The Study Of Effects Of Irradiation Defects On Mechanical Properties In Iron-based And Uranium Alloy Using Molecular Dynamics Method

The development and safe application of nuclear energy mainly depend on the performance of the structural materials in the reactor.The mechanical property degradation of irradiated metals has been confirmed to be closely related to interaction between dislocations and irradiation defects.Understanding underlying mechanisms at atomic scale has a significant effect on the development of radiation resistant materials in future.The ferritic/martensitic steel is one of structural materials used in the nuclear reactors.α-Fe is a fundamental phase of ferritic/martensitic steels.Understanding interaction mechanisms between dislocations and irradiation defects in the pure iron is significant for the development of radiation resistant ferritic/martensitic steels.Due to the fact that at low temperature the plastic deformation of pure iron is mainly controlled by the screw dislocation,the interaction of screw dislocations with dislocation loops is investigated in this work.Firstly,the effect of temperature,Burgers vectors of dislocation loops and the dislocation loops size on the interaction mechanism between1/2[111] screw dislocations and 1/2<111> dislocation loops is analyzed using molecular dynamics methods.Three interaction mechanisms are found: the formation of the helical configuration,cross-slip mechanism and the diffusion of screw segments in dislocation loop core.Detailed analysis suggests for a given screw dislocation,the specific Burger vector of a 1/2<111> interstitial dislocation loop and its size determine the dominated mechanism of line-loop interaction.It is also found that the critical stress for a screw dislocation to cross a 1/2<111> dislocation loop is varied due to the interactions involved in these new mechanisms.Secondly,the influence of temperature,shear strain rates and loop positions on the reaction between the 1/2[1 1 1] screw dislocation and the 3 nm interstitial dislocation loop with the Burgers vector of 1/2[1 1 1] in α-Fe has been investigated by means of molecular dynamics methods,using a potential developed for body-centered cubic Fe.Three different kinetic mechanisms also have been identified.Molecular dynamics(MD)simulations indicate that the transformation of the dislocation loop type,crossslip movement,and glide on primary slip plane during pinning and unpinning process are primary features related to three new interaction mechanisms.Detailed analysis suggests that for a given screw dislocation line,the shear strain rate,temperature and the loop position determine the dominated mechanism for a line-loop interaction.However,the most primary influential factors are the loop position and temperatures.The main influence of the temperature may be to change the interaction configuration between the dislocation and the dislocation loop by modifying the defect diffusion behavior.Thirdly,for the simulation of the displacement cascade in Fe C alloy,the interatomic potential for Fe C developed by Ackland et al.is verified.Sequentially,the threshold displacement energies for the three principle crystallographic directions are determined.At last,displacement cascades for different cascade energies in Fe C alloy are simulated.The effect of carbon on the ballistic phase,recombination stage and cooling stage is analyzed.The effect of distribution of carbon atoms on the critical stress for a screw dislocation to unpin is analyzed.Finally,the influence of radiation defects on the thermo-mechanical properties of UO2 within 600–1500 K is studied using the molecular dynamics method.Two types of point defects have been investigated in the present work: Frenkel pairs and antisites with concentrations of 0 to 5%.The results indicate that these point defects reduce the thermal expansion coefficient(α)at all studied temperatures.The elastic modulus at finite temperatures decreases linearly with the increase in concentration of Frenkel defects and antisites.The extent of reduction(R)in elastic modulus due to two different defects follows the trend Rf > Ra for all studied defect concentrations.The interaction between a screw dislocation and dislocation loops in pure iron has been investigated,which provides the reference for the predication of the response of mechanical properties in irradiated metals.The study of displacement cascades in Fe C alloy performed in this work would provide guides for the predication of primary damage state and mechanical properties of Fe C alloy.Besides,the study of the effect of radiation defects on thermo-mechanical properties in uranium implies that Frenkel pairs and antisite defects could degrade the performance of UO2 and should be seriously considered for estimation of radiation damage in nuclear fuels used in nuclear reactors.