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P5.04 - Modeling Phase Transformations in NiTi Shape Memory Alloy for Energy Harvesting Applications 
April 8, 2015   2:30pm - 2:45pm

Shape Memory Alloys (SMA) can exist in different phases based on the temperature and stress. When thermo-mechanically loaded, they can deform to a different shape. This feature makes them good candidates for sensors and actuators. A novel idea is to combine them with a piezoelectric material for harvesting thermal energy [1]. When a piezoelectric matrix embedded with SMA fibers is subjected to a heating-cooling cycle, the SMA converts the temperature gradient into a strain change stimulating the piezoelectric matrix to produce a voltage. To verify the feasibility of the idea using a computational model, the first step is to predict the behavior of the SMA accurately. We focus on NiTi since its transformation temperatures lie close to room temperature. Experimental evidence shows that SMA�s oscillate between two shapes during thermal cycling [2]. This phenomenon, known as two-way shape memory effect, occurs due to a transformation between the austenite and the single-variant martensite phase. This property of the SMA will be utilized in the harvester. Recent tests revealed the occurrence of length scale effects on martensitic transformations in NiTi at the nano-scale [3]. The harvester we intend to prototype uses SMA fibers with diameters in the order of 500 nm, making size effects relevant. In this study, the transformation temperatures, stress and size effects in the two-way shape memory phenomenon of cylindrical NiTi nano-wires are, for the first time, simulated and analyzed using molecular dynamics (MD) simulations. Preliminary results from MD simulations of the one-way shape memory effect indicate the formation of a higher density of martensite variants and variant interfaces and a reduction in the de-twinning stress, as the diameter of the nano-wire increases. Size dependency in the transformation to austenite, proved in nano-spheres [4], has also been observed in nano-wires. An asymmetry in the stress-induced transformation properties and mechanism during tension and compression is also seen. This analysis will be extended to the two-way shape memory effect. The results obtained from this atomistic study will be used in a larger scale model, to predict the thermo-electro-mechanical behavior of the harvester. References: [1] N. Nersessian, G. P. Carman, and H. B. Radousky. Energy harvesting using a thermoelectric material, US Patent 7,397,169 B2, 2008. [2] N. G. Jones, and D. Dye. Martensite evolution in a NiTi shape memory alloy when thermal cycling under an applied load. Intermetallics 19, 1348-1358, 2011. [3] T. Waitz, T. Antretter, F. D. Fischer, N. K. Simha, and H. P. Karnthaler. Size effects on the martensitic phase transformation of NiTi nanograins. Journal of the Mechanics of Physics of Solids 55, 419-444, 2007. [4] D. Mutter, and P. Nielaba. Simulation of the thermally induced austenitic phase transition in NiTi nanoparticles. Eur. Phys. J. B 84, 109-113, 2011.

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