Fig. 5 shows typical indentation-depth profiles of the unirradiated
CW 316 SS sample where 8 single indents were measured
for each specimen. It is worth mentioning that the distance
between indentations was about 50 lm, which exceeds the grain
size of CW 316 SS (40 lm). In the case of the depth less than
50 nm, the measured hardness values were inaccurate because of
the uncertainty in the indenter tip geometry, strain rate effects
around the tip, and other surface artifacts such as surface contamination
films [27]. For values in the depth larger than 50 nm, the
nanoindentation hardness for the unirradiated specimen decreases
slightly with penetration depth. The model developed by Nix and
Gao based on the concept of geometrically necessary dislocation
may explain this [28]. This model predicts the hardness-depth profile
as follows: