Hyun Kwang Seok

Controlling the textures of the metal strips via the continuous confined strip shearing(C2S2) process

Abstract A new concept process, a so-called as C2S2 process based on equal channel angular pressing, for fabricating the A1 alloy sheets with a high formability is introduced. The C2S2 machine was designed so that it can feed the metal strip in a continuous manner at a relatively high speed in the range of ∼20 m/min. A significant amount of the shear deformation could be achieved by passing the sample through the ECAP die with Φ = 120°. The pole figures were calculated both to demonstrate the evolution of the textures of the 1050 A1 alloy sheets and to elucidate the feasibility of the proposed process as a means for enhancing the formability of the metal strips.

Texture evolution of the strip cast 1050 Al alloy processed by continuous confined strip shearing and its formability evaluation

Abstract Studies on texture evolutions were conducted on strip cast 1050 Al alloy sheets to assess the feasibility of equal channel angular pressing (ECAP) and subsequent heat treatments as a means for enhancing the formability while reducing the earing of the sheets. ODFs were calculated to investigate texture evolutions due to ECAP and subsequent heat treatments. The r -value and Δr-value were determined based on the measured pole figures to judge the formability and the earing of the samples. Experiments showed that the production of the 1050 Al alloy sheets with high formability ( r =1.05) and low earing (Δr=0.05) could be possible by ECAP and the heat treatment at appropriate temperature.

Prediction of Si contents to suppress the interfacial reaction in the SiCp/2014 Al composite

Abstract Equilibrium Si contents required to prevent the interfacial reaction in the SiC p /2014 Al composite were predicted using both theoretical calculations and experimental method. According to the results, equilibrium Si contents increased with increasing temperature; When the composite is exposed at temperatures below the solidus of the matrix, less than 1xa0at.% of Si was found to be sufficient in order to prevent the interfacial reaction. However, Si contents required to prevent the interfacial reaction were observed to increase substantially at temperatures near 600°C such that more than 5xa0at.% of Si was required to prohibit the interfacial reaction even at 620°C. Application of such results for designing the matrix alloy and selecting process parameters for the semi-solid forming were demonstrated.

Biocompatibility and strength retention of biodegradable Mg-Ca-Zn alloy bone implants.

The biocompatibility and strength retention of a Mg-Ca-Zn alloy were studied to evaluate its efficacy for osteosynthesis applications. Mg-Ca-Zn alloy and self-reinforced poly l-lactide (SR-PLLA) bone screws were implanted into New Zealand rabbits for radiography analysis, micro computed tomography analysis, histomorphometry, hematology, serum biochemistry, histopathology, and inductively coupled plasma mass spectrometry analysis. Bending and torsion tests were performed on intact specimens to find the initial mechanical strength of these Mg-Ca-Zn alloy bone screws. Strength retention of the Mg-Ca-Zn alloy implants were calculated from in vivo degradation rates and initial mechanical strength. Based on the animal study, Mg-Ca-Zn alloy bone screw showed absence of subcutaneous gas pockets, characteristic surface erosion properties, faster degradation rate than SR-PLLA bone screw, normal reference range of hematology and serum biochemistry, better histopathological response than SR-PLLA bone screw, and stable concentrations of each constituent element in soft tissues surrounding the implants. The initial strength and strength retention of Mg-Ca-Zn alloy were compared with those of various biomaterials. The initial strength of Mg-Ca-Zn alloy was higher than those of biostable and biodegradable polymers. The strength retention of Mg-Ca-Zn alloy bone screws was similar to those of biodegradable polymer. Therefore, this Mg-Ca-Zn alloy represents an excellent biodegradable biomaterial candidate for osteosynthesis applications.

Facile solvothermal preparation of monodisperse gold nanoparticles and their engineered assembly of ferritin-gold nanoclusters.

Herein, we report a quick and simple synthesis of water-soluble gold nanoparticles using a HAuCl4 and oleylamine mixture. Oleylamine serves as a reduction agent as well as a stabilizer for nanoparticle surfaces. The particle sizes can be adjusted by modulating reaction temperature and time. Solvothermal reduction of HAuCl4 with oleylamine can be confirmed by measuring the product in Fourier transform infrared (FTIR) spectroscopy. The plasmon band shifting from yellow to red confirms a nanosized particle formation. Amide bonds on the surface of the nanoparticles formed hydrogen bonds with one another, resulting in a hydrophobic monolayer. Particles dispersed well in nonpolar organic solvents, such as in hexane or toluene, by brief sonication. Next, we demonstrated the transfer of gold nanoparticles into water by lipid capsulation using 1-myristoyl-2-hydroxy-sn-glycero-3-phosphocholine (MHPC), 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-(methoxy polyethylene glycol)-2000 (DPPE-PEG2k), and 1,2-dioleoyl-sn-glycero-3-N-{5-amino-1-carboxypentyl}iminodiacetic acid succinyl nickel salt [DGS-NTA(Ni)]. The particle concentration can be obtained using an absorbance in ultraviolet-visible (UV-vis) spectra (at 420 nm). Instrumental analyses using transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) analysis, dynamic light scattering (DLS), and FTIR confirmed successful production of gold nanoparticles and fair solubility in water. Prepared gold particles were selectively clustered via engineered ferritin nanocages that provide multiple conjugation moieties. A total of 5-6 gold nanoparticles were clustered on a single ferritin nanocage confirmed in TEM. Reported solvothermal synthesis and preparation of gold nanoclusters may serve as an efficient, alternate way of preparing water-soluble gold nanoparticles, which can be used in a wide variety of biomedical applications.

Alloy design of thixoformable wrought SiC/Al alloy composites

Abstract One of the major issues in fabricating Al alloy composites reinforced with SiC particulates is to avoid interfacial reaction products, i.e., Al 4 C 3 and Si. The formation of such interfacial reaction products can be suppressed by the presence of an adequate amount of Si within the matrix of the composites. The equilibrium Si contents required to prevent the interfacial reaction in various SiC p /wrought Al composites were predicted both using theoretical calculations and experiments. Application of these results for designing the matrix alloy and selecting process parameters for the semi-solid forming was demonstrated.

Polycaprolactone coating with varying thicknesses for controlled corrosion of magnesium

Controlled corrosion of magnesium is critical for its clinical application to orthopedic devices. For this purpose, we coated the surface of Mg with a biodegradable polymer, polycaprolactone (PCL) and attempted to control the Mg corrosion with varied coating thicknesses in a reproducible manner. As we increased the coating thickness from 0 to 13.31xa0±xa00.36xa0μm, the volume of hydrogen gas and amount of Mg ions, the indicators of Mg corrosion, decreased by almost half from 0.57xa0mL/cm2/day and 0.55xa0mg/day to 0.20xa0mL/cm2/day and 0.26xa0mg/day, respectively. However, the elemental compositions on the surface revealed possible detachment of polymer coating and rapid water absorption at the early stage of corrosion for all coating thicknesses. Therefore, the lessons learned from this study suggest pre-treatment of the Mg surface for better polymer–metal adhesion, as well as preparation of the coating with lowered porosity as a stronger water-permeation barrier, to eventually allow precise control on Mg corrosion.

Effect of the gate geometry and the injection speed on the flow behaviors of a semi-solid A356 Al alloy

The effects of the viscosity, gate speed, and gate geometry on the flow behaviors of Al alloys in molten and semi-solid states were studied by direct observations of the flow patterns during die filling. Two different fluids, thixotropic fluid (semi-solid Al alloy) and Newtonian fluid (molten Al alloy), were chosen as the model fluids for monitoring the differences in flow patterns during die filling. High-speed photography was employed to record the real time flow patterns. Qualitative assessments on the critical gate speed were made based on the experimental results. The experimental results were also compared with results predicted based on the Reynolds number. Although the critical gate speed varies depending on the gate geometry, it is less than ∼2.5 m/sec in semi-solid forming, while it ranges from 0.3∼0.5 m/sec in squeeze casting.

Load-bearing capacity and biological allowable limit of biodegradable metal based on degradation rate in vivo.

In this study, a newly developed Mg-Ca-Zn alloy for low degradation rate and surface erosion properties was evaluated. The compressive, tensile, and fatigue strength were measured before implantation. The degradation behavior was evaluated by analyzing the microstructure and local hardness of the explanted specimen. Mean and maximum degradation rates were measured using micro CT equipment from 4-, 8-, and 16- week explants, and the alloy was shown to display surface erosion properties. Based on these characteristics, the average and minimum load bearing capacities in tension, compression, and bending modes were calculated. According to the degradation rate and references of recommended dietary intakes (RDI), the Mg-Ca-Zn alloy appears to be safe for human use.

In vitro dynamic degradation behavior of new magnesium alloy for orthopedic applications

We report on methodologies for use in the design of a biodegradable Mg alloy appropriate for load-bearing but temporary orthopedic implant applications. Comparative studies of Mg-5Ca and Mg-5Ca-1Zn were conducted to explore the effects of a combination of minor alloying and hot extrusion, on the alloys mechanical properties and corrosion resistance. The extruded Mg-5Ca-1Zn exhibited high ultimate compressive strength of 385 MPa and suffered no significant structural degradation even after immersion in simulated body fluid for 30 days. Mg-5Ca-1Zn alloy showed the mechanical strength and controlled corrosion rate to be considered as an ideal candidate for biodegradable orthopedic implant material.