Influence Of Alloying With Bi On The Deformation Behaviors Of Magnesium

Magnesium alloys have been widely used in lightweight structural materials due to their low density and high specific strength.The low symmetry HCP structure and limited deformation systems of magnesium lead to low strength and ductility.The sharp basal texture and mechanical anisotropy restrict the further application.Alloying is a common approach to modify the mechanical properties of magnesium.It is found that alloying with Bi in Mg could refine the grains,improve the ductility and weaken the basal texture.The high melting point intermetallic phase Mg3Bi2(821℃)could improve the high temperature mechanical properties.The systematic study on the deformation mechanism of Mg-Bi alloy is therefore essential to further improve the strength and ductility of Mg,so the sophisticated Mg alloy design is possible.In this thesis,Optical Microscopy,X-Ray Diffraction,Scanning electron microscopy,Transmission Electron Microscopy,Electron Backscatter Diffraction,microhardness test and tensile tests were employed.In casted Mg-xBi(x=1,3,6,9 wt.%)alloys,the grain size is found to reduce from 713 μm to 186 μm with increasing of Bi content.The independent model shows that the grain refinement efficiency of Bi is in between Al and Zr.The tensile yield stress of extruded sample reaches the maximum of 183 MPa when 6%Bi is added and its critical resolved shear stress for prismatic slip is estimated as 84 MPa.On the other hand,the elongation of all the alloys studied is less than 5%,which is caused by the large intermetallic particles Mg3Bi2.It is also found that the annealing heat treatment could weaken the basal texture due to the<1012>oriented nucleation and growth.To avoid the early fracture of materials,a lean Mg-2Bi alloy is then investigated.The variation of microstructure,strength and texture of the alloy with temperature from 100 to 410℃were systematically studied.It is found that the grain size of Mg-2Bi alloy increases from 1.3 μm to 22.4 μm from 100℃ to 410℃.The tensile yield and ultimate strength reach the highest of 176.9 MPa and 236.5MPa respectively at 200℃.And the alloy extruded at 100℃ exhibits the best elongation of-35%.The texture is found not influenced by the temperature change.The intragranular misorientation axis(IGMA)analysis shows that the basal<a>slip,prismatic<a>slip and pyramid<c+a>slip are activated during extrusion at 100℃.The main deformation mechanism during tensile test for corresponding Mg-2Bi alloy is grain boundary sliding according to TEM analysis.A Mg-2Bi-0.45Ca alloy is finally studied to explore the effect of Ca addition on the texture and mechanical properties of magnesium for 4 different temperatures.It is found that the grain size of Mg-2Bi-0.45Ca extruded at 105℃ is 0.8 μm and increases to 26.9 μm when the temperature reaches 410℃.The basal texture is weakened in all alloys and we find a near[1011]fiber texture for alloy extruded at 105℃.As increasing the extrusion temperature,the strength and ductility of alloys decrease.The yield and ultimate tensile strength of the material extruded at 105℃ are the largest of 238 MPa and 293 MPa respectively.The corresponding elongation is～15.4%.The intragranular misorientation axis(IGMA)analysis showed that only basal<a>slip and pyramid<c+a>slip are activated in the selected grains during extrusion at 105℃.And the yield stress shows the strain rate sensitivity for this alloy,which is further confirmed that the dominant deformation mechanism is grain boundary sliding during the tensile test.