NEW X-RAY IMAGING TECHNIQUE REVEALS CRYSTAL DEFECTS.

The deformation of metals takes place primarily by the motion and interaction of defects in the crystal structure known as dislocations. Quantitative understanding of this deformation, needed for modeling of metal-forming processes, has been hampered by a lack of knowledge about the complex configurations assumed by these defect microstructures when the metals are deformed.

A new experimental technique, ultra small angle x-ray scattering (USAXS) imaging, has been shown by NIST to hold promise as a useful tool for studying microstructures in situ. The technique is based on forming an image of the sample using x rays which have been scattered from defects in the crystal structure. Although these x rays scattered from defects are very weak, the USAXS technique allows them to be isolated from the high-intensity background to form an image of the defects only. The scattering from these components can be detected down to around one seventh to one tenth of the intensity of the main transmitted x-ray beam. Preliminary tests of this technique were made in March 2000, using copper samples in which defects had been produced by a very slow deformation treatment. A more thorough study was conducted in May 2000. The tests were very successful and microscopic damage was imaged using several different scattering conditions, A basic theory for the image formation process in USAXS imaging has bee n worked out and it was validated by experiments completed in September 2000. The technique is believed to be a major breakthrough with broad potential applications, including the study of the defects which control metal deformation behavior. A paper on USAXS imaging has been accepted for publication.