Differences in the accumulation of ion-beam damage in Ni and NiFe explained by atomistic simulations

Following low-dose irradiation with a 3 MeV beam of Au ions, i.e. less than one displacement per atom, a lower number of defects were experimentally observed in NiFe than in pure Ni. At higher doses, more damage is observed in NiFe than in pure Ni. Also, at these high doses, defect structures are observed deep in the material, far from the region where ions are implanted, more so in Ni than in NiFe. In this study, these experimental results are explained using atomistic modeling. Sequences of overlapping displacement cascades with intervening defect aging are simulated. Evidence is provided that nanosecond aging at 900 K can be used as a surrogate for long-time, room-temperature aging. Then, using this procedure, it is shown that the low defect diffusivity of NiFe leads to less aggregation and recombination events between each displacement cascade in a given volume than in Ni. Variations in the local defect chemistry in NiFe produces a broad spectrum of defect formation energy, leading to the trapping of defects at energetically favorable sites: this explains the low defect diffusivity. Also, this low diffusivity explains why, at high dose, defects in NiFe do not propagate as deeply in the material than in pure Ni.