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NONEQ.STEEL Report Summary

Project ID: 306292
Funded under: FP7-IDEAS-ERC
Country: Netherlands

Final Report Summary - NONEQ.STEEL (Controlling Non-Equilibrium in Steels)

In this project, we have discovered unexpected mechanisms affecting and controlling nucleation and growth of non-equilibrium phases in steels. Not only that, we have designed new ways to study these processes. In particular, big achievements have been reached through molecular dynamics (MD) simulations. We have developed a model to analyse nucleation events from MD simulations based on the classical nucleation theory. That analysis have led for the first time to the observation of non-classical nucleation mechanisms during homogeneous and heterogeneous nucleation events, namely step-wise nucleation and aggregation of nucleation clusters. These simulations open up an unprecedented insight on the way we understand nucleation in metals. Also MD simulations have allowed to understand the role of defects such as twin boundaries and stacking faults on the fcc-to-bcc phase transformation. Furthermore, through MD simulations we have observed simultaneous occurrence of diffusional and diffusionless phase transformation mechanisms during growth of bcc in an fcc matrix, which may explain specific morphologies observed during the growth of bcc phases in steel.

We have also investigated the formation of non-equilibrium phases in steels experimentally. With respect bainite, we have created a model for the kinetics of its formation which, for the first time, considers independently the activation energy for nucleation at austenite/austenite interfaces (grain boundary nucleation) and the activation energy for nucleation at austenite/bainitic ferrite interfaces (autocatalytic nucleation). This means that this model takes into account the physics behind the transformation kinetics and the actual morphology that bainite develops during its formation. With respect martensite, we have observed that the size of martensitic features can be reduced by thermal cycling. This grain size reduction is affected by the strengthening of austenite, the reduction of available variants and by the manner in which martensite forms. There is also a consequent increase on the strength of the material.

We have now a better understanding of the interactions between non-equilibrium phases such as martensite, bainite and austenite in steels. Results of our investigations have brought to the proposal of different ways to increase the kinetics of bainite formation in steels based on these interactions: by adding a small fraction of martensite to the structure, which contributes to the formation of bainite by the addition of new nucleation sites; and by preconditioning austenite grain boundaries with a short annealing at high temperatures. Much research has been also devoted to understand the carbon partitioning from martensite to austenite and competing processes such as carbon redistribution within martensite. Experiments have shown more carbon within the bcc phase than thermodynamically accepted. In our experiments we have found that this is caused mainly to segregation of carbon to defects. We have also proposed a new model for spinodal decomposition of ferrite in Fe-C systems accounting for the interstitial nature of carbon. The distribution of carbon in bcc controls important properties of steel such as strength, ductility and toughness and plays an important role in common processing routes such as tempering and bake hardening.

In this project we have observed that martensite/austenite microstructures, crucial in current steel developments, can be further improved for better ductility without reduction of strength by a controlled reheating in which the fractions of phases do not change. By means of high-energy synchrotron X-ray diffraction, we have observed that the strength ratio between the different phases in these microstructure plays a crucial role in the onset and rate of mechanically induced decomposition of retained austenite. These results are capable to be extended to other materials containing composite microstructures with metastable phases.

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