The Effect of Aluminum Additions on the Microstructure and Thermomechanical Behavior of NiTiZr Shape-Memory Alloys
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TRODUCTION
DURING the past half century, extensive research characterizing the microstructure and mechanical properties of shape-memory alloys (SMAs) has been conducted. Named for their unique behavior of recovering their original shape at a higher temperature after being deformed at a lower temperature, SMAs are of great technological importance because of a wide range of commercial applications, such as aircraft couplings, robotic actuators, cellular phone antennas, orthodontic wires, eyeglasses frames, and stents.[1] Since the discovery of the shape-memory behavior in equiatomic nickeltitanium alloy (nitinol) by William Buehler in 1962, NiTi-based alloys became the most prominently studied SMA system leading to many of their current uses.[2] However, existing commercial applications are limited to near room-temperature usage because the maximum forward transformation temperatures of pure NiTi ranges from 333 K to 353 K (60 C to 80 C).[3] Compared with the well-established NiTi alloys that operate at low temperatures, a limited amount of work has been done on the development of high-temperature
DEREK HSEN DAI HSU, Graduate Student, and MICHELE V. MANUEL, Assistant Professor, are with the Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611. Contact e-mail: [email protected]fl.edu TAISUKE T. SASAKI, formerly Postdoctoral Fellow, with the Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487, is now Researcher, with the National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan. GREGORY B. THOMPSON, Associate Professor, is with the Department of Metallurgical and Materials Engineering, The University of Alabama. Manuscript submitted March 30, 2011. Article published online May 22, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
shape-memory alloys (HTSMAs) for industrial and commercial purposes. Alloying additions and their effects on NiTi alloys are well documented in literature.[3] Specifically, only a few alloying elements have the effect of increasing the transformation temperatures of NiTi alloys: Zr, Hf, Pd, and Pt.[3] Past research on NiTiZr alloys suggests that they have the potential to be used as a HTSMA as a result of the high transformation temperatures up to 723 K (450 C) with increasing Zr content.[4] However, one concern with Zr addition is that high Zr content can cause brittleness in the alloy. Feng et al.[5] have observed brittleness in a Ni50.6Ti46.4Zr3 (at. pct) alloy during hot forging and hot rolling. Analogous to the NiTi2 phase that affects the strength and workability of NiTi binary alloys, the Ni(Ti,Zr)2 phase and the NiTiZr Laves phase may be responsible for the brittleness shown in NiTiZr alloys.[6] The existence of these two phases along with the Ni(Ti,Zr) matrix phase has been identified by Hsieh and Wu.[6] Nevertheless, NiTiZr alloys remain a promising candidate for hightemperature actuator applications as a result of their low raw material cost. On the contrary, the marte
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