title: Zig-zag twins and helical phase transformations.
abstract: We demonstrate the large bending deformation induced by an array of permanent magnets (applied field ∼0.02 T) designed to minimize poles in the bent state of the crystal. Planar cantilevers of NiMnGa (5M modulated martensite) ferromagnetic shape memory alloy deform into an arched shape according to theory, with a zig-zag microstructure that complies with the kinematic and magnetic compatibility between adjacent twin variants. A general theory of bent and twisted states is given, applicable to both twinning and austenite/martensite transformations. Some of these configurations achieve order-of-magnitude amplification of rotation and axial strain. We investigate also atomistic analogues of these bent and twisted configurations with perfect interfaces between phases. These mechanisms of large deformation, induced by small magnetic fields or temperature changes, have potential application to the development of new actuation technologies for micro-robotic systems.
authors.affiliation: Department of Aerospace Engineering and Mechanics, University of Minnesota, Minneapolis, MN 55455, USA.
authors.forename: Yaniv
authors.initials: Y
authors.lastname: Ganor
authors.affiliation: Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455, USA.
authors.forename: Traian
authors.initials: T
authors.lastname: Dumitrică
authors.affiliation: Department of Aerospace Engineering and Mechanics, University of Minnesota, Minneapolis, MN 55455, USA.
authors.forename: Fan
authors.initials: F
authors.lastname: Feng
authors.affiliation: Department of Aerospace Engineering and Mechanics, University of Minnesota, Minneapolis, MN 55455, USA james@umn.edu.
authors.forename: Richard D
authors.identifier: http://orcid.org/0000-0001-6019-6613
authors.initials: RD
authors.lastname: James
keywords: Ni2MnGa; bending; continuum mechanics; ferromagnetic shape memory; martensitic phase transformation