Papers by Author: Hyo Tae Jeong

Abstract: Strengthening method for the Fe-36Ni based Invar alloy for power transmission wire was investigated in this study. High strength of 1300 MPa could be obtained in this alloy through solution hardening, precipitation hardening and strain hardening by cold working. Phase equilibrium of the Invar alloy was calculated using FactSage®, revealing that thermodynamically stable phases are Mo2C, MoC, M23C6-type FeCrMo carbide, and M6C-type FeMo carbides. Aging treatments were carried out at temperatures ranging from 400 to 900oC for time intervals from 3 min to 30 hrs. Peak aging condition was obtained as 400oC and 1 hr. With temperature increased, peak hardness was decreased abruptly. Microstructure observation was conducted by optical microscopy, scanning electron microscopy, and transmission electron microscopy. By using the result from aging treatment, high strength above 1300MPa was obtained in the cold rolled Invar alloy plate.

Abstract: Texture evolution and superplastic deformation behavior of a quasi-single phase Zn-0.3wt%Al have been investigated. It was attempted to produce a stable and fine-grained microstructure in a dilute Zn-Al alloy through a proper thermomechanical treatment process (TMTP). The grain size of about 1 µm was obtained in the Zn-0.3 wt.% Al alloy and a relatively coarse grain size of 10 µm was also obtained through a subsequent aging treatment. The fine-grained material showed typical rolling texture with basal poles tilted about 30 degrees away from the ND toward RD, while the coarse-grained material showed a typical recrystallization texture with basal poles parallel to ND. A series of load relaxation and tensile tests were conducted at room temperature. According to the internal variable theory of structural superplasticity, the grain boundary characters of fine and coarse-grained materials were different from each other. A large elongation of about 1400% was obtained in fine-grained material at room temperature.

Abstract: The softening of fine-grained ECAPed AZ31 Mg alloys could be ascribed to the texture modification during ECAP. Lower ECAP temperature is more effective in refining the microstructure. The strength of the ECAPed AZ 31 Mg alloys increased with decrease in grain size when they have similar texture.

Abstract: Microstructure and texture evolution in the AZ31 Mg alloy subject to equal channel angular pressing (ECAP) have been investigated and correlated with the mechanical properties. When AZ31 Mg alloy was ECAPed up to 8 passes following the route Bc, grain refinement occurred effectively. Texture was also changed during ECAP. The original fiber texture of the extruded AZ31 Mg alloy changed to a new texture component of ] 1 3 2 5 )[ 1 1 01 ( , and the texture of ] 1 3 2 5 )[ 1 1 01 ( orientation was rotated to ] 0 2 5 7 )[ 6 4 13 ( orientation after 6-pass ECAP process. The variation of the strength with the pass number was explained by the texture and grain size. The strength data of AZ31 Mg alloys followed the standard Hall-Petch relationship when the similar texture was retained during the ECAP process. Otherwise the effect of texture on strength was dominant over the strengthening due to grain refinement.