Research On The Formation Of The Acicular-shape Rare Earth Phase In AZ31+1%RE Magnesium Alloy

Although magnesium alloy has some advantages such as light quality, highspecific strength and high specific stiffness, some disadvantages such as low strength,poor plasticity and poor corrosion resistance limit its applications. As the most widelyuesed commercial magnesium alloy, Mg-Al based magnesium alloy has all abovedisadvantages too. Although, by additing RE to improve Mg-Al based magnesiumalloy has showed good results, but there exist a common phenomenon that RE and Alform acicular-shape rare earth phase distributing along the grain boundary. As theseacicular-shape rare earth phases are high temperature stable phases, its state can’t bechanged by the conventional heat treatment. Because these acicular-shape rare earthphases cut apart the magnesium alloy matrix during deformation, magnesium alloy isvulnerable to stress concentration, this case severely reduces the processability ofmagnesium alloy and limits the improve of its properties.In this paper, considering AZ31+1%RE magnesium alloy as the object of study,Bsed on the standpoint of thermodynamics and kinetics the precipitation reaction ofrare earth phase is studyed, and then the formation mechanism of these acicular-shaperare earth phase is confirmed. In thermodynamic analysis, combining Mg-Al andAl-RE binary phase diagrams, Miedema Model, the calculation model of activity,Toop Model and the standard gibbs free energy change model are used to study theprecipitation reaction of rare earth phase in AZ31+1%RE magnesium alloy. In kineticanalysis, the structure from different stages during solidification is obtained by theuse of the modified liquid quench experimental device and studyed by OpticalMicroscopy（OM）, Scanning Electron Microscope（SEM）,Energy DispersiveSpectrometer （EDS）and X-ray Diffraction（XRD）. The main conclusions in thispaper are as follows:The results of thermodynamic analysis indicats that as the adherence between Aland RE is strongest，the rare earth phase is inevitable in AZ31+1%RE magnesiumalloy. Furthermore the precipitation temperature of Al11Ce（3Al11RE3）inAZ31+1%RE magnesium alloy is qualitatively calculated，it is976.81K, so rare earth phase ispriority to precipitate. The calculation of crystal orientation between Mg matrix andAl11Ce3（Al11RE3）shown that prior precipitated rare-earth phase can’t be theheterogeneous nucleant of Mg matrix, therefore the rare earth phase distribute alongthe grain boundary. Due to the inhibition of these rare earth phase, the alloymicrostructure is refined by adding RE. the phase composition and phaseprecipitation sequence can be qualitatively predicted by thermodynamics empirical orsemi-empirical model, and this provide the direction to the selection of thenodularizer which is used to refined the acicular-shape rare earth phase.the research of the solidification process of AZ31+1%RE magnesium alloy showthat the precipitation of rare earth phase consists of two stages: first, as the primaryphase, rare earth phase is priority to precipitate at high temperature, and because ofthe anisotropy of crystal growth, the morphology of primary rare earth phase ismainly small size irregular flakes. In the eutectic reaction zone, rare earth phaseprecipitate by Al-Al11Ce3（Al11RE3） eutectic reaction, with the decreasing oftemperature, the size of rare earth phase become lager, ultimately, becomeacicular-shape. So we infer the cause of rare earth phase is eutectic reaction. In orderto confirm our judgment, the Al-Re binary alloy and its solidification process isstudyed. The results show that the second phase of as-cast Al-Re binary alloy isconsisted of plate and acicular-shape rare earth phase, the plate rare earth phase isprimary phase and acicular-shape rare earth phase is eutectic phase. The resultsbetween AZ31+1%RE and Al-Re binary alloy are consistent, therefore the reason foracicular-shape rare earth phase is eutectic reaction. Determine the reason for theformation of acicular-shape rare earth phase makes the further study to improve itsmorphology more targeted.The Jackson factor of rare earth phase is calculated, the result show that theJackson factor a is less than2, and the solid-liquid interface of rare earth is rough,therefore the type of Al-Al11Ce3（Al11RE3）eutectic reaction is rough interface-roughinterface. The result of the solute distribution rule on the forming process ofacicular-shape rare earth phase in the Al-Re binary alloy show that the reason foracicular-shape rare earth formed is the lateral solute distribution between the eutectic phases. In the further study, by changing the solute distribution rule during theforming process of acicular-shape rare earth phase to improve its morphology maybefeasible.