Enabling molecular dynamics simulations of helium bubble formation in tritium-containing austenitic stainless steels: An Fe-Ni-Cr-H-He potential

Abstract

Formation of helium bubbles impacts mechanical properties of materials used in nuclear applications. An Fe-Ni-Cr-H-He quinary potential capable of direct molecular dynamics simulations of nucleation and growth of helium bubbles from randomly-born helium interstitial atoms has been developed. This is accomplished by incorporating helium into an existing Fe-Ni-Cr-H quaternary potential while addressing three challenging paradoxes characterizing helium in austenitic stainless steels: (a) interstitial He atoms form tightly bound dimers and larger clusters in the lattice but He atoms are only bound by weak van de Waals forces in the pure gas phase, (b) He atoms diffuse readily in host metals yet significantly distort the lattice causing large volume expansions, and (c) He atoms prefer tetrahedral interstitial sites as opposed to the larger octahedral sites despite large repulsive interactions with metal atoms within the lattice. We demonstrate that our potential reproduces density functional theory results on important properties relevant to helium bubble nucleation and growth. In addition to molecular statics validation of static properties, molecular dynamics simulation tests establish that our potential leads to the nucleation of helium bubbles from an initial random distribution of He interstitial atoms while at the same time capturing the equation of state in the pure He phase.