Multiscale Simulations Of The Effect Of Helium And Dislocation On The Mechanical Properties Of HR-2 Alloy

The multiscale simulations of the effect of helium and dislocation on the mechanical properties of HR-2 alloy are carried out in this paper. At the microscale, the modified embedded atom method (MEAM) and molecular statics (MS) are used to calculate the formation energies and the atom configurations of the helium clusters. And, the kinetic Monte Carlo (KMC) is used to simulate the evolution of helium clusters. At the mesoscale, the three-dimensional discrete dislocation dynamics (3DDD) is constructed and used to model the dislocation sweep the helium clusters. The constitutive equation for describing the thermally-activated stress of helium cluster is fitted. At the macroscale, based on the evolution of dislocation density, a physically based constitutive model of the HR-2 alloy is constructed. The thermally-activated stress of helium clusters is added into the constitutive model for describing the effect of helium on the mechanical property of HR-2 alloy. In the above simulation, the models and the numerical methods are investigated. In addition, the mechanics and the dislocation configuration of HR-2 alloy are investigated for discussing the micro-mechanism of the mechanical behavior. Main conclusions are described as following:a) The MS calculation results indicate that the tetrahedral interstitial position inγ-Fe orα-Fe is the most stable site for interstitial helium atom (HeI). The formation energies of the He-V clusters are increase with increasing the values of the He/V. The vacancy can stabilize the He-V cluster, but the HeI added into the cluster will increase the cluster energy and bring about the self-trap action.b) The KMC simulation demonstrates that thermal vacancies and thermal activation energy have different effects on the helium behavior. At the high temperature, plenty of thermal vacancies will trap the most of HeI, which make the HeI can not congregate and can carry out self-trap. At the low temperature, as the thermally-activated energy increases with elevating the temperature, the bigger He-V clusters are formed because the different self-trap actions are carried out.c) The 3DDD model can accurately simulate the critical shear stress and dislocation configuration of the Frank-Read dislocation source, which are consistent with the experiment. The mechanical behavior and the dislocation evolution in FCC crystal are simulated. The simulation results indicate that our numerical algorithm can efficiently simulate the dislocation dipole and mechanical behaviour under the low strain rate loading.d) The differential equation group consisted of dislocation evolution and mechanical state has serious stiff property, namely the external stress changes more quickly than dislocation evolution, which is consistent with the property of the real material.e) The Langevin force is used to describe the thermally-activated effect of temperature on the dislocation. The thermally slip of dislocation overcoming the Peierls stress is simulated, and the activation energy of Peierls stress is fitted, the result accords with experiment data.f) Due to the barrier energy of the dislocation slipping is relative low, HR-2 alloy has the evident strain strengthen, strain rate strengthen and temperature soften. The temperature and the strain rate have different effects on the evolution of dislocation. With the temperature elevating, the configuration of dislocation changes from the wall to the cell. While with the strain rate increasing, the dislocation mostly plane slips. If the strain rate is enough high, the deformed twins are appeared in order to satisfy the deforming rate.g) According to the forest dislocation cutting mechanism of FCC metal, a physically based constitutive model of HR-2 alloy is developed, based on the dislocation density evolution. The model parameters are consistent with the microstructural properties of the HR-2 alloy, and this model can accurately predict the complicated mechanical behavior of the alloy in a rather wide range of temperatures and strain rates.h) Based on the simulations of KMC and 3DDD,, the thermally-activated constitutive equation of the dislocation overcoming the helium clusters is fitted. Induced the thermally-activated stress caused by the helium clusters into the HR-2 constitutive equation, the hardening effect of helium on the HR-2 alloy is described. The result indicates that using the thermally-activated constitutive equation to evaluate the hardening effect is better than using the critical resolved shear stress (CRSS).In this paper, a serial multiscale model is used to simulate the effect of helium on the mechanical properties of HR-2 alloy. That is using the mircroscale simulation to obtain the distribution of helium clusters, and using the dislocation bynamics to model the interaction between the dislocation and helium cluster for obtaining the thermally-activated stress caused by the helum cluster, lastly added this stress into the physically based constitutive model for describing the effect of helium on the mechanical property. This method gives attention not only to the describing ability of the physically based constitutive equation for the mechanical property, but also to the properties of the microstructures simulated by the micro- or meso-scale models, such as dislocation dynamics.