Phase Field Study of the Diffusional Paths in Pearlite-Austenite Transformation
In the development of advanced steel,accurate and detailed knowledge about the kinet-ics of phase transformations and microstructure formation is critical.The critical issue in pearlite-austenite transformation is the consideration of diffusional paths of the alloy element.Simulation has been an avail-able method to study the diffusion of alloy elements and the migration rate of the phase boundary in the complex morphological evolution of austenite growth.The isothermal pearlite-austenite transformations at 720 and 740℃ in Fe-0.6%C-2%Mn(mass fraction)alloy were studied by phase-field methods based on MICRESS.At different temperatures,the effects of diffusional paths on the austenite transformation were discussed.To achieve a semiquantitative verification of the simulated results,the migration rates of the austenite/pearlite boundary at 720 and 740℃ were estimated from the experimental kinetics curves by fitting the JMA equation.By measuring the Mn profile in austenite,the modes of the austenization at 720 and 740℃ can be verified as partitioned local equilibrium(PLE)and non-partitioned local equilibrium(NPLE)modes.The heterogeneous distribution of Mn in austenite at 740℃ can be observed with STEM-EDS.However,the homogenous distribution of Mn can be found near the pearlite/austenite boundary in austenite at 720℃.The cases considering the γ,α,and interface-diffusional paths were simulated by phase-field methods to compare with the migration rates of the austenite/pearlite boundary.Because car-bon is an interstitial element in steel and has an interstitial diffusional mechanism,it can be speculated reasonably that the diffusion of C mainly proceeded in austenite and ferrite through the considerations of the atom-size of carbon and the experimental results.Phase-field methods were used to study Mn diffu-sion in the lamellar pearlite-austenite transformation.With the analysis of the experimental estimations,the interface-diffusional path was observed as the dominant path for the Mn diffusion.It is because the Mn atoms have greater diffusivity in interfaces than in γ or α-diffusional paths.Furthermore,the diffusional activation energy is closely related to the diffusivity of Mn at the interface.Moreover,compared with the γ-diffusional path,the diffusional flux of Mn in ferrite is much larger than that in austenite.Thus,it can be concluded that the contribution of the α-diffusional path to the migration rate of the pearlite/austenite boundary is larger than that of the γ-diffusional path.As a result,considering the α-diffusional path in the thermodynamics analysis under NPLE mode makes more sense.However,ignoring the interface-and α-diffusional path,which is different from the traditional cognition in PLE mode,will result in a magnitude er-ror for the thermodynamics analysis.
pearliteaustenizationphase field simulationkinetics of phase transformationelement diffusion
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