• Published January 1st, 2017

D. J. Forbes, J. E. Schaffer
Fort Wayne Metals Research Products Corp, Fort Wayne, IN, USA

Introduction

At Fort Wayne Metals, we are applying our knowledge of Nitinol alloys and processing to decrease the stress-temperature sensitivity of Nitinol and increase the temperature range where superelasticity is possible. The superelastic (SE) properties of Nitinol are generally used in moderate temperature environments. Room temperature and body temperature are the most common. In these environments, designers are usually limited to the so-called “superelastic window” spanning about 100°C [1]. Too cold, or below the active austenite finish temperature (Af) and loading may lead to deformation that is irrecoverable until warm-up, acceptable for shape memory (SM) but not for SE. Too warm (above martensite decist, Md) and the alloy will deform conventionally and irreversibly. Even within the superelastic window, temperature fluctuation will drive changes in the stress-induced-martensite onset stress according to the Clausius-Clapeyron relation.

Materials and Methods

NiTi-Co (FWM NiTi#3 [2]) grade wire was cold drawn to 1.5 mm diameter and isobarically annealed in Argon at about 400°C using a reel-to-reel tube annealer. The heat straightened material was tested in uniaxial extension using an Instron 5966 tester equipped with an environmental chamber.

Results

Figure 1 shows the uniaxial stress-strain response of the 1.5 mm NiTi#3 wire tested from -130°C to 90°C. We observed greater than 5% fully recoverable strain throughout the temperature range while maintaining axial strain to fractures exceeding 10% engineering strain.

Figure 1: Uniaxial Stress-Strain curves for a NiTi alloy wire tested at 90°C, 22°C, -20°C, -75°C and -130°C in air.

References:

1. Stoeckel, Dieter, Alan Pelton, and Tom Duerig. "Self-expanding nitinol stents: material and design considerations." European radiology 14.2 (2004): 292-301.
2. http://www.fwmetals.com/services/resource-library/nitinol-wire1/
3. Ahadi, Aslan, and Qingping Sun. "Effects of grain size on the rate-dependent thermomechanical responses of nanostructured superelastic NiTi." Acta materialia 76 (2014): 186-197.

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