Papers by Author: Joong Kuen Park

Abstract: The Cockcroft-Latham (CL) damage factor has been calculated, using FEM simulation, during equal channel angular extrusion (ECAE) of strain hardening materials. A high CL damage factor was predicted inside the billet and the damage was increased with the increasing strain hardening coefficient. This was closely related to a generation of increasingly high tensile stress in the deformation zone inside the billet due to an inhomogeneous deformation. The possibility of cracking inside the ECAE billets was confirmed in the ECAE processing of Ti-0.34O-0.95Fe and Ti-0.5Fe billets. The simulation further predicted that CL damage factor can be reduced by applying back pressure or friction force.

Abstract: The effect of carbon addition on the grain growth and ordering kinetics of FePt film has been experimentally studied by sputter-depositing a monolithic FePt-20at.%C film of 24 nm. Carbon addition of 20at.% to FePt thin film in a form of FePt (20 nm)/Cn (4 nm) (n = 1, 4) significantly reduced both the grain growth and ordering kinetics. Reducing the thickness of carbon layer, i.e. from n = 1 to n = 4, led to a much finer grain size distribution as well as to a finer grain size. The Monte Carlo simulation study indicated that the decrease of grain growth and ordering kinetics is primarily due to a continuous decrease of the mobility of order – disorder inter-phase with the progress of ordering reaction. This can eventually lead to a stable 2-phase grain structure inter-locked by low mobility inter-phases and is responsible for the formation of a fine grain size distribution in the FePt/Cn film with n = 4.

Abstract: The effect of deformation route on the mechanical properties of grade-1 CP-Ti deformed by equal-channel angular pressing (ECAP) was studied using tensile testing, TEM observation, Xray pole figure measurement, and ODF calculation. Route Bc showed high yield stress and comparatively large uniform elongation, thus high ultimate tensile strength, because of its fine grain structure with high angle grain boundary. The reason for this was because of the occurrence of prismatic slip in route Bc, in addition to the commonly occurring basal slip. Route C showed surprisingly high yield stress despite of its unfavorable grain structure because of the significant contribution of texture hardening.

Abstract: The titanium alloys containing the Nb transition elements have been investigated as the Ni-free shape memory and the biomedical alloys with a low elastic modulus. The mechanical properties of the alloys depended upon the meta-stable phases like the α`, α``, ω. To study the martensitic transformations from the β to α`` or α` the Ti-xNb (x=0 to 40 wt%) alloys were melted into the button type ingots using a VAR, and followed by the water-quenching after the soaking at 1000oC for 2hrs. The crystallography of the martensitic phases in the water-quenched alloys was analyzed using a XRD. The diffraction peaks of the orthorhombic martensites were identified by the crystallographic relationship with the bcc matrix. The lattice parameters of the orthorhombic martensites were varied continuously with the contents of the Nb elements. The martensitic transformations of the alloys were studied using the phenomenological theory of Bowles and Mackenzie.

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.