Investigation Of Diffusion Coefficient And Simulation Of Diffusion Behavior In The Multi-Al Alloys

As lightweight structural materials, Al-based alloys are of a good combination of mechanical properties, corrosion resistance, thermal conduction and cast ability, and thus have found extensive application in the fields of microelectronics, automotive and aerospace. Knowledge of both thermodynamic and diffusion kinetic characteristics of Al-based alloys is of critical importance in optimizing alloy composition and understanding how temperature, time and compositions affect the microstructure during heat treatment. So far, reliable thermodynamic databases for a wide variety of multicomponent Al alloys have been constructed, but these are not the cases for diffusivity, especially for ternary and higher-order alloys. The experimental diffusivities are usually very limited for multicomponent alloys, and the data from different sources are often not mutually consistent. It has been well established that the diffusivity matrix in a multicomponent system can be constructed by using atomic mobilities of various elements in phases and thermodynamic factor. The atomic mobility can be optimized from the experimental diffusivities in binary and ternary systems.A key system, Al-Cu-Mg-Mn-Ni-Zn, in the multicomponent Al alloy was chosen as the target in the present thesis, aiming at establishing the accurate atomic mobility databases of some key systems. The main contents are:(1) Various experimentally measured diffusivities of fcc Al-Mg, Cu-Mg and Al-Cu-Mg alloys available in the literature are critically reviewed in the present work. The first-principles calculation coupled with a semi-empirical correlation is employed to derive the temperature dependence of impurity diffusivity for Mg in fcc Cu. Atomic mobilities for the above fcc alloys are then evaluated as a function of temperature and composition by means of DICTRA (DIffusion Controlled TRAnsformation) software. The concentration profiles and diffusion paths are predicted with the mobility parameters in a series of binary and ternary diffusion couples. Comprehensive comparisons between calculated and measured diffusivities and diffusion behavior show that most of the experimental data can be well reproduced by the presently obtained atomic mobilities.(2) On the basis of the critically reviewed experimental diffusivities available in the literature, the atomic mobilities of Cu, Mn and Ni in fcc Cu-Mn-Ni alloys, Cu, Mn and Zn in fcc Cu-Mn-Zn alloys and Ni, Mn and Zn in fcc Ni-Mn-Zn alloys were assessed as a function of temperature and composition by means of DICTRA software. Comprehensive comparisons between the calculated results and the experimental data show that a good agreement is obtained for various diffusivities in binary and ternary systems, including impurity diffusion coefficients, tracer diffusion coefficients and interdiffusion coefficients. The obtained atomic mobilities can also describe various diffusion phenomena in a series of binary and ternary diffusion couples, such as concentration profiles, interdiffusion flux and diffusion paths. Besides, the principle of the diffusion couples’Kirkendall effect was analyzed and Kirkendall velocity construction and Kirkendall plane shift were calculated.(3) The interdiffusion coefficients and various of diffusion behaviors in fcc Cu-Mn-Ni-Zn system were collected and critically reviewed. Due to the lack of accurate experimental diffusivities for the Cu-Mn-Ni-Zn alloys, extrapolation based on ternary information was performed. The obtained mobility parameters, in conjunction with thermodynamic description, have been successfully used to predict a series of Cu-Mn-Ni-Zn quaternary diffusion couple experiments, such as concentration profiles and diffusion paths in3-D view. The phenomena of up-hill diffusion and zero flux plane were reasonably analyzed...