The Phase Field Modeling Of The Morphology And Growth Kinetics Of The Product Phase For The Austenite To Ferrite Transformation In Fe-Calloys

In the development of the new generation steel, the microstructure control in the solid-state phase transformation during the heat treatment process has become one of the most effective ways to improve the performances of steels. The austenite to ferrite transformation is one of the most important structural transitions during the cooling process of steels. The grain size and morphology of the ferrite have important influence on the mechanical properties of steels. The ferrite might have different morphologies during its precipitation from the undercooled austenite, including the grain boundary ferrite allotriomorph, massive ferrite, intragranular ferrite idiomorph, acicular Widmanst?tten ferrite and so on. Those microstructures play an important role in the hardenability and mechanical properties of steels. Therefore, the systematical analyses for the thermodynamics and kinetics of the austenite to ferrite transformation and the deeper understandings in the morphology and growth kinetics for different ferrites have important significance to study the quantitative relationship between the heat treatment process and microstructures and design the suitable chemical composition to obtain the desired performances of steels.In this paper, the morphology and growth kinetics of different ferrites have been systematically investigated by employing the phase field simulation for the austenite to ferrite transformation, where the different dominant transition factors due to the different transition environments for various ferrites have been taken into account, including the anisotropy of the kinetic parameters for grain boundary ferrite allotriomorphs, the anisotropy of the interfacial energy for intragranular ferrite idiomorphs and the anisotropy of the elastic energy for Widmanst?tten ferrites. More specifically.(1) The morphology and growth kinetics of the product phase during the austenite to ferrite transformation at grain boundaries have been simulated under the influence of both the chemical energy and isotropic interfacial energy. During the long-range diffusion transformation, the interface compositions obey the local equilibrium condition and the grain growth obeys the parabolic law. With decreasing the isothermal temperature, the growth velocity of ferrites accelerates and the parabolic growth coefficient increases. Two phenomena, interface acceleration and solute enrichment, have been studied during the impingement of two growing ferrite nucleus. During the massive transformation, the simulations have illustrated its typical transition characteristics, including the short-range carbon diffusion, the quick interface movement and the same composition in ferrite and austenite at both sides of the interface. Moreover, the anisotropy of the kinetic parameters has been taken into account for the short circuit diffusion phenomenon at grain boundaries, and then the simulated morphology becomes an ellipse, which is more consistent with experimental observations compared to the isotropic case. This clarifies that the short circuit diffusion is the key factor for determining the shape of the grain boundary ferrites.(2) The morphological evolutions of the product phase during the austenite to ferrite transformation in the interiors have been simulated under the influence of both the chemical energy and anisotropic interfacial energy. Moreover, the steady-state shape in the Allen-Cahn phase-field model(AC-PFM) including both the anisotropic interfacial energy and the isotropic driving force(ΔFmc) has been systematically investigated, where the competition between the anisotropic shrink and the isotropic growth has been discussed, and it has well explained the similarity between the steady-state shape of intragranular ferrites and the Wulff shape. Through numerical simulations and theoretical analyses, three types of evolution behaviors that depend on the strength of ΔFmc have been concluded: shrink resembling Wulff, growth resembling Wulff and growth deviating from Wulff. In other words, in the limit of ΔFmc → 0, the steady-state shape follows Wulff, which has been proven based on a numerically verified interface normal velocity model. But it deviates from Wulff when ΔFmc grows substantially, where the “critical” ΔFmc for marked deviation estimated from order analyses of the system evolution equation is very closed to the critical point for the Fisher phenomenon(ΔFFisher). Moreover, the similarity between the steady-state shape of intragranular ferrites and Wulff, which is a shape with the minimum interfacial energy, is because that the actual driving force of Fe-C alloys is far less than the theoretical critical value of the transition from similarity to dissimilarity.(3) The morphology and growth kinetics of a Widmanst?tten microstructure during the austenite to ferrite transformation have been simulated under the influence of the chemical energy, isotropic interfacial energy and anisotropic elastic energy. The simulation results suggest that the anisotropy of elastic energy, resulting from the lattice distortion between the ferrite precipitate and the austenite matrix during the phase transformation, has an important effect on the processes of the nucleation and growth of the Widmanst?tten ferrite and the evolution kinetics in different directions. Based on the principle of minimum energy, an ellipse nucleus is most possible to be produced when the volume is fixed, and the growth morphology of the ferrite precipitate would be plate-like which is consistent with the experimental one. The growth of the ferrite precipitate follows completely different kinetic laws in different directions, i.e., parabolic thickening in the direction of the plate thickness and linear lengthening in the direction toward the plate tip. The chief reason for the former is that the moving of the plate broad sides may be regarded as a migration of straight interfaces in the diffusion-controlled phase transformation; the latter is because that the plate tip can maintain a constant radius of curvature during the phase transition after a transient initial stage, which results in that the concentration gradient around the tip remains unvaried. Moreover, with the increase of undercooling or supersaturation, the driving force for the phase transition increases and the lengthening velocity accelerates.