Investigation Of Microscopic Mechanisms Of Mechanical Responses Of Typical Metallic Materials Under Shock Loadings

In the field of aerospace engineering,key structural components made of metallic materials often work under shock-induced extreme conditions in terms of ultra-high temperature,ultra-high pressure and ultra-high loading rate,which result in severe deformation,damage and catastrophic failure of the key structures.Essentially,macroscopic deformation and failure of typical metallic materials are determined by the evolutions of microstructures and micro-defects.Under ultra-high temperature,ultra-high pressure and ultra-high loading rate conditions,complicated evolution and interaction mechanisms of those microstructures remain important scientific issues as they make it very difficult to understand the strength features of metallic materials.In this dissertation,very large-scale non-equilibrium molecular dynamics simulations,combined with microstructural analysis and visualization,were conducted to study the correlation between macroscopic mechanical responses and microscopic plastic deformation,spallation and phase transition of typical metallic materials like body-centered cubic Ta and face-centered cubic Cu.The main research contents and results are summarized as follows:(1)The correlation between macroscopic mechanical responses and deformation mechanisms of polycrystalline Ta were studied.The effects of internal grain size and external shock velocity on the plastic deformation behavior were revealed.Under weak shock compression,a transition from grain boundary-mediated plastic deformation to twinning and dislocation slipping is discovered,as the grain size of Ta increases.The critical grain size of the twinning-slipping transition is determined,and the plasticity mechanism such that perfect dislocation is preferred to be emitted from grain boundaries is also revealed.Under strong shock compression,the plastic flow is dominated by twinning-detwinning and amorphization-recrystallization processes,as the shock velocity increases.The deformation mechanism,shock wave structure and flow strength all present a weak grain size dependence.(2)The dynamic fracture and damage behaviors of polycrystalline Ta under shock loadings were studied.The effects of internal grain size and external shock velocity on the spallation and micro-spallation mechanisms were revealed.It is found that under weak shock loading,the spallation mechanism switches from localized intergranular fracture to cavitation failure in the whole tensile region,as the grain size of Ta increases.For the spallation mechanism dominated by cavitation failure,uniform void nucleation and growth can effectively inhibit damage localization and rapid expansion of the spall plane,thus improving the spall strength of the material.Under strong shock loading,the spallation mechanism switches to micro-spallation,which is dominated by thermodynamic path.The damage mechanism,shock wave structure and spall strength all show a weak grain size dependence.(3)The plastic deformation mechanisms of Ta under complex loading paths,including double,single shock and quasi-isentropic compression,were studied.New mechanisms of twin-dislocation conversion under double shock compression and multistage detwinning under quasi-isentropic compression were unveiled.It is found that the plastic deformation mechanism of Ta under reshock compression is dominated by detwinning,multiplication and slipping of dislocations.Under single shock compression,the plastic deformation mechanism of Ta is dominated by amorphization-recrystallization process.Under quasiisentropic compression,the plastic deformation mechanism of Ta is dominated by twinningdetwinning process.The multistage detwinning mechanisms involve two sequential stages,i.e.the interaction between incoherent and coherent twin boundaries and the spontaneous degradation of incoherent twin boundaries.(4)The plastic deformation and multistage phase transition of textured nanotwinned Cu under shock compression were studied.A coupling mechanism of plasticity-phase transition caused by twin boundary sliding and its strengthening effect were revealed.It is found that under shock compression wave,twin boundary sliding is triggered in textured nanotwinned Cu,which results in a FCC-BCC-HCP multistage phase transition phenomenon.The new mechanism of phase transition significantly enhances the yield and spall strength of the material.The exponential relationship between the twin lamella thickness and the threshold pressure of phase transition is discovered.In summary,the researches in this dissertation reveal the mechanical behavior and microstructural evolution mechanism of typical metallic materials under complex loading conditions such as high-speed shock,quasi-isentropic and double shock.These works establish the intrinsic correlation between macroscopic thermodynamic responses(e.g.shear stress release at shock wave front,strengths of dynamic yielding and spallation),and microscopic mechanisms related to grain boundary,twin,dislocation and void.The effects of nanotwin and nano-grain boundary on shock-induced plasticity,phase transition and spallation behaviors are unveiled.These results can promote the understanding of complex flow,damage and phase transition behaviors and their underlying mechanisms of metallic materials under extreme conditions,providing new insights of developing advanced nanomaterials with high resistance to shock loadings.