Characterization of Ternary NiTiPd High-Temperature Shape-Memory Alloys under Load-Biased Thermal Cycling
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I.
INTRODUCTION
HIGH-TEMPERATURE shape-memory alloys (HTSMAs) have received increased attention as the demand for lightweight, compact, and efficient actuators has grown in recent years. Of the known NiTi-based HTSMAs (alloys containing one or more of the alloying elements Au, Pd, Pt, Hf, or Zr), NiTiPd has been one of the most extensively studied[1] and is considered a prime candidate for use as a solid-state actuator in the aerospace, automotive, and energy exploration industries, where space is generally limited, the environment can be harsh, and alloys with a high transformation temperature are a necessity.[2] Originally investigated in 1980 by Boriskina and Kenina, (Ni,Pd)Ti was identified as a stoichiometric pseudobinary system between NiTi and TiPd.[3] In the ternary (Ni,Pd)Ti system, the high (>773 K (500 C)) transformation temperature of TiPd[4] was shown to decrease linearly with decreasing Pd content to a minimum value near 10 at. pct Pd.[5] Later, Shimizu et al. determined that, as in binary NiTi, transformation temperatures in NiTiPd were also extremely sensitive to the (Ni,Pd):Ti ratio on the Ti-lean side of stoichiometry, while alloys with Ti-rich compositions GLEN S. BIGELOW, Materials Engineer, SANTO A. PADULA II and RONALD D. NOEBE, Materials Research Engineers, are with the NASA Glenn Research Center, Cleveland, OH 44135. Contact e-mail: [email protected] ANITA GARG, Senior Research Associate, is with the University of Toledo, Toledo, OH 43606. DARRELL GAYDOSH, Senior Research Associate, is with the Ohio Aerospace Institute, Cleveland, OH, 44142. Manuscript submitted February 16, 2010. Article published online July 22, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
had transformation temperatures similar to those of stoichiometric compounds.[6] Despite good unconstrained shape recovery in NiTiPd of up to 6 pct for alloys near 30 at. pct Pd and high transformation temperatures,[5,7] unconstrained shape recovery is poor for alloys with higher Pd contents, decreasing to 4 pct for Ni13Ti50Pd37 in tension and less than 0.5 pct for alloys with Pd contents above 40 at. pct.[8] Several methods were proposed for improving the shape recovery of these alloys by strengthening the material against plastic deformation. These included alloying with boron,[9] heat treatment resulting in the precipitation of Ti2Ni type phases,[6] and thermomechanical processing, which consisted of either cold rolling and subsequent heat treatment[10] or isothermal mechanical cycling.[11] Despite all the work performed on the NiTiPd system, there has been limited research investigating the shapememory response of these alloys under an applied load, which would be much more relevant to their use as solid-state actuators than measuring stress-free recovery. The most notable exceptions are more recent studies by Cai et al.[12] on Ni19.4Ti50.6Pd30, Noebe et al.[13] on Ni19.5Ti50.5Pd30, and Kumar et al.[14] on Ni10Ti50Pd40. These studies were predominantly limited to higher Pd containing alloys. Consequently, in this investigatio
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