Papers by Author: Hee Young Kim

Abstract: The effect of oxygen content on the stability of α′′ martensite phase of the Ti-20mol%Nb alloy was investigated by differential scanning calorimetry. The reverse martensitic transformation temperature increased with increasing oxygen content, and this means that α′′ martensite phase was stabilized by oxygen addition. The mechanism of α′′ martensite stabilization by oxygen atoms was discussed based on the changes in the lattice parameters of α′′ martensite.

Abstract: The effects of Zr addition on martensitic transformation and the lattice parameters of α” (orthorhombic) martensite and β (bcc) phase were investigated in Ti-3mol%Mo-6mol%Sn based alloys containing up to 4mol%Zr using θ-2θ X-ray diffraction measurement (XRD) and differential scanning calorimetry (DSC). It was found by XRD that orthorhombic α” martensite phase is formed when Zr content is 0 to 2mol% while bcc β phase also existed in the alloy containing 2 to 4mol%Zr. Based on the lattice parameters in α” martensite and β parent phases evaluated, the transformation strains between α” and β phase calculated become slightly small with increasing Zr content. DSC revealed that, with increasing Zr content, reverse martensitic transformation start and finish temperatures decreased down to 410K with a rate of-30K/mol%Zr. It is concluded in the Ti-Mo-Sn alloy system that Zr addition stabilizes β phase and that Zr addition is effective to control martensitic transformation temperature without changing the transformation strains largely.

Abstract: The stability of shape memory properties was investigated by thermal cycling tests in Ti-23Ta-(2-4)Sn alloys. The martensitic transformation temperature decreased considerably after aging at 573 K for the Ti-23Ta-2Sn alloy. The decrease in the martensitic transformation temperature by the aging effect was suppressed by the addition of 3at.%Sn and more to the Ti-23Ta alloy. The Ti-23Ta-3Sn alloy exhibited better cyclic stability compared with a Ti-32Ta alloy which has a similar transformation temperature.

Abstract: The self-accommodation microstructure of a -titanium shape memory alloy (SMA) was investigated by conventional transmission electron microscopy (CTEM) observation. There were two distinct minimum units for the self-accommodation microstructure. One was the V-shaped pair of two habit plane variants that were bounded by the {111}TypeI twin as reported in some previous studies. The other was the bundle of the two habit plane variants that were bounded by the <211>TypeII twin. The later one had not been recognized as the self-accommodation microstructure in -titanium SMAs.

Abstract: Stress amplitude dependence of internal friction (tanδ) of a Ti-base shape memory alloy, Ti-24mol%Nb-3mol%Al, with a well developed texture is investigated by a dynamic mechanical analyzer (DMA) in a tensile mode. In the martensite (C-orthorhombic) phase, tan was 0.01~0.05 and was much higher than that of the parent phase in all the test conditions. This means that the martensite/martensite interfaces move under the external sinusoidal stress. In addition to this high background, tanδ-peak appeared at around 153K. The tanδ-peak height clearly depended also on the stress amplitude (10~50MPa). In addition, a threshold stress for the appearance of the tanδ-peak existed in the DMA test and it was in good agreement with the yield stress (0.2% proof stress) in a tensile test at 153K. These results mean that a long-range motion of twin-boundaries is necessary for the appearance of the tanδ-peak.

Abstract: The basic characteristics of TiNi-based and Ni-free Ti-based shape memory alloys are reviewed. They include the crystal structures of the parent and martensite phases in both the alloys, the recoverable strain associated with the martensitic transformation, the transformation temperatures, the temperature and orientation dependence of deformation behavior, etc. The sputter-deposited Ti-Ni thin films are also reviewed briefly because of their possibility of expanding into micromechanical system applications as the most powerful microactuator.

Abstract: Internal friction of Ti-24mol%Nb-3mol%Al (Ti-24Nb-3Al) shape memory alloy with a well-developed texture was investigated by dynamic mechanical analysis (DMA) in a tensile mode with a frequency of 1Hz and a temperature range of 123~423K. Ti-24Nb-3Al alloy fabricated by Ar arc-melting was cold-rolled with a reduction in thickness of 99% and then heat-treated at 1173K for 3.6ks in vacuum to develop a recrystallization texture. The martensitic transformation temperature and internal friction (tan δ) were evaluated under the stress amplitude of 36MPa. The tensile direction was selected to be the rolling direction (RD) and the transverse direction (TD). The specimen was β-single phase with a well-developed {112}β<110>β texture at room temperature and exhibited high damping during martensitic transformation and in the martensite phase. The relationship between internal friction and the tensile direction is discussed on the view point of the crystallography of the transformation.

Abstract: Rolling texture of α"(C-orthorhombic) martensite phase in Ti-22mol%Nb-3mol%Al β-titanium based shape memory alloy was examined using X-ray pole figure measurement. The alloy was dual phase of α'' and β(bcc) at room temperature (RT) and was cold-rolled with reduction rates of 40%~99% at RT. Stress-induced martensitic transformation occurred during the rolling and then the material was fully transformed into martensite phase. Subscript α" and β indicate martensite and parent phase hereafter. The major texture was close to {411}α"<173>α" when the reduction rate was lower than 60%. This texture is a result of a preferential formation of specific variants which have a higher interaction with the macroscopic deformation. As the reduction rate was increased, ND moved toward {101}α" whereas RD was around <010>α" regardless of the reduction rate. {101}α"<010>α" texture was confirmed when the reduction rate was 99%.

Abstract: Effects of rotation speed on the microstructure and transformation behavior were investigated for Ti-48at%Ni melt-spun ribbons. The Ti-48at%Ni shape memory alloy ribbons were fabricated by the melt-spinning method with various rotation speeds ranging from 2500rpm to 7500rpm. The melt-spun ribbons were completely crystallized to TiNi B2 phase during solidification at lower rotation speeds less than 4000rpm. Amorphous and B2 phase coexisted in the melt-spun ribbons fabricated at higher rotation speeds of 5000rpm and 7500rpm. The martensitic transformation behavior was investigated both in the as-spun and heat-treated ribbons. The transformation temperature increased with decreasing cooling rate and increasing heat-treatment temperature.

Abstract: The microstructure and shape memory behavior of Ti-rich Ti-Ni melt-spun ribbons with various Ni-contents were investigated. Ti-xNi(x=40~48at%) ribbons were fabricated by the melt-spinning method at the rotation speed of 5000rpm. They were heat-treated at 1073K for 3.6ks. It was found that the Ti-40at%Ni as-spun ribbon exhibited almost complete amorphous structure, while the specimen with more Ni-content, such as 48at%Ni ribbon exhibited the coexistence of amorphous and crystalline structures. It was also found that the maximum shape recovery strain increased with increasing Ni-content. The Ti-40at%Ni ribbon was very brittle because of many Ti2Ni precipitates formed.