Some crystalline grains hiding as deep as 1 millimeter (mm) beneath the mosaic of grains making up the surface of a hot-rolled polycrystalline aluminum sample have lost their privacy. Several ORNL scientists have spied on them using three-dimensional, or 3D, X-ray vision.
Taking advantage of the rainbow spectrum of laser-like X rays from the Advanced Photon Source (APS) at the Department of Energy’s Argonne National Laboratory, this ORNL group is the first to obtain a nondestructive 3D X-ray diffraction pattern of materials at a spatial resolution of less than a micron (a millionth of a meter). As a result, they can obtain 3D information on the sizes of various grains, their orientations, and their distortions in response to stresses induced by, say, forming a material at different temperatures.
Previously, X-ray mapping of crystalline structure in a material’s surface has been performed at the micron scale but only in two dimensions (e.g., on thin films). But, thanks to a clever addition to their apparatus, ORNL researchers have developed an X-ray technique that adds a new dimension to microstructure characterization. This ORNL achievement of 3D X-ray structural microscopy with submicron resolution was reported in a letter in the February 21, 2002, issue of Nature.
Gene Ice in ORNL’s Metals and Ceramics Division and Ben Larson of the Solid State Division (SSD) are co-principal investigators of this research funded by DOE’s Office of Basic Energy Sciences. Together with SSD’s John Budai and Jon Tischler and postdoctoral associate Wenge Yang of the Oak Ridge Institute for Science and Education, significant hardware and software developments have made it possible for these researchers to observe deep inside materials with submicron precision without dissection.
At the APS a collimated “white” beam consisting of the total range of X-ray wavelengths (colors) is passed through ORNL’s 3D crystal microscope, which focuses the X rays into a beam less than a micron in diameter. The focusing of the beam to a submicron spot is achieved through the use of differentially deposited mirrors, developed by Ice and Beamline Technology Corporation. These advanced elliptical mirrors earned Ice and Beamline Technology an R&D 100 Award in 2000 from R&D magazine as one of the 100 most significant innovations of the year.
The focused white beam strikes a large group of grains off which the X rays reflect. X rays of one wavelength (color) will reflect off grains oriented in one direction, and X rays of a different wavelength will reflect off grains pointed in another direction, and so forth. The X-ray diffraction pattern is captured by a charge-coupled device (CCD) X-ray area detector, which is carefully calibrated to measure the precise angles and intensities of the reflected X rays. From the CCD pattern, the computer determines the size, orientation, and distortion of the stressed grains.
The ORNL technique gained its third dimension through the use of an absorbing platinum wire that moves in small steps to block a few of the reflected X rays at a time. This knife-edge depth profiler, conceived by Larson in March 2000, is like a traveling pinhole camera in the way it controls which reflected X rays reach the CCD. By comparing CCD pictures and using triangulation, the computer resolves the depth of the grains. Larson and Wang developed the software that enables the analysis.
Although this ORNL research is fundamental in nature, it will affect practical areas such as the U.S. aluminum beverage can industry. To protect the environment and reduce cost, the industry continues to make drink can walls thinner and stronger using melted-down, recycled aluminum. The industry is interested in knowing how to get the right texture throughout the sheets for can making while reducing scrap.
“We generate about 2 gigabytes of 3D X-ray data an hour looking at the microstructure in materials,” Larson says. This information on microstructure changes is useful for computer simulations and modeling to optimize the property-determining microstructure of materials and to more accurately predict whether a process change would favorably or adversely affect microstructure. Theorists modeling aluminum microstructure include Gorti Sarma and Balasubramaniam (Rad) Radhakrishnan, both of ORNL’s Computer Science and Mathematics Division.
“Such detailed studies of the mesoscale features and dynamics in technologically important materials will enable more precise understanding of the mechanisms of fracture, such as a crack in a single-crystal turbine blade,” Ice says. Complemented by electron and neutron scattering, ORNL’s new 3D X-ray vision will make it harder for subsurface grains to keep their activities a secret.
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