Study Of Crystal Nucleation Incubation Times On The Basis Of Configuration Heredity In The Isothermal Crystallization Of Undercooled Liquid Metal

Nucleation in melt controls the formation and growth of as-cast metal microstructure it has an important effect on the mechanical and physical properties of materials.In this paper,the microstructure evolution of metal during solidification and nucleation was analyzed by using molecular dynamics simulation method,coupled with the pair distribution function（g（r））,Honeycutt-Andersen（H-A）bond-type index method and cluster-type index method（CTIM）.The critical nucleus was analyzed and identified based on the cluster structure heredity by atomic trace backward tracking method.The nucleation characteristics of liquid metal Al during isothermal crystallization process under different undercooling degrees were analyzed by statistical analysis of critical nucleus information.The nucleation stage of isothermal crystallization was distinguished by analyzing the time of genetic characteristics before the formation of critical nucleation.The thesis mainly includes the following two aspects:First of all,the isothermal crystallization process of liquid metal in different undercooling（from 0.340 to 0.378）was simulate by molecular dynamic（MD）.The evolution of microstructure was analyzed by the system energy,the FCC atoms and the FCC crystal clusters number.and then,the FCC critical nucleus could identified from many of the FCC atoms using atomic trajectory reverse tracing method.The size,number and distribution of critical nuclei in different undercooling were obtained,and the average size of critical nuclei was about 28～32 atoms.This result is consistent with the previous theoretical and experimental results.The nucleation rate was calculated by analyzing the change of critical nucleation number with crystallization time.It is found that the steady nucleation rate increases first and then decreases with the nucleation undercooling,and reaches its maximum value when the undercoolingΔT=0.362,which is approximatelyI₀=3.34×10³⁵m^-3s^-1.This trend is close to the critical undercooling of nucleation derived from classical nucleation theoryΔT^*=0.4.Then,according to the transient genetic characteristics of crystal clusters before the formation of critical crystal nuclei,an effective method for determining the time of crystal nucleation and incubation is proposed,i.e.the nucleation incubation timeτ_cconsists of embryo incubation timeτ_eand embryo effective growth timeτ_g^effduring the isothermal crystallization of Al.According to whether the clusters in each stage have the characteristics of heritability and the change of the number of critical nuclei,the nucleation process of the system can be divided into four stages:structural relaxation,crystal embryo incubation,unsteady nucleation and steady nucleation.By comparing the difference of incubation time between steady state nucleation and unsteady state nucleation under different undercooling degrees,it is found that the embryo growth time of nucleation is longer than that of steady nucleation,and the embryo incubation time of non-steady nucleation is less than that of steady nucleation.Further analysis of the nucleation incubation timeτ_cof all critical nucleation nuclei under different undercooling shows that the average nucleation incubation time（?）_cand the steady nucleation rateI₀are mirror symmetric.