Microstructure Evolution And Mechanical Properties Of Mg-15Zn-34Y Alloy Solidified Under High Pressure

The influence of solidification pressure on the microstructure and phase composition of the alloy is studied in this paper focusing on Mg-15%Zn-34%Y.The crystal structure of the new phase Mg₆₄Zn₁₅Y₂₁1 is revealed and the competition mechanism of different phases in alloy Mg-15%Zn-34%Y is analyzed.Also,the formation mechanism of Mg₆₄Zn₁₅Y₂₁is explained and the mechanical properties of Mg-15%Zn-34%Y alloy and Mg₆₄Zn₁₅Y₂₁alloy under high pressure solidified condition is studied.The microstructure and phase analysis of Mg-15%Zn-34%Y alloy under different pressures show that the solidification pressure significantly affects the microstructure and phase composition of Mg-15%Zn-34%Y alloy.The alloy is composed of staggered 14 H lamella phase(Mg₁₂Zn Y),W phase?Mg₃Y₂Zn₃?and Mg solid solution under normal pressure while in the alloy under 1 Gpa,the W phase disappears and a composition of Mg64.093±0.004Zn15.355±0.002Y20.552±0.005?Mg64Zn15Y21?is generated which is different from the known ternary phase Mg-Zn-Y alloy.The Mg-15%Zn-34%Y alloy solidified under 2GPa pressure is composed of a new phase Mg₆₄Zn₁₅Y₂₁and an eutectic structure(Mg₆₄Zn₁₅Y₂₁phase and Mg solid solution)and the Mg₆₄Zn₁₅Y₂₁phase is in a flower-like state.After 3GPa pressure solidification,the size of Mg₆₄Zn₁₅Y₂₁in the alloy and the volume fraction increase.After analyzing the new phase Mg₆₄Zn₁₅Y₂₁,it can be shown that the Mg₆₄Zn₁₅Y₂₁phase is a cubic system with a space group of Fm-3m?no.225?and the space constant is 7.16567?27??.The Mg atom has three positions,the Y atom has one position and the Zn atom has one position.Only position?0.5,0.5,0.5?is totally occupied by Mg atom while position?0,0,0?and?0.25,0.25,0.25?are all mixed positions which is occupied by Y/Mg and Zn/Mg with the probability ratio of0.882/0.178 and 0.307/0.693,respectively.The analysis shows that the eutectic spacing of Mg-15%Zn-34%Y alloy increases with the increase of solidification pressure.This paper establishes the relationship between eutectic spacing and solidification pressure based on the improved J-H model.The change of eutectic spacing under high pressure can be predicted by this model.The competition mechanism of each phase in high pressure solidified Mg-15%Zn-34%Y alloy is studied.After comparing the parameters of nucleation work,nucleation rate and the growth rate,it can be found that the nucleation work of Mg₆₄Zn₁₅Y₂₁1 phase is lower than that of 14 H and W phases,and the Mg₆₄Zn₁₅Y₂₁phase embryos are more easily transformed into nuclei when the solidification pressure is higher than a critical value.Also,the Mg,Zn and Y content of Mg₆₄Zn₁₅Y₂₁1 phase is close to the nominal composition of the Mg-15%Zn-34%Y alloy,and all the atoms required for the nuclei are more easily from the alloy melt.The Mg₆₄Zn₁₅Y₂₁1 phase provides suitable attachment growth stage for the melt atoms which is conducive to increasing the growth rate.Mg₆₄Zn₁₅Y₂₁1 phase is prior to nucleates and the nucleation rate is higher than that of 14 H phase and W phase.During the growth,the latent heat of solidification will be released to the surroundings to reduce the local subcooling of the melt and make the 14 H and W phased harder to nucleate.It inhibits the nuclei from growing,even makes it disappear.The nano-indentation test,first-principle calculation and analysis of Mg₆₄Zn₁₅Y₂₁1 phase are carried out.The nano-hardness of Mg₆₄Zn₁₅Y₂₁1 phase is4.55 GPa and the Young's modulus is 66.09 GPa.The three independent elastic constants of Mg₆₄Zn₁₅Y₂₁1 phase are C₁₁=52.53633 GPa,C₁₂=27.06729 GPa,C₄₄=38.99479 GPa,respectively.The bulk modulus is 35.5569 GPa according to the elastic constants.The shear modulus is 24.92971 GPa and the Poisson's ratio is0.21585.The toughness of Mg₆₄Zn₁₅Y₂₁1 phase is judged by Pugh criterion which is hard and brittle phase.Mg₆₄Zn₁₅Y₂₁1 phase is anisotropic and<111>is the largest atomic density crystal orientation.The effect of solidification pressure on the mechanical properities of Mg-15%Zn-34%Y alloy is investigates.It shows that high-pressure solidification makes it different for the Mg-15%Zn-34%Y alloy's hardness,tensile properties and compressive properties.The tensile strength of 2GPa solidified of Mg-15%Zn-34%Y alloy increases to 149.72 MPa which is nearly 90%higher than that of the alloy solidified at atmospheric pressure.And the compressive strength is 645.26 MPa which is about 3 times than that at atmospheric pressure.The mechanism of solidification pressure on the mechanical properites of Mg-15%Zn-34%Y alloy is put forward based on the comprehensive analysis.The pressure affects the mechanical properities of the alloy by changing the solid solubility of Zn,Y and Mg solid solution,forming a new phase and changing the eutectic spaceing.The solid solubility of Zn,Y and Mg solid solution increase with the increase of pressure and the the strength of the Mg solid solution phase increases.Mg-15%Zn-34%Y alloy formed Mg₆₄Zn₁₅Y₂₁high-strength phase which is conducive to the alloy resistance to deformation and this is the main reason for the increase in alloy strength.The relationship between the solidification pressure and the eutectiv phase strength??_j?is established.The eutectic spacing of the eutectic phase in the alloy becomes larger and the eutectic phase strength decreases which is not conducive to the improvement of the alloy strength with the increase of the solidification pressure.