First-principles Study On The Structural Correlation Between Liquid And Solid Phases In Mg-based Alloys

In this thesis,both the first-principles calculation and ab initio molecular dynamics simulation on Mg alloys have been performed to investigate the influence rule of alloying elements on the structures of Mg and structural correlation between liquid and solid phases in Mg-Li,Mg-Al binary alloy system.The effect of alloying elements on the structure and mechanical properties of Mg has been studied by first-principles calculation.Compared with pure Mg,the volumetric change rates △V%and the energy change rates △E%of substitutional solid solutions Mg15X(X represents the alloying element)are calculated to obtain the AV%-AE%coordinate system of Mg15X.The distribution of the alloying elements in the△V%-△E%coordinate system reflects that the mxaximum equilibrium solid solubility of alloying element is intrinsically related to the "damage tolerance" of Mg.Furhter,the arrangement of alloying elements is periodic in the △V%-△VE%coordinate system,and the periodic arrangement of the main group elements shows that the electronegativity has a more significant influence on the structure of substitutional solid solution compared to the atomic size.Moreover,the mechanical properties of Mg15X are analyzed according to the classified quadrants in △V%-△E%coordinate system.Combined with the electron density difference,the relationship among alloying elements,structural properties and mechanical properties is roughly established.The local structural,dynamical,and thermodynamic properties of liquid Mg-Li alloys at 973 K have been investigated using ab initio molecular dynamics simulation.The local atomic structures have been characterized by the coordination number,bonded pair,and Voronoi polyhedron analysis.A structural transition region is found in the composition range from 12.5 at.%Li to 31.25 at.%Li:the close packing local atomic structures begin to transform to bcc type,which are dominated by the structural changes around Mg atoms on the effect of Li addition.The inflection point of calculated mixing enthalpy curve is also located in this region,supporting the transition of local atomic structures.Owing to the structural transition,the self-diffusion coefficients of Mg and Li change with different tendencies as Li content increasing.In addition,the interaction between Mg and Li atoms is evaluated by the quantity of charge transfer in liquid Mg-Li alloys.According to the quasi-chemical approximation model,charge transfer is confirmed as the key-factor of interaction energy.The structural evolution of liquid Mg-Al alloys has been investigated with composition variation from pure Mg to Al at 973 K using ab initio molecular dynamics simulation.The local structures are characterized in terms of pair correlation functions,coordinate numbers,and bond pairs.The atomic interactions are evaluated by the electronic structures to reveal the formation mechanism of the liquid structures.As a result of the fact that the formation of Mg-Al bonds is actually restrained by Al-Al bonds with lower energy,the initial local structures around Mg trend to be maintained as the fragments during the structural evolution,and the discrete Al atoms transforms to be segregated around Mg in the composition region from 18.75 to 31.25 at.%Al.Based on these structural characteristics,the liquid structures are modeled as the Al bonded networks surrounding Mg fragments.Further,the dynamical properties are reflected by the self-diffusion coefficients.The results show that the diffusion of Mg atoms is actually limited by Al bonded networks,which confirm the reliability of the model.It is also worth expecting that the structure model may provide a new viewpoint for further study on the complex metallic alloys in solid Mg-Al system.Based on the liquid structural model and in an effort to reveal the common structural feature associated with the off-stoichiometry in Mg-Al intermetallic compounds,the occupation mechanism of anti-site defects in Mg17Al12 and MgAl2-C36 are investigated via the first-principles method concentrated on both energetics and electronic structures.Combined the energetic effect of anti-site defects on the structural stability with bonding characteristics in the off-stoichiometric compositions,the common structural feature is exhibited by the structural motif that Al bonded skeleton perpendicular to the Mg hexagon.Further,such structural motif is embedded into both fcc-Al and hcp-Mg to construct Mg-Al supersaturated solid solution system,and the obtained formation energies in these cases are favored over the corresponding special quasi-random structures.Thus,we suggest that the structural feature shared by the two off-stoichiometric compounds might represent a universal local structural behavior in Mg-Al alloys,from the intermetallic compounds to supersaturated solid solutions.