Molecular Dynamics Investigations On The Thermal And Mechanical Properties Of Palladium At High Temperatures

The melting properties and phase transitions of materials under high temperature and pressure are a fundamental issue in the field of condensed matter physics,and also a hot topic in high pressure science.As a basic form of condensed matter,metal materials are the most widely used and irreplaceable materials in modern industrial manufacturing,which is always been essential for the rapid development of the country and the wellbeing of human beings.In the important period of strategic opportunities for the rapid development of national high-tech industries,we gradually have higher requirements for the service conditions of available metal materials.The new phenomena and effects presented by materials under extreme conditions reveal physical phenomena and patterns that are not available under normal pressure.It plays an important role in the deeper understanding of the properties of matter and its laws of change.As a metal element with a special status among the platinum group elements,palladium(Pd)has a wide range of applications in aerospace,aviation,navigation,weaponry and nuclear energy,and its service conditions are accompanied by extreme conditions of high temperature and pressure.Benefiting from its excellent structural stability under high pressure,Pd is often used as a pressure standard in high pressure.However,there is a lack of research on the high-pressure properties of Pd,and its thermodynamic properties and structural change processes under high temperature and pressure are still unclear,only a few experimental and theoretical studies have investigated its evolution under the simultaneous action of temperature and pressure,which make that carrying out research on the thermal and mechanical properties of Pd under extreme conditions of great importance.Using molecular dynamics(MD)simulations,the high-pressure melting curve of Pd is derived by the two-phase method,which takes into account the efficiency and accuracy.In view of the lack of research on the solid-solid phase transition of Pd under impact loading,we further simulated the dynamic response of Pd under different impact loads using the multi-scale impact technique(MSST),and revealed the structural phase transformation under impact loading in more detail from the atomic scale pathways.A series of structural features are observed in the pressure interval 0-375 GPa.The results suggest that from the initial face-centered cubic(FCC)structure to the stacking faults body-centered cubic(BCC)structure with hexagonal close-packed(HCP)structure,and finally complete melting.In addition,we find that the phase transition depends on the initial crystal direction of impact.Also,the introduction of defects will increase the phase transition pressure of FCC-BCC by 10-30 GPa compared to perfect crystals,which is verified in conjunction with the distribution of stress and potential energy inside the material.In conclusion,we find the FCC-BCC phase transition pressure to be much lower than the static compression by a factor of several under dynamic shock wave loading.Based on MD simulations,the high-pressure melting properties of Pd and the solid-solid phase transition process under shock loading are discussed in detail in this paper,which provides new theoretical insight into the application of high-pressure experiments in the future.