Woo Yang Jang

Generation and control of two way shape memory effect for SMA coil spring

There has been no doubt that a closed coil can not be further compressed. We found out that compression of a closed coil is achieved by reversing coil orientation and subsequently extending the coil. The new technology makes many applications of coil springs feasible that have been regarded as impossibility. Control and generation of two way shape memory effect (twsme) of shape memory alloys are taken here as examples.

Effect of Grain Size on the Shape Recovery in a Melt-Spun Fe-24%Mn-4%Si-5%Cr-5%Co SMA Ribbon

Effect of grain size on microstructure, transformation characteristics and shape recovery has been studied in Fe-24%Mn-4%Si-5%Cr-5%Co shape memory ribbons fabricated by melt spinning. Mean grain size of the ribbons could be controlled by wheel speed; in case of ribbon manufactured in the wheel speed of 10 m/sec, mean grain size was more or less 20 μm, while the mean grain size in 50 m/sec was about 1 μm. Thermal ε-martensite in the shape of plate could be observed in austenite matrix and the volume fraction of that decreased in smaller grains because of grain constraint. As a grain size decreased, one-way and two-way shape recoveries were increased. A change in shape recovery of ribbons was closely related to deformation mode i.e., transformation-induced plasticity or slip according to grain size; it could be confirmed that reversible ε-martensite was induced in preference to irreversible slip in the ribbon with smaller grains during deformation by X-ray diffractometry. Not like in bulk specimen, ε-martensite being very thin in the width of 10 ~ 20 nm and lots of stacking faults being inferred from streaks of SAD patterns, were revealed in melt-spun ribbons manufactured in the wheel speed of 50 m/sec.

Microstructural Evolution in CuAlNi Alloy with Ageing

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.

Effect of surfactant on the morphology of silver nanoparticles

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.

Microstructural Change and Martensitic Transformation in a Rapidly Solidified Cu-Al-Ni Based SMA Ribbons with Heat Treatment

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.

Effect of Microstructure on Martensitic Transformation and Cyclic Deformation Characteristics in a Cu-Al-Ni Based Alloy with Unidirectionally Solidified Structure

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.