Phase Field Simulation Of Martensitic Transformation In Low Carbon Steel By The Finite Element Method

Based on the phase field dynamics model of coupled linear micro elasticity theory,taking martensitic transformation as the starting point,this paper studies the growth law regulated by different parameters in the transformation process,and explains the mechanism of autocatalytic nucleation and self coordination effect between variants in martensitic transformation from the perspective of energy change and stress analysis.The phase field dynamic equation is solved by finite element method.It's described that the selection of medium parameter elements,numerical integration rules,the selection of approximate solution method and the postprocessing process of solution results.The main conclusions are as follows:Martensitic transformation is tended to produce heterogeneous nucleation at the defects in the crystal,because the lattice distortion between the defect and the matrix will lead to elastic strain energy.The process of nucleation is the process of releasing elastic strain energy.The nucleation barrier of transformation is affected by temperature.The size of barrier will increase under high temperatures.The driving force of system is not enough to cross the nucleation barrier,then the transformation cannot occur.By adjusting the dimensionless parameters “?" related to temperature can control the initial temperature of transformation and nucleation behavior to achieve the desired results,which has a certain guiding role for practical production.The interface thickness 7)7)is positively correlated with the interface gradient energy.By reasonably adjusting the parameters G and L related to the interface gradient energy,the movement of the interface can be changed and able to affect the rate of two stages which is martensitic deformation nucleation and growth.The "swelling" structure at the edge of martensitic lath will be caused.In the case of a single variant,martensite reduces the elastic strain energy by forming an appropriate shape.The applied strain will change the habit plane in the direction conducive to the reduction of strain.Different variants of martensite tend to form parallel twin structures to reduce the surrounding elastic stress field and high-level elastic strain energy,which is the fundamental reason for the self-accommodation effect.The phenomenon of autocatalytic nucleation in martensitic transformation is that the stress in the transformation process is concentrated at the edge of martensite,and the residual austenite in the matrix is transformed by the stress gradient between different martensitic laths.Autocatalytic nucleation is not only related to the defects in the matrix,but also related to the density of defects.There are many nucleation positions of martensite in the parts with dense defects.The autocatalytic nucleation of martensite in the system will further expand the martensite lath in the same time and coarsen the martensite.