Effective fitting of nanohardness data in two different ferritic steels irradiated with He ions

A damage plateau (4000 appm/0.25 dpa) was produced in T92 and MA956 by multi-energy He ion irradiation. Nanoindentation was used to characterize the irradiation hardening effect. For the pristine samples, the hardness at the region shallower than 200 nm deviated evidently from the relation predicted by the Nix-Gao model. The hardness of the damaged layer produced by ion irradiation also located in this region. Therefore, a modified model by introducing a maximum allowable density of GNDs proposed by Ruiz-Moreno was used to analyze the hardness data. The fitted result indicated that the hardness of the damaged layer was overestimated by 28.5% for T92 and 48.4% for MA956 in contrast to that obtained from the Nix-Gao model, and a hardening fraction of 64.5% for MA956 and 103.6% for T92 was also obtained. A lower hardening fraction of MA956 was considered to be related to the existence of oxide particles in the matrix.