Theoretical studies of boron and transition-metal carbonitride additions in steels

Doctoral thesis, 2010

Martensitic 9-12%Cr steel alloys are today widely used as critical components in fossil-fired steam power plants with steam temperatures up to 600°C, where sound oxidation resistance and excellent creep strength are required. Their strength and long-term creep resistance are closely connected to the presence and stability of Cr-rich M23C6 (M = Cr, Fe) carbides and densely distributed MX precipitates (M = transition metal, X = C or N), which effectively act as obstacles for dislocation migration. In this thesis, the impact from additions of boron and transition-metal carbonitrides to 9-12%Cr steels are studied from a theoretical perspective by employing first-principles density functional theory (DFT) calculations. In the first part of this thesis we have demonstrated that boron enters predominately as substitutional solute atoms in α-Fe. However, at elevated temperatures the interstitial occupancy is not negligible and the diffusion of boron in α-Fe is under equilibrium conditions found to be governed by the interstitial mechanism. This mixed occupancy explains previous inconsistencies in experimental measurements. Further, the findings for the boron diffusion were combined with a coarsening model to address an observed beneficial impact of boron on the coarsening rate of M23C6 precipitates in 9-12%Cr steels. The outcome of the model showed that the fast interstitial diffusion in pure α-Fe was too high to explain the enhanced stability of the precipitates, whereas a substitutional mechanism, favored by vacancy generation during dissolution of M23C6, was concluded to be a possible rate-limiting effect for the coarsening process. In the second part of this thesis, we have performed a systematic investigation of the electronic and atomic structure of coherently strained and semicoherent interfaces between α-Fe and nacl-structured MX films. The coherently strained interfaces were treated directly with DFT calculations, whereas the semicoherent interfaces were address by using a Peierls-Nabarro framework. The results showed that the chemical and elastic contributions can be of equal importance for the interface energetics, and hence, it is essential to accurately include both aspects in the interface description. Furthermore, by utilizing the obtained results for Fe/MX interfaces we continued and investigated the interface energetics of coherently strained nacl-structured MN and tetragonal CrMN films in α-Fe, with the aim to improve the understanding of observed nucleation difficulties of CrMN precipitates in 9-12%Cr steel alloys. Analyzes of the chemical interactions across the interface and the elastic energy costs for the coherency strains lent support to that interface energies could provide a fundamental understanding of the early nucleation stage. Taken altogether, the results in this thesis demonstrate that first-principles DFT calculations can provide valuable information about the impact of boron and transition metal carbonitride additions in steels.