Microstructural Response During Isothermal and Isobaric Loading of a Precipitation-Strengthened Ni-29.7Ti-20Hf High-Temp
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TRODUCTION
THE unique property of shape memory alloys (SMAs) to recover their original undeformed shape when heated has made them useful multifunctional materials for the development of adaptive engineering structures. This distinctive ability is a result of a displacive, solid-to-solid martensitic phase transformation between a high-temperature, higher symmetry austenite phase (cubic B2), and a lower temperature, lower symmetry martensite phase (e.g., orthorhombic B19 or B33, monoclinic B19¢, trigonal R-phase, etc.). Through this martensitic transformation, SMAs can generate recoverable shape changes of several percent strain even when opposed by large stresses (e.g., up to 500 MPa), resulting in high work output that is competitive with, or superior to, conventional hydraulic, O. BENAFAN, formerly Graduate Student, with the Advanced Materials Processing and Analysis Center (AMPAC), Mechanical, Materials and Aerospace Engineering Department, University of Central Florida, Orlando, FL 32816, and is now Materials Research Engineer, with the NASA Glenn Research Center, Structures and Materials Division, Cleveland, OH 44135. R.D. NOEBE and S.A. PADULA II, Materials Research Engineers, are with the NASA Glenn Research Center, Structures and Materials Division. R. VAIDYANATHAN, Associate Professor, is with the Advanced Materials Processing and Analysis Center (AMPAC), Mechanical, Materials and Aerospace Engineering Department, University of Central Florida. Contact e-mail: [email protected] Manuscript submitted August 25, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS A
pneumatic, or electromagnetic actuators.[1,2] In addition, SMAs can function as both the sensor and actuator in a single mechanism, reducing engineering complexities. Hence, SMAs are a promising alternative to conventional actuators in aerospace and automotive applications.[3–6] However, broad commercial success of SMA actuators has remained elusive. This is largely attributed to the fact that commercially available SMAs are primarily limited to binary NiTi alloys that generally exhibit phase transformations at temperatures from slightly below room temperature to around 373 K (100 C),[7] severely restricting the environment in which they can be used. Furthermore, binary NiTi alloys can be dimensionally unstable, displaying significant irrecoverable strains during repeated thermomechanical cycling.[8,9] Various methods were proposed to address both limitations, i.e., the relatively low transformation temperatures and poor dimensional stability often associated with many SMA systems. The addition of Pt, Pd, Zr, or Hf to binary NiTi was shown to increase transformation temperatures.[10] However, none of these systems are without disadvantages, including the rising cost of precious metals, limited transformation strains, and the aforementioned issues associated with dimensional stability. In an attempt to improve the shape memory behavior of these high-temperature systems, various approaches such as solid-solution strengthening,[11,12] thermomechanical cycling/tra
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