Acta Metall Sin  2019, Vol. 55 Issue (5): 593-600    DOI: 10.11900/0412.1961.2018.00506
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Effect of Mn Composition on the Nanometer Cu-Rich Phase of Fe-Cu-Mn Alloy by Phase Field Method
Baojun ZHAO,Yuhong ZHAO(),Yuanyang SUN,Wenkui YANG,Hua HOU
1. School of Materials Science and Engineering, North University of China, Taiyuan 030051, China
Abstract

The precipitation of nanometer Cu-rich phase can be observed in Fe-Cu alloy systems during isothermal ageing. The existence of Cu-rich phase is one of the reasons for the embrittlement of reactor pressure vessel (RPV) steel. The phase-field method applies a set of field variables defined by functions of space and time to describe the temporal evolution of composition and structural parameter, characterizing microstructure evolution during phase transformation. This work uses phase-field model to simulate the three-dimensional morphology, the volume fraction, number density and average particle radius of Cu-rich phase in Fe-Cu-Mn alloy at 823 K. The chemical free energy is derived from the thermodynamic database of the calculated phase diagram (CALPHAD), so the microstructure evolution of precipitation changes are directly corresponded to phase diagram of the real alloy system. The simulation results show that nanometer Cu-rich phase are formed by the spinodal decomposition mechanism in the early stage of phase separation. Meanwhile, Mn atoms segregate to the center of the Cu-rich phase. During the process of Ostwald coarsening, Mn atoms migrate from core to the interface of Cu-rich phase, finally forming Mn-rich ring distributed in the exterior of Cu-rich phase. Its existence can decrease the rates of diffusion growth and coarsening of Cu-rich phase. The Cu-rich phase is bcc structure and disperses in the matrix with spherical shape in the early stage of ageing. As the Cu-rich phase continues to grow, it will transform into fcc structure with ellipsoid or rod shapes. Meanwhile, increasing Mn content of Fe-Cu-Mn alloy accelerates the precipitation of Cu-rich phase and facilitates the growth and coarsening of Cu-rich phase.

 ZTFLH: TG292
Fund: National Natural Science Foundation of China(U1610123);National Natural Science Foundation of China(51674226);National Natural Science Foundation of China(51574206);National Natural Science Foundation of China(51774254);National Natural Science Foundation of China(51701187);Science and Technology Major Project of Shanxi Province(MC2016-06)
Corresponding Authors:  Yuhong ZHAO     E-mail:  zhaoyuhong@nuc.edu.cn