Making It Last

Materials that last longer are a primary need of industry.

Neutron scientists have published numerous papers on how to improve industrial materials, including materials used in the paper industry itself.

Neutron scattering studies of steel tubes at ORNL provided essential data that helped improve the safety and longevity of the paper industry's recovery boilers, saving millions of dollars. An ORNL effort recently identified alternative tube materials and recommended new operating procedures to prevent the cracking observed in stainless steel–carbon steel tubing used in the boilers. Dozens of kraft paper mills in North America adopted ORNL's recommendations, avoiding the need for additional, costly shutdowns for inspections.

Using neutron diffraction at ORNL's High Flux Isotope Reactor, members of the Diffraction and Thermophysical Properties Group measured tube stresses as a function of temperature and processing parameters. "We found that tensile stresses were induced in the tubes' 304L stainless-steel clad layer, and that these stresses contributed to the stress corrosion cracking," says Xun-Li Wang, a materials scientist with the Spallation Neutron Source's Experimental Facilities Division.

Additional neutron diffraction studies and other tests showed that materials higher in nickel, such as alloys 825 and 625, were far more resistant to cracking than 304L stainless steel, the industry standard. Paper mills began installing tubes made of these super alloys—a success story for neutron scattering research.

The study of mechanical behavior represents a newer application of neutron scattering research. "Consumers want materials that last," Wang says. "The study of mechanical behavior tells researchers what makes materials strong. By measuring the change in spacing between atomic planes, we can ‘see' how materials deform at the microscopic level."

Wang leads the development of VULCAN, an engineering diffractometer at SNS, which will be used for more realistic studies of changes in the strength and stability of materials when heated in a furnace or placed under an applied load. The Canada Foundation for Innovation is funding VULCAN's construction. A National Science Foundation grant supports the development of the sample environment, including a load-frame, a furnace, and an electrochemical cell.

An ORNL team helped demonstrate the feasibility of conducting an in-situ welding experiment on a neutron-scattering instrument. They successfully mounted and operated a friction stir welding machine on an engineering diffractometer at the Los Alamos Neutron Science Center. Using a method developed at ORNL, the team was able to determine the weld temperature and stress from the in-situ data. Their exploratory research is supported by ORNL's Laboratory Directed Research and Development Program.

"We hope to advance this type of experiment at VULCAN to understand better how stresses induced by welding alter the weld's microstructure," Wang says. He explains that in the VULCAN experiments, a focusing neutron guide will increase the intensity of neutrons striking the samples, allowing experimenters to resolve how materials change over time under temperature and applied load. Adding a small-angle neutron scattering (SANS) capability to VULCAN will further allow researchers to characterize structure changes at two different length scales—as small as a few atoms to as large as several grains of metals.

Oxide-dispersion-strengthened ferritic alloys may be studied at VULCAN after it becomes operational in 2008. ODS alloys, which contain both nanoclusters and microparticles, are candidates for high-temperature structural materials in future energy production systems, such as turbines and nuclear reactors. Using an in-situ SANS facility at Hahn-Meitner Institute in Berlin, Wang and his ORNL colleagues found that nanoclusters formed during fabrication of the ODS ferritic alloy are stable up to 1400°C, explaining the alloy's extraordinary ability to resist creep deformation at high temperatures.

"We are studying how stable these nanoclusters are in the ODS alloy at high temperatures," Wang says. "Soon, we hope to use VULCAN to learn what happens to the nanoclusters under applied load."

Wang's work, along with that of his colleagues at the SNS, will provide a lengthy new chapter to the library of materials science.

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