| dc.description.abstract | Since the addition of Hf or Zr provides a cost-effective alternative to NiTi alloyed with Pd, Pt and Au, due to the high cost of the latter, NiTiHf and NiTiZr alloys have recently attracted considerable attention [1]. In Ni-rich NiTiHf and NiTiZr alloys, it is possible to form nano-scale Ni-rich precipitates upon heat treatment, which strengthen the matrix by acting as barriers against plastic deformation [2-6].The increase in strength due to precipitation also permits reversible martensitic transformation at higher stress levels [2, 3, 7, 8].Furthermore, nano-precipitation in Ni-rich NiTiHf and NiTiZr alloys allows for precise control of transformation temperatures through the change in Ni content of the matrix as a function of precipitate volume fraction [4]. Consequently, the possibility to tailor these properties through nano-precipitation make the Ni-rich NiTiHf SMAs promising candidates for practical applications that involve elevated temperatures and high stresses. Despite the aforementioned advantages brought about by nanoprecipitation, many of the envisaged applications also require a stable fatigue response and long fatigue lives, and thus, it is of utmost importance to assess the cyclic deformation response of NiTiHf and NiTiZr high temperature SMAs (HTSMAs) at elevated temperatures. Even though numerous studies have been conducted on superelastic fatigue properties of SMAs [9-13], only a very few reports are available in the literature addressing their actuation fatigue response [14-17], and to the best of the authors' knowledge, none on the actuation fatigue response of HTSMAs. The current study was undertaken with the motivation of addressing these problems, and thereby establishing a thorough understanding of the effect of microstructure, upper cycle temperature, applied stress level on thermo-mechanical cyclic behavior and fatigue response of Ni50.3Ti29.7Hf20 and Ni50.3Ti29.7Zr20 alloys Overall, the current findings constitute the first systematically obtained set of results demonstrating the microstructure dependence of actuation strain, irrecoverable strain, thermal hysteresis, transformation temperatures and fatigue performance of the Ni-rich Ni50.3Ti29.7Hf20 and Ni50.3Ti29.7Zr20 HTSMAs, demonstrating the importance of controlling H-phase precipitate size. Furthermore, the work presented herein focused on the roles of UCT and applied stress levels on the actuation fatigue performance of the Ni-rich Ni50.3Ti29.7Hf20 Ni50.3Ti29.7Zr20 HTSMAs.
The experimental results reported herein not only revealed the optimum UCT and applied stress levels for desired high-temperature actuation fatigue performance, but also constitute the first set of data towards building a comprehensive database for HTSMA actuation performance to guide the efforts in designing durable and stable HTSMA actuators with the maximum possible work output. | en |
