Dissolution of nitride precipitates in iron by low-dose neutron irradiation

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Abstract
The microstructural changes brought about by irradiating α-iron containing a distribution of platelet α″-nitrate precipitates with fast neutrons at 355 K are described. With increasing irradiation dose these underwent radiation-induced dissolution. By a dose of 4 × 1022 neutrons m-2(E> 1 MeV) many had been replaced by rafts of small precipitates apparently lying within the pre-irradiation boundaries of the original precipitates. Others were apparently unaffected, while a small number had partially dissolved and appeared as regions of undisturbed platelet surrounded by rafts of small precipitates. At higher doses, an increasing fraction of the precipitates were affected. At a dose of 7.5 × 1023 n m -2 no larger precipitates remained and a homogeneous population of small dislocation loops and precipitates was observed, whilst at 8.4 × 1023 n m -2 no visible damage or precipitation remained. Post-irradiation annealing at 593 K of foils irradiated to a dose of 4 × 1021 n m-2 gave rise to dislocation loops and precipitates. At annealing temperatures below 546 K no damage was observed at this dose. This complex pattern of behaviour is discussed in terms of cascade dissolution of the large precipitates, followed by the formation of nitrogen-point defect complexes, and the re-precipitation at room temperature after irradiation of any nitrogen that is not trapped.