Study On The Precipitation Behavior Of Mg-Gd-Y Alloy During Aging Treatment

Magnesium alloy is one of the lightest structure materials in the world. With addition of a small amount of rare earth(RE) element, the strength of the alloy at both room and elevated temperature can be improved obviously. Gadolinium and yttrium are the most common used rare earth elements in magnesium alloy. Aging treatment is a specific method to Mg-RE alloy due to their aging strengthening behavior.The present work is aimed at figuring out the aging precipitation sequence of the Mg-Gd-Y-Nd-Zr alloy. Mg-7Gd-5Y-lNd-0.5Zr alloy is used for the present study. And the main results of the experiments are as follows:The eutectic phase, which has a face centred cubic(f.c.c) structure with a lattice parameter of a=2.22nm, is found of a big volume fraction in the as-cast ingot. In the as-cast alloy, a square shaped precipitate is also present. And the square shaped precipitate has a f.c.c structure with a lattice parameter of a=0.53nm. After a two-stage homogenization process, the eutectic phase is then dissolved. Only the square shaped precipitates exist in the homogenized ingot.During aging treatment, the supersaturated rare earth atoms would precipitate from the matrix. As a result, the Vickers-hardness of the alloy would increase, hold and then decrease slowly as the aging process went on. The β’ precipitate, which has a base centred orthorhombic (b.c.o) structure, is present in significant fraction in aged samples. In the 200℃ aging process, β1 phase and β phase are not found. Atomic-resolution high angle annular dark-field (HAADF) images are taken to characterize the structure of β’phase. The zigzag arranged arrays in the high angle annular dark-field images demonstrate that the β’phase has a Mg7RE type structure. Some small precipitates with different structures are found at the neck of β’ precipitates in the 300h-aged alloys. Concentration of the rare earth atoms in these small precipitates is higher than that in β’phase, the β1 is more likely to nucleate at these part.The β’precipitate would transform into β1 phase during aging treatment at a even higher temperature. β1 phase has a f.c.c structure with a lattice parameter of a=0.74nm. The orientation relationship between β1 and the a-matrix is:[110]β1//[0001]a, (111) β1//(1120)a. The equilibrium stable β phase has the same crystal structure to β1 phase with the lattice parameter three times of β1 phase.