Study Of Phase Separation Kinetics In Fe?Cr Alloys By Three-dimensional Phase-field Under Applied Stress

Ferrite and austenite stainless steel is the important material of modern plant structure parts,for its excellent mechanical properties and corrosion resistance.The ferrite phase in the alloy serving in the range of 300-550 oC is decomposed into Cr-riched phase(?'),which results in increased strength,the deterioration of properties such as corrosion resistance and toughness of the alloy,thus affecting the service life and the safety of the alloy.And stress can accelerate the phase separation,therefore,it is of theoretical significance to study the changes of ?' phase in size,shape and quantity under external strain with the increase of aging time,to study the effect of external strain on the microstructure and kinetics evolution of ?' phase in Fe?Cr alloy,and to predict the microstructure evolution and mechanical properties under external strain.Based on Cahn-Hilliard diffusion equation and Khachaturyan microelasticity theory,this paper introduces external stress and strain into free energy,establishes a three-dimensional phase field model of Fe?Cr alloy,and studies the precipitation,growth and coarsening process of ?' phase in Fe?Cr alloy under external stress combined with transmission electron microscope(TEM)experiment.Also,the effects of two-dimensional and three-dimensional simulation,different applied stress conditions,sizes,states and temperatures on the morphology evolution,phase separation rate and coarsening behavior of ?' phase in Fe?Cr alloy were revealed.The volume fraction and average particle radius of ?' phase in TEM experiment agree well with the 3D simulation.In two and three dimensional phase-field simulation under external strain,the particle orientation and the change trend of kinetic process of are consistent.Under the action of external stress,the orientation of ?' phase is obvious,the elongation direction is parallel to the tensile stress direction and perpendicular to the compressive stress direction.And the component boundary range of ? phase and ?' phase becomes wider under external stress.The particle size distribution of ?' phase under external stress increases in width and decreases in peak value,the aspect ratio of particles increases,therefore,the particle size was not concentrated,while the effect on the microstructure under compressive stress is more significant.The effect of smaller external stress 20 MPa is unobvious on the volume fraction,particle number density,average particle radius and concentration evolution of ?' phase.With the increase of external stress to 200 MPa,the volume fraction and particle number density at the initial phase separation of ?' phase change rapidly.And the initial phase separation changes faster under compressive stress.With the increase of aging temperature,the influence of external stress on initial phase separation rate and the directional elongation of ?' phase is also significantly increased.But the coarsening effect on ?' phase is reduced under high temperature and external tensile stress,while the coarsening is fast under compressive stress.