First-principles Calculation And Experimental Study Of Mg-Al And Mg-Y Alloys

As the lightest metal structural materials in the industry,magnesium and its alloys have significant value and broad application prospects in various fields,because of the low density and high specific strength.Magnesium alloys have less independent slip systems at room temperature due to the hexagonal closepacked(HCP)structure,thus,the disadvantages such as poor plastic deformation ability and low formability have limited the engineering application.It is necessary to improve the plasticity of magnesium alloys while expected strength should be ensured at low temperature,which has been one of the most important and urgent problems to be solved.First-principles calculation,experimental test and visco-plastic self-consistent(VPSC)simulation were used to study the effects of alloying elements(Al and Y)on mechanical properties and plastic deformation mechanism of magnesium alloys.First,based on generalized stacking fault energy(GSFE),the effects of Al and Y on critical resolved shear stress(CRSS)of several slip systems were analyzed,which provided a guidance for experimental research.Secondly,the microstructure and mechanical properties at room temperature of Mg-Al and Mg-Y alloys were studied by experimental tests,thus verifying the first-principles calculation results.Finally,the plastic deformation processes of Mg-Al and Mg-Y alloys were numerically fitted and analyzed by visco-plastic self-consistent model,then the results of Al and Y on critical resolved shear stress of magnesium were discussed together with first-principles calculations results.This paper got a comprehensive view on the effects of alloying elements on mechanical properties and deformation mechanism of magnesium at the microscopic and macroscopic levels.Therefore,it provided theoretical gist for the composition design of magnesium alloys with high properties,as well as a certain reference for the development of new magnesium alloy materials.The contents of this dissertation mainly include the following aspects.(1)First-principles calculations and experimental results show that smaller atoms of Al element decrease the lattice constant of magnesium,and the effect of Y atom is just the opposite.Even though,atoms of both Al and Y elements are able to reduce the axial ratio of magnesium alloys,which is beneficial to improve the symmetry of the lattice and the plasticity of Mg alloys.(2)As for the basal slip system({0001}<112?0> slip system)and non-basal slip systems({101?0}<112?0> and {101?1}<112?0> slip systems),alloying elements of Al and Y have the ability to reduce the GSFE and thus it has a positive contribution to the plasticity of metallic materials.For pyramidal {112?2}<112?3> slip system,Al increases the ratio of stable stacking fault energy to unstable stacking fault energy,so <c+a> dislocation tends to exist in the form of full dislocations.On the contrary,Y makes it fall off,and <c+a> dislocation have the tendency to decompose into movable partial dislocations,which benefits the plastic deformation.(3)According to the Peierls-Nabarro model,the CRSS values can be deduced from GSFE results of various slip systems.Both Al and Y are able to lower the CRSS of basal and non-basal slip systems,especially,Y significantly reduce the CRSS of the prismatic slip system.What’s more,Al and Y elements have shorten the difference value of CRSS between non-basal slip systems and basal slip systems,which enables non-basal slip to activate more easily.Theoretically,Al and Y elements have been proved to show the effect of solid solution plastifying.(4)Tensile yield strength,ultimate tensile strength and elongation of Mg-Al alloys are significantly improved by comparing with pure magnesium,and they increase with the increase of Al contents.The yield strength of Mg-Al alloys increases by more than 88.2 % compared with that of pure magnesium,and elongation at fracture increases by more than 11.7 %.Tensile yield strength and ultimate tensile strength of Mg-Y alloys are also enhanced,the elongation at fracture has markedly improved.The yield strength of Mg-Y alloys increases by more than 31.5 % compared with that of pure magnesium,and elongation at fracture increases by 127.4 % at best.The experimental results show that Al and Y could improve the strength and plasticity of Mg alloys,which verifies that critical resolved shear stress calculated by the first-principles calculation.(5)The results of visco-plastic self-consistent simulation are in good agreement with the experimental results,and the changing trend of CRSS values obtained by visco-plastic self-consistent simulation are consistent with first-principles results.These two methods demonstrate that Al and Y elements can shorten the difference value of CRSS.Therefore,the research method is feasible and can be applied widely.