Deformed Microstructure And Mechanical Properties Of Directionally Solidified Mg-Gd(Zn)-Y Alloy

Magnesium as a close-packed hexagonal structure only has two independent slip systems at room temperature,which cannot meet the Von-Mises criterion equiaxed polycrystalline materials have at least 5 independent slip systems during uniform plastic deformation),so the room temperature plasticity is poor.The transverse grain boundary is the weak link of deformation and should be minimized.On the one hand the columnar magnesium alloy prepared by directionally solidified has less transverse grain boundaries,on the other hand it has a structure similar to bicrystals(three grain boundary constraints).Therefore,the columnar polycrystalline magnesium alloy was prepared by directionally solidified technique and was subjected to tensile deformation at room temperature and high temperature.The EBSD testing method was used to characterize the deformed microstructure and study the effects of initial orientation and deformation temperature on mechanical properties、deformation mechanism of directionally solidified magnesium alloy.The experimental results are as follows:The deformation mechanism was mainly basal slip at room temperature.If the columnar crystal grains were in "soft orientation"(basal slip Schmid Factor>0.3),the deformation was coordinated by {1012} tensile twin,because the {1012} tensile twin has low Critical Resolved Shear Stress(CRSS)and expand easily,which can alleviate the stress concentration during the deformation process,so the columnar polycrystalline Mg6.0Gd-0.5Y alloy with initial orientation of "soft orientation" grains had lower ultimate tensile strength(104.0 MPa)and high elongation(32.8%).If the columnar crystal grains were in "hard orientation"(basal slip Schmid Factor<0.3),the deformation was coordinated by {1011} contract twin and {1011}-{1012} double twin,because contract twin has large CRSS and expand difficultly,so the columnar polycrystalline Mg-4.78Zn0.45Y-0.10Zr alloy with initial orientation of "hard-oriented" grains had high ultimate tensile strength(213.4 MPa)and low elongation(6.4%).Tensile twins and compressive twins were generated when the columnar polycrystalline Mg-9.0Gd-0.5Y grains with "soft orientation" and "hard orientation deformed,so their tensile strength(127.2 MPa)was lower than Mg-4.78Zn-0.45Y-0.10Zr alloy and the elongation(16.9%)was lower than Mg-6.0Gd-0.5Y alloy.When the columnar polycrystalline Mg-6.0Gd-0.5Y alloy stretched at 150 ℃～200℃,the stress-strain curve was similar to the deformation at room temperature,and the work hardening was still dominant at 250℃,its corresponding ultimate tensile strength and elongation were 73.6 MPa and 55.5%.The columnar polycrystalline Mg4.78Zn-0.45Y-0.10Zr alloy showed obvious dynamic recrystallization characteristics when deformed at 250℃(tensile strength and elongation were 76.2 MPa and 26.9%).It showed excellent resistance to high temperature deformation of the columnar polycrystalline Mg-6.0Gd-0.5Y alloy.It exhibited obvious dynamic recrystallization characteristics when stretched at 350℃,and its ultimate tensile strength and elongation were 40.9 MPa and 111.0%at 350℃.In summary,the columnar polycrystalline Mg6.0Gd-0.5Y alloy had both good room temperature plastic deformation ability and excellent resistance to high temperature deformation.