Theoretical Study Of The Dislocation Sliding Mechanisms Of Magnesium And Its Alloys

Magnesium(Mg)and its alloys have found extensive applications in automobile,aircraft,and electronic components,due to their high strength,lightweight,and good electric properties.However,their broader applications in modern industry are severely limited by their low tensile strength and poor plasticity.The limited room temperature ductility of Mg is due to their hexagonal close packed(HCP)structure and dominant basal<a>slip slip mode,which possesses only two independent slip systems,far from satisfying the five independent slip systems required by the von Mises criterion for an arbitrary homogeneous straining.Therefore,the activation of additional deformation mechanisms,especially shear deformation with a component along the c axis,is significant for enhancing the ductility of Mg at room temperature.Intuitively,pyramidal<c+ a>slip in HCP metals is ideal for accommodating c-axis strain and provides sufficient slip systems.However,the Burgers vectors associated with such slip is larger than those of<a>slip.Accordingly it is critical to understand the origin and behavior of such dislocations to improve the plasticity of Mg alloys at room temperature.Recent experiments have demonstrated that alloying rare earth elements can significantly improve the plasticity of Mg at room temperature by activating the pyramidal II<c + a>slip.However,the formation mechanisms of such dislocations remain largely unexplored and under debate.Including the experiment research,there are a lot of theoretical calculation work set out to study the formation mechanisms of different slip system in magnesium alloys.The slip behavior of dislocations are closely related to the generalized st,acking fault energy(GSFE)curves,which show both stable and unstable stacking fault energy(SFE).The unstable SFE determines the behavior of slip modes and the mobility of dislocations.The stable SFE,which is associated with the stable position on the slip direction,significantly affects the dissociation of dislocations.However,recent GSFE curve calculation results predicted that superimposed Y atoms can impede the pyramidal II<c +a>slip,while experiments found promoting influence.First-principle calculations are widely used to calculate the SFE and GSFE curves.However,it is known that the standard first-principle calculations,including both LDA and GGA,cannot capture the long-range van der Waals(vdW)interactions for nonhomogeneous electron densities.This may lead to serious errors in the prediction of the structure,stability,and function of materials.Although first-principle calculations can be observe the microstructure from atomic level,due to the limitation of calculating condition,the models of first-principle calculations are smaller than that of the actual crystal.Based on the classical Newtonian mechanics,molecular dynamics(MD)simulations could study the larger atomic system,get the information of size effect and boundary effect of materials under different loading,observation of the formation and dissociation process of dislocation.In this dissertation,we use vdW included first-principle calculations,in combination with MD simulations,systematically studied the slip modes and plasticity of Mg and its alloys.The major conclusions can be summarized as follows:(1)We have carried out first-principle calculations to systematically study the slipping mechanism of pure Mg slabs.Our optB88-vdW calculations find that the vdW interactions play a significant role in the slipping process.For the unstable stacking fault configuration of pyramidal-II,inclusion of the vdW reduced the SFE value strongly by up to 69 mJ/m2,and the related restoring stress may be lowered by up to 0.74 GPa.In addition,the disembrittlement D value increase significantly from 2.0 to o2.3 after considering the vdW interactions,which indicates the prominent role of vdW forces to the plasticity of Mg.Our findings indicate the importance of vdW corrections in the accurate prediction of mechanical properties of metals.(2)The origins of pyramidal II<c + a>dislocations and their dissociation mechanisms were investigate by using the dispersion-inclusive density functional theory,in combination with molecular dynamics simulations.We find that the superimposed Y atoms decrease both the unstable stacking fault energy(SFE)and stable SFE of pure Mg,facilitating the formation of pyramidal II<c+a>dislocations.Importantly,a flat potential-energy surface(PES)exists around the position of stable SFE,which allows cooperative movement of atoms within the slip plane.Alloying Mg with Y atoms increases the range of the PES,and ultimately promotes different sliding pathways in the Mg-Y alloy.These findings are consistent with experimentally observed activation of the py ramidal II<c+a>slip system in Mg-Y alloys,and provide important insight into the relationship between dislocation structure and macroscopic enhancement of plasticity.(3)The energies of deformation fault(12)and twin-like fault(T2)of thirteen binary Mg alloys were studie.d using density functional theory.Results shown that the faulted regions are energetically favorable for solute segregation,and the reduction of SFE was caused by charge redistribution.We define a charge redistribution factor,F,to quantify the solute-induced charge redistribution.An analytical model was established to calculate SFE from F.Furthermore,the GSFE curves of Mg alloys were calculated.We conclude that both the range of potential energy surface and unstable SFE have significant influence on the formation of pyramidal II<c+ a>dislocations.This results would help with understanding the sliding modes of different slip system,and will provide theoretical guiding for further design high strength and plasticity magnesium alloy structural materials.