Papers by Author: Yont Tai Lee

Abstract: Dynamic compressive tests were conducted on Zr-based amorphous alloys and amorphous matrix composite containing dendritic β phases. Dynamic compressive tests were conducted using a compressive Kolsky bar and then the test data were analyzed in relation to microstructure and fracture mode. Under dynamic loading, the maximum shear stress and ductility of the amorphous alloy and composite were considerably lower than those under quasi-static loading because of the decreased resistance to fracture. Deformation under dynamic loading lowered strain and compressive strength because of reduced fracture resistance, and the alloy containing dendritic β phases showed better compressive strength and ductility than the monolithic alloy.

Abstract: New titanium alloys with a low elastic modulus have been developed for biomedical applications to avoid the stress shielding effect of an artificial prosthesis. The newly developed alloys contained the transition elements like Zr, Hf, Nb, Ta which were non-cytotoxicity elements and β stabilizers. In the present paper the elastic moduli of Ti-xM containing Zr, Hf, Nb, Ta were evaluated by measuring the velocity of supersonic wave (Pulse Echo Overlap). The effectiveness of the alloying elements for lowering the elastic modulus was investigated. In addition, the dominant factors for the low modulus were discussed. Ta was the most effective in lowering the elastic modulus of the alloys. The effectiveness of Hf was not acceptable for decreasing the elastic modulus. The dominant factor was the lattice parameter for Zr, and the poisson's ratio for Nb, Ta, respectively, in lowering the elastic modulus of Ti.

Abstract: Effect of silicon content on the creep properties of Ti-6Al-4Fe-xSi was studied. Creep resistance of Ti-6Al-4Fe-xSi alloys was superior to that of Ti-6Al-4V. Ti-6Al-4Fe-0.5Si alloy exhibited the highest rupture strength and creep resistance among the Ti-6Al-4Fe-xSi alloys investigated. The minimum creep rate of the alloys decreased with increasing silicon content up to 0.5wt.% and then it increased again when the silicon content was higher than 0.5wt.%. TiFe precipitates were formed mainly at the β phase area of Ti-6Al-4Fe-xSi alloys by consuming titanium and iron in β phase, when the alloys were thermally exposed at 500 and 600°C during the creep test. During the creep test, microvoids were induced at the TiFe/α phase interfaces and the cracks were formed along the TiFe/α phase interfaces by the coalescence of the voids. Those cracks were finally connected each other through the α phase.

Abstract: New titanium alloys with a low elastic modulus have been developed for biomedical applications to avoid the stress shielding effect of the artificial prosthesis. The newly developed alloys contained the transition elements like Nb, Ta, Zr which were non-cytotoxicity elements. These elements produced β, ω, and α'' phases with heat treatment conditions in titanium alloys and determined the elastic modulus of the alloys. However, the clear mechanism of the low elastic modulus alloys has not been known. In the present paper, the total energy and elastic modulus of β and α'' phases were calculated using a first principle calculation employing the generalized gradient approximation (GGA). The mechanism of the low elastic modulus was discussed with calculated values.

Abstract: A new high strength titanium alloy system with low cost alloying elements, such as Al, Fe, has been recently developed. In present study the expensive V was replaced with Fe, and Si was added from 0 to 7.5wt.%. The effect of Fe and Si on the microstructure and tensile properties of Ti-6Al-4Fe-xSi (x=0, 0.1, 0.25, 0.5, 0.75wt.%) alloys was investigated. The room and high temperature mechanical properties of Ti-6Al-4Fe alloys were better than those of the Ti-6Al-4V. It was mainly due to the phase boundary strengthening at ambient and high temperature. The strength and elongation of the developed alloys depended upon the Si contents. The Si elements made the grain boundary and colony size fine, and increased the strength of the developed alloys by solid solution and precipitation hardening. The tensile strength variation with the Si contents at room temperature and 400°C, and at 450°C and 500°C showed a similar behavior, respectively.