Neutron Sciences 2008 Annual Report - 17.79 MB - Spallation ...

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EDUCATION AND OUTREACH 2008 ANNUAL REPORT
ORNL NEUTRON SCIENCES neutrons.ornl.gov
Summer student Meaghan Riemer presents
a poster at the Neutron Sciences student
poster session.
Student Internships
Every year the Neutron Sciences sponsors
internships for high school and
college students. In 2008, we hosted
a record 42 students. Applications
from each student are reviewed, an
interview is conducted, and selected
students are assigned to areas best
suited to their paths of study and
interest. Each student is assigned
a mentor, who is responsible for
overseeing the student’s work and
for providing opportunities for the student to learn
and grow from the experience.
Postdoctoral appointments are also made throughout
the year. During the past year, five postdoc assignments
were made.
Contact: Bob Martin (martinrg@ornl.gov)
neutrons.ornl.gov/jobs
Summer Student Amazed by Neutron Scattering Results
If a pin is rotated against a metallic alloy 10 times at pressures perhaps 10 to 100 thousand times
the ambient pressure range, this rubbing effect can cause severe plastic deformation, compressing the microstructure
beneath the pin to a nanostructure that can be measured using SANS. This new method of metal grain refinement, called high-pressure
torsion (HPT), can reduce metal grains, ranging in size from 20 to 200 micrometers, to as tiny as 20 nanometers—or 1,000 to 10,000 times smaller than
the initial grain size.
Meaghan Riemer, a summer student from Clemson University, collaborated with Xun-Li Wang, Sheng Cheng, Ken Littrell, and Ducu Stoica, all of the Neutron Scattering Science
Division, to answer this question: if nanometer-sized grains of different nickel alloys are subjected to HPT, how will the microstructure change? To determine the answer,
the team conducted SANS experiments at HFIR.
“We experimentally addressed questions about different alloy samples with different grain sizes to determine how their internal structure changed after high-pressure torsion,”
says Littrell, the instrument scientist for the General-Purpose SANS instrument at HFIR. “Within a few HPT cycles, the microstructures of all the samples, which
initially had different grain sizes, looked identical.”
“The upshot of this study is that there appears to be an optimum grain size in a metal or alloy subjected to high-pressure torsion, regardless of the material’s initial microstructure,”
says Xun-Li Wang, a materials scientist at SNS. “It appears that HPT crushes large grains to smaller sizes. When the grain size is too small—in the nanometer
range, for example—the grains are unstable and tend to grow under deformation.” He added that more experiments are needed to get a complete picture.
Reached at Clemson University, Meaghan Riemer said, “I’m really impressed with the capabilities of the SANS instrument at HFIR to capture the effects on metal grain sizes
of high-pressure torsion. More experiments may uncover some interesting applications of HPT in the material sciences, such as improving metal strength. I found it exciting
to observe the flexibility of SANS for doing different experiments. I hope to return to participate in additional experiments using SANS. My summer at ORNL was truly an
enriching experience.”