Molecular dynamics simulations of the two-way shape-memory effect in NiTi nanowires
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Molecular dynamics simulations of the two-way shape-memory effect in NiTi nanowires
Prashanth Srinivasan1 , Lucia Nicola2 , Barend Thijsse2 , Angelo Simone1 1 Faculty
of Civil Engineering and Geosciences, Delft University of Technology Delft, The Netherlands
2 Faculty
of Mechanical, Maritime and Materials Engineering, Delft University of Technology Delft, The Netherlands
ABSTRACT Shape memory alloys (SMAs) exist in different phases depending on temperature and stress level. Experimental evidence shows that SMAs oscillate between two shapes during thermal cycling. This phenomenon, known as two-way shape-memory effect, occurs due to a transformation between the austenitic phase and the martensitic phase. The two-way shape-memory behavior is studied here by molecular dynamics simulations in NiTi nanowires of different diameter to understand the effect of loading on the size-dependent behavior. Thermal cycling is performed while holding the system at zero stress and at a fixed compressive stress. At zero stress, the martensite structure formed on cooling depends on the wire diameter. However, when cooling is performed at a sufficiently large constant compressive stress, the formation of a single martensitic variant is observed for all diameters. INTRODUCTION Shape memory alloys are unique materials which are increasingly being used in various applications at the micro- and nano-scale [1]. Temperature-dependent response imparts special properties to these alloys. The two-way shape-memory effect (TWSME) is the ability of the SMA to remember two shapes at two different temperatures. This arises due to a transformation between the high temperature austenitic phase and a low temperature martensitic phase. Upon cooling, bulk austenite forms a multi-variant martensitic structure if not stressed, whereas it forms a single variant martensite under tension or compression [2, 3]. Jones and Dye [4] studied experimentally the microstructural evolution of the martensitic phase in 1.5 mm × 1.5 mm × 19 mm NiTi specimens when they were thermally cycled under an applied load. They concluded that a particular martensitic variant grows along the load axis upon cooling to accommodate the applied strain, thereby leading to the TWSME. The austenitic phase of NiTi has a cubic B2-type crystal structure. Upon cooling bulk NiTi at zero stress below a particular transformation temperature, the B2 structure transforms into a B19’ type monoclinic martensitic twinned structure. However, in nano-systems, it was reported that the
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B2 structure upon cooling at zero stress transforms into an orthorhombic B19-type martensite [5–8] dominated by a single variant. The effect of size on martensitic transformation was also studied using transmission electron microscopy by Waitz et al. [9] in nano-crystalline NiTi. They observed that thermally-induced multi-variant martensitic transformation does not occur in grains smaller than 60 nm. Since the nano-crystals in this experiment were not free but part of a polycrystal, it is unknown what was their stress
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