Diffusion Kinetics Of The Mg-Al-Zn-Sn Alloys And The Microstructural Evolution Simulation During Aging Precipitation

With advantages of light weight,high specific strength,abundant reservation,magnesium alloys have broad application prospects in transportation,aerospace,3C devices,medical equipment,and other fields.Age-hardening is a common way to improve mechanical properties by composition design and optimization of heat treatment process.The conventional alloy design depends on trial-and-error experiments and usually takes for large amount of time and money.The newly proposed ICME(Integrated Computational Materials Engineer)brings the dawn for the alloy design,which combines key experiments with computational simulations,and obviously cut down the R&D cycle.CALPHAD(CALculation of PHAse Diagram)method is a vital part in ICME can perform simulation and guide alloys design based on reliable thermodynamic database,atomic mobility parameters and thermophysical parameters combined with key structural information.The reliable database is the premise and guarantee for the simulation.However,the kinetic database is still in lack.This work aims to perform the following research on age-strengthening Mg-Al-Zn-Sn alloys:(1)The composition-distance profiles of HCP＿A3 phase in Mg-Al-Sn and Mg-Zn-Sn systems at various temperatures were determined by diffusion couples.Novel numerical inverse method was primarily validated through comparing the calculation results with those by traditional Matano-Kirkaldy method.The interdiffusivities within HCP＿A3 phase region along the composition-distance profiles were efficiently calculated based on efficient number inverse method.(2)The atomic mobility of HCP＿A3 phases in Mg-Al,Mg-Zn and MgSn systems were critically assessed and optimized based on the review of diffusivities on pure elements,binary and ternary systems.After that,atomic mobility of the HCP＿A3 phase in Mg-Al-Sn,Mg-Zn-Sn and MgAl-Zn ternary systems were optimized by CALTPP software combined with interdiffusivities results.The activation energy and pre-exponential factor along the composition was calculated by Arrhenius equation.The atomic mobility of Mg-Al-Zn-Sn system was constructed and validated by the predictions of diffusion behaviors and the calculated diffusivities.The effects of the addition elements on diffusivities of HCP＿A3 matrix are also analyzed.(3)Some deviations were observed for solubility in Mg-Sn system between experiment results and the calculated phase diagram.Therefore,the present work critically reassessed the experimental data and reoptimized the thermodynamics descriptions.New heat-treatment process for Mg-5.5Al-5Sn(wt.%)and Mg-2Zn-8Sn(wt.%)alloys were designed based on thermodynamic calculations.After that,microstructure and hardness evolution were analyzed for various alloys to investigate the agehardening process.(4)Microstructural evolution of the Mg-Al-Zn-Sn system was simulated based on the thermodynamics description,atomic mobility,thermophysical parameters and key experimental information combined with KWN(Kampmann-Wagner Numerical)model.The number density,volume fraction and mean radius were quantitatively calculated during the simulation.Besides,the precipitates morphology and aspect ratio information were also considered.Good agreements were obtained when comparing the experimental results with simulations.The machine learning model was trained for the relationship of composition-age precipitation condition-microstructures-hardness and the hardness of multi-component alloys can be predicted.This thesis contains 99 figures,29 tables and 232 references.