Energy landscape of defects in body-centered cubic metals

The structural materials in nuclear reactors are subjected to severe irradiation conditions, leading to changes in their mechanical properties. The aging of these materials raises important issues such as those related to the safety of existing plants and future reactors. In many cases, materials with body-centered cubic bcc crystal structure are used with iron, tungsten, vanadium and tantalum as base metal. Collisions between irradiating particles and atoms constituting materials generate point defects whose migration leads to the formation of clusters responsible for aging. In this thesis, we studied the energetic properties of point defects in the bcc metals mentioned above at the atomic scale. Modeling point defects at the atomic scale can be achieved with different methods that differ only in the quality of the description of the interaction between atoms. Studies using accurate atomic interactions such ab initio calculations are computationally costly making it impossible to directly study clusters of large sizes. The modeling of atomic interactions using semi-empirical potentials reduces the reliability of predictive calculations but allow calculations for large-sized clusters. In this thesis we have developed a unique energy model for dislocation loops as well as for three-dimensional interstitial cluster of type C15. The resulting model has no size limit and can be set entirely by ab initio calculations. To test its robustness for large sizes of clusters we also set this model with semi-empirical potentials calculations and compared the predictions of the model to atomic simulations. With our development we have determined: (i) The relative stability of interstitial dislocation loops according to their Burgers vectors. (ii) The stability of the clusters C15 compared to the type of cluster loop. We showed that the C15 type clusters are more stable when they involve less than 41 interstitials in iron. (iii) In Ta we were able to show the same stability till 20 interstitials. The experiments involving iron show that depending on the irradiation temperature, highly mobile dislocation loops of Burgers vector 1/2 <111> or loops with Burgers vector <100> are formed. Considering formation mechanisms under irradiation as a function of temperature, formation of the <100>-type clusters lacked an acceptable theoretical explanation for about 50 years. In this thesis, the accuracy of our energy model enabled validation of several theories proposed in the last 50 years. In particular we have shown that the formation of loops <100> at high temperatures can be formed from C15 clusters which may be created directly in the irradiation process. These clusters are immobile and can grow. Beyond a certain size, the C15 clusters dissociate into loops 1/2 <111> or <100>. We have extended our model to free energy calculation of defect formation allowing for finite temperature predictions which is further compared to atomic simulations. The laws established in this thesis using our model to calculate the free energy of formation of the cluster size functions were then used in a cluster dynamics simulation. On comparison with experiments involving post-irradiation Oswald ripening in a sample of iron exposed to an atmosphere of helium, our energy model showed significant improvements over older energy laws, such as the capillary law widely-used in multi-scale computation cluster dynamics or Monte Carlo kinetics. We conclude that the new laws established from our calculations are essential to predict the concentration of dislocation loop under irradiation, depending on their sizes. The success of such an approach encourages extension of a similar study in more complex materials. (author)