Mechanical and Phase Transformation Behavior of Plastically Strained NiTi-based Shape Memory Alloys
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Mechanical and Phase Transformation Behavior of Plastically Strained NiTi-based Shape Memory Alloys Peng Yu, and Qizhen Li Chemical and Metallurgical Engineering, University of Nevada, Reno, Reno, NV, 89557 ABSTRACT This paper examines the effect of plastic deformation on the strength and transformation heat and temperatures of the superelastic Nitinol samples. Two types of compression tests and differential scanning calorimetry tests were conducted. The first type of compression test deformed the samples to final fracture, and the second type included two loading –unloading cycles with or without plastic deformation. The mechanically tested samples were analyzed using differential scanning calorimetry.
INTRODUCTION NiTi-based SMAs (shape memory alloys) attracted great research interest and had broad applications due to their good mechanical properties, since Buehler et al. discovered shape memory effect of NiTi alloy in 1963[1]. The various applications include actuators, electric switches, pipe couplings, mobile phone antennas, eyeglass frames, dental braces, and endovascular stents. Extensive researches were performed to understand phase transformations, shape memory and superelasticity effects, and thermomechanical properties of SMAs, as thoroughly reviewed by Otsuka et al [2]. Recently, researchers start to study fatigue and fracture of SMAs[3-10], since cyclic loading is often encountered. The studies are mainly constrained to have only the elastic deformation and phase transformations during cyclic loading. However, little research has been done to investigate the plastic deformation effect on the following mechanical property and transformation behavior. In the above-mentioned applications, there are the cases that the structures experiencing plastic deformation during the service life. This paper studies the influence of plastic deformation on the strength, and transformation heat and temperatures of the superelastic Nitinol samples using compression tests and differential scanning calorimetry tests.
EXPERIMENT Room temperature compression tests and differential scanning calorimetry tests were performed on the samples of superelastic Nitinol alloy. The Nitinol SE508 wire with the 1.6mm diameter was obtained from NDC (Nitinol Devices and Components). The material composition is 55.8wt% nickel and 44.2wt% titanium. The wire was cut to make the samples with the aspect ratio of 1.8-2.2 for room temperature compression tests at a cross-head speed of 0.05mm/min. There were two types of compression tests. First, the samples were compressed till final fracture. Second, the samples were tested to either right before the initiation of plastic deformation or a small plastic deformation, followed by unloading, reloading to either right before the initiation of
plastic deformation or a small plastic deformation and unloading. Differential scanning calorimetry tests were run on the as-received sample and the mechanically tested samples. The temperature range is from -90 to 200oC, and the heating/cooling
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