Diffusion Homogenization Of Rare Earth In EW75Alloys

Magnesium alloys is the most light structure material which is used more and more widely. With the quickly developing of society industry, not only the magnesium alloys with much higher properties were needed, but also the magnesium panels with much bigger size. But during the solidification progress of the large magnesium ingots, the rare earth elements would get together in the grain boundary easily and form the imbalance eutectic organizations. It’s very bad phenomenon for the deformation of the large magnesium ingots. Homogenization heat treatment can eliminate this kind of segregation phenomenon and make the alloy elements distribution homogeneous, what’s more, the deform properties of the magnesium alloys could be improved. But now there is no suitable quantified model to check the homogenization progress. It’s necessary to study the diffusion behavior of rare earth elements in the magnesium alloys for the establish of corresponding mathematical theory quantified model. OM, SEM, XRD, EPMA, DSC and mechanical properties test were used to study the diffusion of the rare earth elements.Observing the microstructure of the large EW75magnesium alloy ingot, the result shew that:the grain size of the as-cast alloy was very big (nearly165μm average); the grains were equinoctial crystals; Y, Gd, Nd elements got together on the grain boundaries where the concentration of them was much higher then the certain of the grains and distributed periodically. According to this characteristic and combining with the solution of Fick law, the boundary segregation dynamics equations for rare earth elements in the magnesium alloys were put forward:Shewman model: Researching on the diffusion couple tests, the diffusion parameters such as D, Do and Q were got. And then put the parameters of Y into the diffusion homogenization models, the homogenization dynamics curves of EW75alloy was got. Through the curves, it was easily known that the best homogenization regulation is535℃×18～40h.According to the predict result of homogenization models and the analysis of DSC curve, the different regulations for the homogenization of large as-cast EW75alloy were made. After homogenization heat treatment, the microstructure and tested the mechanical properties of the EW75alloy were observed. Compared with the results of three homogenization models, it showed that:Shewma、S.L.Semiatin models were consistent to the experiment result of homogenization heat treatment, Guass model could also predict the homogenization progress, but not very well. After535℃×20h heat treatment:the imbalance eutectic organizations disappeared; grain boundary segregation were eliminated; what’s more, the rare earth elements distributed homogeneously in the matrix; the mechanical properties of EW75alloy were improved.