Multi-Scale Investigation of Plastic Deformation and Dislocation Behavior in Uranium Mononitride

Uranium mononitride (UN) is considered an advanced nuclear fuel candidate due to its superior thermal conductivity and high fissile material density. However, its mechanical response, particularly under conditions relevant to nuclear reactors, is not yet fully understood. In this work, molecular dynamics (MD) simulations are utilized to explore the deformation mechanisms and dislocation dynamics in UN. There are two interaatomic potentials of UN: the Kocevski EAM potential and the Tseplyaev angular-dependent potential. Our results show that the Kocevski potential identifies the primary slip system as 1/2 [110] (110), consistent with experimental observations, while the Tseplyaev potential suggests slip on 1/2 [110] (111). MD simulations of stress-strain behavior further enable estimation of nanoindentation hardness, with the Kocevski potential delivering accurate predictions despite limitations in modeling dynamic plastic deformation. Complete dislocation mobility functions have been derived for both edge and screw dislocations across thermally activated and phonon-drag regimes. At 300 K, the linear mobility of the edge dislocation, as predicted by the Tseplyaev potential, is 817 Pa^-1 s^-1, while that of the screw dislocation, as predicted by the Kocevski potential, is 4546 Pa^-1 s^-1. At intermediate stress levels, subsonic motion of the edge dislocation is periodically interrupted by velocity surges, with speeds reaching the material's average sound velocity. The threshold Schmid stress is calculated in the range of 179-197 MPa, providing an upper bound for the uniaxial yield stress of polycrystalline UN, estimated at 548-603 MPa. Moreover, the dislocation mobility functions have been implemented in ParaDiS, a discrete dislocation dynamics code, to simulate dislocation behavior of UN at the mesoscale level.