Papers by Author: Kwang Koo Jee

Abstract: The present authors have invented a new method of changing coil orientation of a coil spring. The major merit of coil orientation change for SMA are reduction of space and control of two way shape memory effect. For instance, when a coil is heated above Af temperature with a load, the coil contracts. There is unrecovered strain which is attributed to elastic deformation by the load. The elastic strain can be eliminated by changing coil orientation. By removing elastic strain, the necessary space can be reduced. Besides generation and control of two way shape memory effect is presented using the reverse of coil orientation.

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: The shape memory effect (SME) and magnetic shape memory effect (MSME) Fe-Pd thin film are using the film curvature method. The corresponding residual stress change due to theSME and MSME in Fe-Pd film is measuredduring thermal cycling and magnetic field changing. AFe-Pd thin film with a lateral composition gradient is deposited onto micromachined x7 cantilever beam arraysubstrate,such that each of the cantilever beams is coated with a film of different composition.There is abrupt stress change in only .1 at % Pd as the temperature of the film is cycled, and the corresponding stress change was measured as 0.16 GPa. The film with .4 at % Pd showsthe abrupt stress change at 0.7 Tesla, which means that the composition has the MSME.

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.

Abstract: A clad composite of five plies of STS/Al/Al/Al/STS was produced by roll-cladding at 350°C from ferritic stainless steel (STS) and aluminum (Al) sheets. In order to clarify the strain state during roll-cladding, the evolution of textures at different through-thickness layers in the roll-clad composite was investigated. Because the deformation mainly occurred in the Al sheets during roll-cladding, the present investigation was focused on the Al sheets. In the STS/Al/Al/Al/STS composite, the mid Al sheets between STS and center Al displayed pronounced texture gradients with shear textures and plane strain rolling textures, while the center Al sheet depicted a plane strain texture throughout the whole sheet thickness. Simulations with the finite element method (FEM) disclosed that a large variation of shear strain rate during roll-cladding led to the formation of these texture gradients.

Abstract: Orientation rotation behaviors taking place in aluminum single crystal and polycrystal were studied during uniaxial tensile deformation at room temperature. Specimens were elongated in an in-situ deformation stage equipped on a high resolution electron backscattered diffraction (HREBSD)system. A series of crystallographic evolution and deformation behavior of concerned areas in the specimens during deformation was directly analyzed using HR-EBSD.