Toward first-principles understanding of carbonitride precipitation in steels
The most important and widely used group of metallic alloys is steels, which
are alloys of the element iron together with carbon and usually other elements
such as B, N, V, Nb and Cr. The strength in these alloys is closely connected
to the distribution of point defects, precipitates and grain boundaries which
effectively act as obstacles for dislocation migration. In the present thesis
we aim to obtain a fundamental understanding of the precipitation of transition
metal carbonitrides in steels.
In particular, we investigate the effect of boron diffusion on the coarsening
rate of M23(C,B)6 precipitates (M = transition metal) using density
functional theory (DFT) calculations. The results show that boron predominantly
enters as a substitutional solid solution in the iron matrix at low temperatures.
At elevated temperatures the interstitial population can however not be neglected
which leads to that boron diffusion under equilibrium conditions will be governed
by the interstitial mechanism. Further, the corresponding diffusion rate is shown
to be too fast in order to explain the coarsening rate of M23(C,B)6 and
it's concluded that other possible mechanisms must be explored.
In addition, the energetics for semicoherent interfaces between the iron matrix
and nacl structured MX precipitates (X = C, N) are studied using DFT in
combination with a Peierls-Nabarro model. The electronic structure at the interface
is characterized by covalent Fe(3d)-X(2p) and metallic Fe(3d)-M(d) bonds, where the
strength of the metallic interaction is connected to the relative position of the
M d-band and Fe d-band centers. In addition, it is shown that the elastic energy
contained in the dislocation network, due to the lattice misfit at the interface,
gives a significant contribution to the interface energy.
Density functional theory