Classical thermodynamic approach to void nucleation in irradiated materials
Abstract One of the major problems in building a future fusion power station is the development of suitable structural materials. These materials will be exposed to high energy neutron bombardment, with consequent changes in their mechanical properties – embrittlement, hardening and swelling, for example. A missing link in modelling these effects is an effective treatment of the nucleation of voids under irradiation. These voids are initially stabilised by transmutation helium but, once formed, grow by vacancy accumulation. In this paper, a quasi-chemical model is developed to calculate the entropy of a steel/helium system. Although a substantial contribution from quasi-chemical effects might be expected, the steady state concentrations of dissolved helium are found to be too low for such effects to manifest. The steady state concentration of dissolved helium is low in absolute terms, but the resulting supersaturation is very high, making it reasonable to assume that all available nucleation sites are instantaneously activated.
A quasi-chemical treatment of interstitial solid solutions: It application to carbon austenite
Abstract A first order quasi-chemical treatment of binary interstitial solid solutions has been given. The enumeration of the nearest neighbor pairs is simplified by only considering those pairs whose number depends on the atomic configuration of the solution. Simple expressions have been derived for the partial Gibbs free energy and thermodynamic activity of an interstitial solute atom in terms of the pairwise interaction energy between two nearest neighbor solute atoms. It is shown that the equations yield the appropriate results in the limits when the binding energy between solute pairs becomes zero or infinite. Finally the equations are shown to be compatible with the known activity of carbon in austenite and a computer fitting technique has been used to derive the value of the solute-solute binding energy concomitant with the closest fit of the experimental data fo the theoretical activity equation.
A Possible Mechanism for Electron‐Bombardment‐Induced Loop Punching in Helium‐Implanted Materials
The recently proposed mechanism for the punching of dislocation loops by overpressurized helium bubbles in molybdenum is studied quantitatively. According to this mechanism, under the electron beam of the transmission microscope, He atoms are excited or ionized and the resulting excited species (excited He atoms and free electrons) are responsible for the pressure rise in the gas beyond the threshold for loop punching. In the model, the pressure increase is attributed to EL reduction of the effective volume accessible to the gas due to the formation of a cavity around each excited species. The radius of this cavity is evaluated and, also, the excited fraction required to reach the threshold is discussed in terms of excitation life times. 
 
On examine quantitativement le mhcanisme, recemment propose, pour I'emission de boucles de dislocation par des bulles d'helium surpressurishees dans le molybdhe. D'apres ce mecanisme, sous l'effet du faisceau electronique du microcsope a transmission, les atomes d'helium sont excites ou ionises et les especes excites resultantes (atomes He excites et electrons libres) sont responsables d'une elevation de pression dans le gaz suffisante pour depasser le seuil necessaire a l'emission d'une dislocation par la bulle. Dans notre modele, l'e1evation de pression est attribuee a une diminution du volume effectif accessible au gaz par suite de la formation d'une cavite autour de chaque espece excitee. Le rayon de cette cavite est evalue et la fraction excitee requise pour atteindre le seuil d'emission est discutee en fonction de la duree de vie des excitations.