Atomistic Simulation Of Fracture And Coupled Grain Boundary Motion In Nanocrystals

Extensive investigations have been carried out upon nanocrystalline materials with many excellent properties.However,the low ductility of nanocrystals significantly restricts their applications.Instead of the dislocation mechanisms in coarse-grained polycrystalline materials,it is well known that the plastic deformation of nanocrystalline materials are dominated by grain boundary(GB)mediated mechanisms.Therefore,in-depth analyses are required to describe the fracture behavior and coupled GB motion in nanocrystals under different loading conditions.These researches are of great significances in the understanding of deformation mechanisms and optimization of nanocrystals with superior performances.In this dissertation,the fracture behavior of nanocrystals with grain size gradients and coupled GB motion in nanocrystals under shear loading are investigated using molecular dynamics method.The main creative research achievements are introduced as follows.(1)The fracture behavior of pre-cracked nanocrystals with grain size gradients is simulated.It is found that,a nanocrystal with a larger grain size gradient not only mobilizes more atoms to resist fracture,but also transforms the fracture mode from intergranular to intragranular.Thus,it could elevate the crack propagation.On the contrary,intergranular fracture can always be kept in nanocrystals with a small grain size gradient.Observation of the atomistic configuration,calculation of the Schmid factor,statistic of the percentage of defect atoms,analysis of the energy dissipation,and calculation of the stress intensity factor are carried out to investigate the deformation mechanisms behind the fracture behaviors.(2)Coupled GB motion in monotonically sheared nanocrystals is investigated.Due to the pinning effect of triple junctions,saturation of coupled GB motion is observed in nanocrystals,which is not observed in bicrystal.The elementary structure units in nanocrystals gradually transform from in-order to out-of-order during the GB motion.The two components of coupled GB motion,normal GB migration and tangential GB motion,initiate and saturate at nearly equivalent shear strains.Both the normal GB migration and tangential GB motion are affected by the grain size,but the coupling factor is slightly affected by it.Besides,the effects of material and temperature on coupled GB motion are investigated.(3)The deformation behavior and GB response of nanocrystal under cyclic shear loading are investigated.The GB migration hysteresis phenomenon in symmetric tilt GBs is observed for the first time.During cyclic loading,the elementary structure transformation in the middle of the GB is reversible,while the ordered GB structure at the two ends segments transform irreversibly and produced a disordered state.Both the disordered GB length and the residual GB migration displacement increase with increasing cyclic strain range.As the cyclic count increases,GB migration hysteresis vanishes after the entire GB becomes disordered.Cyclic loading has a cumulative fatigue effect on the GB migration behavior.Besides GB motion,both dislocation retraction and nucleation may occur during unloading.(4)The maximum strain and cyclic strain amplitude are treated as the two parameters characterizing the cyclic loading performance.Their effects on the structure transformation and GB motion in nanocrystals are studied.For fixed cyclic strain amplitude,the increase in the maximum strain monotonously lengthens the disordered segments at two ends of the GB.It even removes GB migration hysteresis.For fixed maximum strain with a small strain,the nucleation,motion,and accumulation of GB disconnections are rarely affected by dislocations.Therefore,the cyclic strain amplitude slightly affects the GB migration hysteresis.For fixed maximum strain with a large strain,the impinging of dislocations not only disturbs the ordered GB structure but also changes the pinning points of GB motion.The cyclic strain amplitude significantly affects the ordered GB length and the GB migration hysteresis.