Toward first-principles understanding of carbonitride precipitation in steels

Licentiate thesis, 2008

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.