Damping characteristics of TiNi shape memory alloys
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I.
INTRODUCTION
TiNi alloys are known as the most important shape memory alloys (SMAs) because of their many applications based on the shape memory effect (SME) and pseudoelasticity (PE). This comes from the fact that TiNi alloys have superior properties in ductility, strength, fatigue, corrosion resistance, recoverable strain, etc. It is also well known that TiNi alloys can exhibit high mechanical damping. I~-41 However, our understanding in this area is incomplete. No systematic investigation has been reported on the damping characteristics of TiNi SMAs. Damping mechanisms, in general, involve the stressinduced movement of defects. For high-damping metals, the major mechanisms are the stress-induced movement of dislocations or planar defects, tS1 Most of these mechanisms can be phenomenologically split into three classes: dynamic hysteresis, static hysteresis, and transformation mechanisms. Dynamic hysteresis is produced by the stressaided ordering of defects overcoming local barriers by thermal activation and yields damping that is frequency dependent and amplitude independent. Static hysteresis appears due to the stress-induced "unpinning" or "breakaway ~ process of the defects I5,6,7] and yields damping that is frequency independent and amplitude dependent. Some metals exhibit a high level of damping in the region of a transformation, for example, in the temperature range of a thermoelastic martensitic transformation, tS'm Such thermoelastic damping is frequently amplitude independent and proportional to ITI/T, where ITI is the heating or cooling rate and f is the frequency of vibration. In this study, by using both resonant-bar and lowfrequency inverted torsion pendulum techniques, the damping capacity of TiNi alloys was investigated in the high-temperature cubic B2 parent phase (B2), the lowtemperature monoclinic B I9' martensite phase (M), and H.C. LIN, Associate Professor, is with the Department of Materials Science, Feng-Chia University, Taichung, Taiwan 400, Republic of China. S.K. WU, Professor, is with the Institute of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan 106, Republic of China. M.T. YEH is with the Materials R&D Center, Chung-Shan Institute of Science and Technology, Lung-Tan, Taiwan 325, Republic of China. Manuscript submitted February 15, 1993. METALLURGICAL TRANSACTIONS A
the intermediate rhombohedral R phase (R). The dominant damping mechanisms occurring in these phases and the characteristics associated with the thermoelastic transformations of TiNi SMAs are also discussed.
II.
EXPERIMENTAL PROCEDURES
A. Materials The conventional tungsten arc-melting technique was employed to prepare Tia9.sNis0.2 and Ti49Nisl alloys. Titanium (purity, 99.7 pct) and nickel (purity, 99.98 pct), totaling about 150 g, were melted and remelted at least six times in argon atmosphere. Pure titanium buttons were also melted and used as a getter. The mass loss during melting was negligibly small. The as-melted buttons were homogenized at 1050 ~ for 72 hours and quenched in w
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