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A Thermo-Magneto-Mechanical Free Energy Model for NiMnGa Single Crystals

Published online by Cambridge University Press:  01 February 2011

Phillip Morrison
Affiliation:
morrison.phillip@gmail.com, North Carolina State University, Dept. Mech. & Aero. Eng., 3211 Broughton Hall, Raleigh, NC, 27695, United States
Stefan Seelecke
Affiliation:
stefan.seelecke@ncsu.edu, North Carolina State University, Dept. Mech. & Aero. Eng., 3211 Broughton Hall, Raleigh, NC, 27695, United States
Manfred Kohl
Affiliation:
manfred.kohl@imt.fzk.de, FZ Karlsruhe, Karlsruhe, N/A, Germany
Berthold Krevet
Affiliation:
berthold.krevet@imt.fzk.de, FZ Karlsruhe, Karlsruhe, N/A, Germany
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Abstract

The paper extends the authors' recent model for one-dimensional rate-dependent magneto-mechanical behavior of NiMnGa single crystals to account for temperature-dependent effects including austenite/martensite and ferro-/paramagnetic phase transitions. The magneto-mechanical model is based on the Helmholtz free energy landscape constructed for a meso-scale lattice element with strain and magnetization as order parameters. This two-dimensional energy landscape includes three paraboloidal wells representing the two easy-axis and one hard-axis martensite variants relevant for the structurally one-dimensional case. Phase transformations resulting from applied stresses and magnetic fields follow from a system of evolution laws based on the Gibbs free energy equations and the theory of thermally activated processes, which in the low-thermal-activation limit appropriately reproduce the athermal transformation behavior observed in these materials. The phase fractions subsequently determine the macroscopic strain and magnetization of a sample of NiMnGa by means of a standard averaging procedure. To account for the first-order phase transitions to austenite, additional temperature-dependent wells representing the stable states of austenitic NiMnGa are introduced into the Helmholtz energy landscape. The transition from ferromagnetic to paramagnetic states is modeled as a second order transformation based on the gradual degeneration of the ferromagnetic wells with increasing temperature.

Type
Research Article
Copyright
Copyright © Materials Research Society 2008

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References

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