Simulation Study On Evolution Of Microstructures During Rapid Solidification Of Liquid Mg-Zn Alloys

The development of the rapid cooling theory and the history of the moleculardynamics simulation are briefly reviewed in this thesis firstly. The solidificationprocesses of liquid metal Mg-Zn under different conditions are simulated. By meansof the different microstructural description methods of the pair distribution function,bond-type index method, and cluster-type index method (CTIM), and the technique oftracing cluster based on the CTIM, the formation properties and evolutionmechanisms of microstructures during the solidification processes of liquid metalunder different conditions are deeply studied.A simulation study has been performed for the rapid solidification process of liquidMg70Zn30alloy to research the formation mechanism of the large clusters. The resultsshow that the bonding probability between Zn-Zn atoms is increased obviously during thesolidification process at cooling rate of1×1012K·s-1. The amorphous structures are formedmainly with the1551,1541and1431bond-types, and the characteristic1551bond-type isdominant for metal glasses due to low energy. The icosahedron cluster plays a key role informing amorphous structure, and becomes the main body of the large clusters which areformed by the combination of some middle and small clusters with distinctly differentsizes, through mutual competition by unceasing annex and evolution in a seesawmanner.The initial melt temperature and alloy composition can leads to discrepancies inmicrostructures during the solidification processes. The effects of six different initialmelt temperatures and nine alloy compositions on the microstructure evolution duringthe solidification process of liquid Mg-Zn alloys have been investigated by usingmolecular dynamics simulation. Simulation results show different initial melttemperatures have different degrees of effects on the amorphous characteristics of thesolidification structures (such as the1551bond-type, icosahedron cluster, averagecoordination number, etc.). It is found that the degrees of effects are nonlinearlyrelated to the initial melt temperatures and fluctuated in a certain range, and thefluctuation range decreases gradually with the decline of the initial temperature. Butthe average energy per atom for different initial temperatures indicates that thechanges of average atomic energy are linearly related to the initial melt temperatures.The higher the initial temperature is, the lower the average atomic energy of theamorphous structure finally formed, that is, the more stable the amorphous structure is, and the stronger the amorphous forming ability. The first peak of partial gZn-Zn(r) willrise above the partial gMg-Mg(r) and cross over each other at the glass transition regionfor nine Mg-Zn alloy compositions during the solidification processes. And thenumbers of icosahedron cluster have a much difference among nine Mg-Zn alloycompositions.It is found that icosahedron cluster plays a key role in the Mg-Zn metallic glass andthe second peak splits in the pair distribution function curve. We study the effect oficosahedron cluster to the second peak splitting in the pair distribution function curve.From analyzing the geometry of icosahedra, it is known that the side length oficosahedron is larger than the circumradius of icosahedron for a little, and why themost of central atoms of icosahedra is the smaller Zn atoms for Mg70Zn30metallicglasses can be easily understood. Furthermore, the IS number increases remarkablyand become the dominant cluster in the supercooled liquid and glass structures. Theintercross-sharing (IS) linkage between two icosahedra increases remarkably andbecome the dominant cluster among the four linkages in the glass structures. As welinked the vertex of IS clusters to some other neighbor points, it can be correspondingto each peak of the pair distribution function curve. So, it can be reasonably drawnthat the second peak splitting of the pair distribution function curve is mainly causedby icosahedron in the system.To investigate the formation and evolution of icosahedral nano-clusters duringrapid solidification, a molecular dynamics (MD) simulation study has been performedfor a system consisting of10000000atoms of Mg70Zn30alloy. The results show thatthe icosahedral nano-clusters exist in the system with different probabilities, thenumbers of icosahedral nano-clusters with high-probability demonstrate an odd magicnumber sequence in turn as19,23,25,27,29,31,33,35,37,39,41,43,45,47,49,51,53,55… and so on. The magic number clusters can be classified into four types:chain-like, triangle-tailed, quadrilateral-tailed and pyramidal-tailed by differentgeometry structures of their central atoms contained in clusters. The heredity andevolution of icosahedral nano-clusters are further investigated by means of an inversetracking method. For the IS-ICO nano-clusters formed of1-3icosahedra, the perfectheredity is of dominant position during the solidification processes. The replacementheredity indicates that the evolution of icosahedral nano-clusters gathers or scattersby pentagonal pyramid formed of6atoms, quadrangle formed of4atoms and triangleformed of3atoms. For the IS-ICO nano-clusters formed of4and5icosahedra, thereplacement heredity emerges unconspicuous. It shows that the less number of atoms for same number icosahedra corresponds to higher perfect heredity.