Relationship between the electronic structure and the precipitation of FeTiP in interstitial-free ferritic steels

Optimal properties of modern interstitial-free ferritic steels are achieved by appropriate additions of certain elements such as Ti. The latter triggers the precipitation of carbides, sulphides and nitrides. The precipitation of FeTiP is sometimes observed but cannot be understood because of the lack of any thermochemical data. This raises the question of the unexpected stability of FeTiP relative to the other phosphides. We have performed ab initio electronic structure calculations to elucidate the origin of this stability. Our calculations show that the position of the Fermi level in the d bands of the various phosphides plays a crucial role in determining their relative stability. In the case of FeTiP, the Fermi level is situated in a region where the bonding states of the intermetallic compound are nearly filled while the antibonding states remain empty. This is quite similar to the case of the pure intermetallic compound FeTi where also the bonding states are filled while the antibonding states remain empty and the Fermi level falls in a pseudogap in the densities of states. This results in a maximum in the cohesion for this compound. Thus the increased stability of the ternary phosphide relative to those of the binary phosphides can, in part, be attributed to the formation of strong metallic bonds, and this increase in cohesion can be qualitatively explained in terms of a simple Friedel-type tight-binding model. The increased metal-P interactions also play a role owing to reduced metal-P distances in the ternary phosphide.