Molecular Dynamics Investigation Of Shock-induced Isostructural Phase Transition In Metallic Cerium

Dynamic behavior of condensed materials under shock compression is one of the multidisciplinary key issues involving mechanics,physics,and materials science.During shock compression,the phase transition in materials can be caused by the extreme condition such as high pressure and high temperature,motivating various and complex mechanical behaviors to happen.Cerium(Ce)is a rare earth metal element with a wide range of applications.In the range of low temperature and low pressure,there are two face-centered-cubic(fcc)phases(?-Ce and?-Ce)and a double-hexagonal-close-packed phase(?-Ce)for metallic Ce.At ambient temperature and about 0.7 GPa pressure,Ce undergoes ??? phase transition with a volume shrink of 14%-17%discontinuously.There is a long lasting debating about the mechanism of this phase transformation among Mott transition,Kondo volume collapse and entropy driven model.In former studies,because of the ??? transition in Ce under shock compression,the shock front in cerium exhibits a 3-wave configuration:elastic precursor,plastic shock wave in y-Ce,and phase transition wave corresponding to the ??? phase transformation according to the experimental observation.The former studies were carried at macroscale based on the EOS of multi-phase materials or at microscale based on electro structure theory,and not able to describe the evolution of the microstructures in materials during nonequilibrium process such as the a-y phase transition in Ce.Molecular dynamics(MD)is a common used method can provide information in such temporal and spacial scale in computational materials science.In this work,the large-scale MD simulations were carried to investigate the mechanical behavior of single crystal Ce under shock compression along three crystallographic orientation:[001],[Oil],and[111].1.In this work,a new embedded-atom method(EAM)potential compatible for ?-Ce and?-Ce was developed.This EAM potential has been employed to study several basic properties of cerium in these two fcc phases,such as equilibrium lattice constants,cohesive energies,and elastic constants.These results showed good accordance with experiments and first principle calculations.Thus the newly developed EAM potential could provide a reasonable description of fcc Ce.The lattice defects have been studied with the formation energy calculations of vacancies,interstitials,surfaces,stacking faults,and twinning defects in ?-Ce and y-Ce lattice.The lattice dynamics of ?-Ce and ?-Ce have been analyzed using our EAM potential.The lattice vibrational entropy was calculated and plotted as functions of temperature for each phases.The vibrational entropy change across the ?-? phase transition showed to be?0.67 kB per atom at ambient temperature.2.Using molecular dynamics simulation with the EAM potential in this work,several isotherms and radial distribution functions were calculated.These isotherms and radial distribution functions demonstrate a first order phase transition between two fcc structures,corresponding to ?-Ce and y-Ce,with a critical point sets at Tc?550 K and Pc?1.21 GPa.Thus the newly developed EAM potential could provide a reasonable description of the?-? phase transition of Ce within the scale of classical molecular dynamics simulation.3.The newly developed EAM potential for fcc Ce is employed in the large-scale MD simulations of shock loading onto single crystal Ce to study its dynamic behavior,especially the shock-induced ??? phase transition,and the orientation dependence with[001],[011]and[111]shock loading.MD simulation results showed single-wave or multi-wave configuration for shock wave profiles.Under the shock loading along the[001]or[011]crystallographic orientation,the shock wave possesses a 2-wave structure:an elastic precursor and a phase transition wave,while under shock loading along the[111]crystallographic orientation,the obtained shock wave shows a 3-wave profile as observed experimentally.Thus the shock wave structure is obviously dependent on loading orientation.The Hugoniot data obtained in MD simulation show good agreement with the experimental results.The shock loading MD simulation shows lower phase transition pressure than hydrostatic loading,indicating an accelerant role of the deviatoric stress played in the shock induced ??? phase transition in Ce.4.The local lattice structure before and after shocked are recognized with polyhedral template matching and confirmed with radial distribution functions.Under the[011]and[111]loading,the lattice structure maintains the fcc before and after the shocks,and experiences a collapse during the last shock(the second shock for the[011]loading and the third shock for the[111]loading).The lattice structure also maintains fcc before and after the first shock for the[001]loading,while after the second shock the structure type is considered to be body-centered-tetragonal(bct)which is a meta-stable structure resulting from the used EAM potential for Ce.