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Direct mesoscale measurements of radiation-induced changes in the mechanical properties of bulk materials remain difficult to perform. Most widely used characterization techniques are either macro- or microscale in nature, focusing on overall properties or overly small areas for analysis. Linking the atomic structure of irradiated materials directly with their radiation-affected properties remains one of the largest unmet challenges in radiation materials science. By measuring the change in surface acoustic wave speed as a function of relative orientation on metallic single crystals, we demonstrate that transient grating (TG) spectroscopy experiments have the sensitivity necessary to detect radiation-induced material property changes. We also show that classical molecular dynamics (MD) simulations can be used to accurately simulate orientation-based changes in surface acoustic wave speed in TG experiments, by comparing with experimental measurements and theoretical predictions. The agreement between theory, simulation, and experiment gives confidence in classical MD as a predictive tool to simulate defect-based changes in elastic properties, which cannot yet be fully treated by theory. This ability is of critical importance for the informed use of TG spectroscopy to measure material property changes induced by radiation damage, which may vary by amounts formerly too small for reliable in situ detection. Finally, our MD simulation framework is used to study the effect of an imposed vacancy population on the acoustic response of several materials. The results of these studies indicate that TG experiments are well suited to the ex situ and in situ study of radiation-induced material property changes., National Science Foundation (U.S.) (Grant 1122374), National Science Foundation (U.S.) (Grant CHE-1111557), Transatomic Power (Award 023875-001), U.S. Nuclear Regulatory Commission (MIT Nuclear Education Faculty Development Program. Grant NRC-HQ- 84-15-G-0045)

The ability to study radiation damage in situ will directly enable the rapid innovation and qualification of materials for nuclear applications by allowing direct observation of the effects of radiation damage accumulation. This is a challenging task, as the measurement technique must be noncontact, nondestructive, rapid, and still allow for online irradiation without interference. Applicable methods of mechanical spectroscopy are surveyed, noting their potential usefulness for characterizing radiation-induced microstructural changes in situ. The transient grating (TG) spectroscopy technique appears most suited for these studies, due to its noncontact, nondestructive nature, its ability to rapidly probe materials to the depth of ion irradiation, and the large number of deconvolvable components extractable from its signal. Work is proposed to separate the individual mechanisms of irradiation damage using in situ and ex situ TG spectroscopy, through a suite of single-effect and integrated experiments., National Science Foundation (U.S.). Graduate Research Fellowship Program (Grant No. 1122374), National Science Foundation (U.S.) (Grant No. CHE- 1111557)