A Thermo-Magneto-Mechanical Free Energy Model for NiMnGa Single Crystals
- PDF / 522,887 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 61 Downloads / 138 Views
1050-BB03-04
A Thermo-Magneto-Mechanical Free Energy Model for NiMnGa Single Crystals Phillip Morrison1, Stefan Seelecke1, Manfred Kohl2, and Berthold Krevet2 1 Dept. Mech. & Aero. Eng., North Carolina State University, 3211 Broughton Hall, Raleigh, NC, 27695 2 FZ Karlsruhe, Karlsruhe, Germany ABSTRACT The paper extends the authors’ recent model for one-dimensional rate-dependent magnetomechanical behavior of NiMnGa single crystals to account for temperature-dependent effects including austenite/martensite and ferro-/paramagnetic phase transitions. The magnetomechanical 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. INTRODUCTION Ferromagnetic shape memory alloys (FSMA) are of great interest because of their potential for fast actuation, particularly compared to conventional shape memory (SMA) actuators which rely on purely thermal actuation. However, the magnetic field strength necessary to trigger the martensitic transformations behind the actuation typically requires large electromagnets leading to a rather bulky overall system design. Recently, Kohl and co-workers [1,2], proposed a novel NiMnGa thin-film actuator, which is only several millimeters in size. The device is designed as a small cantilever beam with a mirror attached to its free capable of performing, e.g., optical scanning tasks at a frequency of up to 200Hz while achieving scanning angles of almost 180 degrees.
The key to the design is that the actuator only requires a small permanent magnet by making use of not only the conventional shape memory effect but also the temperature-dependent transition from ferro- to paramagnetic state. It is the objective of this paper to introduce a one-dimensional model for magnetization and strain in response to a longitudinal stress component, a transversally applied magnetic field and varying temperature that will capture the fully coupled thermo-magneto-me
Data Loading...
