Papers by Author: Gon Seung Yang

Abstract: Titanium-based nitride coatings on cutting tools, press molds and dies can prolong their life cycle because of superior corrosion and oxidation resistance of coatings. In this study, TiAlN/ZrN and TiCrN/ZrN multilayer coatings were prepared by R.F magnetron sputtering, and microstructural evolution and corrosion resistance of the coatings were investigated during heat treatment. The TiAlN/ZrN and TiCrN/ZrN multilayer coatings were degraded by heating up to 600 oC with formation of oxides particles on the surface. During the heat treatment, the TiCrN/ZrN and TiAlN/ZrN multilayer coatings showed the lowest corrosion current density and the highest polarization resistance at temperature range of 400 oC and 500 oC. Consequently, the TiAlN/ZrN and TiCrN/ZrN multilayer coatings showed good corrosion resistance at temperature range of 400°C and 500°C during heating.

Abstract: This research has been performed to investigate the effect of ageing onto the grain refinement of 82wt%Cu-13.5wt%Al-4.5wt%Ni-0.1wt% misch metal alloy with ageing temperature and time. The variation of transformation temperature with ageing has been found to be heavily dependent on ageing time and temperature. In the second reversed transformation cycle, the temperature of Ms point decreases with ageing time at 300
, but no variation at 100
. The temperature of As point increases with ageing time at both temperatures. After the second reversed transformation cycle, the transformation temperature does not show any significant changes in martensitic phase with ageing, the temperature of Ms point being the same afterwards. Parent phase after ageing appears to show M18R and N2H martensitic phases depending on the ageing time and temperature. It is also found that α and γ2 phases are formed only at the condition of 300
and 24h.

Abstract: Series of TiCrN wear resistant coatings have been known to be superior to other coatings in protecting tools which may be damaged corrosion, oxidization, abrasion and hardness of mold materials. The corrosion and oxidation resistance of TiN/CrN(TiCrN) multilayer coatings is better than that of single TiN and CrN coatings. TiCrN coated layer was formed by coating a layer of TixCr1-xN by non-stoichiometry. In this study, TiCrN coated layer by R.F magnetron sputtering was executed heat treatment each other temperature by tube furnace. And then surfaces of coated layer and cross sections were observed by FE-SEM and XRD. Corrosion resistance tester measured characterizations of coated layers. When the temperature increased, TiN was partially transformed to Ti2N and TiNX, and CrN was also transformed to Cr2N. The formation of non-stoichiometric phases decreased corrosion resistance.

Abstract: The effect of transformation temperature and phase transformation characteristics by alloying method of CuZnAl shape memory alloy with a small of misch metal and Zr contents were investigated. The addition of misch metal and Zr was very effective for reducing the grain size. After solution treatment, the specimens were post-quench aged or step quenched at 373K to 623K for variation of hardness value. It was found that the hardness value was very increased at 473K and 523K. The fracture mode has been changed from trans granular brittle fracture to ductile fracture with void formation and coalescence by the addition of misch metal and Zr. Ageing of the β-phase decreased the Ms temperature, but that of the martensite phase increases the As temperature. The change in As temperature with post-quench aging can be attributed to recovery of order in the β-phase.

Abstract: The Arc melting technique was employed to synthesize the type I clathrate of Ba8Al16Si30 compound. Phase transformations during synthesis and homogenization treatment were investigated using X-ray diffraction (XRD) and thermal analysis (TG/DSC). Thermoelectric properties as functions of temperature and homogenization treatment time were evaluated in this study. Maximum ZT was 0.14 at 590K for homogenized at 1173 K for 168hrs specimen and it is strongly expected to show higher value above 600K.

Abstract: CuAlNi shape memory alloy with Cu-13.5Al-4.5Ni(wt%) composition was prepared by cross-rolling method and the aging effect, phase transformation characteristics, microstructural variation were investigated. Transformation temperature was greatly increased in aged specimens at 250°C. Transformation temperature was not changed after the second reversed transformation, and Ms point was same in most specimens with third reversed transformation cycle. The variation of Ms point was not seen with aging at 100°C, but it was decreased with aging at 250°C. Transformation temperatures appear to be constant, to an measurable extent, in specimens prepared by both hotrolling and cross-rolling at 900°C. Plate shaped-specimen with the thickness of about 1mm was prepared by cross-rolling treatment at 350°C. The transformation temperatures did not change after the second transformation-reverse transformation cycling, and specially transformation start temperature was the same in most specimens experiencing third thermal cycle and thereafter. Undesirable tweed-like structure was observed in 250°C-aged specimen after a cross-rolling at 350°C. On aging the specimen prepared by cross-rolling, G.P zone was formed easier than that of γ2 phase. This phase was transformed to plate-like θ phase during aging.

Abstract: The effect of the betatizing conditions on microstructure, transformation characteristics and pseudoelastic behavior has been studied in an unidirectionally solidified Cu-Al-Ni based alloy. The coarse γ2 phase is precipitated in the cast-rod betatized at 650
 and the volume fraction of it is decreased by increasing the betatizing temperature above 700 . However, a small amount of γ2 phase is left behind despite betatizing at 800 . No martensitic transformation takes place in the cast-rod betatized at 650, because the precipitation of γ2 phase leads to stabilization of β1 parent phase. However, β1→β1′ martensitic transformation occurs in the cast-rod betatized at 700  and both β1→β1′ and β1→γ1′ martensitic transformations concurrently undergoes in the cast-rod betatized above 750  because of the slope of Al concentration nearby γ2 phase. The loaddisplacement loop of the as-cast rod betatized at 750  is linear, but those of the as-cast rods betatized at 800  and 850  are non-linear showing the typical pseudoelastic loop and the deformation is completely recovered upon unloading.