Essentially all structural materials have a regular crystal structure. For example, depending on the elements with which the iron is alloyed, steels may be either body-centered-cubic or face-centered-cubic. To a great extent, the physical and mechanical properties of these materials are determined by the nature and concentration of defects in the crystals. The importance of these defects is reflected in a comment from Sir Charles Frank, “Crystals are like people: it is the defects in them that make them interesting.”
The defects of interest include point defects such as vacant lattice sites (vacancies), one-dimensional line defects called dislocations, and internal interfaces such as twins and grain boundaries. Only vacancies would be present in thermal equilibrium, but finite concentrations of the other defects exist in all practical materials as a result of the processes used in fabrication and processing. The initial defect structure will evolve in service due to thermal and mechanical loads, particularly if the material is exposed to neutron or ion irradiation which can lead to the formation of a complex catalog of non-equilibrium defects. The study of defect physics refers to research aimed at understanding how these defects form and interact to create both the desirable and undesirable properties of these materials. Materials engineering can be viewed as the effort to use this knowledge to create new materials or to mitigate undesirable property changes.
Research in defect physics at ORNL includes a broad array experimental and computational approaches. Experimental work involves neutron scattering and diffraction measurements at both the Spallation Neutron Source and the High Flux Isotope Reactor (HFIR), analogous x-ray measurements at the Advanced Photon Source, and extensive use of transmission electron microscopy. The effects of radiation on defect structure and corresponding material properties are investigated in experiments using the HFIR. Computational research includes tools to address the full range of relevant time and length scales, from large-scale ab initio calculations (~104 atoms) and atomistic simulations (~108 atoms, fs to ns) to mesoscale microstructural models and finite-element and continuum models (macroscopic, years). ORNL has an extensive program in understanding defect physics, including the Center for Defect Physics, a DOE Energy Frontier Research Center.
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