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Microstructural characterization of shape memory alloys on the atomic scale
The unique properties of shape memory alloys (SMAs), such as the pseudoelastic effect, are caused by a solid-to-solid phase transformation. These materials can undergo large strains in a fully reversible way, due to a change from a cubic austenitic to a highly twinned martensitic phase. However, the effect is subject to fatigue under cyclic loading and it is assumed that accumulation of dislocations impedes the phase transformation. Furthermore, an orientation dependent mechanical response to uniaxial loading of SMA single crystals has been observed experimentally. Here, molecular dynamics simulations of nanopillar compression are performed, in which the interaction between dislocations and phase-transformation are studied in detail by the application of advanced data analysis methods. By virtue of these atomic scale methods, the macroscopic material response is immediately related to the constantly changing microstructure in the material. The active deformation mechanism of the pillars, detected by atomistic simulation, ranges from ideal pseudoelasticity to severe plastic deformation, depending purely on the pillar orientation.