Experimental Study And Numerical Simulation Of Diffusion And Growth Behavior Of Multicomponent Compounds In Mg-Y/Mg-Al-Zn Alloy

As an important lightweight structural material,magnesium alloy has broad application in transportation,electronic communication,aerospace and other fields.In order to meet the needs of different service environments,the microstructure of magnesium alloys is usually adjusted by alloying or changing processing methods in order to improve the mechanical and physical properties.Previous studies have pointed out that powder metallurgy or laser additive can improve the mechanical properties of magnesium alloys owing to the change of distribution and growth behavior of intermetallic compounds.However,the existing research lacks in the understanding of the diffusion growth behavior of these intermetallic compounds.For this reason,the present thesis uses diffusion couple technique and numerical inversion method to analyze and discuss the diffusion growth kinetics of typical binary or ternary compounds in the Mg-Y and Mg-Al-Zn alloy systems.The results are expected provide basic information to the alloy design and performance optimization of powder metallurgy magnesium alloy materials.The detailed contents are as follows:For the Mg-Y rare earth magnesium alloy system,the formation and growth behavior of ?-Mg24Y5 and ?-Mg2Y intermetallic compounds have been studied in previous experiment.The interdiffusion coefficient of solid solution phase and intermetallic compound phase depending on composition and temperature are computed by the numerical inversion method based on this information.The results indicate that the diffusion coefficient of ?-Mg24Y5 is about 4 times that of ?-Mg2Y.At the same time,the calculated diffusion coefficient can be used to quantitatively simulate the relationship between the element diffusion distribution,the diffusion flux and the growth thickness of the intermetallic compound layer measured in the diffusion couple experiment.For the Mg-Al-Zn ternary alloy system,the growth constant of ? phase is determined to be 8.587×10-15 m2/s by detecting the Mg-? diffusion couple with different holding times at 360?.At the same time,the composition range of the ? phase in the isothermal section and the diffusion path of the Mg-? diffusion couple are determined.The in-situ observation method is used to study the change of the thickness of ? phase with diffusion time,and the composition distribution and growth constant of the diffusion couple are also determined.The numerical inversion method is used to calculate the inter-diffusion coefficient of solid solution phase related to the components in the diffusion path.The experimental data and the numerical inversion method are further combined to calculate the diffusion coefficient of ternary intermetallic compound,and the result can reproduce the diffusion growth behavior of the ? ternary intermetallic compound.For the ?-Mg5Al2Zn2 phase in the Mg-Al-Zn system,the temperature-dependent dynamic behavior of the ?-Mg5Al2Zn2 phase is obtained by combining the diffusion couple technique and the numerical inversion method.By experimentally studying the diffusion growth behavior of ? phase in the temperature range of 330?-360?,the Arrhenius formula of growth constant and interdiffusion coefficient is obtained:?kp=5.48×10-5exp(120010/RT)m2/s,?DMgMg Al=3.48×10-10exp(50420/RT)m2/s and ?DZnZn Al=2.87×10-7exp(91440/RT)m2/s.And the experimental data can be verified by subjecting these parameters into diffusion governing eauqtions.By comparing the experimental and calculated results,the temperature-dependent diffusion growth rate of the ? ternary intermetallic compound is mainly controlled by the mobility of Zn element in the Mg-Al-Zn system.