Research On The Formation Of The Spherical -shape RareEarth Phase In AZ31+1%RE+1%Sb Magnesium Alloy

Due to the advantages of magnesium alloy, such as high specific strength, specific stiffness, good vibration damping performance, easy to recycle, etc, it is one of the most important members on the market of the lightest metal.Forever, low absolute strength, poor plasticity processing capacity, and poor corrosion resistance of faults also limits the applications of magnesium alloys. Adding RE and Sb in the magnesium alloy containing aluminum,can improve the microstructure, change the type and morphology of the second phase, make the acicular rare earth phase disappear, generate, enhance the mechanical properties of aluminum magnesium alloys.But the mechanism of the nucleation and growth of the spherical rare earth phase is not yet clear, which limits the improvement the comprehensive performance of magnesium aluminum alloy by rare earth alloying.Taking AZ31+1%RE+1%Sb alloy as the main research object, the thermodynamics, dynamics and crystallography analysis has been taken to explain precipitation of second phase in the alloy system and the formation mechanism of spherical rare earth phase.In the study of thermodynamics, the using the Miedema model and the model of activitycombined with the phase diagrams, the research of the RE-Sb phase, the Mg-Sb phase, the Al-RE phase, the Mg-RE phase and the Mg-Al phase in the alloy system precipitation have been done. In the kinetics study, through the liquid quenching experiment which simulates the actual solidification process,the study to the alloy at different stages of solidification has been finished using with OM, SEM, EDS and other testing means,nuclear morphology evolution law in the process of growth of the RE-Sb phase and Mg-Sb has been found respectively, In the crystallography study, using test results in electron backscattered diffraction (EBSD), analysis of the crystal structure data of the second phase has been done to study the the effect RE and Sb in the alloy and CeSb growth mechanism.Main conclusions are as follows:The results of thermodynamic analysis indicatsthat as the adherence between RE and Sb is strongest,the spherical rare earth phase is inevitable in AZ31+1%RE+1%Sb magnesium alloy The order of second phase precipitation in the alloy system has been taken:RE-Sb> Al-RE>Mg-Sb> Mg-RE> Mg-Al.In kinetics study, there are four stages in the melting progress:the 15minites holding on the 780℃;the cooling progress from 780℃to 720 ℃; the 20 min holding on 720℃temperatures; the water-cooled progress.Liquid quenching experiment simulate the four stages. Through liquid quenching experiments, the precipitation of second phase has been studied. It is in the second stage that the RE-Sb phase begins to nucleate and grow, maintains the granular feature, finallydistributes within the a-Mg matrix grain. In the third and fourth phases of the smelting process, Mg-Sb phase nucleates and gradually turns to strip shape phase.As the substrate nucleation substrate of a-Mg, CeSb phase distributed in the matrix grain internal and refine grains.Mg3Sb2 and Al4Ce phase can be used as a Mg17Al12 phase nucleation substrate, distributed on the grain boundary, refine grain and break Mg17Al12 phase mesh structure up.It is the the crystal structure, not the external environment of CeSb that decides to CeSb phase morphology. Comparing CeSb phase crystal structure with the other phases’crystal structure, it has a relatively complete crystal symmetry, the order of is {111}>{100}>{110}, according to the crystal surface density And three crystal Jackson factor of the three low index faces were greater than 2, which indicates that crystal growth will be in accordance with the growth of the small plane way.According to the law of Bravais-Friedel,{1,1,1} plane is most likely to grow, finally makes the phase surface of a polyhedron shape CeSb.