Papers by Author: Woo Yang Jang

Abstract: Hydroxyapatite (HA) ceramics were prepared from the bone ash which is mostly used as animal feeds or raw materials. Dissolving behaviors of the HA in buffered water were investigated and compared with commercial HA for further use as biomaterials. HA ceramics were prepared by soaking the bone ash in 0.1 M of NaOH solution at 80°C for 4 h. The ash was calcined at 800°C for 1 h to completely remove organics, and then sintered at 1200°C for 1 h under moisture protection. The bone ash derived-HA consists of mostly HA and small amounts of α-tricalcium phosphate, calcium oxide phosphate, magnesium oxide and calcium oxide. After polishing the HA ceramics, they were immersed in buffered water at 37°C for 3 and 7 days. We previously found that grain boundaries of synthetic HA were intensively dissolved in buffered water. On the other hand, the bone ash derived-HA shows high stability in liquid environment with immersion time compared with commercial HA.

Abstract: The main drawbacks of SMA actuators are a slow response and a waste of electric power. The power should be continuously supplied for SMA elements to remain in the austenite state until the actuator begins to move to the other position. The problems are more serious when batteries and thick SMA elements are used. We develop a new SMA actuator which consumes little energy and responds very quickly. The actuator consists of a specially designed bias spring structure, and two SMA elements which exert the force in opposite direction to each other. The bias spring, unlike a common spring which has one stable position, has two stable positions. A SMA1, for instance, contracts on heating by electricity, the actuator takes one stable configuration. No further power supply is necessary to maintain the stable position, providing enough time for the SMA1 to cool. When the other SMA2 is heated, it contracts with enough force to move the bias spring and to expand the already-cooled SMA1. Power supply stops immediately after the actuator takes the other stable configuration, causing power saving and quick response.

Abstract: A small amount of misch metal was added to CuZnAl alloy in order to study the effect of grain refinement and mechanical properties, phase transformation behavior and stabilization of martensite. It was found that the addition of misch metal was very effective for reducing the grain size. The coarse grains over 1000 μm have been refined to the size of 30 μm by the addition of 0.43wt% misch metal. The grain size of thermo-mechanically treated alloys was barely affected by cold working. The fracture strength and ductility have been significantly increased with the increase of misch metal content when tensile test is carried out below Mf temperature. Also, the fracture strength has been more increased in the case of post-quench ageing treatment than the direct quench ageing treatment. The fracture mode has been changed from transgranular brittle fracture to ductile fracture with void formation and coalescence by the addition of misch metal.

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: Nano-sized silver particles are considered to apply a silver paste for electrode because of its high conductivity on sintering at low temperature. In this study, silver nanoparticles as seeds were prepared by chemical reduction method with capping agent. Silver particles were prepared using SDS (Sodium dodecyl sulfate) as a surfactant and silver nanoparticles as seeds and reacted with ascorbic acid as a reduction agent. The silver seeds with 10-20 nm in size with uniform distribution were formed and the size and shape of silver particles were strongly dependent on the concentration of surfactant.

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: 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.

Abstract: The effects of flash-heating and aging on the microstructure and transformation characteristics has been studied in Cu-Al-Ni based melt-spun ribbons. When the as-spun ribbon, stabilized due to higher Al content and fast cooling, is flash-heated at 750 °C, the β1 parent phase is partially retained. Even though flash-heating temperature is increased upto 850 °C, a little β1 parent phase with lager grains above 10 μm is still retained. The Ms temperature is increased with increasing flash-heating temperature due to the decrease of grain constraint within grown grains. However, the greater part of β1 parent phase with smaller grains of 2 ~ 3 μm is transformed to martensite in the initial stage of aging at 200 °C. The Ms temperature of the ribbon with smaller grains is higher than that of the ribbon with larger grains. It is considered that the increase of Ms temperature is caused to the disappearance of lattice defects introduced during melt spinning by aging at 200 °C rather than grain growth.