单晶Ce冲击相变的分子动力学模拟

Molecular dynamics simulation of shock-induced isostructural phase transition in single crystal Ce

金属Ce在室温条件下当压力达到约0.7 GPa时会发生一阶相变,体积突变减小14%-17%,相变前后两相分别为γ-Ce和α-Ce,均为面心立方结构.实验中发现冲击波在Ce中传播,其波形存在明显的多波结构依次为γ-Ce弹性前驱波、γ-Ce塑性波、γ-Ce→α-Ce相变波.基于新发展的金属Ce的嵌入原子势,对单晶Ce的冲击相变行为进行了分子动力学模拟.模拟结果表明,在一定强度下,单晶Ce中的冲击波阵面分裂为多波结构,波形结构与加载晶向明显相关:在[001]和[011]晶向加载下表现为双波结构,依次为前驱波和相变波;在[111]晶向加载下波阵面分裂为弹性前驱波、γ-Ce塑性波、γ→α相变波,与已有的实验观察相一致.冲击波速的Hugoniot关系在低强度加载下与实验符合得较好.同时在此冲击相变过程中,应力偏量对相变起促进作用,相较于静水压加载,冲击加载的相变压力条件更低一些.

Cerium(Ce),a rare earth metal,undergoes a significant(14%-17%) and discontinuous volume shrinkage when subjected to ~0.7 GPa compression at ambient temperature:there happens a first-order isostructural phase transition from γ-Ce phase to α-Ce phase(these two phases are both face-centered-cubic(fcc) phase).Because of the α→γ transition in Ce under shock compression,the shock front in cerium exhibits a 3-wave configuration:elastic precursor,plastic shock wave in γ-Ce,and phase transition wave corresponding to theγ→α transition according to the experimental observation.In this paper,a recently developed embeddedatom-method(EAM) potential for fcc Ce is employed in the large-scale molecular dynamics simulations of shock loading onto single crystal Ce to study its dynamic behavior,especially the shock-induced α→γ phase transition,and the orientation dependence with [001],[011] and [111] shock loading.The simulation results show 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 γ→α phase transition in Ce.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 bodycentered-tetragonal(bct) which is a meta-stable structure resulting from the used EAM potential for Ce.The fcc lattice rotation after shock is observed in the [011] and [111] loading after the phase transition,while no reorientation occurs in the [001] loading.