Molecular dynamics (MD) simulations are being carried out on grain boundaries (GBs) in Ni-Cr alloy with a wide range of characters (e.g., misorientation, tilt vs. twist, free volume) and under various radiation damage conditions as part of a separate work. These simulations will be used to identify the GBs most likely to roughen during in situ experimentation (i.e., GBs with minimal required dose for roughening). Electron transparent foils containing these ideal GBs will be irradiated in situ at the Intermediate Voltage Electron Microscope (IVEM) at Argonne National Laboratory using 500 keV Ne ions. High-resolution imaging of GBs before and after irradiation will be used to directly confirm and measure the degree of roughening, for use in a roughening vs. GB character data set, while in situ videos will be collected throughout the irradiations to better understand the evolution to steady state.



To date, GB roughening has been thought of and modeled as a high temperature phenomenon. This has led to few experimental studies analyzing these roughening transformations, despite the well-recognized changes in GB properties. However, consideration of driving forces such as point defect concentration gradients during irradiation leads to the potential for roughening at low homologous temperatures. While recent transmission electron microscopy (TEM) studies of GB evolution during room temperature, in situ irradiation have observed roughening of pre-existing GB facets, these works covered only three types of GBs and only to relatively small radiation doses. It is thus currently unknown how irradiation-induced roughening varies with GB character or whether the roughened state represents a new steady state GB phase which will survive greater damage. Further, such steady state GB phases are not expected to persist in the absence of irradiation, making ex situ study impractical. The existence of radiation-induced, steady state GB phases would be significant to our understanding of numerous GB phenomena under irradiation, including creep, radiation-induced segregation and precipitation, dislocation channeling, and irradiation-assisted stress corrosion cracking. Direct, in situ confirmation of GB roughening and any dependence on GB character will have implications for materials modeling, GB engineering for radiation environments, and current material lifetimes in nuclear energy systems and would prompt re-examination of well-studied materials – common and complex.



The total work – including sample preparation, IVEM experiments, image analysis, and manuscript preparation – is expected to last approximately five months. (The MD simulations are being performed as part of a separate work and will be completed prior to the estimated award date.) The work will produce a final report for NSUF RTE, at least one peer-reviewed journal publication, and a roughening vs. GB character data set from which the phenomenon can be modeled for future work.

The proposed work will observe and confirm radiation-induced grain boundary (GB) roughening transitions in situ and will generate a roughening vs. GB character data set for future model development. Because roughening represents a significant change to GB properties (e.g., sink strength, mechanical strength, impurity mobility), the work is highly relevant to DOE NE’s Light Water Reactor Sustainability (LWRS) and Advanced Reactor Technologies (ART) programs. For example, irradiation assisted stress corrosion cracking (IASCC) is known to limit the lifetime of components in light water reactor systems and to depend locally on dislocation channel transmission across GBs. While the distribution in GB character has been shown to correlate with a material’s susceptibility IASCC, the link is not straightforward or simple. Better understanding of GB character evolution and steady states would certainly be relevant to this LWRS program challenge. Similarly, next-generation reactor systems are expected to experience extremes in radiation dose, temperature, and corrosion. Taking molten salt corrosion as an example, recent work has linked GB migration with the development of extended, intergranular corrosion networks in metals. It seems reasonable that predicting such degradation, which stretches much farther than surface erosion, would be improved by an understanding of roughening – given the known connection between GB character and migration. Such improvements would be relevant to ART program goals. These two examples are a small sampling of the existing knowledge gap regarding roughening, which leaves researchers with an incomplete picture of microstructural evolution for critical structural and cladding materials. The proposed work thus aims to provide new insights similar to past gains from in situ vs. ex situ irradiation and from multi- vs. single-factor (e.g., irradiation with corrosion vs. irradiation then corrosion) experimentation.