A Simulation Study On The Identification Of Critical Nuclei And Features Of Nucleation In The Rapid Solidification

Homogeneous nucleation,as the typical feature of rapid solidification of metals and alloys,has a significant influence on the final solidified structure and the properties of materials.Therefore,the nucleation mechanism and its control methods have always been a interesting topic in the research fields of materials science and condensed matter physics.To reveal the nucleation features of the rapid solidification,the foremost task is to have a comprehensive and accurate understanding on the size,geometry and interface morphology of the critical nucleus.As the nucleation is a stochastic dynamic process which involves exceedingly small time and length scales,molecular dynamics（MD）simulation can provide a unique insight into the microscopic aspects of nucleation,while it is difficult to directly obser ve and track the formation of critical nucleus experimentally.In this thesis,we investigate the nucleation process of liquid metals by MD simulations.By means of structural analysis methods such as pair distribution function,H-A（Honeycutt-Andersen）bond-type index method and CTIM（cluster-type index method）,the evolutions of microstructures of the simulated systems during crystallization are analyzed.By the reverse tracing of atomic trajectory,the critical nuclei are identified based on the structural heredity of different clusters.Further,the characteristics of nucleation of the metals are revealed under rapid solidification and deep supercooling.Firstly,on the basis of CTIM,a structural analysis method is proposed to distinguish fcc（face-centered cubic）,hcp（hexagonal closest packed）and bcc（body-centered cubic）single-crystal,poly-crystal and hybrid-crystal clusters according to the linkage modes among basic clusters.The fcc medium-range-order,which is composed of fcc basic clusters linked by intercross-sharing（IS）modes,is defined as an fcc single-crystal cluster.The fcc extended cluster,which is composed of fcc basic clusters linked by vertex-sharing（VS）,edge-sharing（ES）or face-sharing（FS）besides IS modes,is defined as an fcc poly-crystal cluster.The fcc extended cluster,which is composed of fcc（dominant）,hcp and bcc basic clusters linked by IS modes is defined as an fcc hybrid-crystal cluster.The internal structures and interface characteristics of different types of extended crystal clusters are characterized and illustrated in an example of the isothermal crystallization of quenched amorphous Ag.The simulation results show that the size of fcc single-crystal cluster is the smallest,followed by fcc poly-crystal cluster,and fcc hybrid-crystal cluster are the largest.The geometric configurations of internal atoms of the fcc single-crystal,poly-crystal and hybrid-crystal clusters are all non-spherical shape.The shell atoms of fcc single-crystal and poly-crystal cluster contain a small amount of hcp and bcc atoms,while the shell atoms of hybrid-crystal cluster are all amorphous and liquid atoms.Further,a unique method to distinguish critical nuclei from embryos is proposed based on the structural heredity of crystal clusters.The extended crystal cluster which has continuous heredity is defined as a nucleus,while the one which has no continuous heredity is called as an embryo,and the nucleus corresponding to the onset point of continuous heredity is identified as a critical nucleus.The rapid solidification process of millions-atom of metal Al is simulated,and the formation and evolution of fcc single-crystal clusters is studied.Hundreds of critical nuclei are further identified by analyzing the continuous heredity of fcc single-crystal clusters during rapid solidification,and the crucial information such as critical size,geometric configuration and interfacial structure of nuclei are directly obtained.The first critical nucleus contains 37 atoms,which have a spherical-like shape.The main size distribution range of critical nuclei is 10～30,and the average size of critical nuclei is about 26,which is completely consistent with the critical size calculated by the equal-probability method.With the increase of nucleation supercooling,the size of the critical nuclei tends to decrease as a whole,and the nucleation rate first increases and then decreases.Moreover,different critical nuclei at same temperature are not only different in size,but also in internal structure and interface morphology.Most of the critical nuclei have non-spherical and irregular geometrical configurations,and the interfaces of most of nuclei are fcc/hcp-liquid structures.Finally,the homogeneous nucleation limit（HNL）of liquid Al under deep supercooling is studied and its kineti c and thermodynamic characteristics are revealed.The nucleation processes of supercooled liquid Al at different temperatures are simulated,and the HNL is deduced out based on the identification of critical nuclei.The influence of the breakdown of Stokes-Einstein（SE）relation on the HNL is also investigated.The temperature_ks=0.51_m（_m is the theoretical melting temperature of Al）corresponding to the HNL of Al is determined,and the thermodynamic spinodal temperature_ts corresponding to the vanishing of nucleation barrier is found to be0.45_m,which is slightly lower than_ks.It is revealed that the thermodynamic nucleation barrier at_ks is very small but non-zero,and a kinetic critical slowing down phenomenon is found near_ks.Additionally,it is found that the breakdown of SE relation can prolong the structural relaxation time and then promote the occurrence of the HNL,but the breakdown is not a prerequisite for the occurrence of the HNL.At last,the nucleation characteristics of supercooled liquid Al at the HNL are intuitively observed by means of visualization.In this thesis,single-crystal,poly-crystal and hybrid-crystal clusters are strictly distinguished,and the structural characterization of embryos and nuclei wit h nanometer dimensions is improved.A cluster analysis method to identify critical nuclei is proposed,which can accurately identify multiple critical nuclei formed at the same time in large systems or under deep undercooling.Statistical analysis of hundreds of nuclei reveals the size,geometry and interface morphology of the critical nuclei,which is helpful for researchers to have a more comprehensive and clear understanding of critical nuclei and nucleation characteristics under deep supercooling.