Study On The Formation Mechanism Of Tubulose Phase In The AZ80-Sn-Ce Magnesium Alloy

In this paper,the AZ80-2Sn-2Ce magnesium alloy was selected as the test alloy.Continuous solidfication structure in high temeperature wasobtained by applying water-quenching method.With the help of OM?SEM?EDS?FESEM?XRD and tensile property test,the formation mechanism of tubulose Al₄Ce phase was investigated with the purpose of providing the fundamental theory for further control of its amount,size and distribution and eventually improve the tensile properties of the magnesium alloy.The results show that the as-cast AZ80-2Sn-xCe magnesium alloy is mainly consisted of?-Mg,Mg₁₇Al₁₂,Al₄Ce and Mg₂Sn phase.The Al₄Ce phase is a high temperature phase,which can change continuous Mg₁₇Al₁₂ phase into semicontinuous and discontinuous with Mg₁₇Al₁₂ phase decreased in amount.The Al₄Ce phase in the as-cast AZ80-2Sn-xCe magnesium alloy shows tubulose morphology,and its hollow structure is filled with?-Mg phase.Whereas in the as-cast AZ80-xCe magnesiumalloy,it shows acicular morphology.During the growth of the Al₄Ce phase,Sn element enriches in the center of non close-packed plane,which causes undercooling as to restrain its growth.According to the Berg efffect,the difference of Al and Ce concentration between the edge of non close-packed plane and the center of non close-packed plane increases.When the growth rate and the size of the Al₄Ce phase come to a critical value,the undercooling degree?in the center of non close-packed plane reduces to a certain extent.Under the effect of two above aspects which can surppress the growth,the growth in the center of non close-packed plane stops and a small pit comes into being.In contrast,the diffusion of solutes in the vicinity of the edge of non close-packed plane is faster,thus the crystal edge grows quickly and expands the small pit in the center of the non close-packed plane into hollow structure.In the crystal structure of the Al₄Ce phase,the?001?plane is the non close-packed plane,it has the lowest density of atoms.On the basis of the Bravais law,there is very high growth rate in[001]direction.Along with the high temperature in melt,the Al₄Ce phase grows rapidly in[001]direction and finally forms the tubulose morphology.It corresponds with the experimental phenomenon that the Al and Ce elements show differental concentration in the distribution in axial direction of the tubulose Al₄Ce phase which can make a rapid growth in the axial direction.Meanwhile,Sn element enriches in the radial direction of the tubulose Al₄Ce phase,which causes undercooling to surppress its growth.As a result,the axial growth of the tubulose Al₄Ce phase is enhanced.Sn element can lower the peak temperature of primary crystal nucleation and its temperature interval,which prolongs the growth time of Al₄Ce phase in high temperature liquid melt.In other words,it provides more time for the growth of Al₄Ce phase.To sum up,Sn element promotes the formation of tubulose Al₄Ce phase in the test alloy.And this explains why the Al₄Ce phase shows tubulose morphology in the AZ80-2Sn-xCe alloy while it shows acicular morphology in the AZ80-xCe alloy.The reasons above can explain the formation mechanism of tubulose Al₄Ce phase in the as-cast AZ80-2Sn-xCe magnesium alloy.It can be observed that some acicular Al₄Ce phase exists in the test alloy.This is because that the local temperature and the composition are not completely uniform in the melt,so the growth condition for different Al₄Ce phase does make a difference.Part of the Al₄Ce phase tures into complete tubulose morphology because of the better growth condition.Part of the Al₄Ce phase grows into incomplete tubulose morphology as a result of worse growth condition.However,some Al₄Ce phase doesn't reach the critical value of size so that no hollow structure is formed in the center of non close-packed plane.As a result,these Al₄Ce phase exhibit acicular morphology.In the melt of test alloy,the early formed tubulose Al₄Ce phase eventually has big size as a result of owning the long growth time and the lager growth rate.In contrast,the lately formed tubulose Al₄Ce phase shows smaller size because the remaining amount of the Al and Ce atoms after the previous formation of a large number of Al₄Ce phase reduce dramatically along with the shorter growth time and the less growth rate.In the as-cast AZ80-2Sn-xCe magnesium alloy,with the Ce adition increased,the yield strength increase,but the elongation decreases.The fine-grain strengthening and dispersion strengthening effect in the matrix lead to the increase of the yield strength.However,tubulose Al₄Ce phase can separate the?-Mg matrix to some extent,makes it more easily to generate stress concentration in Al₄Ce/?-Mg interface,which is harmful for the elongation of the test alloy.The Mg₂Sn phase possesses very good dispersion strengthening effect,weakens the unfavorable effect brought by the tubulose Al₄Ce phase.As a result,the as-cast AZ80-2Sn-xCe magnesium alloy possesses higher ultimate strength,yield strength and elongation than the as-cast AZ80-xCe magnesium alloy.