Theoretical Study Of The Reinforcement Effects Of Ternary Laves Phase Mg₂Cu₃Si And Its Native Point Defects In Mg-Cu-Si Alloys

To improve fuel economy and reduce environmental pollution,Mg alloys have been attracted extensive attention in automotive and aeronautical industries due to their excellent physical properties,e.g.low density,high stiffness,excellent creep resistance and good vibration performance,etc..Currently the limited formability and tensile strength as well as the low ignition resistance are major hinder for the application of Mg alloys,especially the restrained mechanical properties at elevated temperature.Therefore,to bypass the limitations of mechanical properties of Mg alloys as engineering materials,and tremendous effort has been conducted toward developing new Mg alloys with higher strength.Mg alloys with age hardening response and lightweight have been widely studied.The main strengthening mechanisms of Mg alloys include solid solution strengthening,precipitation strengthening and dispersion strengthening.And addition of alloying elements is an effective approach to optimize the microstructure and improve the mechanical properties of Mg alloys.Another,the native point defects play a significant role in determining the mechanical,physical and functional response of solid materials.For instance,the existences of antisite and vacancy defects in the Laves phase are beneficial to synchro-shear deformation and improve the tenacity of Laves phases.Hence,in this study,the reinforcement effects of ternary Laves phase Mg2Cu3Si and its native points defects in Mg-Cu-Si alloys are deeply studied by ab initio calculation,the main contents of this study are as follows:Firstly,the main reinforcement effects of precipitation phases Mg2Cu3Si,Mg2Si and MgCu2 on Mg-Cu-Si alloys are studied by predicting and comparing the phase stabilities and mechanical properties of these compounds from ab initio calculations.The lowest formation enthalpy and cohesive energy indicate that Mg2Cu3Si has the strongest alloying ability and structural stability.The elastic modulus indicates that Mg2Cu3Si has the strongest resistance to reversible shear/volume distortion and has maximum hardness.The characterization of brittle/ductile behavior manifests that MgCu2 has favorable ductility.Meanwhile the evaluation of elastic anisotropy indicates that Mg2Si possesses almostly elastic isotropy.Debye temperature prediction shows that Mg2Si and Mg2Cu3Si have better thermal stability.Electronic structure reveals the strong Cu-Si and Si-Si covalent character in Mg2Cu3Si,the strong Mg-Mg metallic character and Cu-Cu covalent character in MgCu2,and the strong Mg-Si covalent bonding in Mg2Si.The current work studies hardness,ductility and thermal stability of the key precipitation phases Mg2Cu3Si,Mg2Si and MgCu2,and gains the comprehensive insights into main reinforcement effects of these precipitation phases on Mg-Cu-Si alloys.Secondly,the formation and structure of nine possible native point defects in ternary C14 Laves phase Mg2Cu3Si have been studied by ab initio calculation.A two dimensional phase diagram of chemical potentials is determined for calculation of defects formation energy as a function of chemical potentials.The energetic results indicate that the most stable defect with the lowest formation energy is antisite defect CuSi except(Cu)Si-rich conditions where antisite defect CuMg is the most energetic,whereas antisite defect SiMg is the most difficult except Si-rich condition where vacancy defect VSi is the most unfavorable.These results are consistent with experimental observation of the non-stoichiometry in Mg-Cu-Si phase diagram,and also show clearly that the species of point defects can be tailored by controlling the chemical environment.The local distortions around point defecs increase from antisite defect SiCu to SiMg,showing that the polyhedral symmetry and coordination number of defect sites affect the local structure of point defects.The calculated density of states and differential charge densities show the strong(Cu,Si)-Si covalent bonding,and antisite CuMg is active because the weak Mg-Cu bonding is replaced by the strong Cu-Cu bonding,whereas MgCu and MgSi are unfavorable because the strong Cu-Si bonding is replaced by the weak Mg-(Cu,Si)bonding.