A Two-Way Shape Memory Study on Ni-Rich NiTi Shape Memory Alloy by Combination of the All-Round Treatment and the R-Phas
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JMEPEG DOI: 10.1007/s11665-017-3059-y
A Two-Way Shape Memory Study on Ni-Rich NiTi Shape Memory Alloy by Combination of the All-Round Treatment and the R-Phase Transformation Z.H. Wu, D. Vokoun, C.C. Leu, and C.T. Hu (Submitted May 4, 2017; in revised form September 12, 2017) Ni4Ti3 precipitates are formed in Ni-rich NiTi shape memory alloys (SMAs) after a certain heat treatment. Such a treatment then results in the changed phase transformation behavior of the alloy switching from the one-step reversible phase change B2 M B19¢ to the two-step, B2 M R M B19¢, phase change. The present study reports a two-way shape memory (TWSM) due to an all-round treatment followed by an R-phase constraint aging at room temperature. The enhanced TWSM behavior was observed upon temperature cycling between 273 K (R-phase) and 77 K (B19¢ phase). The effect of various constraint strains in R-phase aging by employing different diameters of the constraint ring was studied. However, the TWSM effect due to constraining the R-phase will be eliminated after a temperature rise of specimen to the fully parent phase (373 K). Keywords
all-round treatment, constraint aging, NiTi, R-phase, shape memory effect
1. Introduction NiTi shape memory alloys (SMAs) have been receiving a great deal of attentions due to their non-conventional thermomechanical properties applicable in wide range of applications (Ref 1-3). Shape memory (SM) phenomena in NiTi and other properties, such as the superelastic behavior (Ref 4, 5) or generation of recovery stresses (Ref 6, 7), are related to the reversible thermoelastic martensitic phase transformation (Ref 8). The two-way shape memory effect (TWSME) refers to the reversible shape change of SMA samples through thermal cycling at zero external loading. Unlike the one-way SM effect, the TWSME is not usually an inherent property of SMAs. However, the TWSME may build up after certain thermomechanical training procedures. During training processes, an aligned stress field is introduced and distributed in SMA samples. The resulting residual stress field assists the formation and growth of preferentially oriented martensite variants when the martensitic transformation proceeds on cooling (Ref 9-12). Our previous report indicated another TWSME related to the Rphase transformation (Ref 13). The TWSME due to the R M B2 transformation is discussed also in Ref 14. Preferentially oriented R-phase variants can be formed in one single training cycle. Once the preferentially oriented R-phase variants are built in NiTi, further cooling may result in the formation and Z.H. Wu and C.T. Hu, Department of Material Science and Engineering, National Tsing Hua University, Hsinchu 30043, Taiwan; D. Vokoun, Department of Functional Materials, Institute of Physics of the ASCR, Na Slovance 2, 182 21 Prague, Czech Republic; and C.C. Leu, Department of Chemical and Materials Engineering, National University of Kaohsiung, Kaohsiung 811, Kaohsiung, Taiwan. Contact e-mail: [email protected].
Journal of Materials Engineering and Performance
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