Toward first-principles understanding of carbonitride precipitation in steels

The most important and widely used group of metallic alloys is steels, whichare alloys of the element iron together with carbon and usually other elementssuch as B, N, V, Nb and Cr. The strength in these alloys is closely connectedto the distribution of point defects, precipitates and grain boundaries whicheffectively act as obstacles for dislocation migration. In the present thesiswe aim to obtain a fundamental understanding of the precipitation of transitionmetal carbonitrides in steels.In particular, we investigate the effect of boron diffusion on the coarseningrate of M23(C,B)6 precipitates (M = transition metal) using densityfunctional theory (DFT) calculations. The results show that boron predominantlyenters as a substitutional solid solution in the iron matrix at low temperatures.At elevated temperatures the interstitial population can however not be neglectedwhich leads to that boron diffusion under equilibrium conditions will be governedby the interstitial mechanism. Further, the corresponding diffusion rate is shownto be too fast in order to explain the coarsening rate of M23(C,B)6 andit\u27s concluded that other possible mechanisms must be explored.In addition, the energetics for semicoherent interfaces between the iron matrixand nacl structured MX precipitates (X = C, N) are studied using DFT incombination with a Peierls-Nabarro model. The electronic structure at the interfaceis characterized by covalent Fe(3d)-X(2p) and metallic Fe(3d)-M(d) bonds, where thestrength of the metallic interaction is connected to the relative position of theM d-band and Fe d-band centers. In addition, it is shown that the elastic energycontained in the dislocation network, due to the lattice misfit at the interface,gives a significant contribution to the interface energy