The Strengthening Effects Of Alloying Elements On Mg Alloys From First-principles Calculations

Magnesium alloys are considered as the lightest metallic structure materials, have beenwidely concerned in various fields owing to their low density, high strength-to-weight ratio,excellent electrical and thermal conductivity, good machining performance, vibrationreduction performance and easy recycling. With the development and applications ofmagnesium alloys, more excellent mechanical properties are required. Therefore, how toimprove the strength of magnesium alloy has become a hot point in the world.Elemental alloying is considered as an efficient way to improve the tensile strength ofmagnesium alloys by mechanisms of solid-solution strengthening, precipitationstrengthening and fine-grain strengthening. However, there is still lack of systematic studyon the effects and mechanisms of alloying elements on Mg alloys. It is known that Al, Zn,Sn, Ca, Zr and RE are recognized as the common alloy elements for magnesium. In thispaper, a first-principles method was performed to study the strengthening effects ofmagnesium alloys with16alloy elements and Zn+Ca codopings for acting as references ofdesigning magnesium alloys. Furthermore, the mechanism of strengthening was discussedin detail by means of charge density, density of states and chemical bond analyses.For the cases that alloying elements disperse uniformly or segregatively, fewresearches have been done to study which case is more beneficial to strengthening. Hence,the strengthening effects of the distribution of alloying elements on Mg alloys need furtherresearch. In addition, it’s quite difficult to achieve perfect models which doping elementsdistribute uniformly or segregatively in conventional experimental methods. Therefore, inthis paper, a first-principles plane-wave pseudopotential method is used to overcome the challenges to alloy design. Following is the main conclusions drawn in this work:(1) We simulated the tensile processes of Mg54and Mg53X1(X=Al, Zn, Mn, Si, Li, Sn,Ti, Mo, Zr, In, Tl, Pb, Y, Na, K and Ca) models using first-principles calculations toevaluate the strengthening effects of the alloying elements during the yield stage ofmagnesium alloys. The results indicate that there are obvious strengthening effects of thealloying elements on magnesium alloys, especially Ti, Al, Zn, Pb and Ca elements havemore significant influences. The atoms whose atomic radiuses are smaller than Mg have thestronger strengthening effects than the solutes with bigger atomic radiuses than Mg duringthe0.04-0.08strain. When the valence electron count of alloying element is bigger than Mg,the charges transfer from Mg to doping atom, the charge transfer phenomenon strengthensthe bonds at the doping sites but weakens the surrounding Mg-Mg bonds compared with theundoped situation. Furthermore, the strengthening effects of alloying elements onmagnesium matrix will weaken with the number of valence electron increasing. When thevalence electron count of alloying element is smaller than Mg, the charges transfer fromdoping atom to Mg. The strengthening effects of alloying elements on magnesium matrixwill weaken with the number of valence electron decreasing.(2) A first principles method was performed to study the strengthening effects of Znand Ca atoms on Mg matrix. First, we calculated the formation enthalpy (H f) of Mg30Zn2,Mg30Zn2and Mg30Zn1Ca1to judge which doping configuration is stable. Second, we builtMg52Zn1Ca1, Mg52Zn2and Mg52Ca2models depending on the formation enthalpycalculation results. Last, we simulated the tensile and fracture processes of Mg52Zn1Ca1-I,Mg52Zn1Ca1-II, Mg52Zn2and Mg52Ca2models to investigate the strengthening effects of Zn,Ca atoms into Mg-Zn, Mg-Ca and Mg-Zn-Ca alloys. For the ideal tensile strength:Mg52Ca2> Mg52Zn1Ca1> Mg52Zn2. The results explicitly illustrate that the strengtheningeffect of Ca doppings is biggest, the strengthening effect of Zn doppings is smallest, and thestrengthening effect of Zn and Ca codoppings is in the middle. Ca atoms show betterstrengthening effects than Zn atoms. The calculation results also indicate that the dopingatoms whose atomic radiuses is bigger than Mg show better improvement on thestrengthening effects than the doping atoms with smaller atomic radiuses. Zn and Ca codoppings in magnesium matrix offset the lattice distortion caused by Zn doping or Cadoping.(3) We focusd on the effects of the distribution of Al and Zn elements on thestrengthening potency of magnesium alloys. The aim of this work is to provide a referencefor the alloy design and prove the necessity of homogenization heat treatment. Accordingly,the tensile processes of Mg-Al, Mg-Zn and Mg-Al-Zn alloys were simulated using thefirst-principles method. Based on the simulated stress-strain curve, the ideal tensile strengthfor Mg51Al3(I), Mg51Zn3(I) and Mg51Al2Zn1(I) possessing uniform distributions is largerthan that for Mg51Al3(II-IV), Mg51Zn3(II-IV) and Mg51Al2Zn1(II-IV) where the alloyingelements distribute segregatively. Consequently, the doping atoms dispersinghomogeneously show better strengthening effects, which verify the necessity ofhomogeneity heat treatment. Al atoms show better strengthening effects than Zn atoms. Thesimulated results show that Mg51Al3(I-IV) models all break between Mg-Mg layerssurrounding the doping atoms. It indicate that the addition Al to Mg reinforce the Mg-Albonds and weaken the Mg-Mg bonds.