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My research activities center around
VULCAN,
the materials science and engineering diffractometer at the SNS. Current
research topics include the following.
 | Mechanical properties. The macroscopic
mechanical properties are determined by the microstructure. Neutron
diffraction is well suited for the determination of strain, stress, and
texture. Dislocation density can be determined by analyzing the detailed peak
shape profile of broadened diffraction peaks. Limited information on grain
size and shape may also be obtained from the peak shape analysis. When the
grains of interest are of nanometer size, small angle neutron scattering will
become a useful tool. Simultaneous diffraction and small angle scattering
measurements, as would be realized with the
VULCAN
diffractometer at the SNS, will give an added advantage for characterizing the
microstructure of deformed grains.
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| Phase transformation. Controlled
decomposition of bulk metallic glass precursors offers a promising approach
for synthesis of bulk nanostructured materials in large quantities. The
decomposition of bulk metallic glass involves multi-stage phase separation and
amorphous-crystalline phase transformation. By studying the kinetics of the
phase transformation, with small angle X-ray and neutron scattering, we hope
to understand the fundamental physics underlining decomposition process in
bulk metallic glass.
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| Magnetism. Recent interests in amorphous Nd-Fe-Al
were prompted by the discovery of novel magnetic properties that these
materials possess. Nd60Al10Fe20Co10,
for example, is a hard magnet whose magnetic properties relate closely to the
crystallization states of the material. In the as-cast (amorphous) state, the
material is a hard magnet with a coercive field greater than 300 kA/m. After
annealing at T>740 K, the room temperature coercive field decreases
drastically and vanishes for samples annealed at T=770-780 K. We are
interested in how the magnetic correlation is affected by the extent of
crystallization. New grain structures are formed after annealing, as revealed
by recent in-situ small angle scattering experiments with synchrotron X-ray.
It is also interesting to see how the change in magnetic correlation length is
related to the new grain structures.
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| Neutron Scattering Instrumentation. A
design goal for VULCAN is to enable spatial mapping with 0.1 mm resolution.
Because the targeted applications often involve the use of large samples or
special environment, slits cannot be used for this purpose. Methods are being
developed to achieve 0.1 mm spatial resolution. For the incident beam, a new
compact neutron lens is proposed. The device will be made of a stack of bent
silicon wafers, each having a reflective multilayer (supermirror) deposited on
one side and a neutron-absorbing layer on the other side. Full theory for the
design has been worked out and verified with Monte-Carlo simulation. For the
diffracted beam, imaging devices made from thick packets of diffracting bent
silicon wafers (known as the Bragg Mirrors) will be used. |
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