Effects And Mechanisms Of Alloying Elements On The Stacking Fault Energy And Twin-boundary Segregation Energy Of Mg

Mg possesses many advantages, such as low density, high specific strength and stiffness,however the number of slip systems at room temperature is pretty limited (only have twoindependent slip systems), resulting in poor plastic deformation ability. Traditional Mgalloys cannot satisfy the urgent need of high usage performance in different environmentsand work conditions, which greatly restricts its applications in industry. Accordingly, thedevelopment and preparation of new high performance Mg alloys has become the focus ofoversea and domestic researchers.Element alloying can promote the activation of additional deformation modes, such asnon-basal slips and twinning, and therefore is regarded as an effective method to improve themechanical properties of Mg alloys. However, there is still lack of systematic study on theeffects and mechanisms of alloying elements on Mg alloys. To design new Mg alloys, it is inurgent need to build a “database” that can suggest more potential alloying elements. Notethat the generalized-stacking fault (GSF) energy and the twin-boundary segregation energyare two very important physical parameters in the database, which have great significance inpredicting the mechanical properties of Mg alloys. Specifically, the decrease in GSF energyimplies that the probability to initiate dislocations and twins is increased, which is conduciveto initiating extra deformation modes. Moreover, the decrease in twin-boundary segregationenergy means that the segregation potency of solutes into twin-boundary is increased, andthe effects of solutes on improving twin properties become more pronounced.In this work, the effects and mechanisms of the alloying elements on the GSF energyand twin-boundary segregation energy of Mg alloys are investigated in regard to the typesand contents by the first-principles calculations. The work here can provide a reference and abasis for the design of new Mg alloys with high performance. Following is the mainconclusions drawn in this work:(1) The variation of GSF energies is studied in Mg alloys with21kinds of alloyingelements. Based on the contribution of basal and non-basal slip systems to the plasticity, aweighting model for the total GSF energy is built, which can act as a criterion to evaluate the overall deformation ability of Mg alloys. A common rule is revealed that the larger thedifferences in atomic radii between Mg and alloying elements X, the lower the GSF energies.When the atomic radii are close, the larger the electronegativities of elements, the lower theGSF energies. Moreover, the dependence of total GSF energies on the atomic radii is largerthan that on the electronegativities. The results here provide a reference to the compositiondesign for the new Mg alloys with high performances.(2) A distribution map based on the total GSF energy is built, which provides a basisfor predicting the plasticity of Mg-based alloys. It is revealed that In, Li, Sn and Bi elementsresult in a decrease in GSF energies, and possess large solubility in Mg. Therefore, they areconducive to the ductility enhancement of Mg alloys. The alloyings of Cd, Al, Ag, Ga, Znand Pb elements decrease the GSF energies to a relatively large extent, while they result inlattice distortions at the same time. Accordingly, the plasticity of such Mg alloys (alloyedwith Cd, Al, Ag, Ga, Zn and Pb) greatly depends on the contents of doping atoms. Moreover,Be, Si, Ge, Cu, Na, Sr and K elements though decrease the GSF energies efficiently, theyhave poor solubility in Mg, and therefore they have little effects on improving thedeformation ability of Mg alloys. In addition, the co-doping of Al and Sn elements lead to adecrease in the I2and T2SFE, and promote the activation of partial dislocations and twins,which is beneficial to improving the plasticity of Mg alloys. It is found that the SFE in thebasal slip system is lower in HCP Au than that in Mg, leading to the ease of phasetransformation to the FCC structure.(3) Typical Al and Zn elements are selected to study the effects of alloying contents onthe GSF energies of Mg alloys. With the Al or Zn contents increasing, the GSF energies inthe basal and non-basal systems decrease gradually; meanwhile, the GSF energies of morelocations decrease, implying that the chances of basal slips increase. It is revealed that theprobabilities to initiate basal and non-basal stacking faults increase with an increase in Al orZn contents, which is beneficial to the plasticity improvement of Mg alloys; meanwhile, thelattice distortions become more severe, which suppresses the extension of dislocations andresults in the solid-solution strengthening effects. Thereby, it is predicted that there shouldbe a critical value for the alloying contents in theory, when the value is exceeded, the alloysexhibit “strengthening” effect; in reverse, present “softening” effect.(4) The mechanisms of Al and Zn elements on the GSF energies of Mg–Al–Zn alloysare revealed, and a criterion is further provided to predict the slip location that may formstacking faults most possibly. The decrease in GSF energies presumably originates from theenhanced flowability of charges and the weakened Mg–Mg bonds on slip planes with the additions of Al and Zn elements. Moreover, it is predicted that the basal <a> and pyramidal<c+a> slips are probable to initiate at the locations containing alloying elements andbetween the Mg layers adjacent to alloying planes; the pyramidal <a> slips may formbetween the atomic layers with doping atoms; while the prismatic <a> slips are promotedbetween the Mg layers. These results provide a reference to the prediction of plasticity in Mgalloys with multiple elements.(5) The variation of twin-boundary segregation energy is studied in Mg alloys with19kinds of alloying elements. It is found that the solute atoms with smaller atomic radius thanPb (include Pb; exception: Ti and Zr for the {1011} twin) tend to segregate at thecompressed sites of {1012} and {1011} twin boundaries, while the ones larger than Pb aremore probable to occupy the extended sites. It is revealed that the stronger the chemicalbonds are and the larger extents that the strains alleviate at twin boundaries, the largersegregation potencies the solute atoms possess. Moreover, a common rule is revealed thatthe twin-boundary segregation energies decrease with the increasing difference in the atomicsizes (atomic radii and equilibrium volumes) between Mg and solute atoms. Meanwhile, thedependence of twin-boundary segregation energies on the atomic radii is larger than that onthe electronegativities. Moreover, an approximately linear relationship is presented when the{1012} twin-boundary segregation energy is summarized against the equilibrium volume(exception: Ti, Zr and Cd), and when the {1011} segregation energy is against the atomicradius (exception: Ag, Zr, Bi and Be). These results provide a guidance to further tailor thetwin properties of Mg.(6) A distribution map in regard to the twin-boundary segregation energy is built, whichprovides a basis for predicting the twin strengthening effects of Mg-based alloys. It isrevealed that Ga, Zn, Al, Ag, Cd, Bi and Ca elements result in a decrease in segregationenergies, and possess moderate solubility in Mg. Therefore, they have potentials to improvethe twin strengthening effect; Be, Si, Ge Sb, Na and Sr though decrease the segregationenergies efficiently, they have poor solubility in Mg, and therefore they have little effects onimproving the twin properties of Mg alloys. It is revealed that the co-segregation of Zn(compressed site) and Ca (extended site) decrease the segregation energies to a larger extent,resulting in a stabilizing effect on twin structures. Moreover, with the Al or Zn contentsincreasing, the segregation energies decrease gradually, and the elastic stains reduce, leadingto an increase in the segregation potency. These results are conducive to selecting multipleelements, which are usable to tailor twin properties.Overall, the effects of alloying elements on the GSF energy and twin-boundary segregation energy are studied in this work; moreover, the distribution maps in regard to theGSF energies and segregation energies are built, which can provide a reference for theprediction of plasticity and twin segregation potencies. Based on the electronic structure andstain field, the mechanisms of alloying elements on the GSF energies and twin-boundaryenergies are revealed. This work provides a basis for further understanding and tailoring thealloying elements in Mg alloys, and has a great significance to build the “database”, whichwill facilitate the design and preparation of new Mg alloys with high performance.