Investigation On The Deformation,Damage And Fracture Of SiC Under Extreme Conditions

The dynamic mechanical behavior of materials has always been an important research field of shock dynamics,and the phenomena and mechanisms of materials under extreme conditions such as high temperature and high pressure and high strain rate are the hotspots that have attracted more and more attention in recent years.Research on the shock response of materials or structures includes theoretical analysis,simulation studies,and experimental studies.In this paper,the deformation,damage and fracture behavior of SiC ceramic materials under extreme conditions are studied comprehensively using molecular dynamics simulations and related theoretical analysis.The dynamic process and the microscopic mechanism involved are analyzed in detail.The main research contents are as follows:（1）The shock response characteristics of SiC single crystals and nanocrystalline at room temperature are studied by molecular dynamics simulations.The effects of shock intensity on deformation,damage and spallation damage are studied by varying the particle velocities.The effects of crystal orientations on Hugoniot state,Hugoniot elastic limit and plastic deformation,and phase transition are discussed in detail.The influence of temperature rise caused by shocks on the transformation of spallation failure mechanism is analyzed in detail.It is found that with the increase of shock intensity,the shock compression changes from elastic to plastic and structural phase transition,and the shock induced spallation gradually changes from the classical spallation to the micro-spall.（2）The shock response characteristics of SiC single crystals at high temperature are studied by large-scale molecular dynamics simulations.The effect of initial high temperature on the kinetics of high melting SiC materials is investigated by changing the initial ambient temperature of the material.From the shock Hugoniot curve,shock induced plastic deformation and shock induced structure phase change and shock induced spall,the influence of initial high temperature on the material is analyzed and the difference of crystal anisotropy at high temperature and normal temperature is found.The study found that the initial high temperature reduces the Hugoniot stress level,inhibits the formation of deformation twinning,reduces the structural phase transition pressure and the tensile strength of the material.（3）The effect of strain rate on the tensile strength of SiC single crystals and nanopolycrystals is studied by large-scale molecular dynamics simulation.The quasi-isentropic loading method is proposed to study the“fracture/spallation”behavior of the material approximately and equivalently,and the variation of tensile strength of SiC material in the wide strain rate range(10⁷-10¹² s^-1)is realized in the study.At the same time,the anisotropy of plasticity and damage and quasi-isentropic tensile strength and failure mode under quasi-isentropic compression in single crystal SiC are studied.The effects of different initial compressive strains on the subsequent quasi-isentropic tensile fracture behavior are also analyzed.It is found that the tensile strength of single crystal SiC shows anisotropy and is almost independent of strain rate below 10¹⁰ s^-1,while nanocrystalline SiC shows weak strain rate sensitivity.Above this strain rate,the strain rate sensitivity of strength increases significantly.The initial compression deformation and damage have a significant effect on the tensile fracture behavior of the material.The[001]crystal orientation shows a unique octahedral cleavage failure,while the[110]and[111]crystal orientations tend to fail in the vertical loading direction.Based on the molecular dynamics results and macroscopic experimental data,the strain-rate related strength model of tensile strength of SiC polycrystals in a wide strain rate range is fitted and proposed.The model is in good agreement with the prediction of the dynamic tensile strength of the material at the experimental strain rate level.（4）The ultra-large-scale molecular dynamics simulation is used to study the effects of grain size on the shock behaviors of nanocrystalline SiC.Based on supercomputers,the simulations of shock response of materials with grain sizes ranging from 2 nm to 32 nm are achieved through ultra-large-scale systems with atoms up to 200 million.The effects of grain size on the physical and mechanical behaviors of nanocrystalline SiC ceramics such as Hugoniot curve,shock induced plastic deformation and shock induced structural phase transition and shock layer cracking are analyzed.