Microstructure Evolution And Mechanical Properties Of Microalloyed Az80

Microalloying is one of the effective ways to research and develop low-cost,high-performance magnesium alloys.By investigating the microstructure and mechanical properties of micro-alloyed magnesium alloys,the influence of micro-alloyed elements on the microstructure and mechanical properties of magnesium alloys was explored,and a correlation model of composition/process-structure-properties of micro-alloyed magnesium alloys was established.While ensuring cost advantages,the mechanical properties of magnesium alloys were improved and the application of magnesium alloys in civil applications was promoted.In this thesis,two types of micro-alloyed AZ80 magnesium alloys were designed using mixed rare earth elements（La,Ce,Y）with high solid solution differences and non-rare earth elements Ag:AZ80-0.3La-0.2Ce-0.1Y and AZ80-0.3La-0.2Ce-0.1Y-0.1Ag.Further,the effect of RE and RE+Ag microalloying element combinations for the AZ80 on the evolution of solidification and deformation microstructure.Simultaneously,the effect of microalloying on the mechanical properties of AZ80 as-cast and as-extruded was investigated to reveal the strengthening and toughening mechanisms.The above work has revealed the following four points.1.Microalloying changed the morphology and the number of secondary phases of the AZ80 alloy and promoted the growth ofα-Mg along the dendrites.In addition,the rare earth elements and Al elements in the alloy formed Al₄RE intermetallic compounds and transformed the Al₈Mn₅ phase into the Al₈REMn₄ phase,thus changing the composition of the second phase in the alloy.At the same time,rare earth elements promoted the transformation of theβ-Mg₁₇Al₁₂ phase from the eutectic structure to the dissociated eutectic,transforming the network eutectic structure in the alloy into a broken network,increasing the volume fraction of the massive anisotropic eutecticβ-Mg₁₇Al₁₂ phase in the alloy,and changing the morphology of the second phase of the alloy.Further,the combination of RE+Ag microalloying elements reduced the solid solution of Al elements inα-Mg and increased the volume fraction ofβ-Mg₁₇Al₁₂ phase in the alloy.2.The mechanical properties of AZ80 alloy were effectively improved after microalloying.The yield strength,tensile strength and elongation of AZ80 alloy increased from 61 MPa,128 MPa and 1.5%to 83 MPa,170 MPa and 1.9%after rare earth microalloying.The combination of RE+Ag microalloying elements further increased the tensile strength,yield strength and elongation of the alloy to 187 MPa,105 MPa and 2.9%.Quantitative analysis of the strengthening mechanism showed that the increase in strength was mainly due to an increase in the volume fraction of theβ-Mg₁₇Al₁₂ phase in the bulk,while the increase in plasticity was mainly due to the transformation of the reticular eutectic structure into a fracture network,which hindered the crack expansion.Additionally,the microalloying element combination of RE+Ag facilitated the occurrence of non-basal slip in the alloy,further enhancing the plastic deformability of the alloy.3.Microalloying altered the dynamic recrystallisation behavior of the AZ80 alloy,while promoting the dynamic precipitation of theβ-Mg₁₇Al₁₂ phase in the alloy.The combination of RE+Ag microalloying elements effectively reduced the stacking fault energy of AZ80 alloy,promoted the transformation of dynamic recrystallisation from discontinuous to continuous dynamic recrystallisation,and brought down the average grain size from 25.74μm to 19.94μm.The trace rare earth and Ag elements reduced the solid solution of the Al element inα-Mg,providing a greater driving force for the nucleation of theβ-Mg₁₇Al₁₂ phase.This resulted in an increase in the volume fraction of the second phase in the alloy from 1.1%to 3.2%and 5.8%,respectively.Moreover,the precipitation of theβ-Mg₁₇Al₁₂ phase shifted towards discontinuous precipitation to continuous precipitation.In addition,the Al₈REMn₄ and Al₄RE phases formed in the alloy after microalloying were able to act as heterogeneous cores for theβ-Mg₁₇Al₁₂phase,increasing the number of heterogeneous cores in the precipitated phase.4.Microalloying effectively enhanced the mechanical properties of the extruded AZ80 alloy.The rare earth microalloying increased the yield strength and tensile strength of the extruded AZ80 alloy from 236 MPa and 338 MPa to 288 MPa and 387MPa,respectively.The combination of RE+Ag microalloying elements further allowed the yield strength,tensile strength and elongation of the alloy to be increased to 334MPa,423 MPa and 12.8%.The combined mechanical properties were close to those of conventional high content rare earth alloyed and vigorously plastic deformed magnesium alloys.At the same time,quantitative analysis of the strengthening mechanism indicated that the increase in the overall mechanical properties was mainly due to the large increase in the dynamically precipitatedβ-Mg₁₇Al₁₂ phase.In addition,the combination of RE+Ag microalloying activated the columnar slip,which promoted the coordination of AZ80 intergranular deformation and effectively enhanced the plastic deformation capacity of the alloy.