# Atomistic Investigation of Plastic Deformation and Dislocation Motion in Uranium Mononitride
## 4 Discussion  
In this work, we demonstrated that MD simulations can provide accurate predictions of the nanoindentation hardness of UN. We believe this method can be extended to other nuclear fuels, particularly in high-temperature regimes, where oxidation layers may limit experimental nanoindentation measurements. The key requirement for an interatomic potential in such simulations is the accurate prediction of the elastic properties and softening behavior. Our findings show that the Kocevski potential gives excellent predictions for nanoindentation hardness, despite its limitations in modeling plasticity dynamics in UN. This is acceptable for simulations where brittle failure dominates, as the plastic deformation dynamics become less relevant. Our earlier work [12] has further shown that the Kocevski potential provides a reasonable description of elastic behavior, which is the focus in such brittle regimes. The Kocevski and Tseplyaev potentials yield different predictions for the principal slip systems in UN. The Kocevski potential predicts that the principal slip system is \(\frac{1}{2}\langle 110\rangle\langle 110\rangle\), based on screw dislocation studies, while the Tseplyaev potential suggests \(\frac{1}{2}\langle 110\rangle\langle 111\rangle\), as derived from edge dislocation studies. Experimental data [9] on deformation in UN indicates that the \(\{110\}\) planes are the principal slip planes, which aligns with the predictions of the Kocevski potential. This study also noted that screw dislocations are more prevalent in UN [9]. This experimental observation combined with our finding--that screw dislocations exhibit larger linear mobility and smaller transition stress to the phonon-drag regime compared to edge dislocations--leads us to recommend the screw dislocation mobility function for use in discrete dislocation dynamics (DDD) simulations of UN. The discrepancy in the predictions of the two potentials may arise from the fact that the Tseplyaev potential likely overestimates the covalent nature of bonding in UN. Another explanation for the differing predictions is that the dominant slip planes in UN may vary with temperature, a phenomenon observed in other materials such as Sr-doped KCl [48]. For instance, while \(\{111\}\) planes are the primary slip planes in UC, this conclusion is based on high-temperature measurements (1680-2200 K) [47]. Since our simulations are conducted at 300 K, further experimental and DFT studies are required to clarify the principal slip systems in both UN and UC across a range of temperatures. Dislocation properties, both static and dynamic, are highly sensitive to the chosen interatomic potential. Thus, quantitative conclusions drawn from atomistic models should be considered approximations [15; 36]. For instance, the Peierls stress of UN, calculated as 2.98 GPa, aligns with the order of magnitude reported for UO\({}_{2}\) by [PERSON] and [PERSON], which ranged from 2.7 to 12.9 GPa depending on the interatomic potential used [13]. Based on preliminary testing, we found that the Peierls stress decreases by approximately 1 GPa when the dislocation line length increases from 40 A to 80 A, though this change does not affect the relative ordering of Peierls stress across the \(\frac{1}{2}\langle 110\rangle\) slip planes for the edge dislocation. To ensure comparability, it is crucial to evaluate Peierls stress values under consistent simulation conditions, including the same potential, supercell size, and strain rate. This work presents the first complete mobility functions for \(\frac{1}{2}\langle 110\rangle\) edge and screw dislocations in UN, providing a foundational step toward parameterizing plasticity dynamics in this material. One limitation of our model is that it is based on simulations at a single temperature (300 K). Key parameters, such as transition stresses and velocities between different regimes, as well as the phonon drag coefficient, are known to vary strongly with temperature [18; 35].