Multiscale Modelling of Irradiation Induced Effects on the Plasticity of Fe and Fe-Cr

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

Molecular dynamics modeling of cavity strengthening in irradiated iron

One of the most important problems in the field of nuclear industry is the relationship between irradiation-induced damage and the resulting induced mechanical response of the target metal and in particular ferritic base steels. In this work molecular dynamics simulation is used to simulate the nanoscale interaction between a moving dislocation and a defect, such as a cavity, as void or He bubble. The stress–strain curves are obtained under imposed strain rate condition using the atomic potentials based on the Fe potential of Ackland et al. 1997 for a void and He bubble as a function of He content and temperature. It appears that a 2 nm 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. With increasing temperature the escape stress decreases and at the same time there is increasing degeneracy in the type of interaction

Influence of the stress field due to pressurized nanometric He bubbles on the mobility of an edge dislocation in iron

Voids and He bubbles are strong obstacles to dislocation, which induce hardening and loss of ductility. In Fe, molecular dynamics simulation is used to investigate the basic mechanisms of the interaction between a moving edge dislocation and a void or He bubble, as a function of its He content, temperature, interatomic potentials and interaction geometry. Different interatomic potentials for Fe-Fe and Fe-He interactions are used. It appears that 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. The mobility includes the cross-slipping of these segments, which leads to the formation of a jog. It appears 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

Molecular dynamics study of strengthening by nanometric void and Cr alloying in Fe

Ferritic steels are the main candidates for the structural components of future fusion reactors. Because of the complexity of their structure, most of the simulation works are focused on the base phase of these materials such as alpha-Fe or Fe-Cr alloy. In this work molecular dynamics (MD) simulation is used to investigate the influence of chromium on the plasticity of bcc Fe-Cr alloy. Recent interatomic potentials for the Fe-Fe interactions are compared, namely Chiesa 2009 and Malerba 2010, with a comparison to Ackland 1997, Mendelev 2003 and Ackland 2004, widely used in this field. For the Fe-Cr system the potentials of Ackland 2004 for Fe-Fe and of Olsson 2005 for Fe-Cr and Cr-Cr interactions are used. Firstly, the recent Fe-Fe interatomic potentials are assessed by studying the interaction of the edge dislocation with a 2 nm void at 10 K. Secondly, the impact of Cr in Fe on the mobility of an edge dislocation for contents between 0 and 100 at.% is studied. It appears that its glide stress increases with Cr content up to about 60 at.%, and then decreases. The high flow stress observed at 60 at.% Cr does not relate to Cr intrinsic mechanical properties, but to the induced lattice distortion. (C) 2013 Elsevier B. V. All rights reserved

Obstacle strength of binary junction due to dislocation dipole formation: An in-situ transmission electron microscopy study

We report the experimental observation of the 1/2 edge dislocation dipole formation and annihilation in ultra-high purity Fe using transmission electron microscopy (TEM) in-situ straining. The observation is confirmed by TEM image simulations. The edge dipole is formed by the interaction of a moving screw dislocation with an obstacle of dislocation character. It results from the glide of the two arms of the dislocation on two different glide planes, which stabilizes the dipole that is closed by a jog. The dipole is later released from the obstacle and disappears, presumably by gliding of the dipole's edge segments along their Burgers vector and freeing the mobile screw dislocation from the jog. This mechanism. leads to enhanced obstacle strength of the immobile dislocation well above Orowan critical stress, promoting forest strength. (C) 2015 Elsevier B.V. All rights reserved

In situ transmission electron microscopy of the interaction between a moving dislocation and obstacles of dislocation character in pure iron

In this study, we report our observation of the interaction between a moving dislocation and an obstacle of dislocation character in pure bcc-Fe using a TEM in situ straining experiment at room temperature. Our observations reveal that the interaction of a 1/2111 dislocation with a dislocation debris leads to a dislocation loop at the obstacle by Orowan mechanism. Hence, the strength of these obstacles is estimated to be in the range of Gb/L, which is higher than the strengthening induced by different nanometric defects of similar size