Molecular Dynamics Simulations For Plastic Properties Of Nano Twinned/Polycrystalline VN Coatings

Ultrahard nano multilayered films and nano twinned materials,as two kinds of the most important coating materials have been receiving extensive application and attention due to their excellent mechanical properties,among which the transition metal nitrides with B1 structure are the most attractive ones.However,the inelastic behavior,as well as the interactions between the defects in such kinds of materials remain unclear,especially at atomic level,which limits the design,development and applications of such kinds of materials.In this dissertation,the elastic-plastic properties of B1 phase twinned/polycrystalline VN coatings was investigated with molecular dynamics simulations.A suitable interatomic potential for V-N system was developed,the strengthening/softening mechanisms were explored,the dependences of elastic/plastic behavior of the materials on strain-rate,stress-state,grain size/twin thickness were researched.The main work and conclusions in this dissertation are listed as follows.?I?The parameters in 2NN MEAM potential determined for B1 structured V-N system with the results by first principle calculations and experimental results,which can satisfactorily reproduce the mechanical and physical properties of the V-N system with B1 structure and other metastable phase structures.Using the concept of generalized stacking fault energy,the sequence of the possible slip systems was predicted.?II?The responses of VN samples under indentations on their?001?and?111?surfaces using a cylindrical indenter were simulated to study plastic deformation behaviors and the effects of in-plane anisotropy in VN.The main results are as follows.?1?Three kinds of plastic deformation mechanisms were found,including full dislocation glide,Shockley partial dislocation glide and twinning.?2?The formation mechanism of deformation twin was proposed,i.e.,the nucleation of a partial dislocation and its propagation form a stacking fault?SF?,which is the premise of deformation twin,and the sequential nucleation and propagation of partial dislocation on adjacent parallel{111}planes contribute to the thickening of deformation twins.?3?The in-plane anisotropy of the deformation behavior was observed,the cylindrical indenter along different orientations can induce different deformation mechanisms,such as partial dislocation glide and twinning.?III?We performed the MD simulations for the nanoindentations on VN{001},{111}and{110}using a spherical indenter to study the initial plasticity,dislocation reactions and the evolution of the dislocation loops?DLs?,and achieved the following main results.?1?The plastic behaviors in all the cases,can be attributed to the nucleation and glide of partial dislocations,which form{111}SF;but the initial defects are quite different,which is dislocation flower for{001},SF tripyramid for{111},and symmetric inclined SF for{110};?2?The reactions between the main defects include:the movement of two parallel partial dislocations nucleated successively to an equilibrium state,forming an extended dislocation,and the intersection of two unparallel SFs,forming a stair-rod dislocation;?3?Four types of DLs were observed:embryonic DLs,ribbon DLs,?V?shape DLs,and the prismatic DLs;?4?The Thompson tetrahedron can be used to illustrate the evolution of microstructures during the nanoindentations on different surfaces,attributed to different angles between the indented surfaces and Thompson tetrahedron.?IV?The MD simulations for tension/compression,2D and 3D nanoindentations on nanotwinned VN with different kinds of twin boundary?TB?atoms were performed to study the effects of TBs.The following main results are obtained.?1?Under tension,brittle fracture is the main failure mechanism without obvious strengthening;but for compression,nucleation and glide of dislocations are the main yield mechanism and distinct strengthening can be observed.However,the migration of TBs in the V-TB cases would result in softening.?2?There four kinds of reactions related to TBs:the migration of TB,the block against PDs by TB,full dislocations and PDs nucleated from TB.?3?Slips confined,dislocations repelled and the elastic property enhanced by TBs may account for the strengthening of the VN coatings.On the other hand,the migration of TBs,the partial slips parallel with TBs and the dislocations nucleated from TBs may result in weakening.?4?A new explanation was proposed to describe the relation between hardness and twin thinness,which takes into account the strengthening and weakening mechanisms mentioned in?3?.?V?The MD simulations for the tension/compression of polycrystalline VN were performed to explore the strain rate effect,tension/compression asymmetry and grain size effects.The main results obtained are listed as follows.?1?When strain rates are less than 1E9 s^-1,the tensile/compressive stress-strain curves tend to converge,which can be considered as quasi-static loading.?2?Elastic tension-compression asymmetry was observed,which could be ascribed to the higher friction in compression,the asymmetry of interatomic potential,and the effect of cutoff distance in MD simulations.While plastic tension-compression asymmetry could mainly result from additional different kinds of deformation behaviors,i.e.,intergranular fracture and dislocation glide.?3?The generalized self-consistent method can well describe the variations of elastic properties against grain size of polycrystalline VN.?4?The critical tensile/compressive strain increases with the decrease of grain size,indicating the enhancement of ductility.?5?The critical tensile stress is insensitive to grain size,but the critical and flow stresses strongly depend on grain size d,which decrease linearly with 1/d,showing an inverse Hall-Petch relation,which should be ascribed to the dominant role of the grain boundaries,which serve as a softer phase,at small d.The results obtained in this dissertation is of great importance to understand the strengthening and hardening mechanisms of ultrahard nano-multilayered coatings and nanotwinned ceramics,and provide theoretical support to the optimization of the microstructuresandthe design anddevelopmentofhigh-performance nano-multilayered and nanotwinned ceramics coatings.