Study On Preparation Processes And Microstructure Of Mg-xGd-Zr Alloys

Usually the rare earth magnesium alloys contain a variety of rare earth elements, such as Y, Nd, et al. There are few researches on the effects of Gd as a single element added in alloys on microstructure and mechanical properties, especially from the perspective of solidification of alloys. A perfect melting process can not only improve the rare earth magnesium alloy’s casting microstructure and the comprehensive performances but also reduce the cost of preparation of the alloy. The density of Gd is far higher than Mg, so the gravity segregation is easily formed and difficult to eliminate in the subsequent processing. The as-cast microstructure of rare earth magnesium alloy plays an important role for the subsequent preparation process. So this paper intends to use Mg-xGd-Zr rare earth magnesium alloys as the research objects for getting a kind of rare earth ally with a less macro-segregation and a controllable gravity segregation by changing the content of Gd and optimizing the preparation processes, at the same time, reveal the diffusion mechanism of Gd in liquid Mg and have a reference function for melting processes of other species of rare earth elements in the liquid Mg.This paper studies on the effects of melting process (melting temperature, stirring and cooling rate, method of Gd added) on solidified microstructure of Mg-Gd-Zr alloy.The result shows that a faster cooling rate or an ampler stir or a higher melting temperature is advantage to get a smaller gain size in the solidification structure. When studying the effect of temperature, the gain size is comprehensively affected by the cooling rate and the content of Gd. The optimization of preparation process for melting is with a melting temperature of 800℃, an ample stir, and a quenching in 15℃ water during solidification.The action mechanism of pure Gd as raw materials is the diffusion-dissolution pattern in liquid Mg. The action mechanism of Mg-30Gd intermediate alloy as raw materials is the melted-eutectic decomposition pattern. The different patterns result in the difference of the aggregation degree of initial Gd atoms and the liquid structure and formation. The pure Gd’s initial diffusion-dissolution process takes most time of smelting process. However, the Gd atoms can transform into free states just by decomposition of eutectic structure, and it shorten the whole smelting process largely.This paper also study on the effects of the content of Gd and heat treatment on microstructure and mechanical properties. The results show that Gd element has segregation in eutectic structure in the as-cast microstructure. The volume fraction of eutectic structure increases with the growth of content of Gd. The gain size decreases with the growth of content of Gd. And the as-cast alloy is brittle fracture. After the solid solution treatment, the eutectic structure decomposed and the gain size grew up, so the strength of the alloy decreased. The solid solution alloy is intercrystalline fracture. After the aging treatment, strength of the alloy increased due to the precipitation strengthening effect. The aging alloy was transcrystalline fracture. The Rm, Rp0.2, and HB of the alloy is proportional to the content of Gd, the elongation of the alloy is inversely proportional to the content of Gd.This paper put forward a model of Gd element diffusion-dissolution in liquid Mg. The processes of Gd element dissolution in liquid Mg is mainly divided into three stages: the state of atomic activation, the state of steady-state forming, and the state of steady-state broking and reforming. In the solidification structure, the distribution of compounds around solid Gd is influenced by temperature and content of Gd. In the height direction, the diffusion distance of Gd atoms is very short and the proportion of segregation is very serious due to the density difference between Gd and Mg element. In the horizontal direction, the diffusion distance of Gd atoms and the content of Gd in the melt increase with the growth of melt temperature, at the same time, the segregation degree of solute Gd atoms is lower. Under the same conditions, the inhibition for the growth of dendrites weakens.