Investigation On Diffusion Behaviors Of Alloy Elements In Magnesium

Magnesium alloys are being increasingly concerned in the automotive and aerospace industries for weight saving. The majority of Mg alloys derives their mechanical properties from precipitation hardening, while the study of precipitation process demands accurate diffusion data. Diffusion is critically important to the macroscale properties of Mg alloys, such as castability, creep resistance and strength. However, diffusion coefficients of various solutes in Mg the kinetics of Mg has been studied to a far lesser extent. Due to the issues related to corrosion, surface oxidation, and contamination of impurities during sample preparation in diffusion measurements, few experimental data are available in the literature for diffusion coefficients of solutes in Mg.The diffusion behaviors and intermetallic compounds formation were studied via scanning electron microscopy?SEM?. The intermetallic compounds in the diffusion reaction layers were identified by energy dispersive spectroscopy?EDS? and an X-ray diffractometer?XRD?.The solid-state and solid/liquid diffusion reaction between Mg-40 Al and Mg-X?X=20Ca, 20 Ce, 20 La, 30 Nd, 30Y? were studied in the 350-400°C and 475-525°C temperature range. During the whole diffusion process, Al was the dominant diffusing species, and it substituted for Mg of the Mg-X?X=20Ca, 20 Ce, 20 La, 30 Nd, 30Y? substrate. Intermetallic compounds of Al-X?X=Ca?Ce?La?Nd?Y? were formed via the reaction. The enthalpy of formation of binary intermetallic compounds in the Mg-Al-X?X=Ca?Ce?La?Nd?Y? ternary system was rationalized using the Miedema model at the soild state diffusion, Al-X?X=Ca?Ce?La?Nd?Y? has the lowest enthalpy of formation in all binary intermetallic compounds. The Gibbs free energy of binary intermetallic compounds in the Mg-Al-X?X=Ca?Ce?La?Nd?Y? ternary system was rationalized using the Modified Quasichemical Model?MQM? at the soild/liquid diffusion, Al-Ca?Al-Nd?Al4X?X=Ce?La?Y?,Al11X3?X=Ce?La?Y??Al3X?X=Ce?La?Y? and Al2X?X=Ce?La?Y? have the lowest Gibbs free energy in Mg-Al-X?X=Ca?Ce?La?Nd?Y? ternary system, respectively. The growth kinetics was well conformed to the parabolic law, so the growth of the diffusion reaction layers was diffusion controlled in the studied temperature range. At the solid-state diffusion couple, thicknesses of diffusion reaction layers following the trend:d_Y>d_Nd>d_Ce>d_La>d_Ca,and the diffusion coefficient of Al element in diffusion reaction layer following the trend:D_Nd^Int,Total>D_Y^Int,Total>D_Ce^Int,Total>D_La^Int,Total>D_Ca^Int,Total?At the solid/liquid diffusion couple, thicknesses of diffusion reaction layers following the trend:d Y>d Nd>d_Ce>d_La>d_Ca,and the diffusion coefficient of Al element in diffusion reaction layerfollowing the trend:D_Y^Int>D_Nd^Int>D_Ce^Int>D_La^Int>D_Ca^Int, the magnitude of the diffusion coefficient is 10-10¹0-11.The Mg-40 Al and the atomic radius of different intermediate alloy Mg-20X?X=Cu?Y?Nd?Ce?La?Gd?Ca?Sr? effect of atomic size of magnesium alloy diffusion behavior were study. When r Cu<r Al, Cu was the dominant diffusing species, Cu depleted region was formed at the?Mg-40Al?/?Mg-20Cu? diffusion couple. When r_Al<r_Y<r_Nd<r_Ce<r_La<r_Gd<r_Ca<r_Sr, Al was the dominant diffusing species, Al depleted region was formed at the?Mg-40Al?/?Mg-20X?X=Y?Nd?Ce?La?Gd?Ca?Sr??. With the increase the atomic radius of alloying elements:d_Y>d_Nd>d_Ce>d_La>d_Gd>d_Ca>d_Sr, the diffusion reaction layer thickness gradually decreased, and the diffusion reaction layer growth constants and the atomic size to meet the lnk=A+exp?B-Cr?. The activation energy increased linearly.The diffusion behavior of Fe and Cu in magnesium was investigated by diffusion couple method. The mechanism of Mn removal from the melt was revealed, and a more comprehensive diffusion data of Cu in Mg was obtain. With the increase of Mn content, the diffusion layer thickness increases, the diffusion coefficient of Fe in the magnesium melt decreases. The diffusion reaction layer effectively prevents Fe atoms diffused into the melt. In view of the above mechanism, the use of 45# steel crucible smelting Mg process to the Mg to add the 2wt.%Mn element, it was found that the addition of Mn element can effectively reduce the Fe content in the Mg melt. The Fe content can be reduced effectively by using ZGMn13 crucible melting Mg. Mg Cu2 and Mg2 Cu were formed in the Mg/Cu diffusion couples at the temperature range of 400-460°C. The growths of Mg Cu2 and Mg2 Cu followed the parabolic kinetics. The interdiffusion coefficients were evaluated as functions of compositions in Mg Cu2 and Mg2 Cu intermetallic phases by Heumann-Matano method. The impurity diffusion coefficient of Mg in Cu(^MgD_Cu* ) and Cu in Mg(^CuD_Mg* ) were evaluated by Hall method.