Peering Through Steel: Lab Scientist Develops New Testing Technique

May 15, 1998

By Lynn Yarris, [email protected]

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

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.

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