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Lee, D.J.,Yoon, E.Y.,Ahn, D.H.,Park, B.H.,Park, H.W.,Park, L.J.,Estrin, Y.,Kim, H.S. Elsevier Science 2014 Acta materialia Vol.76 No.-
The paper is concerned with large strain deformation behavior of metallic materials as exemplified by copper under high-pressure torsion (HPT). To that end, the evolution of microstructure was considered in terms of a dislocation density-based constitutive model embedded in a finite element code. The variation of the specimen geometry, the hydrostatic pressure state, the equivalent strain and the dislocation density were examined by numerical simulations. The concurrent variation of the average dislocation cell size, which was identified with the emerging new grain size of the material, was also traced. The simulated results for the dislocation density and the grain size were shown to be in good agreement with the experimental data for commercial purity copper. It was concluded that the dislocation density-based constitutive model is well placed as a tool for describing and predicting the evolution of microstructure during severe plastic deformation, particularly HPT, using the finite element method.
Jung, H.D.,Sun Park, H.,Kang, M.H.,Lee, S.M.,Kim, H.E.,Estrin, Y.,Koh, Y.H. North-Holland 2014 Materials letters Vol.116 No.-
We report successful fabrication of polyetheretherketone/magnesium (PEEK/Mg) composite with a Mg content of 30vol% selectively coated with hydroxyapatite (HA). The in vitro bio-corrosion and biocompatibility of the composite were examined for potential biomedical applications. A PEEK/Mg composite included compression molding at 370<SUP>o</SUP>C, in which Mg particles were well dispersed in a PEEK matrix. In addition, the surfaces of Mg particles exposed to the matrix material were selectively coated with a HA layer by treatment in an aqueous solution. This HA coating considerably enhanced the in vitro bio-corrosion resistance and biocompatibility of the PEEK/Mg composite without sacrificing its excellent mechanical properties.
Microstructure and corrosion properties of ultrafine-grained interstitial free steel
Hadzima, Branislav,Janed,ek, Miloš,Estrin, Yuri,Kim, Hyoung Seop Elsevier 2007 Materials science & engineering. properties, micro Vol.462 No.1
<P><B>Abstract</B></P><P>The microstructure and the corrosion behaviour of ultrafine-grained interstitial free (IF) ferritic steel processed by equal channel angular pressing (ECAP) at room temperature following route C were investigated. Already after the first pass of ECAP, the microstructure was refined by a factor of approximately 200. On further passes, ECAP was found to lead to continuous grain refinement, while elongated grain structure produced in the first pass was retained throughout all processing cycles. After 8 passes, the microstructure comprised bands of elongated grains of average length of 500–1000nm and average width of 200–300nm. Corrosion characteristics of a non-deformed coarse grained specimen and an ultrafine-grained specimen that had undergone 8 passes of ECAP were investigated using electrochemical potentiodynamic tests. ECAP induced grain refinement does not appear to influence the electrochemical characteristics in neutral NaCl solutions. This encouraging finding demonstrates that ECAP, while enhancing mechanical characteristics, does not compromise corrosion resistance of IF steel. A protective oxidic layer is formed in alkaline NaCl solution, which reduces the corrosion rate significantly.</P>
Kim, S.W.,Jung, H.D.,Kang, M.H.,Kim, H.E.,Koh, Y.H.,Estrin, Y. Elsevier 2013 Materials science & engineering. C, Materials for Vol.33 No.5
This paper reports a new approach to fabricating biocompatible porous titanium with controlled pore structure and net-shape. The method is based on using sacrificial Mg particles as space holders to produce compacts that are mechanically stable and machinable. Using magnesium granules and Ti powder, Ti/Mg compacts with transverse rupture strength (~85MPa) sufficient for machining were fabricated by warm compaction, and a complex-shape Ti scaffold was eventually produced by removal of Mg granules from the net-shape compact. The pores with the average size of 132-262μm were well distributed and interconnected. Due to anisotropy and alignment of the pores the compressive strength varied with the direction of compression. In the case of pores aligned with the direction of compression, the compressive strength values (59-280MPa) high enough for applications in load bearing implants were achieved. To verify the possibility of controlled net-shape, conventional machining process was performed on Ti/Mg compact. Compact with screw shape and porous Ti scaffold with hemispherical cup shape were fabricated by the results. Finally, it was demonstrated by cell tests using MC3T3-E1 cell line that the porous Ti scaffolds fabricated by this technique are biocompatible.
Li, Y.,Pang Ng, H.,Jung, H.D.,Kim, H.E.,Estrin, Y. North-Holland 2014 Materials letters Vol.114 No.-
Commercial purity titanium (Grade 4) was processed by encapsulation-aided equal-channel angular pressing. Encapsulation in a copper cartridge made it possible to carry out the process repeatedly, up to 8 passes, at a relatively low temperature of 300<SUP>o</SUP>C. Extremely high values of yield strength and tensile strength close to record literature values for this titanium grade and matching, or even surpassing, the levels for conventional Ti-6Al-4V alloy were obtained. A bi-modal grain structure observed may be responsible for a favorable combination of strength and ductility the processed material exhibits.
Kang, M.H.,Jung, H.D.,Kim, S.W.,Lee, S.M.,Kim, H.E.,Estrin, Y.,Koh, Y.H. North-Holland 2013 Materials letters Vol.108 No.-
Biodegradable porous magnesium (Mg) with hydroxyapatite (HA) coating suitable for biomedical applications was fabricated. A blend of Mg and NaCl particles was sintered by spark plasma sintering (SPS), and then the NaCl was dissolved to obtain a porous structure. Different levels of porosity (50%, 60% and 70%) were achieved by adjusting the volume fraction of NaCl, while preserving high pore interconnectivity with a large pore size of~240μm. In addition, a dense HA coating layer comprised of needle-shaped HA crystals was formed on the surface of the porous Mg by treatment in an aqueous solution. Both bare and HA-coated porous Mg specimens with a porosity of 60% exhibited ductile behavior under compressive loading and similar levels of ultimate compressive strength (~15MPa). However, HA coating significantly enhanced the corrosion resistance of porous Mg.