Effect of Aging on Microstructure and Shape Memory Properties of a Ni-48Ti-25Pd (At. Pct) Alloy
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NI-TI based alloys have been extensively studied and are the only successfully commercialized shape memory materials because of their excellent balance of physical, mechanical, and functional properties.[1,2] Much of the research related specifically to shape memory behavior has focused on the Ti-rich and equiatomic Ni-Ti compositions because these alloys have higher transformation temperatures ~363 K– 373 K (~90 C–100 C[3,4]) as compared with Ni-rich binary alloys, where transformation temperatures can be well below room temperature.[5] Though Ni-rich NiTi alloys have lower transformation temperatures, they exhibit superior dimensional stability during pseudoelastic stress cycling[6] and load-biased thermal cycling,[7,8] compared to their Ti-rich counterparts.[9,10] In this case, dimensional stability refers to the amount of residual strain generated during repeated transformation cycles, with little or no residual strain generation TAISUKE T. SASAKI, formerly Post Doctorate Fellow, Department of Metallurgical and Materials Engineering, University of Alabama, Tuscaloosa AL, and is now a Researcher, National Institute for Materials Science, Tsukuba, Japan. B. CHAD HORNBUCKLE, Graduate Student, MARK L. WEAVER and GREGORY B. THOMPSON, Professors, are with the Department of Metallurgical and Materials Engineering, University of Alabama, Tuscaloosa AL. Contact e-mail: [email protected] RONALD D. NOEBE, Senior Materials Research Engineer and GLEN S. BIGELOW, Materials Engineer are with the Structures and Materials Division, NASA Glenn Research Center, Cleveland, OH. Manuscript submitted November 22, 2011. Article published online November 8, 2012 1388—VOLUME 44A, MARCH 2013
being preferred. Good dimensional stability is the most important property relevant to actuator applications where multiple cycles occur. If stability is poor, the material usually fails prematurely because of low cycle fatigue, or the actuator fails as the SMA changes dimension forcing the device out of tolerance. Slight deviations in stoichiometry toward Ni-rich binary Ni-Ti compositions result in the formation of Ni4Ti3 precipitates upon aging at intermediate temperatures.[11] The precipitation of the secondary phase plays a crucial role in increasing transformation temperatures and improvement in the shape memory properties of binary alloys. A moderate increase in martensitic transformation temperatures in aged Ni-rich alloys is associated with the enrichment of Ti within the matrix because of the formation of the Ni-rich precipitates and the effect of local stress fields at the precipitate interface on the martensitic transformation.[12,13] The improvement in dimensional stability for these alloys is also attributed to the presence of the Ni4Ti3 precipitates. In general, the best dimensional stability is observed when the precipitate size is on the order of 10 to 20 nm.[6–8] At this size, the coherent precipitates are very effective in resisting slip processes, which in turn minimizes the accumulation of residual strain during cycling. T
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