Magnetostrictive Galfenol (Fe-Ga) is a promising new active material. Single crystals of Galfenol have been shown to
exhibit up to 400 ppm magnetostrictive strains with saturating fields of several hundred oersteds. Its robustness and
ability to actuate in either tension or compression allows for new actuator and sensor designs. However, due to the high
permeability of Galfenol, it needs to be in thin sheet form for many device applications to minimize eddy current losses.
Work is underway to develop conventional rolling processes to produce large quantities of thin Galfenol sheet, while
retaining a preferred <100> crystallographic texture to optimize magnetostrictive performance. Knowledge of
deformation behavior at elevated temperature is crucial to understanding formability and crystallographic texture
evolution during rolling. In this work, the high-temperature plasticity and the deformation behavior of polycrystalline
Galfenol were investigated using conventional axial compression tests and rolling experiments. As the temperature
increased, significant softening of the material occurred in the temperature range from about 450°C to 800°C. The
results also suggested that significant dynamic recovery and recrystallization occurred during deformation at above
Galfenol (Fe-Ga) is a promising and mechanically robust magnetostrictive actuator material. However, due to its high conductivity, it needs to be in thin sheet form to avoid excessive eddy current losses. Work is underway to develop conventional rolling processes to produce large quantities of thin Galfenol sheet, while retaining a preferred <100> crystallographic texture to optimize magnetostrictive performance. Knowledge of high temperature polycrystalline plasticity is crucial to understanding formability and crystallographic texture evolution during rolling. The deformation behavior of polycrystalline Galfenol at high temperatures was studied. Preliminary results suggest that significant dynamic recovery and/or recrystallization occur during deformation, resulting in a random texture. In-situ neutron diffraction experiments are being developed to obtain qualitative and quantitative information on the high temperature plane strain deformation of Galfenol. These experiments will be used to identify the slip systems that contribute to plastic deformation, and their dependence on temperature. Simultaneously, models of large-scale polycrystal plasticity are being developed to predict internal strains and texture evolution during deformation, which will be validated against the data obtained from the neutron diffraction experiments. Ultimately, the models will be used to develop thermo-mechanical treatments to optimize texture evolution during rolling.
NiMnGa-based magnetic shape memory (MSM) alloys have attained magnetic-field-induced strains up to approximately 10%, making them very attractive for a variety of applications. However, for applications that require the use of an alternating magnetic field, eddy current losses can be significant. Also, NiMnGa-based MSM alloys' fracture toughness is relatively low. Using these materials in the form of particles embedded in a polymer matrix composite could mitigate these limitations. Since the MSM effect is anisotropic, the crystallographic texture of the particles in the composites is of great interest. In this work, a procedure for fabricating NiMnGa-based MSMA/elastomer composites is described. Processing routes for optimizing the crystallographic texture in the composites are considered.
The magnetic shape memory (MSM) effect occurs in some ferromagnetic martensitic alloys at temperatures below the martensite finish temperature and involves the re-orientation of martensite variants by twin boundary motion, in response to an applied stress and/or magnetic field. The driving force for twin boundary motion is the magnetic anisotropy. In this study, magnetization measurements as a function of magnetic field were made on several oriented single crystals of Ni-Mn-Ga alloys using a vibrating sample magnetometer. The magnetization versus magnetic field curves were characteristic of magnetically soft materials with magnetic anisotropy consistent with literature estimates for the different martensite structures observed in Ni-Mn-Ga alloys. Differences in the slope of the curves were due to the martensite structure, the relative proportion of martensite variants present, and their respective easy and hard axis orientations. Thermo-magneto-mechanical training was applied in an attempt to transform multi-variant specimens to single variant martensite. Training of the orthorhombic 7M martensites was sufficient to produce a near single variant of martensite, while the tetragonal 5M martensite responded well to training and produced a single-variant state. The strength of the uniaxial magnetic anisotropy constant for single-variant tetragonal 5M martensite, Ni<sub>52.9</sub>Mn<sub>27.3</sub>Ga<sub>19.8</sub>, was calculated to be K<sub>u</sub>=1.8 x 10<sup>5</sup> J/m3, consistent with literature values. To obtain single-variant martensites, heat-treatment of the specimens prior to thermo-magneto-mechanical training is necessary.
In the current work, repeated mechanical and magnetic forces have been applied to Ni-Mn-Ga samples with different compositions and different thermomechanical histories in order to determine the combined effects of these parameters on the magnetic shape memory effects, especially the magneto-mechanical properties, of these alloys. The results demonstrate that prior history has strong influence on the twinning start stress and twinning strain. In addition, heat treatment of the materials seems to increase the amount of strain that can be obtained (up to the theoretical limit). Moreover, there is indication that prior heat treatment may also affect the martensite crystal structure that is formed during cooling. In addition, the dependence of martensitic transformation on composition and prior thermomechanical treatments was also studied by differential scanning calorimetry (DSC) analysis.