Simulation Study On The Microstructure Evolution During Rapid Solidification Of Magnesium And Magnesium-Yttrium Alloy

As"green engineering materials of the 21st century",Magnesium（Mg）alloys have attracted much attention for the advantages of high specific strength,long degradability,and low density,compared with other counterpart alloys.Nowadays,Mg alloys have shown significant promises and prospects as biodegradable materials and lightweight materials.However,Mg alloys prepared by conventional processes have poor plasticity and processing performance,which results from the hexagonal close packed（HCP）structure from the atomic level.And this shortcoming has become the main hurdle to overcome in the application of Mg alloy in biomedical and other fields.To obtain good mechanical performances,it is a necessity to design and optimize the microstructures of this alloy.Many studies have shown that the solidification structure can be controlled by high pressure,which provides a new method for the preparation of high-performance Mg alloys.However,under current experimental conditions,it is difficult to accurately describe the formation and evolution of microstructure during solidification process,and the regulation mechanism of solidification structure under high pressure is still unclear.As with proceedings of computational power and the applications of computational methods on materials science,the simulations provide new insights in understanding the microstructural evolution during solidification.In this thesis,molecular dynamics（MD）simulations have been systematically performed on the solidification processes of Mg and Mg-Yttrium（Y）alloys under pressure.Cluster Type Index Method（CTIM）has been proposed to characterize the formation and evolution of the microstructural clusters during the solidification of the alloys.These results and analysis are promised to provide theoretical basis and deigning pathways to predict the structural and behavioral variations of Mg alloys in different processing conditions.The main research contents are as follows:MD simulations were adopted to investigate the influence of pressure on structure and dynamics properties of Mg during rapid solidification.The dynamics analysis revealed that the increased pressure resulted in the slowing-down dynamics of Mg melt and the increased dynamical heterogeneities,leading to a more dense-packed structure.The increase of pressure led to the reduction of atomic free volume,the obstruction of atomic motion ability,and the increase of the randomness of atomic motion.The effect of pressure on crystallization behavior was studied,and it was found that increasing pressure significantly increased the crystallization temperature T_cof Mg.The atom-level structural analysis by the CTIM suggested that HCP phase was formed in Mg at zero pressure,and Frank-Kasper A15 complex crystal phase was formed at 20 GPa.The formation of A15 phase was favorable at elevated pressures.In addition,the nature of polymorph selection was discussed by analyzing the phonon dispersion of Mg under different pressures.It is found that high pressure could change the stability of different crystal phases of Mg,so that the A15 phase,which was unstable under zero pressure,could be obtained stably under high pressure.It was further indicated that the increase of external pressure could significantly affect the crystallization kinetics and solidification path of Mg melt.To investigate the formation process and heredity characteristics of the A15complex crystal phase in Mg,MD simulation was used to study its structural evolution and heredity characteristics during rapid solidification at a pressure of 20 GPa.The results showed that the phase transformation of liquid Mg took place in the temperature range from 878 to 906K.During the phase transition,the number of two types of clusters（icosahedron and defect-icosahedron）forming A15 crystal phase increased sharply,and their contents at 100 K were 18%and 52%（～1:3）,respectively.The atoms of the two clusters were interlaced to form the A15 phase.Combining heredity characteristics of clusters and three-dimensional atomic snapshot analysis,conclusions could be reached that the crystallization of A15 crystal clusters at 20 GPa pressure could be divided into three stages:（ⅰ）nucleation stage,T∈[915,906]K,where the heritability of Z12 and Z14 cluster atoms were less than 50%and crystal nuclei gradually form and grew up to be stable;（ⅱ）rapid growth stage,T∈(906,878]K,in which the heritability exceeded 50%for the first time and continued to increase,the crystal nuclei began to grow rapidly,and the adjacent crystal nuclei began to merge;（ⅲ）slow growth stage,T（27）878K,the heritability reached about 90%,the number of atoms in the crystal nucleus tend to be stable,and only a few atoms at the grain boundary transformed into A15 phase.The influences of trace the content of Y on the microstructures evolution and mechanical properties of Mg_（100-x）Y_x（x=0.25,0.75,1.5,3,4,5 at.%）alloys during the solidification were also investigated by MD simulation.It is found that the addition of trace Y had a great effect on the microstructures and mechanical properties of Mg alloy.With the increase of Y content,face-centered cubic（FCC）structure was formed in Mg_（100-x）Y_xalloy.The proportion of FCC cluster atoms in Mg_（100-x）Y_xalloy first increased and then decreased,reaching the highest fraction of 56.65%when x=0.75.The mechanical properties of the alloy were investigated by uniaxial tensile simulation.The results showed that the difference of FCC content in the alloy caused by different Y content resulted in different mechanical properties and deformation mechanism of Mg-Y alloy.Mg_99.25Y_0.75and Mg₉₇Y₃alloys reached the highest yield strength of 1.86 GPa and 1.90 GPa,respectively.The analysis on the microstructural evolution showed that FCC content in Mg_99.25Y_0.75alloy had no significant change before and after the deformation.But the HCP cluster atoms gradually decreased,and defect cluster atoms increased significantly.Meanwhile,HCP cluster atoms in Mg₉₇Y₃alloy gradually transformed to FCC cluster atoms,and the final FCC atomic content reached about 74%.It was further proved that the addition of Y led to the difference of FCC content in the solidification microstructure and the different deformation mechanism of Mg-Y alloy.To further investigate the effect of high pressure on the rapid solidification of Mg alloy,the rapid solidification processes of Mg_99.25Y_0.75alloy at different pressures（0～25 GPa）were simulated.It is found that the external pressure had an significant effect on the crystallization of Mg-Y alloys:when P≤10 GPa,the main microstructure was FCC;P=15 GPa,the FCC,HCP,and body-centered cubic（BCC）clusters co-existed in the system;P≥20 GPa,the BCC clusters occupied more than90%.The crystallization temperature T_cincreased almost linearly with the increase of pressure.Especially,in the pressure range of 15～20 GPa,there was an important critical pressure for structural transition,which caused the final microstructure of Mg-Y alloy to change from FCC crystal to BCC crystal.The pressure release stability analysis showed that pressure was an important factor affecting the stability of different crystal phases of the alloy,and the Mg-Y alloy with BCC crystal phase could be obtained only by applying pressure during solidification.