Study On Diffusion Propertiesof Ca、Zn、Al In Mg

The second phase compounds in the magnesium alloy not only affect solidification process, but also have a decisive influence on the strength. In order to study of formation of the second phase compounds, this thesis is based on diffusion experiments, combined with the results of theoretical calculation of the diffusion coefficient. The interdiffusion processes between different alloying elements and pure magnesium are investigated in different test temperatures and different heat preservation time. And the formation, type, morphology and size of the second phase in the diffusion layer are discussed in the thesis. Moreover, the kinetics data, such as growth constant and activation energy, are calculated.The second phase in solid-liquid diffusion layer of Mg-16 Ca and pure Mg is Mg2 Ca. In the temperature of 570℃, 585℃, 600℃, 615℃and 630℃, the diffusion growth constants are 1.84×10-10, 1.97×10-10, 2.40×10-10, 3.31×10-10 and 5.36×10-10m2/s, respectively. And the activation energy is determined to be 111.28 k J/mol in this test temperature range. The second phase in solid-liquid diffusion layer of Mg-30 Zn and pure Mg is Mg7Zn3 and Mg Zn2. In the temperature of 550℃, 565℃, 580℃, 595℃ and 610℃, the diffusion growth constants are 0.56×10-10, 0.93×10-10, 1.53×10-10, 2.45×10-10 and 2.84×10-10m2/s, respectively. And the activation energy is determined to be 170.56 k J/mol in this test temperature range. The second phase in solid-liquid diffusion layer of Mg-40 Al and Pure Mg is Mg17Al12 and Mg2Al3. In the temperature of 475℃, 490℃, 505℃, 520℃ and 535℃, the diffusion growth constants are 1.17×10-10, 1.46×10-10, 1.93×10-10, 2.68×10-10 and 3.69×10-10m2/s, respectively. And the activation energy is determined to be 92.02 k J/mol in this test temperature range. All of the interfaces between pure Mg and the diffusion layer are cellular structure. The morphology of second phase in the diffusion layer is dendritic or mesh, depending on the diffusion temperature, heat preservation time, composition, quantity of second phase and so on.In the temperature range of 680℃~800℃, the liquid diffusion coefficients of Ca, Zn and Al in pure Mg are all of the order of magnitude of 10-8, of which Zn is maximum and Ca is minimum. Meanwhile, the activation energy of Ca, Zn and Al are determined to be 22.91 k J/mol, 30.01 k J/mol and 29.96 k J/mol, respectively. The diffusion coefficient and the activation energy are relevant to the atomic radius of the elements participated in the diffusion. As a result, the smaller difference of atomic radius between alloy elements and pure Mg is, the larger liquid diffusion coefficients and activation energy are. Through the simulation of Matlab software, the diffusion processes of Ca, Zn and Al in pure Mg are researched. When the diffusion time is 60 seconds, the thicknesses of diffusion layer of Ca, Zn and Al at 700 and 800℃ are all between 3mm and 5mm.