Forty-niner wide receiver Jerry Rice, like other
exceptionally durable athletes, has been called an "iron man." Through 12 NFL
seasons he never missed a game because of injury. Last year, however, injuries prematurely
ended his season, demonstrating that even men of steel have their limits.
 Jin Chan (left), Bill Morris and Seung-Hyuk Kang of
Material Sciences are correlating structural changes in steel to subtle changes in its
magnetic properties. Photo by Don Fike
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Steel, the material, has limits, too. Through repeated use, the metal can eventually
fail. Although steel products can be tested for structural integrity, by the time
microcracks or other evidence of pending failure are detected it is usually too late. What
is needed is an early, non-destructive means of evaluating steel and other metals.
J. William Morris, a metallurgist who heads high-performance metals research at
Berkeley Lab's Center for Advanced Materials, may have a solution. He and his group,
working in collaboration with the research groups of John Clarke and Kannan Krishnan of
the Materials Sciences Division (MSD), are investigating a technique whereby changes in
the magnetic properties of a steel sample that resulted from thermal or mechanical stress
are correlated to changes in its microstructure.
"We know that changes in the microstructure of a sample of steel can be detected
through subtle changes in its magnetic properties, but the trick is to be able to detect
those subtle magnetic changes and associate them with a microstructural change," says
Morris.
The Berkeley Lab collaborators are working to achieve this hat trick through the use of
two unique microscopes--a high-Tc SQUID microscope, which permits samples to be studied at
room temperature, and a transmission electron microscope that can be used to characterize
magnetic materials.
"A SQUID-based microscope is enormously sensitive to changes in magnetic fields,
and this one can be transported for in-situ [on-site] inspections," says Morris.
"The electron microscope allows us to evaluate the microstructure at high
resolutions."
A potentially serious problem in the aerospace and automotive industries is the sale or
inadvertent use of previously used parts, parts that are labeled as new but may actually
be only a relatively few (as in a million or so) cyclic loads from fatigue failure.
Metallurgists will tell you that it is not at all unusual to find fatigued steel parts
in the critical components of bridges, buildings, highway overpasses, airplane wings,
turbine blades, and even nuclear reactors.
"It is enormously difficult to distinguish new from old metal unless there are
obvious nucleated cracks," says Morris. "Right now, there is no way to test the
lifetime of a metal in a practical manner. A buyer must accept a seller's word that the
product is new."
Even though metallurgists have been aware that the magnetic properties of steel can
change over time as the material undergoes various forms of stress, these changes were
thought to be too small to have an engineering effect. What was really missing was a
device that was sensitive enough to detect these subtle changes, yet practical to use.
Enter the new a high-Tc SQUID microscope developed by Clarke and his group. SQUIDs
(Superconducting QUantum Interference Devices) are tiny detectors about the size of the
period at the end of this sentence. Used to measure magnetic fields, they are among the
most sensitive detectors known to science.
Whereas other SQUID microscopes use metallic SQUIDs that operate at near absolute-zero
temperatures, meaning they must be chilled with liquid helium, Clarke's new microscope
employs a high-Tc SQUID (for high-critical temperature) fashioned from a ceramic oxide
material that operates at liquid nitrogen temperatures.
The use of liquid nitrogen as the coolant is the key to this microscope's unique
ability to measure samples at room temperatures. Inside a vacuum chamber, a high-Tc SQUID
is mounted atop a can of liquid nitrogen. Separat-ing the SQUID from the world outside the
vacuum chamber is a window of silicon nitride. Samples can be placed on the outside
surface of this window or scanned across it to produce a magnetic image. In either mode,
the sample is at all times outside the vacuum chamber.
In addition to a relatively large scanning area--approximately 2500 square millimeters
(four square inches)--this new microscope is also equipped with a miniature tensile stage
that is capable of applying stress to the sample while the high-Tc SQUID measures the
sample's magnetic properties.
As Tim Shaw, a member of the Clarke research group working on the microscope explains,
"Our aim is to improve on previous studies by recording two-dimensional magnetic
images of steel with a spatial resolution of approximately 100 microns as the material is
being stressed."
The characterization work is being done at the National Center for Electron Microscopy
on the new Philips CM200. This TEM is specially equipped for "Lorentz imaging,"
a microscopy technique that allows scientists to do nanometer-scale resolution studies of
magnetized samples.
"The combination of a highly coherent field emission source, lens editing
software, and an image filter with digital-image capturing capabilities creates a
state-of-the-art instrument for static and dynamic magnetic observations," says
Krishnan, who oversees operations on the CM200.
In a proof-of-principle experiment, the researchers used heat to deliberately deform a
known sample of 1040 steel. The deformed sample was then magnetized, scanned with the
high-Tc SQUID microscope, then characterized with the CM200.
Says Morris: "Preliminary measurements indicate that the magnetization of steels
depends strongly on their thermal and mechanical treatment, and that the microstructure
responsible for the magnetization can be characterized on the surface by a magnetic
etching technique and high-resolution Lorentz imaging."
In addition to shedding light on the underlying physics of magnetic behavior, Morris
and his collaborators would like to eventually create a topology of magnetic signatures
for various types of steel that could be used to predict likely structural failure long
before the appearance of any microcracks.
