Solute-homogenization Model And Its Application In Designing Mg-based Alloys

The present thesis addresses the composition origin of Mg-based solid-solution alloys to study its inner structural characteristics and establish corresponding structure model which is aimed at guiding composition designation and performance development for new magnesium alloys.As all known,alloy components of mature industrial specifications always locates in a narrow interval,and the optimum properties are always exhibited in specific composition proportions,which indicates that some specific structural unit reflects composition specificity must existence in industrial alloys.Based on some achievement in the component-relevant structural unit our research group previously made,this thesis utilizes cluster-resonance model introduced into the short-range-order structure description of Mg-based binary solid solution to calculate the ideal chemical unit and build solute-homogenization model,which establish a theoretical foundation for composition designation of magnesium alloys.Specifically,this thesis mainly covers these following five aspects:?1?The compositions of Mg-Al series industrial alloy specifications are interpreted.By introducing our cluster-resonance model,a chemical structure unit of Mg-Al binary solid solution is established,[Al-Mg₁₂]Mg₁,which represents the characteristic short-range-order structure,with the bracketed part being the nearest-neighbor cluster centered by Al and shelled by 12 Mg and with one glue atom Mg located between the clusters.Based on this chemical formula,typical Mg-Al industrial alloy specifications in ASTM handbook are well interpreted.For instance,the cluster formula of AZ63A magnesium alloy is[Al_0.78Zn_0.16-Mg₁₂]Mg_1.04Mn_0.02and AZ81A is[Al_0.97Zn_0.03-Mg₁₂]Mg_0.98Mn_0.02.And the deviation between cluster foumula of actual alloys and the ideal chemical structure unit is well correlated to the alloy performance,the smaller deviation,the better comprehensive strength and plasticity properties.?2?A solute-homogenization model is proposed.Here we take high strength Mg-Gd series alloy as our studying alloy system.The stable Mg-Gd structural unit is derived,[Gd-Mg₁₂]Mg₆?represented as{Gd}?after a calculation using the cluster-resonance model.Such a local unit is then mixed with the stable unit of pure magnesium,[Mg-Mg₁₂]Mg₃?represented as{Mg}?.Assuming that the{Gd}units are arranged in FCC-or BCC-like lattice points and the{Mg}units in their octahedral interstices,three proportions between the two units are obtained,1:1,2:3,and 1:3,which constitute three solute homogenization modes,{Gd}₁{Mg}₁,{Gd}₂{Mg}₃ and{Gd}₁{Mg}₃.The prevailing Mg-Gd-based alloys are consequently classified into three groups,respectively exemplified by GW103K?Mg-10Gd-3Y-0.4Zr,wt.%?,GW83K?Mg-8Gd-3Y-0.4Zr?,and GW63K?Mg-6Gd-3Y-0.4Zr?.By analyzing the relationship between alloy composition and its performance,we find that the closer the cluster formulas of actual alloys are to the ideal cluster formula,the better plasticity performance the alloys will have.?3?The dual-cluster formula model for the eutectics close to Mg side in Mg-based binary phase diagrams is established.Mg-based solid solution alloys generally are almost eutectic alloys,which are closely related with the eutectics close to Mg side.A dual-cluster formula model was proposed in our previous work,indicated that eutectic liquids can be regarded as being composed of two stable short-range-order structural units from corresponding eutectics phases respectively.Based on this model,the present thesis interprets all Mg-based eutectics composition.The cluster-resonance model is used to calculate the stable short-range-order chemical structural units of Mg-based solid-solution phase left of eutectics,respresented as a cluster formula,[?CN12 cluster?_Mg]?glue atoms?_1⁶.And the short-range-order structural characteristics of eutectic compound phases right of eutectics can be obtained by identifying the pricinple clusters of these compounds,respresented as another cluster formula,[cluster_?]?glue atoms?_1⁶.Then the dual-cluster formula model for the eutectics is established,which further verifies the short-range-order structural characteristics of Mg-based solid solution can be described by cluster-resonance model.?4?Mg-Gd-Y-Zr alloy compositions are designed and experiment research is conducted.Four alloy compositions are designed,three of them follow the three solute-homogenization formulas and one extra alloy,formulated as{Gd}₁{Mg}₂,does not,which was prepared as a comparison,where{Gd}=[Gd_2/3Y_1/3-Mg₁₂]Mg_5.95Zr_0.05 and{Mg}=[Mg-Mg₁₂]Mg₃.According to the experimental analysis,the{Gd}₁{Mg}₂ alloy,does not follow any solute homogenization model,shows irregular performance change rules and inferior mechanical behaviors,its ultimate tensile strength and elastic modulii of{Gd}₁{Mg}₂ are exceptionally low both solutioned?T4?and in aged?T6?states,its plasticity in T4 is also inferior,and its yield ratio is quite high which means a low service safty level,which indicating that an alloy disobeying the solute homogenization model should not be adopted in practice.While another three alloys follow the solute-homogenization model show regular mechanical behaviors change rules with the increasing of solute content.And especially,the{Gd}₁{Mg}₃ alloy?Mg-5.9Gd-1.6Y-0.4Zr,wt.%?is the most promising candidate for industrial applications,for its relatively high strength level?ultimate tensilte strength?_b=305 MPa?in combination with a superior plasticity??=9%?in the T6 state.?5?Mg-Al-Ce alloy composition is designed and experiment research is conducted.Following the ideal chemical structural unit of Mg-Al binary solid solution,the ideal composition formula of high strength Mg-Al-Ce casting magnesium alloy is proposed,[?Al,Ce?₁-Mg₁₂]Mg₁,and is used to design a new alloy,ACe73?Mg-7.1Al-3.0Ce,wt.%?,formulated as[(Al_0.925Ce_0.075)-Mg₁₂]Mg₁.This ACe73 alloy is prepared by permanent mold gravity casting?PM?process,whose cast state is characterized by discontinuous?-Mg₁₇Al₁₁and acicular Al₁₁Ce₃ precipitates evenly distributed in?-Mg matrix.The ultimate tensile strength,yield strength and elongation are 210 MPa,99.3 MPa and 9.0%,respectively.In comparison with existing PM-cast Mg-Al-Ce alloys,this new alloy shows the highest strength.