which is far greater than precipitation strengthening of Cu-Cr at 450°C.

Abstract: Hydroxyapatite (HA) nanostructures may be an advanced candidate in biomedical applications for an apatite substitute of bone and teeth than other form of HA. In contrast, well-defined size and shape control in synthesizing HA nanostructures is always difficult. Solvothermal synthesis method was adopted to prepare highly ordered hydroxyapatite nanostructures. The morphological analysis by FESEM confirms that the preparation conditions greatly influences the morphological characterization. X-ray Diffraction (XRD) measurements indicate the formation of crystalline hexagonal hydroxyapatite. The crystillinity of HA decreased with increasing ethyline glycol.

Abstract: Cu and Al6061 alloys were diffusion-bonded at 530°C with Cu-15 wt. % Ag alloy as an intermediate layer and the interface microstructure were analyzed. The presence of Cu₉Al₄, CuAl and CuAl₂ intermetallic layers were confirmed along with Ag₃Mg and AgMg intermetallics. Ag was found to diffuse into Al, forming needle-shaped Ag₂Al precipitates in Al substrate close to Al/Cu-Ag interface region. Micro-Vickers indentation aimed at the interface regions, CuAl/CuAl₂ and CuAl₂/Ag₃Mg-AgMg lamella revealed that cracks were formed mostly around the indenter mark in CuAl₂ layer more than in CuAl and no cracks in the Ag₃Mg-AgMg lamella region, suggesting CuAl₂ is more brittle than CuAl and Ag₃Mg-AgMg lamella is rather ductile. Ag₃Mg-AgMg lamella was found to be formed at the interface between Al and CuAl₂. The formation of ductile Ag₃Mg-AgMg lamella at the Al interface suggests that the diffusion bonding Cu and Al with Cu-Ag or Ag as a intermediate layer may increases the interface reliability of Cu/Al hybrid alloy.