High Strain Rate Compression of Martensitic NiTi Shape Memory Alloy at Different Temperatures
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DUE to the unique combination of properties, e.g., strength, ductility, corrosion resistance, biocompatibility, and damping capacity, NiTi shape memory alloys (SMAs) have been extensively investigated and widely used in medical, aerospace, automotive, and energy absorption applications,[1–8] as compared with other types of SMAs like copper-based and iron-based SMAs.[9–17] A clear understanding of the mechanical properties and deformation mechanisms of NiTi SMAs are critical to facilitate the increasing number of applications. Most of the previous research on the mechanical properties of NiTi SMAs have been concentrated on low strain rate loading conditions.[18–28] It has been found in recent decades that the mechanical response of NiTi SMAs is also sensitive to strain rate.[29–42] As Nemat-Nasser et al.[38] reported, the transition stress from the austenite-to-martensite phase transformation and work-hardening rate increases at around 1000 s 1. One year later, Chen and Bo[36]
YING QIU, Postdoctoral Fellow, and XU NIE, Assistant Professor, are with the Mechanical Engineering, Southern Methodist University, Dallas, TX 75205. MARCUS L. YOUNG, Assistant Professor, is with the Materials Science and Engineering, University of North Texas, Denton, TX 76201. Contact e-mail: marcus.young@ unt.edu Manuscript submitted July 22, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
determined the stress–strain curves of NiTi SMAs at a strain rate of 430 s 1 under the environmental temperature ranging from 273 K (0 °C) to 323 K (50 °C) and found that (1) the dynamic stress–strain curves of NiTi SMAs exhibited a work-hardening behavior; (2) the specimen temperature changed during dynamic deformation, which was monitored using a small thermocouple; and (3) the higher the deformation temperature [in the range of 273 K to 323 K (0 °C to 50 °C)], the stiffer the stress–strain behavior. In addition to the mechanical properties of NiTi SMAs, the microstructures and phase transformation characteristics after high strain rate loading were also studied. Void nucleation and growth were observed in NiTi SMA specimens after dynamic loading by Miller et al.[43] After impact loading (300 to 1500 s 1), the austenite start and finish temperatures in the first heating cycle were found to be independent of the strain rate.[44] Liu et al.[30] conducted high strain rate compression on martensitic NiTi SMAs and found that the phase transformation characteristics were dependent on the amount of strain instead of strain rate. Recently, a systematic investigation of the mechanical response of NiTi SMAs at high strain rate have been performed by Adharapurapu et al.[31,45,46] However, changes in the microstructure, phase transformation characteristics, and crystal structure after high strain rate deformation have only recently been studied by the authors here.[47,48] In the previous papers, the mechanical behavior under high strain rate and quasi-static loadings
and relevant effects have been investigated. To date, changes in the microstructure, phase tran
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