| FeCrAl alloy as one of the candidate materials for accident-tolerant fuel cladding,exhibits excellent oxidation resistance,radiation tolerance,high mechanical properties,water corrosion resistance,and good compatibility with UO2 fuel.However,compared to other candidate cladding materials,FeCrAl alloy has a higher thermal neutron capture cross-section.Employing a strategy of reducing wall thickness can effectively improve the neutron economy in a reactor,but it also places higher demands on the mechanical properties of FeCrAl alloy.Mo and Nb solutes are often used to control the microstructure evolution of Fe-based alloys,particularly in the precipitation of Laves phases,which play an important role in grain refinement and improving the thermal stability of the alloy.By studying the aging precipitation behavior of Laves phases using phase field methods,it is possible to gain insights into the microstructural evolution mechanism of the alloy and provide practical guidance for improving processing techniques and enhancing alloy performance.In this paper,a phase field model is established to analyze the effects of Mo and Nb solutes on the microstructure evolution of the alloy,considering solute drag effects,aging precipitation of Laves phases,pinning effects,and grain growth.By coupling multiple mechanisms and their interactions,the influence of Mo and Nb solutes on the microstructural evolution of the alloy is analyzed.This simulation study helps in achieving better material properties by accurately controlling the size,distribution,and morphology of Laves phases to improve the strength and toughness of the alloy.It optimizes the microstructural evolution processes such as grain size,phase distribution,and interface morphology.Based on existing solute drag models,this study introduces a multiphase field model and derives a relationship between the phase field parameters and real physical quantities to achieve quantitative simulations.Utilizing solute drag theory,an additional term for solute drag is derived and incorporated into the coupled multi-action phase field model.The solute drag behaviors of Mo and Nb are investigated based on the phase field solute drag model,and the range of their effects is determined.The simulation results show that Mo and Nb solutes exhibit the maximum drag resistance to grain boundaries when the interface migration rate is1μm/s in an α-Fe-based alloy.A two-phase model for FeMoNb alloy,consisting of an α-Fe matrix phase and a C14-Fe2(Mo,Nb)Laves phase,is established in conjunction with the CALPHAD method.Various weight functions are evaluated for their suitability in the system based on the symmetric assumption,and it is found that the weight function ?2(10-15?+6 ?2)is applicable to the FeMoNb two-phase model.The expression form of the control equations for the field variables under the symmetric assumption and the relationship equations for phase field parameters are derived for numerical computations.The aging precipitation behavior of Laves phases is investigated using the nucleation and growth method.The simulation results show that during the growth stage,the precipitation behavior of Laves phases is influenced by nucleation sites and solute concentration,while during the coarsening stage,Laves phases primarily grow through phase coarsening.A multi-mechanism,coordinated and competitive phase field model is developed based on the two-phase model by coupling Zener pinning,solute drag,and grain growth.Using this model,the pinning behavior of Laves phases to grain boundaries is investigated,and a relationship between the contact angle and pinning force is established.A comparison is made between the grain growth and aging precipitation behavior of Laves phases under single-mechanism and multi-mechanism coupling conditions.The simulation results of the multi-mechanism coupling reveal a significant pinning effect of Laves phases on grain growth,especially when Laves phases are located at grain boundary junctions.Large-sized Laves phases provide stable pinning forces,making it difficult for grain boundaries to undergo depinning.Depinning events mostly occur in small-sized Laves phases.The simulation results demonstrate that the multi-mechanism coupling enhances the pinning effect of Laves phases,and solute segregation at grain boundaries promotes the nucleation and growth of precipitate phases at grain boundaries. |
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