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MM: Fachverband Metall- und Materialphysik

MM 41: Topical Session Diffussionless Transformations II

MM 41.6: Talk

Thursday, March 17, 2011, 17:30–17:45, IFW D

Twin boundary rearrangement in magnetic shape memory alloys studied with a phase-field model — •Frank Wendler, Christian Mennerich, Marcus Jainta, and Britta Nestler — IMP, Karlsruhe University of Applied Sciences

The observed large magnetic field induced strain (MFIS) in magnetic shape memory alloys is characterized by a structural rearrangement of martensite variant fractions. The complicated interdependency of microstructure - stress - magnetic field in the alloy Ni2MnGa motivates this numerical study. The displacive phase transition in the free boundary problem is treated by a phase-field model, based on the formulation of chemical, micromagnetic and magneto-elastic free energy densities. For the isothermal situation of martensite nucleation in the parent phase and twin boundary motion in the martensitic state, order parameters for variants and parent phase are introduced and related to their eigenstrain. The parameters are evolved according to Allen-Cahn dynamics, whereas a time dependant wave equation is solved for the field of elastic displacements. The magnetization field is updated by solving the Landau-Lifshitz-Gilbert equation with a geometric integration scheme. First, a verification of the variational approach is given for the subproblems of either magnetic or elastic fields interacting with the variant structure. Then, we focus on the reversible transformation process in single crystals, where the coupled evolution of magnetic domains and twin variants is studied and examples for resulting magnetization and strain reaction are given. An extension of the approach to treat polycrystalline materials is discussed.