NSTI Nanotech 2009

Finite-size effects in Fe-nanowire solid-solid phase transitions

L. Sandoval, H.M. Urbassek
University of Kaiserslautern, DE

Keywords: solid-solid phase transitions, nanowires, elasticity


Recent research on nanoscale systems has shown that nanowires exhibit astonishing mechanical properties. As examples, we mention the effect of the nanowire cross section in the stress-strain relation in Ni, the orientation-dependent non-linear elasticity in Cu, the mechanical response of Au and Pt nanowires under axial tensile deformation at different strain rates. Structural transformations have also been investigated: pseudo-elasticity or shape memory showing a critical temperature in fcc metallic nanowires, the pressure-induced phase transition in AlN and the martensitic phase transformation induced by stress in NiAl. In this work we investigate the solid-solid phase transition from a bcc to a close-packed crystal structure in cylindrical iron nanowires. Using classical molecular dynamics, we study the martensite (and austenite) transition and discuss its dependence on the nanowire diameter, the heating/cooling rate and an applied axial stress. The interatomic potential employed has been shown to be capable of describing the martensite-austenite phase transition in iron, which reproduces qualitatively the bcc-fcc phase transition. Additionally we study the stress vs strain curves for different temperatures and show that for a range of temperatures it is possible to induce a solid-solid phase transition by axial strain before the elasticity is lost; these transition temperatures are below the bulk transition temperature. The two phases have different (non-linear) elastic behavior: the bcc phase softens, while the close-packed phase stiffens with temperature. Our results are of predictive nature and may bear consequences on the design of nanoelectromechanical systems, since nanowires with novel properties might be included as interconnectors, sensors or actuators.
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