Nuclear Science User Facilities 100 The combination of nf-HEDM and ff-HEDM allows analyzing the distri- butions of orientations and strains at the scale of the sub-grain. Because HEDM provides a crystallo- graphic orientation map at microscale resolution, it is possible to obtain distributions of local misorientation in a grain. Local misorientations indicate local strains, usually plastic strains, because dislocations accompany residual strain and alter lattice orientations. Kernel average misorientation (KAM) is the popular approach to calculate varia- tions of intergranular lattice orientations in electron backscatter diffraction, and it can be adapted to HEDM. KAM is numerically defined as the average misorentations between all neighboring kernels within the kernel. In the case that some kernels include grain bound- aries, the kernels with KAM ≥5 degrees, for example, are excluded from the averaging calculation. Positron annihilation studies Positron Annihilation Spectroscopy (PAS) provides a mechanism for probing very small defect structures produced during irradiation.This technique is important since it is able to measure defects with dimensions smaller than those that can be imaged using even the highest resolution TEM techniques. PAS has the ability to identify the presence of these tiny clus- ters, particularly vacancy clusters.This is critical to help develop a basis for understanding the void nucleation and void incubation processes, about which very little is presently known.Thus, PAS can provide an important link to the modeling of damage evolution. In addition to the role in damage struc- ture evolution, the invisible defects, when they exist, can have an important impact on dislocation movement and overall deformation behavior. This research is ongoing with the lowest-dose samples from the ATR irradiations. Accomplishments At present, a series of experimental and modeling work has been accom- plished. Neutron irradiation on both disc specimens and miniature tensile specimens has been completed.The target doses range from 0.01 to 10 dpa (completed in six cycles), and the target temperatures were 300, 450, and 550℃. For PIE on neutron- irradiated specimens, advanced char- acterization and testing techniques have been employed to explore the relationship between irradiated micro- structure and mechanical properties. Several RTE projects and a two-year This spectrum of simple- to-complex Fe–Cr base alloys will provide the basis for assessing the processes underlying radiation performance even in complex alloy systems.