Light elements in iron and steel: application to Boron
Boron is a common additive element in steel production and, although the usual quantity rarely exceeds a few parts per million (ppm), it has a large impact on the mechanical properties of the material. Nevertheless, literature presents an inconsistent and often contradictory picture of boron's tendencies to alter the mechanical properties. The reason is that the actual impact on the mechanical properties depends not only on the boron concentration but also the boron distribution. The latter will be influenced by the chosen fabrication process, e.g. temperature, holding times and quenching rates, which, like the chemical composition of the steel, varies to ensure a material that is specifically tailored to exhibit required properties.
The aim of the project is the investigation of boron within bcc iron and ferritic steel via density functional theory (DFT). As boron is usually added in very small concentrations, we focus on the behaviour of boron defects within a bcc iron matrix. Furthermore, boron is known to form binary/ternary phases with other alloying elements, including iron itself. Therefore, we also investigated the iron-boron system, with foucs on the search for new phases in this system.
The investigation of boron as defect within a bcc iron matrix primary focus on the activation energies needed for boron migration within the matrix, and also on the interaction of boron with other defects within the matrix, namely a second boron, a vacancy and other light elements, which are usually found within steels. However, the situation is further complicated by the different solution behaviour of boron compared to its nearest neighbours carbon, nitrogen and oxygen, as controversely discussed in the literature: contrary to carbon etc., which prefer an interstitial solution, boron is found to be able to occupy both the substitutional and an interstitial site. The investigation will be further extended to boron defects at grain boundaries.
The investigation of the iron-boron system primary focus on the search for new, experimentally not yet observed phases. Iron-boron binary phases are often used as hard and protective coatings on steel surface, and as the hardness of borides tend to increase with increasing boron content we primary focused on the boron-rich site where according to the experimental phase diagram no phases exist, although one study reported the existence of a FeB2 phase of AlB2-type. In collaboration with Dr. Aleksey Kolmogorov (University of Oxford) we were able to identify a set of two new phases in the iron-boron system: oP12-FeB2 and oP10-FeB4. Calculated elastic moduli indicate a superior hardness of these phases compared to the currently used phases for hard coatings. Furthermore, oP12-FeB2 is expected to be the first semiconducting metal diboride while oP10-FeB4 is shown to have the potential for phonon-mediated superconductivity with a high Tc of 15-20K. Experiments are currently carried out in order to synthesize the new phases. Furthermore, efforts to calculate a phase diagram from the obtained DFT data using the CALPHAD methodology are currently ongoing.