Helium bubbles in bcc Fe and their interactions with irradiation

Abstract

a b s t r a c tThe properties of helium bubbles in a body-centred cubic (bcc) Fe lattice have been examined. The atomic configurations and formation energies of different He-vacancy complexes were determined. The 0 K results show that the most energetically favourable He to Fe vacancy ratio increases from about 1:1 for approximately 5 vacancies up to about 4:1 for 36 vacancies. The formation mechanisms for small He clusters have also been considered. Isolated interstitials and small clusters can diffuse quickly through the lattice. MD simulations of randomly placed interstitial He atoms at 500 K showed clustering over the time scale of nanoseconds with He clusters containing up to 4 atoms being mobile over this time scale. He clusters containing 4 or 5 atoms were shown to eject an Fe dumbbell interstitial which could then detach from the He cluster and diffuse with the remaining He-vacancy complex being effectively immobile. Collision cascades initiated near larger bubbles showed that Fe vacancies produced by the cascades readily become part of the He-vacancy complexes. Energy barriers for He to join an existing bubble as a function of the He-vacancy ratio are also calculated. These can be larger than the diffusion barrier in the pristine lattice, but are lower when the bubbles contain excess vacancies, thus indicating that bubble growth may be kinetically constrained.

Effects of Cr and W additions on the stability and migration of He in bcc Fe: A first-principles study

“…Inside the structural materials, He atom is mobile and easily to be trapped by point defects, forming He bubbles [10][11][12][13], leading to void swelling and He embrittlement. Thus, many attentions have been paid to investigate the behavior of single He impurity and He clusters in point defects and grain boundaries using first-principles calculations and molecular dynamics (MD) simulations [10][11][12][13][14][15][16][17][18][19], such as He solution and migration, formation of He-vacancy clusters, and He-to-vacancy ratio in bcc Fe. So far, most theoretical studies focused on He impurities in pure Fe solid.…”

Nanoindentation and TEM to Study the Cavity Fate after Post-Irradiation Annealing of He Implanted EUROFER97 and EU-ODS EUROFER

“…Although the cavity distribution seems to be random, within zone B, cavities attached to the Y 2 O 3 –matrix interface and grain boundaries were observed (Figure 13). Those observations may be attributable to the random nucleation process itself that may take place before the annealing and the low mobility during the annealing (high migration energy [50]) of large vacancy-helium clusters. Therefore, it is not possible to assert if the addition of yttria particles under these experimental conditions enhance the defect suppression.…”

Evaluation of Irradiation Hardening of P92 Steel under Ar Ion Irradiation

“…where ∆σ y is the increment in yield strength, M is the Taylor factor, α is the strength of barrier, µ is the shear modulus, b is the magnitude of the Burger's vector for moving dislocations, N is the number density of obstacles, and d is the obstacle diameter [14]. Therefore, the number density and size of dislocation loops would determine the magnitude of irradiation hardening in the irradiated CLAM steel.…”

“…Inert gas, especially He, created by nuclear reaction has a significant influence on the microstructural evolution of structural materials exposed to neutrons in nuclear reactors. Although simulations recently indicated bubble formation and loss of structural integrity induced by Ar ion and Xe ion were much less compared to the case of He ion in bcc Fe [14], inert gas is still widely applied to ion irradiation experiments. Therefore, in the present work, the Ar ion was selected to create similar irradiation damage as He created in reactors.…”