Solidification And Precipitation Strengthening Behaviour Of Mg-Sn-Al-Zn-Mn Alloy

Magnesium alloys have been increasingly found application in electronic,automotive and aerospace industries,leading to a remarkable effect of weight reduction.Mg-Sn-Al alloys have attracted extensive attention due to excellent formability,high temperature resistance and ageing strengthening.However,few studies have discussed the mechanisms of microstructure evolution during solidification and the relationship between precipitates and mechanical properties during ageing treatment in Mg-Sn-Al-Zn-Mn alloys.In this thesis,eutectic intermetallic compounds（IMCs）formation and ageing precipitation behaviour of Mg-4Sn-3Al-1Zn-0.6Mn（wt.%）alloy have been investigated respectively,using analytical SEM,EBSD,FIB-TEM,and XCT techniques,combined with thermodynamic calculations.The main conclusions can be drawn as follows:（1）Phase existing in the as-cast microstructure of alloy includesα-Mg dendrites,primary phase ofτ-Al Mn and Al₈Mn₅ phases,Mg₂Sn,β-Mg₁₇Al₁₂ andτ-Mg Al Zn eutectics.The three-dimensional morphology for the majority of Mg₂Sn was a mixture of non-faceted and{100}crystallographic faceted faces.β-Mg₁₇Al₁₂/τ-Mg Al Zn tends to nucleate and grow on the growth facets of Mg₂Sn phase,forming fully divorced eutectic multi-phase particles.（2）The relationship between eutectic intermetallic phases within these multi-phase particles was identified,i.e.seven reproducible orientation relationships（OR-AB1,2,OR-AC1,2,OR-BC1,2,3）between Mg₂Sn（A）,β-Mg₁₇Al₁₂（B）,andτ-Mg Al Zn（C）.The OR-ACs between Mg₂Sn andτ-Mg Al Zn phase were found to have the best lattice match.The Mg₂Sn phase might act as an effective nucleation site for the nucleation and growth ofβ-Mg₁₇Al₁₂ andτ-Mg Al Zn phases.（3）The microstructure evolution during T6 heat treatment was also investigated.Theτ-Al Mn phase transformed into Al₈Mn₅ and Al₁₁Mn₄phases during solution treatment.Flake-like Mg₂Sn phases,rod-like Al-Mn phases,and a small amount of plate-likeβ-Mg₁₇Al₁₂ phases precipitated during the ageing process,both intragranular and grain boundary precipitation were observed.Besides,basal and non-basal Mg₂Sn phases precipitated with various ORs between precipitates andα-Mg matrix.（4）The effect of T6 heat treatment on the mechanical properties of alloy was analysed.The yield tensile strength（YTS）of alloy was improved during ageing process,while compromising ductility.The YTS increased by～35%from～72MPa of solution-treated to～97MPa of 300h-peak-aged alloy.The intragranular deformation was strengthened by numerous precipitates,resulting in the increased tensile yield strength.However,the grain boundary precipitates deteriorated the deformation ability of grain boundaries,which causes crack initiation and propagation along the grain boundary.The fracture mode changed from the mixture of quasi-cleavage fracture and ductile fracture in the solution-treated alloy to the typical brittle fracture in the aged alloy.The whole thesis contains 64 figures,8 tables;151 references are cited.