Interaction Mechanisms Between Low-angle Grain Boundaries And Dislocations In Face-centered Cubic Metals:Molecular Dynamics Simulations

Grain boundary(GB)plays a significant role in the ductility,strength,fracture and overall mechanical properties of polycrystalline metals.Of particular interest are interaction mechanisms between GBs and dislocations and GB migration,in which dislocation-GB interaction are critical atomic processes governing the yield and hardening behavior of polycrystalline metals,while GB migration plays a significant role in the microstructural evolution processes such as recrystallization and grain growth.Low-angle grain boundaries(LAGBs)are important dislocation product during work hardening and recovery.Recent research also indicated that LAGBs can stabilizing nanostructures in metals and hence improve the strength effectively.So far,the atomistic mechanism of GB behaviors is still beyond the capacity of current experimental techniques.To this end,computational simulation become an effective means to provide comprehensive insights the microscopic mechanisms of such behaviors.Through molecular dynamics(MD)simulations,the present work mainly focused on the <1 1 0> tilt LAGB in fcc metal formed by an array of Lomer dislocation locks(LLAGBs)and aimed to characterize how they interact with lattice dislocations and how they migrate under shear.Before these simulations,we studied the GB energy of <1 1 0> tilt LAGBs over a wide range of tilt angles and calculated the fstacking-fault energy of Ni and Al to understand the fundamental properties of the metal and verified the correctness of potential files.In addition,interaction between lattice dislocations and {1 1 1} pure twist GBs are also considered to reveal the effect of GB structures on the interaction process.The main content and major findings include:In the study of interaction between dislocations and symmetrical <1 1 0> tilt boundaries in fcc metal Nickle,we found that dislocation reaction and slip transmission depend on the tilt angle of GB,the character of incident dislocation and the particular glide planes inhabiting the incoming slip.For LAGBs with relatively small tilt-angles,a free sliptransmission zone can be identified where dislocations can be forced to penetrate through the LLAGB without inducing dislocation reaction.Otherwise,the incident slip tends to be trapped,triggering a number of dislocation reactions and leading to indirect slip transmission across the boundary.The critical width of the reaction zone depends on the character of inserting dislocations.Screw dislocation exhibits a smaller reaction zone than 60° dislocation for LAGBs of a certain angle.In the study of interaction between dislocations and {1 1 1} twist boundaries in fcc metal Nickle and the effect of twist angles and temperatures on such interaction process are investigated.MD simulations show that screw dislocations can be completely absorbed by twist GBs.While the 60° dislocations may be blocked and pined.The pinning points,which can be identified with the intersection of grain boundary dislocations and incoming lattice dislocation,are centers of stresses.The critical transmission stress decreased with decreasing the twist angles or increasing the temperature.In contrast to tilt LAGBs,the inserting planes have minor influence on the observed mechanisms.Finally,the shear-coupled migration of symmetrical <1 1 0> tilt boundaries in fcc metal Aluminum was studied.The critical shear stresses and energy barriers of GB migration for tilt angles varying from 2.6° to 16.1° were obtained via MD simulation and nudged elastic band method.A kink pair mechanism based on the dislocation model is proposed to describe the migration of LAGBs with tilt angle up to 10°.Above this value,we show that GBs migration changed to the high angle regime in which the mechanism could be replaced by a partial loop mechanism.Current work systematically investigated the Lomer-type LAGBs at the atomic scale,which revealed the microscopic mechanism of the dislocation-LAGB interactions and improved our knowledge on the migration process.These results provide theoretic evidence and prediction for the deformation processes of fcc metals.We anticipate that the mechanisms reported here will be of importance for the study of relevant issues in other low angle grain boundaries and other metals.