| Shear coupled grain boundary(GB)migration(SCGBM)is a common phenomenon in the plastic deformation of nanocrystalline materials.It is one of the main reasons for macroscopic phenomena such as grain growth and recrystallization caused by shear.It is also an important and effective plastic mechanism for metal deformation.Engineering materials in service are rarely defect-free.Therefore,it is of dramatic interests to investigate how SCGBM interacts with cracks.In this paper,molecular dynamics simulation was used to study the mechanism of SCGBM in ?-Fe and the effect of Cr atoms on atomic scale,and the interaction between shear coupled grain boundary migration and microcracks in ?-Fe and Fe-Cr alloys was investigated.The main work included:(1)The shear coupled migration of ?9[110](221)and ?17[110](223)GBs were simulated at 10 K,and the influence of temperature and strain rate on the coupled motion were investigated.Results showed that the coupling coefficients of ?9(221)and ?17(223)GBs were 0.591 and 0.417,and the same structural unit transformation was observed in the two GBs.Temperature has a great influence on the grain boundary migration.The critical stress of migration for both GBs are decreased with the increase of temperature,and the GB sliding events occurs at temperatures higher than 300 K.However,the strain rate has little effect on SCGBM.(2)The interaction between SCGBM and cracks at different temperatures were simulated,and the mechanism of crack healing under GB motion was analyzed.Simulation results showed that crack in the ?9(221)GB can be completely healed under the interaction with GB at 10 K,100K,300 K,600K and 900 K,and the GB curves as a result of the pinning effect of the crack.The healing speed increased and the pinning effect decreased with the rise of temperature.The crack in the ?17(223)GB are difficult to be healed at any temperature,and pinning effect on GB is not obvious.The asymmetric fixation of cracks on grain boundaries plays an important role in the process of crack healing.In the process of crack absorption,the morphology and structure of GBs change and its thickness increases significantly.The dislocation emission and movement from the tip of crack to the free surface are the preconditions of microcrack healing.(3)The shear coupled motion of GBs in Fe-Cr alloys containing from 1 at.% to 9 at.% Cr and the interaction with crack was simulated at 10 K,and the influence of Cr atoms on coupled motion was explored.The results show that the effect of Cr atom on GB motion is achieved by changing the structure of grain boundaries.For the ?9(221)GBs in Fe-Cr alloys,GB migration is hampered and sliding occurs when the content of Cr is over 3%,and the existence of new structural units was demonstrated to be responsible.In contrast with the ?9(221)GB,the addition of Cr atoms has minimal impact on the structure of the ?17(223)GB.Consequently,the motion mode of the ?17(223)GB does not change with the increase of the content of Cr.The cracks in the ?9(221)GBs in alloys of different concentrations can be healed,and the healing velocity increases with the rise of the concentration of Cr.However,the cracks in the ?17(223)GBs in alloys of different concentrations are difficult to be healed. |
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