First-principles Study On Structure And Properties Of Mg-Al Based Strengthening Phase And Solid Solution

As the lightest structural metallic materials, magnesium alloys have been receiving a greatdeal of attention in many fields including automobile, electronics,3C industries because oftheir low density, high specific strength and stiffness, good thermal conductivity andelectromagnetic screening effects. However, with the rapid expansion of magnesium alloysapplications, it has been difficult for the performance of ordinary magnesium alloy to meetthe increasingly stringent environment. It has some serious disadvantages. Among them, thelimited mechanical property has become one of the most notable diasadvantages, such as thepoor high temperature mechanical properties, low room temperature ductility and soon. Alloying is an effective way to improve the mechanical properties of magnesium alloys.Therefore, a study on the effect of alloy elements on the properties of Mg based alloy has animportant and practical meaning.First-principles calculations have been carried out to investigate the electronic structure,elastic and thermal properties of Mg-R (R=Al, Zn, Ca, Li, Y and Sc) alloys; the structural,electronic, elastic and thermal properties of MgxAl4-xSr and Mg17-xAlxSr2phases; thestructural, electronic, elastic properties of binary precipitated phases in Mg-Al-R (R=Ca, Sr, Y,Bi); and the electronic, elastic and thermal properties of Al2Ca, Al4Sr and MgRe (Re=Y, Sc)under pressure. The study will give some theoretical guidance for the optimization and designof Mg alloys, and provide valuable estimation for the properties unavailable in experiments.The electronic structure, elastic and thermal properties of Mg-R (R=Al, Zn, Ca, Li, Y andSc) alloys are studied. The calculated cohesive energies show that Mg-Sc has the higheststructural stability. The densities of states (DOS) and electronic charge density difference are obtained to investigate the chemical bonding of Mg solid solutions. The results show thatMg-Y (Sc) alloys have very strong covalent bonding due to a Y(Sc) addition is associatedwith a hybridization between the d-orbital of Y(Sc) and the p-orbital of the Mg atoms. Thebulk modulus B, shear modulus G, Young’s modulus E and Poisson ratio ν are derived usingVoigt-Reuss-Hill (VRH) approximation. The results show that all the alloy systems canexhibit ductile properties at2.77at.%R, and Mg-Zn(Li) alloys have the better ductility andplasticity while Mg-Sc has the highest hardness. The Debye temperatures of the solidsolutions are estimated from the average sound velocity based on elastic constantsevaluations. And the result shows the mechanical stability of Mg-Sc is the best.The structural, electronic, elastic and thermal properties of MgxAl4-xSr phases have beendetermined. The calculated formation enthalpies and cohesive energies show that between thetwo different lattice sites of Al(4d) and Al(4e) in Al4Sr unit cell, the more preferable site ofsubstitution of Mg in Al4Sr lattice is Al(4d) lattice site. And the alloying ability and structuralstability of MgxAl4-xSr phases gradually decrease with increasing x. The densities of states(DOS) are obtained to reveal the underlying mechanism of structural stability. The resultsshow that the reason of Al4Sr having highest structural stability attributes to Al4Sr phasehaving more covalent bonds below Fermi level. The bulk modulus, shear modulus, Young’smodulus and elastic anisotropy are estimated from the calculated elastic constants. Themechanical properties of these phases are further analyzed and discussed. The results showthat the hardness of MgxAl4-xSr increases, while the plasticity decreases gradually with theMg content increasing. Al4Sr, MgAl(4d)Al7Sr2and Mg2Al(4d)Al6Sr2phases are all ductile. Andthe Mg2Al(4d)Al6Sr2(x=1) phase has more anisotropy. The calculations of the Gibbs freeenergy show that a solid solution is thermodynamically more stable than pure Al4Sr phaseand the thermal stability gradually decreases with the Mg content increasing within298-573K.The calculated Debye temperatures shows the mechanical stability of Mg2Al(4d)Al6Sr2is thebest among three phases.The structural, electronic, elastic and thermal properties of Mg17-xAlxSr2phases have beendetermined. The calculated formation enthalpies and cohesive energies show that the increase of substitution amount of Al atom weakens the alloying ability, and Al has certain solidsolubility in Mg17Sr2. Between the two different lattice sites of Mg(12k) and Mg(12j), themore preferable site of substitution of Al has been confirmed to be Mg(12k) lattice site. Withthe increase of amount of Al atom, the structural stabilities of Mg17-xAlxSr2become thestronger. The calculations of the densities of states show that the reason of the higherstructural stability attributes to having more bonding electron numbers below Fermi level.The calculations of the elastic properties show that the hardness of Mg17-xAlxSr2increases,while the plasticity decreases gradually with the Al content increasing. And Mg17Sr2exhibitsductility while the other ternary solid solutions are brittle. The calculated Debye temperaturesshows the mechanical stability of Mg22Al12Mg(12k)Sr4is the best.The structural stabilities, electronic structures and elastic properties of Mg17Al12, Al2Caand Al4Sr phases have been determined. The optimized structural parameters are in goodagreement with the experimental and other theoretical values. The calculated formationenthalpies and cohesive energies show that Al2Ca has the strongest alloying ability, and Al4Srhas the highest structural stability. The calculated densities of states (DOS), Mullikenelectronic populations and electronic charge density difference show that the reason of Al4Srhaving highest structural stability attributes to Al4Sr phase having more covalent bonds belowFermi level. The calculated elastic properties show that Mg17Al12and Al2Ca both are brittlewhile Al4Sr exhibits ductility. And Al2Ca has the highest hardness while Al4Sr has the bestplasticity. The calculations of the Gibbs free energy show that Al2Ca and Al4Sr phases aremore stable than Mg17Al12phase within298-573K, and Ca or Sr addition to the Mg-Al alloyscan improve the heat resistance.Structural stabilities, electronic structures and elastic properties of Mg17Al12，Al2Y andMg3Bi2phases have been determined. The calculated heats of formation and cohesiveenergies show that Al2Y has the strongest alloying ability as well as the highest structuralstability. The calculations of the density of states (DOS) show that the increase of thestructural stability of Mg-Al alloy with Y or Bi additions attributes to an increase in thebonding electron numbers at lower energy level below Fermi level. The calculations of thermodynamic properties show that Al2Y has the highest thermal stability. Mg3Bi2phase ismore stable than Mg17Al12phase below550K. Y or Bi addition to the Mg-Al alloys canimprove the heat resistance. The estimated melting temperature of Al2Y from elastic constantis the highest. And Al2Y has the highest thermal stability. The calculated elastic propertiesshow that Mg17Al12and Al2Y are both brittle, while Mg3Bi2is ductile. And among the threephases, Al2Y is a phase with the highest hardness, while Mg3Bi2has the best plasticity.Electronic, elastic and thermal properties of Al2Ca and Al4Sr under pressure have beendetermined. The calculated elastic properties show that the values of elastic contants Cij,bulkmodulus B, shear modulus G and Young’s modulus E increase with increasing pressure, andthe hardness increases. The ductility and plasticity of Al2Ca can be improved with increasingpressure. While the ductility and plasticity of Al4Sr first decrease and then increase above10GPa. Furthermore, Al2Ca has the higher hardness while the Al4Sr has the better ductility andplasticity. The calculations of the density of states (DOS) and Mulliken electronicpopulations show that the structural stabilities of Al2Ca and Al4Sr decrease first withincreasing pressure and then increases. But their structure remain stable and there is noinduced structural phase transformation under pressure from0to50GPa. And Al4Sr is morestable than Al2Ca. The ionic interaction of Al2Ca and Al4Sr become weaker with increasingpressure. The calculations of thermal properties show that the thermal stabilities of Al2Ca andAl4Sr decrease and the Debye temperatures increase with increasing pressure. Further, theDebye temperature values of Al2Ca are always bigger than those of Al4Sr.Electronic, elastic and thermal properties of MgRe（Re=Y、Sc）under pressure have beendetermined. The calculated elastic properties show that the hardness, ductility and plasticityof MgRe can be increased with increasing pressure, but excessive pressure can producecertain negative influence on their mechanical properties. Furthermore, MgY has the betterplasticity than MgSc, while MgSc has the higher hardness than MgY below40GPa. Thecalculations of the DOS and Mulliken electronic populations show that the structuralstabilities of MgRe become stronger with increasing pressure, and MgY is more stable thanMgSc. The reason of the higher structural stability attributes to having more covalent bonds and ionic bonds interaction. However, the calculations of Gibbs free energies show that thethermal stabilities of MgRe gradually decrease with increasing pressure. The calculatedDebye temperatures show the Debye temperatures of MgRe increase with increasing pressure,and that of MgSc decreases slightly above40GPa. Further, the Debye temperature of MgScis bigger than that of MgY below45GPa, which attributes to MgSc having the biggerYoung’s modulus and shear modulus.