A Fast Phase-field Model for Polycrystalline Solidification and the Cross-scale Simulation of the Microstructure Evolution in U-based Alloys

Yun Chen ^1*, Tongzhao Gong ¹, Yongpeng Shi ¹, Wenlin Mo ², Jun Wu ², Xing-Qiu Chen ¹, Dianzhong Li²

¹ Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China；

² Institute of Materials, China Academy of Engineering Physics, 621907 Jiangyou, Sichuan, China

ABSTRACT: The phase-field (PF) approach has been emerged as a powerful method to simulate the solidification microstructure evolution. However, because of the low computing efficiency it is always a big challenge to quantitatively study the solidification in scale of experiments for polycrystalline materials. To enhance the computing efficiency, we employed the front-tracking method to calculate the orientation evolution which substantially reduces the equations needed to be solved. Meanwhile, the phase-field model is nonlinearly preconditioned that allows to solve the governing equations on a mesh with much coarser elements than those used in the conventional methods. In addition, the numerical algorithm of the distributed parallel finite element method on an adaptive mesh is developed to further accelerate computations. Simulations of multi-dendrites growth of Al-4wt.%Cu demonstrate that the proposed fast simulation approaches enable quantitative simulations of a large number of dendrites growth in scale of centimeters. In order to simulate the microstructure and microsegregation evolution of U-Nb alloys during solidification, the cross-scale calculations are performed. The crucial physical parameters of the alloys for PF model, such as the thermodynamic data and the diffusion kinetics are obtained by means of atomic-scale calculations using the ab initio molecular dynamics (AIMD) method. Then large-scale simulations which are comparable to the U-Nb alloy experiments are performed to predict the variation of grain size and microsegregation with cooling rate and Nb content. Based on the polycrystalline simulations, a new microsegregation model is proposed which is different from conventional Lever rule and Scheil equation, but predicts a much better solute concentration during solidification. This work was supported by the Science Challenge Project (Grant No. TZ2016004) and the Youth Innovation Promotion Association CAS.

Keywords: phase-field model; solidification microstructure; cross-scale simulation; U-based alloy