SPInodal DEcomposition Reaction in MArteNsitic Fe-X-C alloys – SPIDERMAN

The ambitious approach chosen is a combination of numerical and thermodynamical simulations and microstructural analyses, both pushed to the limit of the atomic scale. Although these two aspects are inseparable in this project, each faces its own limitations and inherent difficulties.
Regarding the simulation aspect, the heart is the project is the recently developed phase field crystal (PFC) simulation. The major advantage of this technique is that it allows taking into account elastic strain in an atomic scale modeling framework. The challenge is to develop, for the very first time, an operative PFC simulation code for binary alloys, and apply it to the Fe-C system.
On the experimental point of view, the aim is mainly to study chemically and structurally the distribution of carbon atoms down to the atomic scale. This is a very sensitive analytical issue, as detection and quantification of carbon at the atomic scale is still not fully controlled. As previously shown, APT is extremely well adapted for such a study, and it is expected that MS (Mossbauer spectrometry) and TEM will bring complementary information. It will result in a major contribution to the metrology of carbon at the nanometer scale, and provide a unique comparison between the latest generation of three dimensional atom probes and aberration corrected electron microscopes.
Concerning the experimental methodology, samples with different carbon contents, but all having Ms temperatures below room temperature will be produced. If, in the framework of this project, only structural investigations are planned, enough material will be left for mechanical tests, which could be conducted as part of a next project. Incorporation of micromechanical models would open the present project to the possibility of predicting the evolution with time of the mechanical properties of martensite, which is obviously a major industrial issue.

The first important result is the characterization by atom probe tomography and transmission electron microscopy of the homogeneous distribution of carbon in virgin martensite, obtained immediately after quench, prior to any tempering.
This result is essential for the project, as far as it confirms the initial hypothesis, that is to be able of following the evolution of the redistribution of the carbon from a homogeneous solid solution.
The second significant result is that this virgin martensite indeed evolves at room temperature, according to a progressive process, compatible with a spinodal decomposition mechanism. The experiments at the atomic scale are currently in progress, aiming at obtaining a precise kinetics of this process.
Finally, concerning more specifically the simulation aspect, the code for crystal phase field is now almost completed, which constitutes a major achievement of the project. It was succesfully applied to relatively 'simple' cases, in particular the martensitic transformation in pure iron. It can now to be applied to the iron-carbon system, the first results tend to confirm a spinodal decomposition mechanism.

The next steps of the project are, from the experimental point of view, the complete characterization at the atomic scale of the spinodal decomposition kinetics at various temperatures, between room temperature and 150°C. It is also planned to analyze overaged samples (curently under ageing), to characterize the final phases at these various temperatures. Finally, ageings on other alloys (with different compositions) are also planned, in order to determine experimentally the limits of the miscibility gap.
Regarding the modelling aspects, the next stages are to apply the crystalline field of phase code to the binary system Fe-C, and in parallel to widen the ab initio calculations to the ternary Fe-X-C system, with X=Ni and/or Mn. Finally, on the basis of ab initio data, to refine the thermodynamic mean field model in order to predict the miscibility gap limits, predictions that will be compare to experimental results.

M. Gouné F. Danoix, S. Allain, O. Bouaziz, Unambiguous carbon partitioning from martensite to austenite, Scripta Materialia 68 (2013) 1004–1007
Allain S. Danoix F., Gouné M., Hoummada K., Mangelinck D. Static and dynamical ageing, Phil Mag Letters 93 (2013) 68-76 DOI 10.108009500839.2012.742590
M. Lavrskyi, H. Zapolsky and A.G. Khachaturyan «Atomic Fragment Theory in Self-Assembly of Complex Structures: from Disorder to Complex Crystals and Double Helix Polymers»,soumis à PRL, arxiV arXiv:1411.5587v2

Steel is the structural material by far the most used by man, and its importance in the global economy is paramount. Among its various forms, martensite is the one with the highest strength. However, martensite is not stable at room temperature, and decomposes into an inhomogeneous structure consisting of Fe and C rich nanodomains. This decomposition is accompanied by an evolution of the mechanical properties of martensite. However, no comprehensive study of this phase transformation mechanism can be found in the literature.
The main objective of this project is to address this lack of basic knowledge related to martensite low temperature (T <150°C) ageing, most likely by spinodal decomposition. We propose to investigate in details the mechanism(s) of phase transformation involved, as well as the final phase(s) generated. The selected approach is modelling of the physical mechanisms by phase field crystal (PFC) simulation, benchmarked by nanoanalysis techniques at the atomic scale, such as atom probe tomography (APT) and transmission electron microscopy (TEM). The input parameters of the model, in particular pair interaction and elastic strain energies, will be derived from ab initio simulation. Microstructural investigations will ultimately validate PFC simulations of isothermal ageing kinetics.
The ultimate goal is to establish a low temperature (T<150°C) phase diagram for Fe-C based systems, including the nature of the final phase(s) and the limits of their existing domain(s), in terms both of temperature and composition of the alloys.