Study Of Point Defects And Related Thermodynamic Properties In MgSc Binary Alloys

Magnesium alloys have a promising application in many fields such as electronic products and construction due to their light weight and high specific strength.MgSc alloys were first experimentally found to have shape memory effect and superelasticity by Ogawa et al.However,the current research on MgSc alloys focuses more on its phase transition mechanism and less on the physical properties related to defects,which are inevitable in alloys.Therefore,this paper explores the mechanisms related to defects in MgSc alloys from the perspective of theoretical calculations.In this paper,based on the density function theory,the vacancy formation energy and the diffusion barrier of vacancies were calculated for different Sc components bcc structure and hcp phase Mg-x at.%Sc(x=20,30,50)alloys using the VASP package combined with the SQS supercell method,and the local chemical environment and Sc components were analyzed on the bcc pahse and hcp pahse Mg vacancies and Sc vacancies The effects of the local chemical environment and Sc components on the Mg vacancies and Sc vacancies of bcc phase and hcp phase were analyzed.The effects of the local chemical environment and Sc components on the kinetic and thermodynamic properties of the Mg vacancies and Sc vacancies in the bcc and hcp phase were analyzed.The calculations show that in the bcc phase and hcp phase MgSc alloys,the alloying of Sc makes it more difficult for the alloys to form vacancies,and the vacancy formation energy and the diffusion barrier of vacancies in both Mg in the alloys are higher than those of the pure metals.In the bcc phase,the local chemical environment has less influence on its vacancy formation energy,and in the hcp phase,the vacancy formation energy shows a strong dependence on the local chemical environment,and the vacancy formation energy of Mg and Sc increases with the increase of nearest neighbor(1nn)Sc.In the bcc structure,the diffusion barriers of Sc vacancies are all higher than those of Mg vacancies for a certain Sc component,which indicates that Mg vacancies are more mobile than Sc in the MgSc alloy.In the hcp phase,?E(the difference between the energy of the final state and the energy of the initial state in vacancy diffusion)increases with the decrease of the initial state 1nn Sc for the same final state because the strength of the Mg-Mg interaction is greater than that of the Mgvacancy interaction,and thus ?E is very large.Based on the calculated results of phonon density of states,the kinetic properties of MgSc alloys were analyzed,and it is known that in bcc phase and hcp phase MgSc alloys,the local chemical environment has less influence on the phonon total density of states and phonon partial density of states of MgSc alloys containing Mg vacancies and Sc vacancies,while the Sc component in the alloy has more influence on the phonon total density of states and phonon partial density of states MgSc alloys containing Mg vacancies and Sc vacancies.The increase of the Sc component in the alloy causes the accumulation of Sc in the alloy,making the electrostatic shielding effect more pronounced,leading to a softening of the local lattice vibration mode and the appearance of a pseudoenergy gap or peak barrier in a certain frequency range.In addition,because the atomic relative mass of Mg is smaller than that of Sc,the acoustic phonons of Sc contribute more to phonon partial density of states in the low vibrational frequency range,and the optical phonons of Mg contribute more to ph-DOS in the high vibrational frequency range.Based on the calculation results of phonon density of states,the thermodynamic properties of MgSc alloys were analyzed and it was found that in MgSc alloys with perfect lattice bcc and hcp structures,the ability of the ions to deviate from the equilibrium position increases continuously with the increase of the temperature,making the vibrations violent and leading to an increasing entropysharply at low temperatures and remains almost constant after the temperature rises to about 300 K.The heat capacity and vibrational entropy increase with the increase of Sc component in the alloy,and the vibrational entropy at high temperature follows the classical Dulong-Petit law of thermodynamics.The local chemical environment of vacancies has a certain influence on the vibrational entropy difference,i.e.,when the Sc component in the alloy is certain and the vacancy is Mg or Sc,the more the nearest neighboring Mg atoms,the smaller the vibrational entropy difference.The influence of the local chemical environment on the difference of heat capacity is small,and the difference of heat capacity under different near-neighbor pairs increases rapidly with the increase of temperature and reaches a maximum at about 100 K.When the temperature is greater than 100 K,the difference of heat capacity decreases rapidly and then tends to a stable value and nearly coincides,which is limited by the Dulong-Petit law.