Orientation-Induced Anisotropy Of Plasticity And Phase Transition In Monocrystalline Tantalum Damaged By Shock Compression

The dynamic behavior of material damage evolution under extreme conditions has attracted continuous attention in national strategic fields such as modern national defense engineering and frontier science.The shock response and damage mechanism of metal materials under strong shock loading are some of key scientific problems in the field of weapon physics and space protection.Damage and failure of ductile materials is a typical multi-scale physical and mechanical issue.To deeply understand the underlying physical mechanism of macroscopic mechanical behavior,relevant macroscopic phenomena could be analyzed from the microscale.Based on the molecular dynamics simulation method,this study aims to reveal the damage and failure of materials at the microscopic scale under strong dynamic loading and focusing on the anisotropy of shock wave response and microstructure evolution in the nonequilibrium process.The main contents are outlined as follows:(1)Shock wave propagation and mechanical response of tantalum in different crystal orientations under strong shock loading were studied.The evolution of physical field is used to trace the propagation of shock wave and the change of waveform.The potential physical mechanism of the anisotropy of spallation behavior is discussed from the perspective of elastoplastic wave.The results show that the shock wave propagation is highly dependent on the impact strength and crystal orientation.When the impact strength is lower than the Overdriven point,the void morphology and distribution characteristics and elastic-plastic wave response of single crystal tantalum show significant anisotropy.However,when the impact strength is above the Overdriven point,the shock wave response in the three orientations presents a single plastic wave structure,and the evolution of the physical field and the impact mechanical response in the sample are no longer sensitive to crystal orientation.By observing the evolution of spall strength with impact velocity in three typical crystal orientations,it is found that a lower spall strength is statistically associated with a more number of voids and a higher void nucleation rate.(2)The orientation dependence of shock-induced plastic deformation and dynamic damage behavior of single crystal tantalum was investigated.Firstly,the evolution of dislocation density with impact strength under different orientations is discussed.The anisotropy of dislocation motion is explored in terms of plastic deformation mode,critical strength of activation dislocation motion,and reaction between dislocations.The results show that the plastic deformation is mainly dislocation sip in the orientation of[110]and[111],and the dislocation is characterized by 1/2<111>.In addition,the critical strength of activation dislocation motion in both crystal orientations is lower than that in[100]orientation.In the case of[100],there more types of dislocations are generated in the sample and the reactions between dislocations are more complex.By quantifying dislocation density and analyzing the spatial distribution of the void,it is found that there is coupling between dislocation motion and cavitation in different crystal orientations.Dislocation motion plays a role in void nucleation and then influences the spall strength of the material.By observation of the spall region,it is found that the entanglement of dislocation lines or the dense dislocation is potential nucleation site of the void.Additionally,the movement of dislocation in the spall regime is affected by the number of void nucleation.(3)The melting characteristics at different orientations and their variations with impact strength are revealed in detail The anisotropy of the transient crystal structure evolution of the material is discussed and the orientation dependence of the structural phase transition under impact compression and tensile state is explored.Combined with the Simon melting curve,the evolution of radial distribution function,and temperature profile,the detailed melting states corresponding to various impact strengths at different orientations are effectively evaluated.Through the analysis of the local atomic structure and the quantitative statistics of micro-defects,the result indicated that a large number of dislocation will lead to the instability of crystal structure and local damage,which is not conducive to the transformation of crystal structure in the system.Besides,it is found that the BCC-HCP crystal structure transformation occurred during the compression process of monocrystalline tantalum along the[100]-orientation under the one-dimensional strain impact loading condition Importantly,the structural transformation of BCC-FCC crystals in the[100]orientation is induced by void nucleation.On the contrary,in the cases of[110]and[111],the large number of microvoids with wide distribution region leads to a significant amorphization in the spall region,and serious lattice distortion inhibits the crystal structure transformation.