Helena Zapolsky

F. Schwab, M. Lavrskyi, G. Demange, R. Patte

Groupe de Physique des Matériaux, Normandie University, UNIROUEN, INSA Rouen, CNRS, Rouen  76000, France.

Insight from the atomistic phase field modelling on fcc->bcc phase transformation and carbide formation

In recent years, significant progress has been made by using molecular dynamic and Monte Carlo modelling to study the physical properties of materials at atomic level. At the same time, recent advances in computational methodologies and massively parallel computers have made it possible to carry out the simulations containing several million of atoms. However, all these advances are not yet well suited to study slowly evolving systems with the typical diffusion time scale. Recently, the new approach, called “Quasi-particles Approach”, based on the Atomic Density Function theory, has been proposed  to study the microstructural evolution in different types of materials at diffusion time scale keeping the atomic scale resolution [1]. In this lecture, I will go back to the basics of this approach, introducing the principal equations and main assumptions. Then, I will showcase and discuss a few examples of applications of the Atomic Density Function method and Quasiparticle approach to study the displacive fcc/bcc phase transformations and the formation of different metastable carbides in the Fe-C system [2,3]. Both of these phenomena are critical for the development of advanced steels.

Displacive fcc/bcc phase transformations play a pivotal role in the physical properties of steels and ferrous alloys and inherently alter their mechanical properties, including fatigue, plasticity, and strength. At the atomic scale, we demonstrate that the Quasiparticle Approach (QA) is capable of quantitatively predicting the structure of fcc/bcc interfaces and identifying the main mechanism responsible for the propagation of this interface.

The carbides also have a decisive effect on the mechanical properties, deformation behavior, and many other properties of steel. According to the metastable phase diagram, at room temperature, one of the more stable carbide is cementite (Θ-Fe3C). However, beside cementite phase many others metastable iron carbides have been observed experimentally or predicted by theoretical considerations. Among others, the η-Fe2C carbide is observed during room temperature aging of martensite. In this lecture, I will discuss the structural transformations that occur during low-temperature aging in Fe-C martensite, leading to the appearance of metastable carbides. The connection between the η-Fe2C phase and other metastable carbides will be explored based on the Fe2n + 1Cn (n = 1, 2, 3, ...) Magnéli series.

At the end, a general discussion will be held regarding the integration of atomistic phase field models in the multiscale modeling approach.

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