Boron substitution induced FCC Fe/Cr23C6 interfacial strengthening: An ab initio study

Boron is added to stainless steels and polycrystalline superalloys to promote ductility, corrosion resistance, and retard creep at high temperatures (above 700 °C). However, the role of local atomic environment(s) and how boron may impact the interfacial properties between the Cr23C6 precipitate, and the matrix are not fully understood. In the present work, we consider the face-centered cubic (FCC) Fe (001)/Cr23C6 (001) interface as a model system to study how the interfacial local atomic structure and energy are affected by boron doping using first-principles density functional theory (DFT) calculations. The atomic structure(s) and energetics of a coherent interface were calculated by including the elastic contribution using linear elasticity theory combined with DFT. Boron has a significant effect on the interfacial energy and the atomic arrangement near the interface region through the increased covalent bonding, thereby ordering the interface and making any diffusion processes less favorable energetically. In turn, this may contribute to the retardation of the Cr23C6 precipitate growth and coarsening. Because of the concurrent precipitation of the NbC, Cr23C6, and Sigma phases, it affects the whole cascade of microstructural evolution processes contributing to creep. More specifically, the ancillary additions of boron to the A-terminated Cr23C6 significantly reduce the interfacial energy (∼0.09 J/m2) between the iron matrix and the Cr23C6: from ∼ 0.38 J/m2 without doping to 0.29 J/m2 with doping. This interfacial energy decrease due to boron additions is an important factor in interface strengthening and retarding creep in austenitic stainless steels. Such B-induced short-range order interface strengthening is facilitated via promoting interface covalence bonding and considerable local lattice distortion due to the atomic size misfit. These results were obtained using the new interface model described in the paper. This new interface model allows to quantitatively distinguish the highly complex asymmetric interface structures and corresponding interfacial energies and surface energies.