First-principles And Molecular Dynamics Studies On The Effects Of Alloying Elements On The Deformation Behaviors Of Magnesium

As the lightest metallic structural material,magnesium is expected to be applied in the areas of transportation and aerospace to reduce fuel consumption and carbon emissions,which benefits the sustainable development of society and the environment.However,the wide application of magnesium in engineering fields is prohibited by its poor plasticity at room temperature,low strength,and low elastic modulus.Being a traditional method to improve the mechanical properties of Mg,alloying treatment has been widely adopted to develop Mg alloys.In order to design novel magnesium alloys more efficiently and economically,it is necessary to clarify the influence mechanism of alloy elements on the elastic and plastic deformation behaviors of magnesium.Currently,the theoretical research on the elastic deformation behavior of magnesium mainly focuses on the influence of alloy elements on the polycrystalline elastic modulus of magnesium,and the research on the three-dimensional elastic modulus of single-crystal magnesium is scarce.In the meanwhile,the theoretical research on the plastic deformation behavior of magnesium mainly concentrates on the changes of stacking fault energy,which could indirectly reflect the influence of alloying elements on the dislocation slip behaviors.Although there is a certain relationship between stacking fault and dislocation,they are different defects in crystals.The nucleation,motion,cross-slip behaviors of dislocations can not be comprehensively explained by the stacking fault energy.This thesis investigated the influence of alloying elements on the physical parameters closely related to dislocations at the atomic scale,in order to summarize the influence of alloying elements on the deformation behavior of magnesium and provide a theoretical basis for selecting appropriate alloying elements to develop novel practical magnesium alloys.The thesis is divided into two major parts.First,the first-principles calculations are carried out to obtain the three-dimensional elastic moduli of single-crystal Mg binary alloys and the polycrystalline elastic moduli to evaluate the influence of alloying elements on the elastic deformation behavior of Mg.Secondly,the first-principles and molecular dynamics methods are performed to simulate the effect of solute atoms on the core structures of basal<a>,prismatic<a>and pyramidal<c+a>dislocations,because the plasticity and strength are closely related to slip mechanisms of dislocation and the characteristics of dislocation are governed by the core structure of dislocation.In the meanwhile,the Peierls stresses of prismatic<a>and pyramidal<c+a>dislocations are calculated on the basis of the classic Peierls-Nabarro model.Based on the above results,the role of solute atoms in dislocation nucleation and motion is analyzed to understand the intrinsic mechanism of alloying treatment that affects the dislocation slip mechanism.The major conclusions are drawn as follows.(1)The effects of 18 alloy elements at the concentrations of 2.78 at.%and 6.25at.%on the elastic moduli of magnesium are calculated.It is revealed that the rare earth(RE)elements would significantly increase the value of Young's modulus(E)along[0001]direction,which causes the spindle-shaped three-dimensional Young's modulus of the Mg-RE systems,thereby intensifying the elastic anisotropy of Mg.Besides,all RE elements would increase the value of the shear modulus(G)on the basal plane(G₍₀₀₀₁₎).The above results regarding the directional elastic moduli show that the RE element can considerably enhance the bonding between the basal planes.In addition,when the concentration is 2.78at.%,only Sc significantly increases the values of polycrystalline elastic moduli G and E with the increments of 2.15 and 4.59 GPa,respectively.Other elements just slightly modify the elastic moduli.Li,Al,Ag,Sc,Y,Tb,Dy,Ho,Er,and Lu would markedly enhance the values of G and E at 6.25 at.%.This information provides fundamental data for choosing alloying elements to design magnesium alloys with high elastic moduli.(2)The changes of the dislocation core structure are simulated when the solute atoms Sn,Al,Ca,and RE(Y,Sc,Gd,Er,and Nd)are located inside or in the vicinity of the<a>screw dislocation core.It is found that the RE and Ca elements with large positive size misfits would reduce the decomposition width of the basal dislocations,which is beneficial to the cross-slip of<a>screw dislocation from the basal plane to the prismatic plane.In contrast,Sn and Al with minor positive and negative size misfits,respectively,exert little influence on the core structure of basal<a>dislocations.All these solute atoms and basal dislocations attract each other,indicating the strengthening effect of solutes on the basal slip.Note that Ca and RE elements generate a stronger effect compared with Sn and Al.In addition,it is noted that the RE additions would promote the core structure of<a>screw dislocation transformed into non-planar core structures implying a strengthening effect on the basal slip.(3)The structural and energy modifications of basal dislocations cores under the local influence of interstitial oxygen atom are simulated.It is found that the interstitial oxygen atom and basal dislocation core mutually repel each other.This repulsion increases the resistance to the glide of basal dislocation,thereby strengthening magnesium.Moreover,the oxygen atom would reduce the energy of the prismatic<a>screw dislocation,making it more stable laying on the prismatic plane,and facilitating the prismatic slip.(4)The Peierls stress of prismatic<a>dislocations under the doping of 23 alloy elements is calculated.It is found that almost all the alloy elements(except Mn)would reduce the Peierls stress,namely,the lattice friction is reduced thus facilitating the nucleation of prismatic dislocation.After taking the competition of<a>dislocation nucleating on the basal or prismatic plane into consideration,it is found that Ag,Ca,Li,Mn,Zn,Zr,and all RE elements would cause the values of the ratio of unstable stacking fault energies of basal and prismatic slip systems(?_usf(B)/?_usf(P))higher than that of pure Mg,indicating these elements can enhance the opportunity of<a>dislocation nucleating on the prismatic plane.Y,Ce,Ca,Al,and Sn are chosen to interact with the prismatic<a>screw dislocation cores.It is also found that the prismatic<a>dislocation and solute atoms Y,Ce,and Ca would attract each other,which shows that these elements would strengthen the prismatic slip.By contrast,Al and Sn would induce the prismatic dislocation core cross-slipping to the basal plane and decomposing,which indicates Al and Sn prefer the basal slip other than the prismatic slip.According to the above results,it is concluded that the key factor in promoting prismatic slip is to increase the probability of<a>dislocations nucleating on the prismatic plane and stabilize the core structure of prismatic<a>dislocations.(5)The Peierls stress of the<c+a>dislocation on the pyramidal?plane in Mg is calculated under the influence of typical alloying elements(Al,Zn,Ca,Sn,Zr,Y,Ce,and Gd)by the first-principles methods.It is found that Sn,Ca,Ce,Gd,and Y would decrease the Peierls stress,which is in favor of the generation of<c+a>dislocation,while Al,Zn,and Zr would increase the Peierls stress making it more difficult to nucleate.The molecular dynamics simulations of compression along c-axis and shearing along<c+a>direction in single-crystal Mg and Mg-3at.%Y alloys show that:1)the glissile<c+a>dislocations extended on the pyramidal?plane would not decompose into sessile states(such as<c>+<a>)without encountering a strong strain field;2)it is observed that the I₁ stacking fault can act as the nucleation source of the<c+a>dislocation;3)glissile<c+a>dislocations extended on pyramidal plane can still be released when the<c+a>dislocation interacts with I₁,I₂,and<c+a>dislocation;4)randomly distributed Y solute atoms have little effect on the decomposition behavior of the<c+a>dislocations;however,Y solute atoms would exert a drag force on the pre-existing edge<c+a>dislocation,thus hindering the gliding of<c+a>dislocation.Moreover,it is inferred that the key factor in promoting the pyramidal<c+a>slip is to significantly reduce the stress level required for the nucleation of<c+a>dislocation.