Seyed Masood Hafez Haghighat
Degradation of mechanical properties due to nanometric irradiation induced defects is one of the challenging issues in designing ferritic materials for future nuclear fusion reactors. Various types of defects, namely dislocation loops, voids, He bubbles and Cr precipitates may be produced in ferritic materials due to high doses of irradiation with the 14 MeV neutrons stemming from the deuterium-tritium fusion reaction. Multiscale modelling methods, namely molecular dynamics (MD) and dislocation dynamics (DD) methods, are used here to study the impact of radiation-induced defects on the mechanical properties of model ferritic material. MD simulation is used to study the state of a nanometric helium bubble in α-Fe as a function of temperature, 10 to 700 K, and He content, 1 to 5 He atoms per vacancy. It appears that up to moderate temperatures the Fe lattice can confine He to solid state, in good agreement with known solid-liquid transition diagram of pure He. However, while for a given temperature and He density range where an fcc structure is expected, He in the bubble forms an amorphous phase. He bubble forms a polyhedron whose morphology depends on either the surface energy or elastic-plastic properties of Fe at either low or high pressure, respectively. At high He contents the bubble surface breaks down at the mechanical stability limit of the Fe crystal, leading to a pressure decrease in the bubble. The basic mechanisms of the interaction in α-Fe between a moving dislocation and a nanometric defect, as a function of temperature, interatomic potentials, interaction geometry and size are investigated using MD simulation. The stress-strain responses are obtained under imposed strain rate and using different interatomic potentials for Fe-Fe and Fe-He interactions. It appears that voids and He bubbles are strong obstacles to dislocation, which induce hardening and loss of ductility. A nanometric void is a stronger obstacle than a He bubble at low He contents, whereas at high He contents, the He bubble becomes a stronger obstacle. It also appears that different potentials give different strengths and rates of decrease of obstacle strength with increasing temperature. Temperature eases the dislocation release, due to the increased mobility of the screw segments appearing on the dislocation line upon bowing from the void or He bubble. Concerning the obstacle size at low content He bubble, where it is penetrable defect, size increase from 1 to 5 nm make them harder in agreement with the elasticity of continuum. At high He contents a size dependent loop punching is observed, which at larger bubble sizes leads to a multistep dislocation-defect interaction. Atomistic simulations reveal that the He bubble induces an inhomogeneous stress field in its surroundings, which strongly influences the dislocation passage depending on the geometry of the interaction. Fe-Cr alloys are also studied using MD simulations as model alloys for the ferritic base steels. Studying the flow stress of a moving edge dislocation in Fe(Cr) shows that the flow stress is not sensitive to temperature and strain rate, while it increases with Cr concentration. The flow stress of a screw dislocation is temperature dependant and its value is much higher than that of edge dislocation. Interaction in a pure Fe matrix of an edge dislocation with a nanometric Cr precipitate having various Cr content inside (Cr/Fe ratio) indicates that with the Cr/Fe ratio the obstacle strength increases, with a strong sensitivity to the short-range order. Temperature induces a monotonous decrease of the strength for all studied Cr/Fe ratios. DD calculation coupled to finite element method (FEM) is used to simulate the interaction of an edge dislocation with a void. DD calculations present a good match to the MD simulation results for the impact of image forces on the dislocation due to the free internal surface of the void. Using DD simulation, hardening due to the presence of nanometric defects, as immobile dislocation segments and spherical defect, are considered. The results of MD simulations for the strength of different defects are introduced in DD simulation. It appears that the presence of both immobile dislocation segments and nanometric defects may lead to the hardening of α-Fe. However, the defect density has more significant importance on the DD simulation, obtained here, than the strength of a single defect. TEM in-situ straining is used to validate simulations, in particular on the dislocation interaction mechanisms. Experiments were performed in ultra high purity Fe specimens. It appears that the microstructure of the material consists mainly in elongated screw dislocations. The in-situ straining tests led to the observation of the formation of screw dislocation dipoles in consistency with the MD simulation results. The interaction between a dislocation and an obstacle, as nanometric defect or immobile dislocations, leading to their shear or to Orowan loop formation were successfully observed. This work shows that using of different simulation methods can assist in the understanding of nano- and meso-scale phenomena occurring due to the presence of irradiation induced defects upon deformation of α-Fe. The information obtained from various microstructure mechanisms at lower scale simulations can be passed to higher scale simulation methods in order to realize the simulated phenomena. Experimental observations may validate what is observed in simulations provided the simulation scale is reachable in experimental observations.
EPFL2010
