Phase-field Simulation And Experimental Investigation On Precipitation In Aging Of Cast Mg-Al Alloy

Cast Mg-Al-based alloy, a lightweight metal material, has a good prospect for engineering application. The strength of Mg-Al-based alloy can be enhanced by aging process, the effect of which is closely related to crystallography, thermodynamics and dynamics of the precipitates. It is a promising strategy to combine phase field simulations and experimental characterizations to investigate the precipitating process of this alloy in providing advices for enhancing the strengthening effect, improving the properties and further broadening its application.A series of heat treatment experiments were performed to investigate the precipitating process of this alloy. The type, morphology and distribution of the precipitates were studied using SEM. The crystallographic feature and morphology of the precipitates were characterized using TEM, based on which the growth kinetics of the precipitates were obtained. Three dimensional morphology of the precipitate were characterized using AFM(Atomic Force Microscopy) and quantitative analysis was also made based on the statistical data of the sizes of precipitates.A phase field model for simulating the continuous precipitation of β-Mg17Al12 phase during aging process has been developed. The chemical free energy was obtained by thermodynamic calculation. Both the interfacial energy anisotropy and the elastic strain energy have been taken into consideration in this model. The effects of the interfacial energy anisotropy were introduced by defining interfacial anisotropy function, and the effects of elastic strain energy were introduced based on the theory of micro elastic strain energy formulated by Khachaturya. The interface mobility coefficient was calibrated by using the growth kinetics obtained from the experiments.A special numerical technique based on two sets of mesh was employed to deal with the mesh-induced anisotropy. The KKS(Kim-Kim-Suzuki) interface treatment and parallel computing were adopted, which made it feasible to perform a three dimensional simulation of the precipitation of multi-variant at micrometer scale.On the basis of the developed phase field model, the evolution of single- and multi-variant precipitates β-Mg17Al12 were simulated in two dimensions and three dimensions, which is in good agreement with experiments. Based on the simulation,the effects of the interfacial energy anisotropy and the elastic strain energy on the growth kinetics and the morphology of the precipitates were investigated, and a mechanism for explaining the morphology features of the precipitates was proposed from the aspects of the contribution of interfacial energy anisotropy and the elastic strain energy in different stages of evolution as well as the strain field of the computational domain. Furthermore, the influence of the interfacial energy and elastic strain energy on the thickness of the precipitates was dicussed, which brought possible strategies and solutions for improving the strengthening effect of the aging process. In addition, by modeling the distribution of elastic interaction energy in the vicinity of the precipitates, the influence of the elastic interaction energy on the nucleation of precipitate near a pre-existing precipitate was also discussed, which supplied a theoretic explanation for the nucleation near the lozenge end of the pre-existing precipitate observed in experiment. A three dimensional simulation of the precipitation of the multi-variant has performed at micrometer scale, and the results of simulation is a good reflection of the morphology and the distribution of multi-variant precipitates.