| Background:Tungsten(W)is the best choice for anode target of X-ray tube because of its high density,high strength,high melting point and corrosion resistance.As tungsten has disadvantages such as poor ductility,irradiation brittleness,impurity atom segregation and poor grain boundary cohesive force,are greatly affect its service life.Therefore,improving the performance of metal tungsten is one of the most important issues in the research field of X-ray anode target.The macroscopic properties of materials are mainly determined by their microstructures,and molecular dynamics simulation is one of the main research methods in the field of materials science.The specific formation and evolution mechanism of crystal nucleus of liquid metal tungsten during nonequilibrium solidification process is still unclear.In order to further clarify the influence of thermodynamic and kinetic factors on the formation and evolution of microstructure during rapid solidification process,molecular dynamics simulation was adopted to calculate the rapid solidification process of liquid metal tungsten under different cooling rates and pressures,and its crystallization mechanism was further studied,so as to provide new ideas for the development of new materials for the anode targets.Objective:By using the molecular dynamics simulation method,the microstructure characteristic and its evolution process of liquid metal W under different solidification conditions are deeply studied,so as to fundamentally understand its crystallization mechanism and physical nature,which will provide a theoretical guidance for the improving of the performance of medical X-ray machine anode targets,and prepared for the development of new anode target materials.Methods:In this paper,the rapid solidification of liquid metal W at six different cooling rates(10 K/ps,5 K/ps,1 K/ps,0.5 K/ps,0.1 K/ps,0.05 K/ps)was firstly simulated using the molecular dynamics simulation method based on the large-scale atomic/molecular parallel simulation software(LAMMPS),and then the solidification process of liquid metal W under different pressures(5 GPa,10 GPa,15 GPa,20 GPa,30 GPa)at a cooling rate of 0.1 K/ps was calculated.Finally,the competitive growth mechanism of different crystal structures was analyzed in depth.And particularly,at the very moment of phase transition during rapid solidification process,the formation and evolution of the crystal was investigated by using pair distribution function,atomic average energy,the largest standard cluster analysis,tracing method and threedimensional visualization.Results:The study of the solidification process of liquid metal W at different cooling rates shows that even at high cooling rates,the system still forms an amorphous structure coexists with topological close-packed(TCP)crystals rather than a pure amorphous structure,and the content of TCP crystals increases with the decrease of cooling rate.Both perfect and defective TCP crystal increase rapidly as the temperature decreases in the early stage of crystallization,and as the perfect TCP crystal structure becomes dominant in the system,the defective TCP crystal structure begins to gradually change to the perfect TCP crystal structure.At the cooling rate of 0.1 K/ps,the β-W structural unit composed of three atoms of Z12,Z14 and Z15,increases rapidly during the rapid solidification process of liquid metal W in the temperature interval of 3310 K≤T≤3360 K,but the structure is unstable;when 3310 K≤T≤3260 K,the critical nucleus of β-W TCP crystal formed and began to grow rapidly;at T≤3260 K,the large nuclei continue to grow slowly by adjusting and optimizing the atomic position,and finally the system forms a relatively perfect β-W phase TCP crystal structure(T refers to temperature).By studying the solidification process of liquid metal W under different pressures,it was found that the higher the pressure,the higher the temperature at which the phase transition of the system starts;since the pressure was applied at a cooling rate of 0.1K/ps,the intrinsic phase of the system was β-W phase TCP crystal.As the pressure increases,the β-W phase TCP crystal gradually decreases,and at 15 GPa,BCC crystals appears and grow rapidly and become dominate,and the BCC crystals increase with increasing pressure until 30 GPa when the β-W phase TCP crystal disappears and the whole system forms a near-perfect BCC crystal structure.By studying the competition mechanisms of different crystalline phase structures during the solidification of liquid metal W under different pressures,it was found that TCP crystal was formed first at high temperatures,and its stability closely depends on the pressure and the degree of the regularity of its own structure.If the standard β-W unit is formed under high pressure,the TCP crystal β-W structure will exist in the final solid;otherwise nearly perfect BCC crystal forms although some defect TCP crystal appeared first in the supercooled liquid.Conclusion:In this paper,the molecular dynamics simulation method is used to calculate the solidification process of the liquid metal W under different conditions.By analyzing the microstructure evolutions of the liquid metal W during rapid solidification,it is found that the cooling rate is a key factor in controlling the crystallization process,in which the TCP crystal structure formation ability is extremely strong,and its nucleation process can be divided into three stages,the incubation of the crystal nuclei,the formation and rapid growth of the critical nucleus,and the slow growing and regulation of the crystal nucleus.Secondly,the pressure is also an important factor in determining the crystallization pathway,and the phase transition onset temperature is closely related to the pressure that increases with the increase of pressure,and the sub-stable β-phase is the optimal phase for liquid tungsten metal in the low-pressure-strong state,and the BCC-phase is the optimal phase in the high-pressure-strong state.When the pressure is greater than or equal to 15 GPa,the effect of pressure is dominant,and the system forms a crystal structure dominated by BCC.When a complete β-W phase unit cell structure is formed in the system,the system will retain the TCP crystal,otherwise it will change to the BCC crystal structure.In summary,this paper deeply studies the microstructure evolution and crystallization mechanism of liquid metal tungsten during rapid solidification,and reveals the competition between TCP crystal and BCC crystal,which not only enriches the nucleation theory under the condition of rapid solidification of liquid metal,but also points out the direction of how to regulate the final solidification structure of metal tungsten. |
PDF Full Text Request