Modeling and simulation with molecular dynamics of the edge dislocation behavior in the presence of Frank loops in austenitic stainless steels Fe-Ni-Cr
Austenitic stainless steels are widely used in the nuclear industry as internals. These structures reside mainly in the reactor vessel and, due to their proximity with fuel assemblies, are subjected to severe operating conditions. These elements are exposed to high irradiation doses which can reach 100 dpa after 40 years of operating, at a temperature close to 350°C. These operating conditions affect the microstructure of steels and their mechanical behavior, which leads to the deterioration of their mechanical properties and their corrosion resistance. The objective of this PhD research work is to establish at the atomic scale a constitutive law describing the edge dislocation motion in a random Fe-Ni10-Cr20 solid solute solution, to bring a comprehensive understanding of the interaction mechanism between the edge dislocation and the Frank loops and to investigate the effect of temperature, alloying random generator, orientation and size of the Frank loop on the mechanical stress. To achieve these objectives, molecular dynamics simulations were conducted with a recently developed FeNiCr potential used to mimic the behavior of austenitic stainless steels. These simulations have been performed in static conditions as well as at 300 K, 600 K and 900 K and the interactions realized for loop sizes of 2nm and 10nm. A constitutive law taking into account the temperature and strain rate is proposed; the interaction between the edge dislocation and the Frank loop revealed 3 kinds of interaction mechanisms: simple shearing, unfaulting and absorption of the loop. Absorption is the most stable mechanism; the analyses of the resulting mechanical properties have shown that the unfaulting mechanism requires the highest stress to make the dislocation overcome the obstacle. On the other hand, contrary to previous studies, the unfaulting of the loop surface occurs only when the dislocation comes into contact with the edge dislocation; for the 2 nm Frank loop size, the coupling between the probability of the outcome of the reaction and the average strength of the obstacle constitutes useful data for Dislocation Dynamics simulations. The observations of the resulting Frank loop configurations following the interaction with the dislocation allow justifying the emergence of clear bands observed in TEM. This work has been partially supported by the European Commission FP7 with the grant number 232612 as part of the PERFORM 60 project.