The overall approach to modeling He behavior in complex structural materials, such as FMS and NFA, that accounts for real microstructures was illustrated in Figure 3 and is briefly described in Section 1.06.6.2 below. The atomistic simulation results detailed above were used to inform the higher spatial — and temporal-scale microstructure evolution models of iron-based structural alloys.

1.06.5.4 Dislocation-Cavity Interactions

MD techniques have also been used to study dislocation-cavity interactions.238 The results of this work, where the cavities range from voids, to underpressurized, equilibrium, and overpressurized He bubbles, can be described in terms of an obstacle strengthening parameter (a) defined as

a = tc(L — d) / Gb [21]

Here, tc is the MD critical resolved shear stress for cavities with a diameter d that is spaced L apart,

G is the shear modulus, and b is the Burgers vector.

In summary, a

• Depends on the helium to vacancy ratio, m/n, and is highest for m/n for near-equilibrium bubbles and is lower for overpressurized bubbles.

• Decreases with increasing temperature.

• Increases with cavity size and at 300 K the peak a increases from about 0.2 to 0.4 in the diameter range of 1-4 nm.