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This page is currently (February 2007) under construction...
Currently, there are four active research programs in our group:
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| Domain Walls and Twin Boundaries in Ferromagnetic Shape Memory Alloys [NSF] | |
Alloys of the type Ni2MnGa exhibit a phenomenon known as ferromagnetic shape memory, meaning that they can change shape under an applied magnetic field. These alloys belong to the class of the multiferroic materials, which can respond to multiple externally applied fields. Upon cooling from the melt, this alloy undergoes two ordering transformations (disordered to B2 and B2 to L21, also known as Heusler ordering), a paramagnetic to ferromagnetic transition at around 380 K, and a martensitic transformation from cubic to tetragonal or orthorhombic at a temperature that depends sensitively upon the alloy composition.  The crystal structures of the cubic (left) and tetragonal (right) phases are shown in the figure on the left; Ni atoms are green, Mn red, and Ga blue. The arrows indicate the atomic magnetic moments, with a dominant contribution from the Mn atoms. The unit cell parameter is about 0.56 nm for the cubic phase, and the tetragonal phase, which has a large magnetocrystalline anisotropy, has a c/a ratio of about 0.94. |
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In this project, we employ Lorentz transmission electron microscopy (LTEM) to study the nature of magnetic domain walls, and the interactions of those walls with other microstructural features, such as |
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Recent Publications:
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| Phase reconstruction and vector field electron tomography [DOE] | |
| The increasing complexity of today's magnetic materials systems must be accompanied by improvements in the methods used to study those materials. In this project, which is a continuation (renewal) of research carried out in a previous DOE grant, we apply phase reconstruction based on the Transport-of-Intensity Equation (TIE) to the three-dimensional study of magnetic field distributions around magnetic samples. The program consists of an experimental component and a modeling component. The modeling component evaluates the accuracy and efficiency of phase reconstructed vector field electron tomography, by using numerical simulations and analytical evaluation of error propagations. This analysis will eventually result in a set of experimental conditions which must be satisfied to carry out successful 3D vector field reconstructions. The experimental work involves acquisition of tilt series phase maps using both the TIE formalism and electron holography. Samples for the experiments will include tips for a magnetic force microscope, patterned microstructures of magnetic material (permalloy and Fe-Co), and natural (magnetotactic bacteria) and man-made (colloidal Cobalt particles) configurations of magnetic nano-particles. The resulting 3D magnetic induction maps will be compared with the results of extensive micromagnetic modeling. The program will also further develop methods for the computation of magnetostatic interactions between nano-particles. | |
| Experimental and Computational Tools for the Digital Representation and Prediction of Microstructures and its Incorporation in the Designer’s Knowledge Base [ONR] | |
| Representation and Reconstruction of three-dimensional microstructure in Ni-based superalloys [AFOSR] | |
| Unfunded Research (stuff we do for fun...) | |
| 1. A Modern 3-D View of an “Old” Pearlite Colony
Original Data: 242 slices of 1 micron thickness each, obtained by serial sectioning and optical microscopy. The dark lines enclose the cementite phase. The original slices were digitized by Milo Kral of the University of Canterbury, New Zealand, and subsequently aligned with respect to each other using cross-correlation techniques. The complete stack of 242 slices was then thresholded to extract the 3D pearlite colony. Subsequently, various rendering methods were used to visualize the intricate lamellar structure of the colony. The full results of this study were published in the December 2006 issue of JOM.
On the left, the nucleation site of the cementite stack is visible; the entire colony started at this point and branched out in two different directions. The other images (b and c) show a single cementite lamella, view from two different directions.
The three images above show the complete reconstructed stack (a), and two portions of the stack, each about one quarter of the total volume.
This image was obtained from two separate reconstructions and can be viewed with red-blue stereo glasses. It shows a 50 micron thick section through the entire pearlite colony. Two different lamellar orientations are clearly observable. |
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| © Marc De Graef, 2007 |
anti-phase boundaries and martensite variant boundaries (twin boundaries). The image to the right shows a magnetization map of an austenitic sample (at room temperature); the width of the image corresponds to a distance of about 4 microns. The colors indicate the direction of the local magnetization vector: red indicates a magnetization pointing from left to right, green from right to left, yellow from top to bottom, and blue from bottom to top. The curved boundaries between the colored regions correspond to anti-phase boundaries, i.e., defects in the lattice ordering of this alloy. The interactions between these anti-phase boundaries and the magnetic domain walls affect the magnetic coercivity of this alloy, and form the subject of intense micro-structural investigations.
Around 1960, Mats Hillert (now at the Royal Institute of Technology, Sweden) performed a serial sectioning metallography experiment on a pearlite colony in a carburized electrolytic iron. His original data set has been digitized and re-analyzed using modern 3-D tools, such as volume rendering and red-blue stereoscopic imaging. The images below show some of the results of this study.
