First Principles Calculations On The Process Of Soluting And Aging For Mg-Al-RE(RE=Gd,Y) Alloys

With its excellent performance at room temperature, Mg–Al alloy is one of the most widely used magnesium alloy series. While the thermal stability of its main precipitated phase β–Mg17Al12 is poor, so its application is still restricted. Adding a small amount of rare earth elements to form second phases with good thermal stability can significantly improve the comprehensive performance of the alloy due to the effect of precipitation hardening.The first-principles calculations based on density functio nal theory calculation method were employed to investigate the phase stability, electronic structure, mechanical and thermodynamic properties of main strengthening phases, that is, Al2 Y, Al3 Y, Al2 Gd and Al3 Gd phases in Mg–Al–RE(RE=Gd, Y) alloys. The calculated results coincide well with the experimental values, which show that the calculation has good reliability. The formation enthalpy showed that the four phases can stay stable and Al2 RE is more stable than Al3RE; Therefore, Al2 RE has the priority to precipitate in the process of alloy solidification. Electronic density of states(DOS) and charge density difference were calculated to analyze the root of its structure stability and interatomic bonding properties. Elastic constants of Al2 Y, Al3 Y, Al2 Gd and Al3 Gd were calculated, and then the bulk modulus, shear modulus, young’s modulus and Poisson’s ratio were deduced. The calculated results showed that the mechanical properties of these four phases are very similar; they have hard and brittle nature and high melting point, so they may have similar reinforcement effect in the alloy. Fina lly, phonon spectrum and phonon density of states were calculated, the thermodynamic properties and Debye temperature were obtained. In conclusion, Al–RE phase has good thermal stability and high strength; therefore, it can significantly improve the comprehensive performance of Mg–Al alloys.On the basis of the physical and chemical properties for Al–Y phases at zero pressure, we further discussed the crystal structure, mechanical properties, electronic structure and thermodynamic properties of Al2 Y and Al3 Y phase under various pressures. With the increasing pressure, cells of the two phases have been continuous contracted, the compression ratio as a function of pressure in accordance with the polynomial change. DOS calculations showed that the hybridized bonding between atoms is abated and the delocalization of electrons becomes stronger as the pressure increases. Elastic constants, modulus, Poisson’s ratio and other mechanical properties of the two phases are increase with the pressure increase, which indicates the decrease of brittleness and increase of plasticity. Debye temperature of the two phases increases with the increasing pressure, suggests that the covalent bond between atoms enhance. Entropy, enthalpy, free energy and heat capacity only have a slight change with the increase of pressure.Finally, the ideal solid solution model of Mg31Al1、Mg31Y1 and Mg30Al1Y1 were built to preliminarily discuss the electronic structure and mechanical properties changes when a Y or Al atom replaces an Mg atom to form solid solution. The calculation showed that the occupied range of electrons near the Fermi level narrows and the hybridized bonding between atoms is abated when the solute atoms solve into pure Mg, this phenomenon is mainly related with the changes of interatomic bonding characteristics. Due to solid solution strengthening effect, shear modulus of the solid solutions have a certain degree of increase, and Y makes the solution harder and more fragile.