A Dislocation Dynamics Study Of Irradiation Hardening Effect

Irradiation hardening,referring to the increase of yield strength after irradiation,is commonly observed in irradiated materials.Experiments show that irradiation hardening effect is highly related with irradiation embrittlement effect such as decrease of elongation and increase of ductile-brittle transition temperature.Therefore,a quantitative prediction of irradiation hardening effect is essential for the safety evaluation of in-service materials in nuclear reactor,and it also helps to screen and design of next-generation nuclear materials.The physical origin of irradiation hardening is that irradiation induced defects act as obstacles to dislocation motion.Traditional irradiation hardening models predict irradiation hardening effect by calculating the critical shear stress required for dislocation unpinning from irradiation defects based on the dislocation line tension approximation.In these hardening models,a parameter usually called strength factor that reflects the ability of the irradiation defect impeding dislocation motion is obtained by fitting experimental data.However,the fitting results of the strength factor for the same type of irradiation defect could vary significantly in different studies.The main reason is that both the hardening model applicable to each type of irradiation defect and the method to superpose the hardening contributions from different types of irradiation defects are still inconclusive.Hence,the traditional hardening models fitted by experimental data can only be interpolated within the range covered by experiments.In recent years,dislocation dynamics method becomes a powerful tool in studying irradiation hardening effect because it is capable of simulating the interaction between dislocation network and irradiation defects and correlating the interaction processes with macroscopic mechanical responses.In this paper,mesoscopic interaction models between dislocation and irradiation defects(including void and dislocation loop)are developed and incorporated in dislocation dynamics framework to simulate dynamic interaction between irradiation defect and dislocation.Taking tungsten(a candidate material for plasma-facing components in nuclear fusion reactor)as the matrix material,void hardening,dislocation loop hardening and their synergetic hardening are studied.Through analyzing dislocation dynamics simulation results,irradiation hardening models and hardening superposition method with zero parameter fitting by experiments are established to quantitatively predict irradiation hardening effect.The main contents are as follows:1.Mesoscopic void-dislocation and dislocation loop-dislocation interaction models are developed and incorporated in dislocation dynamics method to simulate the dynamic interaction between dislocation and irradiation defects.The interaction model between dislocation and irradiation defect is composed of two parts,namely elastic interaction and collision.The elastic interaction force between dislocation and void or dislocation loop is deduced according to elasticity theory,while the collision mechanisms are extracted from the short-range interaction details observed in atomistic simulations.The interaction models are then incorporated in the dislocation dynamics software ParaDiS to simulate the dynamic interaction between dislocation and irradiation defect.The dislocation dynamics simulation successfully reproduces the dislocation configuration evolution observed in the atomistic simulation validating the established interaction models.2.The dislocation dynamics software ParaDiS incorporated with irradiation defect modules is applied to simulate the impediment on dislocation motion from randomly distributed voids or dislocation loops in tungsten,and the hardening by voids or dislocation loops is studied.Simulation results show that the hardening by voids or dislocation loops is proportional to the 2/3 power of number density which coincides with the traditional FKH model.Further fitting results show that the strength factor of FKH model is not a constant but in positive correlation with defect size.By replacing the strength factor of FKH model with a function of defect size,MFKH(modified FKH)model is established which is appropriate to quantify the hardening effect of voids or dislocation loops.3.The process of a dislocation moving in the region where voids and dislocation loops coexist is simulated to investigate the synergetic hardening by voids and dislocation loops.To better quantify the synergetic hardening effect,an effective superposition method that quantifies hardening from different types of irradiation defects is developed based on the traditional empirical superposition method.The related parameter is calibrated by dislocation dynamics simulations which ensures the accuracy of hardening prediction.4.The irradiation hardening models(including MFKH model and effective superposition method)developed based on dislocation dynamics simulations possess explicit expressions with zero parameter fitting by experiments,which predict irradiation hardening effect by simply substituting the sizes and number densities of irradiation defects.18 neutron irradiated tungsten samples are selected from references.The ranges of irradiation dose and temperature are 0.02 ～ 1.54 dpa and 90 ～ 1200 ℃ respectively,and the observed irradiation defects are mainly voids and dislocation loops.Taking the sizes and number densities of voids and dislocation loops as input,the average relative error between predicted hardening and measured hardening is 23%.For comparison,the average relative error between the hardening predicted by traditional hardening model and the measured one is 55%.Therefore,the hardening models developed based on dislocation dynamics simulations own higher precision.In summary,by incorporating the mesoscopic interaction models between dislocation and irradiation defect that are established according to the elasticity theory and the atomistic simulation results,dislocation dynamics method is applied to study the pinning effect of irradiation defects on dislocation motion and the corresponding hardening effect,and based on the dislocation dynamics simulation results,the irradiation hardening model with higher precision is proposed which improves the method of evaluating irradiation effect for nuclear materials.