Simulation Study Of The Influence Mechanism Of High Pressure On The Microstructure Evolution During Solidificaiton For Liquid Metal Zirconium?Nickel And Nickel-zircoium Alloy

In this thesis,at the beginning,the basic knowledge on the solidification of liquid metals,the formation of amorphous allo ys and the computer simulation,and their great significance in scientific research and materials preparation processes have been introduced.Especially,the origin and devel opment of Molecular dynamics?MD?have been explained,from the general principles to specific application process containing the interatomic potentials,periodic boundary conditions,thermodynamic condition and computer algorithms et al.The Cluster Type index Method?CTIM-3?adopted in this paper has been described in greater detail.In this paper,the MD simulation method,several microstructural analysis methods which covering the average potential energy of atoms,the pair distribution function,HA bond-types method and the CTIM-3 have been adopted to systematically investigate the influence mechanisms on the microstructural formation and evolution under different pressure conditions during the rapid solidification process of liquid metal Zirconium?Zr??Nickel?Ni?and Ni-Zr alloy at atomic level.At first,the MD simulations for investigating the microstructural evolution during the solidification process of liquid metal Zr under different pressure conditions?0⁵0 GPa?but at the same cooling rate?1.0×10¹11 K/s?have been performed with the Embbed Atom Method?EAM?Potential.The results reveal that in the system,the critical pressure?P_c,28.75 GPa?has been identified.When the P>P_c,the Zr melts will be vitrifi ed,namely,the amorphous structures formed,and the larger the pressure,the higher the glass transition temperature T_g.While when P<P_c,the Zr melts will be crystallized,and the Ostwald's step rule is applied during the crystallization processes,namely,always the bcc?14 6 0 8 0 0 00 0?structure occurred at first in the system and tranformed to hcp?12 0 0 0 6 6 00 0?structure in the end;the higher the pressure,the lower the onset temperature?T_c?of crystallization.There is only the single-intermediate-state crystallization?SisC?during solidification process under zero pressure,but the multiple-intermediate-state crystallizations?MisC?are usually observed under higher pressures.From structural analysis,it is found that if nucleation is essentially completed at the end of the first crystalline?bcc-dominated?stage,the MisC will occur,otherwise,the SisC occurs.Finally,the thermodynamics and kinetics factors should be comprehensively considered for more correct understanding the effect of higher pressure on crystallization processes.Adopting the MD simulations and EAM potential,the microstructural evolutions of liquid metal Ni during solidification process at the same cooling rate?1.0×10¹11 K/s?under different pressure conditions?0³0 GPa?have been investgatied.The results show that under all the given pressures,the simulation systems are all directly solidified into crystal structures.And t he onset crystallization temperature T_c is enhanced almost linearly with the increase of pressure.The fcc?12 0 0 0 12 0 0 0 0?,hcp?12 0 0 0 6 6 0 0 0?and bcc?14 6 0 8 00 0 0 0?of three clusters consisted by the 1421,1422,1441 and 1661 bond-types play a key role in the microstructure transitions from liquid to crystal structures.Highly interesting,it is found for the first time that there is an important phase transformation point between 20²2.5GPa,as pressure upto this value,the system would be crystallized from fcc to bcc structure during the solidification processes from the same initial liquid system at the same cooling rate.Based on the same EAM potential,MD simulation studies have been performed for liquid Ni₆₄Zr₃₆6 alloy during rapid solidification process under different pressure conditions?0⁵0 GPa?at the same cooling rate?1.0×10¹00 K/s?.The effects of pressure on microstructural evolutions and the relationship between mechanical properties and pressure are studied.It is found that the icosahedral cluster?12 0 12 0 0 0 0 0 0?and the defect icosahedral cluster?12 0 8 0 0 0 2 2 0?,?12 2 8 2 0 0 0 0 0?play a key role during the microstructural transitions.All the solidified structures are of amorphous structures consisting of various basic clusters and their combinations with different sizes.It is worth noting that all the glass transition temperatur e T_g,main bond-types,main basic clusters and elastic constants are almost increased linearly with the increase of pressure.Especially,the amorphous structure formed under 50 GPa would become an excellent mechanical properties of materials with the most tightness microstructural configuration.In summary,the results above mentioned have given a new angle of view,for deeply understanding that the influence of higher pressure on microstructure evolution and mechanical properties during solidification pro cesses of liquid metals of Zr and Ni,and alloy of Ni ₆₄Zr_36,especially for the microstructural mechanisms of the multiple-intermediate-state occurred in systems.There are important effects of the pressure on the formation and evolution of microstructures,especially on the formation and evolution of main basic clusters.Comparing the effects of pressure and cooling rate on solidification structures of system,it is found that the effect of increasing pressure is similar to that of decreasing cooling rate;but their acting mechanisms are different and should be further analyzed.